IEEE Standards - draft standard template - Mentor
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IEEE P3004.8"!/D5.012 Draft Recommended Practice for Motor Protection in Industrial and Commercial Power Systems
Sponsor
Technical Book Coordinating Committee of the IEEE Industry Applications Society
Approved
IEEE-SA Standards Board
Copyright © 2013 by the Institute of Electrical and Electronics Engineers, Inc.
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New York, New York 10016-5997, USA
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This document is an unapproved draft of a proposed IEEE Standard. As such, this document is subject to change. USE AT YOUR OWN RISK! Because this is an unapproved draft, this document must not be utilized for any conformance/compliance purposes. Permission is hereby granted for IEEE Standards Committee participants to reproduce this document for purposes of international standardization consideration. Prior to adoption of this document, in whole or in part, by another standards development organization, permission must first be obtained from the IEEE Standards Activities Department (stds.ipr@ieee.org). Other entities seeking permission to reproduce this document, in whole or in part, must also obtain permission from the IEEE Standards Activities Department.
IEEE Standards Activities Department
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Abstract: This recommended practice covers the protection of motors used in industrial and commercial power systems. It is likely to be of greatest value to the power-oriented engineer with limited experience in the area of protection and control. It can also be an aid to all engineers responsible for the electrical design of industrial and commercial power systems.
Keywords:
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Introduction
This introduction is not part of IEEE P3004.8/D5.012, Draft Recommended Practice for Motor Protection in Industrial and Commercial Power Systems.
This document is an update to IEEE 242-2001 (Buff Book) chapter 10. This recommended practice covers motor protection in industrial and commercial power systems. This document does not cover health care facilities (refer to the IEEE White Book).
Notice to users
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Current interpretations can be accessed at the following URL: HYPERLINK "http://standards.ieee.org/reading/ieee/interp/index.html"http://standards.ieee.org/reading/ieee/interp/ index.html.
Patents
[If the IEEE has not received letters of assurance prior to the time of publication, the following notice shall appear:]
Attention is called to the possibility that implementation of this recommended practice may require use of subject matter covered by patent rights. By publication of this recommended practice, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE is not responsible for identifying Essential Patent Claims for which a license may be required, for conducting inquiries into the legal validity or scope of Patents Claims or determining whether any licensing terms or conditions provided in connection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonable or non-discriminatory. Users of this recommended practice are expressly advised that determination of the validity of any patent rights, and the risk of infringement of such rights, is entirely their own responsibility. Further information may be obtained from the IEEE Standards Association.
[The following notice shall appear when the IEEE receives assurance from a known patent holder or patent applicant prior to the time of publication that a license will be made available to all applicants either without compensation or under reasonable rates, terms, and conditions that are demonstrably free of any unfair discrimination.]
Attention is called to the possibility that implementation of this recommended practice may require use of subject matter covered by patent rights. By publication of this recommended practice, no position is taken with respect to the existence or validity of any patent rights in connection therewith. A patent holder or patent applicant has filed a statement of assurance that it will grant licenses under these rights without compensation or under reasonable rates, with reasonable terms and conditions that are demonstrably free of any unfair discrimination to applicants desiring to obtain such licenses. Other Essential Patent Claims may exist for which a statement of assurance has not been received. The IEEE is not responsible for identifying Essential Patent Claims for which a license may be required, for conducting inquiries into the legal validity or scope of Patents Claims, or determining whether any licensing terms or conditions provided in connection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonable or non-discriminatory. Users of this recommended practice are expressly advised that determination of the validity of any patent rights, and the risk of infringement of such rights, is entirely their own responsibility. Further information may be obtained from the IEEE Standards Association.
Participants
At the time this draft recommended practice was submitted to the IEEE-SA Standards Board for approval, the Protection and Coordination Working Group of the technical Books Coordinating Committee of the Industrial and Commercial Power Systems Department of the Industry Applications Society had the following membership:
DOCVARIABLE "varWkGrpChair" \* MERGEFORMAT , Chair
DOCVARIABLE "varWkGrpViceChair" \* MERGEFORMAT , Vice Chair
Participant1
Participant2
Participant3
Participant4
Participant5
Participant6
Participant7
Participant8
Participant9
The following members of the balloting committee voted on this DOCVARIABLE "txtTrialUse" \*Lower \* MERGEFORMAT DOCVARIABLE "txtGorRPorSTD" \*Lower \* MERGEFORMAT . Balloters may have voted for approval, disapproval, or abstention.
(to be supplied by IEEE)
Balloter1
Balloter2
Balloter3
Balloter4
Balloter5
Balloter6
Balloter7
Balloter8
Balloter9
When the IEEE-SA Standards Board approved this recommended practice on , it had the following membership:
(to be supplied by IEEE)
, Chair
, Vice Chair
, Past President
, Secretary
SBMember1
SBMember2
SBMember3
SBMember4
SBMember5
SBMember6
SBMember7
SBMember8
SBMember9
*Member Emeritus
Also included are the following nonvoting IEEE-SA Standards Board liaisons:
, NRC Representative
, DOE Representative
, NIST Representative
IEEE Standards Program Manager, Document Development
IEEE Standards Program Manager, Technical Program Development
Contents
TOC \t "Heading 1,1,Heading 2,2,Heading 3,3,IEEEStds Level 1 Header,1,IEEEStds Level 4 Header,4,IEEEStds Level 3 Header,3,IEEEStds Level 2 Header,2" 1. Scope PAGEREF _Toc395025685 \h 1
2. Normative references PAGEREF _Toc395025686 \h 1
3. Definitions, abbreviations, and acronyms PAGEREF _Toc395025687 \h 3
3.1 Definitions PAGEREF _Toc395025688 \h 3
3.2 Acronyms and abbreviations PAGEREF _Toc395025689 \h 5
4. General discussion PAGEREF _Toc395025690 \h 7
4.1 Low-voltage systems PAGEREF _Toc395025691 \h 8
4.2 Medium-voltage systems PAGEREF _Toc395025692 \h 8
5. Factors to consider in protection of motors PAGEREF _Toc395025693 \h 8
5.1 Motor characteristics PAGEREF _Toc395025694 \h 9
5.2 Motor-starting conditions PAGEREF _Toc395025695 \h 11
5.2.1 Solid-state reduced-voltage motor start PAGEREF _Toc395025696 \h 13
5.2.2 Autotransformer start PAGEREF _Toc395025697 \h 15
5.2.3 Primary resistor or reactor start PAGEREF _Toc395025698 \h 15
5.2.4 Part-winding start PAGEREF _Toc395025699 \h 15
5.2.5 Wye-delta start PAGEREF _Toc395025700 \h 15
5.2.6 Multi-speed motor start PAGEREF _Toc395025701 \h 17
5.3 Ambient conditions PAGEREF _Toc395025702 \h 17
5.4 Driven equipment PAGEREF _Toc395025703 \h 17
5.5 Power system quality PAGEREF _Toc395025704 \h 18
5.6 Motor importance PAGEREF _Toc395025705 \h 18
5.7 Load-side faults for motor controllers PAGEREF _Toc395025706 \h 1819
5.8 Ground faults PAGEREF _Toc395025707 \h 19
5.9 Maintenance capability and schedule PAGEREF _Toc395025708 \h 20
5.9.1 Continuous monitoring (trending) PAGEREF _Toc395025709 \h 20
5.9.2 Periodic monitoring (trending) PAGEREF _Toc395025710 \h 20
5.10 Service factor PAGEREF _Toc395025711 \h 20
5.11 Application considerations PAGEREF _Toc395025712 \h 21
5.12 Motor and conductor protection PAGEREF _Toc395025713 \h 22
6. Types of protection PAGEREF _Toc395025714 \h 23
6.1 Purpose of motor protection PAGEREF _Toc395025715 \h 23
6.2 Abnormal power supply conditions (undervoltage protection) PAGEREF _Toc395025716 \h 2728
6.2.1 Undervoltage (Device 27) PAGEREF _Toc395025717 \h 2728
6.2.2 Device 27, instantaneous or time delay PAGEREF _Toc395025718 \h 2728
6.2.3 Device 27, with latching contactor or circuit breaker PAGEREF _Toc395025719 \h 2829
6.2.4 Device 27, with ac magnetically held main contactor PAGEREF _Toc395025720 \h 2829
6.2.5 Device 27, with dc magnetically held main contactor PAGEREF _Toc395025721 \h 2930
6.2.6 Device 27, with voltage-sensing relays PAGEREF _Toc395025722 \h 2930
6.3 Phase unbalance protection (Device 46, current) (Device 47, voltage) )(Device 60) PAGEREF _Toc395025723 \h 2930
6.3.1 Devices 46, 47,& 60;purpose PAGEREF _Toc395025724 \h 2930
6.3.2 Devices 46, 47,& 60;single phasing PAGEREF _Toc395025725 \h 3031
6.3.3 Devices 46, 47,& 60;instantaneous or time delay PAGEREF _Toc395025726 \h 3132
6.3.4 Devices 46, 47,& 60; relays PAGEREF _Toc395025727 \h 3132
6.4 Overcurrent protection (Device 51, inverse time) (Device 50, instantaneous) PAGEREF _Toc395025728 \h 3233
6.5 Ground Fault Relay (Device 64) PAGEREF _Toc395025729 \h 3233
6.6 Underexcitation (loss of field) protection (Device 40) PAGEREF _Toc395025730 \h 3233
6.7 Over excitation (volts/hertz) protection (Device 24) PAGEREF _Toc395025731 \h 3233
6.8 Bearing Protection Relay (Device 38) PAGEREF _Toc395025732 \h 3334
6.9 Mechanical Condition Protection Relay (Device 39) PAGEREF _Toc395025733 \h 3334
6.10 Thermal and electronic overload protection (Device 49) PAGEREF _Toc395025734 \h 3334
6.11 Multifunction relay (Device 11) PAGEREF _Toc395025735 \h 3435
6.11.1 Device 11, MV multifunction relay applications PAGEREF _Toc395025736 \h 3536
6.11.2 Device 11, LV multifunction relay applications PAGEREF _Toc395025737 \h 3637
7. Low-voltage motor protection PAGEREF _Toc395025738 \h 38
7.1 Low-voltage motor overcurrent protection PAGEREF _Toc395025739 \h 39
7.1.1 Thermal and electronic overload relays PAGEREF _Toc395025740 \h 39
7.1.2 Time-delay (or dual-element) fuses PAGEREF _Toc395025741 \h 40
7.1.3 Inverse-time circuit breakers PAGEREF _Toc395025742 \h 40
7.1.4 Instantaneous trip circuit breakers PAGEREF _Toc395025743 \h 40
7.2 Low-voltage motor ground-fault protection PAGEREF _Toc395025744 \h 42
7.2.1 Solidly grounded systems PAGEREF _Toc395025745 \h 4243
7.2.2 Low-resistance-grounded systems PAGEREF _Toc395025746 \h 4243
7.2.3 High-resistance-grounded systems PAGEREF _Toc395025747 \h 43
7.3 Low-voltage motor stator winding over temperature (Device 49S) PAGEREF _Toc395025748 \h 43
7.3.1 Thermistor winding over-temperature devices PAGEREF _Toc395025749 \h 4344
7.3.2 Resistance temperature detector (RTD) winding over-temperature devices PAGEREF _Toc395025750 \h 44
7.4 Low-voltage motor undervoltage protection PAGEREF _Toc395025751 \h 44
7.4.1 Undervoltage relays PAGEREF _Toc395025752 \h 4445
7.4.2 Undervoltage sensors for circuit breakers PAGEREF _Toc395025753 \h 4445
8. Medium-voltage motor protection PAGEREF _Toc395025754 \h 4445
8.1 MV motor thermal overload protection (Device 49) PAGEREF _Toc395025755 \h 4850
8.2 MV motor overcurrent protection PAGEREF _Toc395025756 \h 4951
8.3 Fault protection PAGEREF _Toc395025757 \h 5456
8.3.1 Motor current differential element (Device 87M) PAGEREF _Toc395025758 \h 5456
8.3.1.1 Device 87M, Conventional phase differential overcurrent relay PAGEREF _Toc395025759 \h 5456
8.3.1.2 Device 87M, Self-balancing differential using window CTs PAGEREF _Toc395025760 \h 5557
8.3.2 Device 87M, split-winding current unbalance PAGEREF _Toc395025761 \h 5658
8.3.2.1 Purpose PAGEREF _Toc395025762 \h 5658
8.3.2.2 Arrangement of CTs and relays PAGEREF _Toc395025763 \h 5658
8.3.2.3 Evaluation of split-winding current unbalance protection PAGEREF _Toc395025764 \h 5658
8.3.2.4 Application of split winding protection PAGEREF _Toc395025765 \h 5759
8.3.3 Ground-fault protection (Devices 50G, 51G, 50N, 51N) PAGEREF _Toc395025766 \h 5759
8.3.3.1 Purpose PAGEREF _Toc395025767 \h 5759
8.3.3.2 Device 50G, Instantaneous ground-fault protection PAGEREF _Toc395025768 \h 5759
8.3.3.3 Device 51G, time-overcurrent ground-fault protection PAGEREF _Toc395025769 \h 5860
8.3.3.4 Installation of cable for ground-fault protection PAGEREF _Toc395025770 \h 5860
8.3.3.5 Device 51N, residually connected CTs and ground-fault relay PAGEREF _Toc395025771 \h 5860
8.3.3.6 Selection of resistor for low-resistance system grounding PAGEREF _Toc395025772 \h 5961
8.4 Monitors PAGEREF _Toc395025773 \h 5961
8.4.1 Stator winding over temperature (Device 49S) PAGEREF _Toc395025774 \h 5961
8.4.1.1 Device 49S, RTDs PAGEREF _Toc395025775 \h 5962
8.4.1.2 Device 49S, Thermocouples PAGEREF _Toc395025776 \h 6163
8.4.1.3 Device 49S, Thermistors PAGEREF _Toc395025777 \h 6264
8.4.1.4 Device 49S, Thermostats and temperature bulbs PAGEREF _Toc395025778 \h 6264
8.4.1.5 Application of stator winding temperature protection PAGEREF _Toc395025779 \h 6365
8.4.2 Rotor over temperature (Device 49R) PAGEREF _Toc395025780 \h 6365
8.4.2.1 Synchronous motors PAGEREF _Toc395025781 \h 6365
8.4.2.2 Wound-rotor induction motor-starting resistors PAGEREF _Toc395025782 \h 6365
8.4.3 Mechanical and other protection PAGEREF _Toc395025783 \h 6365
8.4.3.1 Motor bearing and lubricating systems PAGEREF _Toc395025784 \h 6365
8.4.3.2 Ventilation and cooling systems PAGEREF _Toc395025785 \h 6466
8.4.3.3 Liquid detectors PAGEREF _Toc395025786 \h 6567
8.4.3.4 Fire detection and protection PAGEREF _Toc395025787 \h 6567
8.4.3.5 Partial discharge detectors PAGEREF _Toc395025788 \h 6567
8.4.3.6 Online motor stator insulation monitoring PAGEREF _Toc395025789 \h 6668
8.4.4 Vibration monitors, sensors, and machinery protection systems PAGEREF _Toc395025790 \h 6668
8.4.4.1 Purpose of vibration monitoring PAGEREF _Toc395025791 \h 6668
8.4.4.2 Transducers PAGEREF _Toc395025792 \h 6769
8.4.4.3 Proximity transducers PAGEREF _Toc395025793 \h 6769
8.4.4.4 Monitors PAGEREF _Toc395025794 \h 7173
8.4.4.5 Diagnostic systems PAGEREF _Toc395025795 \h 7274
8.5 Synchronous motor protection PAGEREF _Toc395025796 \h 7476
8.5.1 Amortisseur (Damper) winding protection PAGEREF _Toc395025797 \h 7577
8.5.2 Field-current failure protection PAGEREF _Toc395025798 \h 7678
8.5.3 Excitation voltage availability PAGEREF _Toc395025799 \h 7779
8.5.4 Pullout protection (Device 55) PAGEREF _Toc395025800 \h 7779
8.5.5 Incomplete starting sequence (Device 48) PAGEREF _Toc395025801 \h 7779
8.5.5.1 Operation indicator for protection devices PAGEREF _Toc395025802 \h 7779
8.5.5.2 Induction motor protection PAGEREF _Toc395025803 \h 7779
8.6 Protection against excessive starting (48, 51LR, 49, 66) PAGEREF _Toc395025804 \h 7880
8.7 Rotor winding protection PAGEREF _Toc395025805 \h 7880
8.7.1 Synchronous motors PAGEREF _Toc395025806 \h 7880
8.7.2 Wound-rotor induction motors PAGEREF _Toc395025807 \h 7981
8.8 Lightning and surge protection PAGEREF _Toc395025808 \h 7981
8.8.1 Types of protection PAGEREF _Toc395025809 \h 7981
8.8.2 Locations of surge protection PAGEREF _Toc395025810 \h 8082
8.8.3 Application of surge protection PAGEREF _Toc395025811 \h 8183
8.9 Protection against overexcitation from shunt capacitance PAGEREF _Toc395025812 \h 8183
8.9.1 Nature of problem PAGEREF _Toc395025813 \h 8183
8.9.2 Protection PAGEREF _Toc395025814 \h 8284
8.10 Protection for automatic reclosing and automatic transfer PAGEREF _Toc395025815 \h 8385
8.10.1 Nature of problem PAGEREF _Toc395025816 \h 8385
8.10.2 Protection PAGEREF _Toc395025817 \h 8385
8.11 Protection against excessive shaft torques PAGEREF _Toc395025818 \h 8486
8.12 Protection against excessive shaft torques developed during transfer of motors between out-of-phase sources PAGEREF _Toc395025819 \h 8486
8.13 Protection against failure to rotate PAGEREF _Toc395025820 \h 8587
8.13.1 Failure to rotate PAGEREF _Toc395025821 \h 8587
8.13.2 Reverse rotation or loss of phase PAGEREF _Toc395025822 \h 8587
9. Protection for AC ASD applications PAGEREF _Toc395025823 \h 8789
9.1 ASD general information PAGEREF _Toc395025824 \h 8789
9.1.1 ASD terminology PAGEREF _Toc395025825 \h 8789
9.1.2 NEMA MG-1 Part 30 and 31 inverter duty PAGEREF _Toc395025826 \h 8789
9.1.3 Selecting drives PAGEREF _Toc395025827 \h 8890
9.1.4 ASD bypass circuit protection PAGEREF _Toc395025828 \h 8991
9.1.5 ASD output filters and reactors PAGEREF _Toc395025829 \h 9193
9.1.6 Overtemperature and overload protection PAGEREF _Toc395025830 \h 9193
9.1.7 Regeneration and dynamic braking PAGEREF _Toc395025831 \h 9395
9.1.8 Protection device monitoring by auxiliary control equipment PAGEREF _Toc395025832 \h 9395
9.2 Low voltage AC ASD motor protection PAGEREF _Toc395025833 \h 9395
9.2.1 Low voltage ASD bypass motor protection PAGEREF _Toc395025834 \h 9496
9.2.2 LV ASD multiple motor applications PAGEREF _Toc395025835 \h 9496
9.3 MV AC ASD motor protection PAGEREF _Toc395025836 \h 9496
9.3.1 MV protection PAGEREF _Toc395025837 \h 9496
9.3.2 Protection commonly included in ASD PAGEREF _Toc395025838 \h 9496
9.3.3 Zone 1 protection PAGEREF _Toc395025839 \h 9799
9.3.4 Zone 2 protection PAGEREF _Toc395025840 \h 98100
9.3.5 Zone 3 protection PAGEREF _Toc395025841 \h 99101
9.3.5.1 Zone 3 overcurrent protection PAGEREF _Toc395025842 \h 100102
9.3.5.2 Zone 3 single-phase input protection PAGEREF _Toc395025843 \h 101103
9.3.5.3 Zone 3 ground-fault protection PAGEREF _Toc395025844 \h 101103
9.3.5.4 Zone 3 CTs and relay harmonics PAGEREF _Toc395025845 \h 101103
9.3.5.5 Zone 3 motor differential protection PAGEREF _Toc395025846 \h 101103
9.3.5.6 Zone 3 switching protection (snubber circuits) PAGEREF _Toc395025847 \h 101103
9.3.6 Additional factors that impact protection PAGEREF _Toc395025848 \h 102104
9.3.7 Multiple motor applications PAGEREF _Toc395025849 \h 104106
9.3.8 Shaft voltage and bearing currents and common mode voltages PAGEREF _Toc395025850 \h 104106
9.3.9 Partial discharge PAGEREF _Toc395025851 \h 105107
10. DC motor protection PAGEREF _Toc395025852 \h 105108
11. DC ASD motor protection PAGEREF _Toc395025853 \h 105108
12. Motor protection for hazardous (classified) locations PAGEREF _Toc395025854 \h 105108
12.1 Class I, Division 2, and Class I, Zone 2 areas PAGEREF _Toc395025855 \h 105108
12.2 Class II areas PAGEREF _Toc395025856 \h 106108
Annex A Bibliography PAGEREF _Toc395025857 \h 107109
Annex B IEEE Device designations PAGEREF _Toc395025858 \h 110112
B.1 Typical motor protection device function numbers PAGEREF _Toc395025859 \h 110112
B.2 Main device letters PAGEREF _Toc395025860 \h 114116
B.2 Other suffix letters PAGEREF _Toc395025861 \h 115117
B.3 Auxiliary Devices PAGEREF _Toc395025862 \h 115117
B.5 Auxiliary Contact Position Definitions PAGEREF _Toc395025863 \h 116118
Annex C Protection setting considerations (informative) PAGEREF _Toc395025864 \h 117119
C.1 Typical motor protection settings PAGEREF _Toc395025865 \h 117119
C.2 Current unbalance and ground fault protection in HRG system PAGEREF _Toc395025866 \h 117119
C.3 Overcurrent protection in fixed capacitor applications PAGEREF _Toc395025867 \h 117119
Annex D Motor Protection Examples PAGEREF _Toc395025868 \h 118120
D.1 LV Instantaneous breaker example PAGEREF _Toc395025869 \h 118120
D.2 LV ASD example, PAGEREF _Toc395025870 \h 118120
D.3 MV fused contactor example PAGEREF _Toc395025871 \h 118120
D. 4 MV breaker start example PAGEREF _Toc395025872 \h 118120
D.5 MV ASD example PAGEREF _Toc395025873 \h 118120
Annex E Partial discharge (informative) PAGEREF _Toc395025874 \h 120122
E.1 PD background PAGEREF _Toc395025875 \h 120122
E.2 PD sensor options PAGEREF _Toc395025876 \h 120122
E.3 HFCT sensor installation PAGEREF _Toc395025877 \h 122124
E.4 Peak PD level and PD activity guidelines PAGEREF _Toc395025878 \h 123125
E.4 Cable PD PAGEREF _Toc395025879 \h 126128
Draft Recommended Practice for Motor Protection in Industrial and Commercial Power Systems
IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or environmental protection in all circumstances. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements.
This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading Important Notice or Important Notices and Disclaimers Concerning IEEE Documents. They can also be obtained on request from IEEE or viewed at HYPERLINK "http://standards.ieee.org/IPR/disclaimers.html" http://standards.ieee.org/IPR/disclaimers.html.
Scope
This recommended practice covers the protection of motors used in industrial and commercial power systems. It is likely to be of greatest value to the power-oriented engineer with limited experience in the area of protection and control. It can also be an aid to all engineers responsible for the electrical design of industrial and commercial power systems.
Normative references
The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies.
API Std 541-2004, Form-Wound Squirrel Cage Induction Motors500 Horsepower and Larger, 4th Edition.
API Std 546-2008, Brushless Synchronous Machines500 kVA and Larger, 3rd Edition.
API Std 547-2005, General-Purpose Form-Wound Squirrel Cage Induction Motors
250 Horsepower and Larger, 1st Edition.
IEC 60947-4-1-20002012, Low-Voltage Switchgear and Controlgear, Part 4-1: Contactors and Motor- Starters, Section OneElectromechanical Contactors and Motor-Starters.
IEEE Std 112TM-2004, IEEE Standard Test Procedure for Polyphase Induction Motors and Generators
IEEE Std 115- 115-2009 - IEEE Guide for Test Procedures for Synchronous Machines Part IAcceptance and Performance Testing Part IITest Procedures and Parameter Determination for Dynamic AnalysisIEEE Std 141-1993 (Reaff 1999), IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (IEEE Red Book).
IEEE Std 241-1990 (Reaff 1997), IEEE Recommended Practice for Electric Power Systems in Commercial Buildings (IEEE Gray Book).
IEEE Std 242-2001 IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
IEEE Std 519TM-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems
IEEE Std 620TM-1996 (Reaff 2008), IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines
IEEE Std 841-2009 IEEE Standard for Petroleum and Chemical IndustryPremium Efficiency, Severe-Duty, Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors Up to and Including 370 kW (500 hp)
IEEE Std 1015-2006, IEEE Recommended Practice for Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems (IEEE Blue Book).
IEEE Std. 1349-2011, IEEE Guide for the Application of Electric Motors in Class I, Division 2 and Class I, Zone 2 Hazardous (Classified) Locations
IEEE Std. 3004.1- , IEEE Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems
IEEE Std. 3004
..
IEEE Std C37.06TM-2009, IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current BasisPreferred Ratings and Related Required Capabilities for Voltages Above 1000 V.
IEEE Std C37.13 TM-2008, IEEE Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures.
IEEE Std C37.16 TM-2009, IEEE Standard for Preferred Ratings, Related Requirements, and Application Recommendations for Low-Voltage AC (635 V and below) and DC (3200 V and below) Power Circuit Breakers.
IEEE Std C37.17-1997, Trip Devices for AC and General Purpose DC Low-Voltage Power Circuit Breakers.
IEEE Std C37.2-2008, IEEE Standard Electrical Power System Device Function Numbers and Contact Designations.
IEEE Std C37.46TM-2010, IEEE Specifications for High-Voltage (>1000 V) Expulsion and Current-Limiting Power Class Fuses and Fuse Disconnecting Switches.
IEEE Std C37.96-2012, IEEE Guide for AC Motor Protection.
IEEE Std C37.110-2007 - IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes
IEEE Std C62.21TM-2003 (Reaff 2009), IEEE Guide for the Application of Surge Voltage Protective Equipment on AC Rotating Machinery 1000 V and Greater.NEMA ICS 2-2000, Industrial Control and Systems Controllers, Contactors and Overload Relays Rated 600 Volts.
NEMA MG 1-2011), Motors and Generators.
NFPA 20-2013, Standard for the Installation of Stationary Pumps for Fire Protection.
NFPA 70-2014, National Electrical Code® (NEC®).
Definitions, abbreviations, and acronyms
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary: Glossary of Terms & Definitions should be referenced for terms not defined in this clause. [BB46]
Definitions
abnormal operating condition: As applied to motors, including, but not limited to, starting, locked rotor, voltage unbalance, overload, and short-circuit. As applied to equipment in classified locations, equipment failure is considered to be an abnormal operating condition.
adjustable speed drive: an electric drive designed to provide easily operable means for speed adjustment of the motor, within a specified speed range. (See IEEE 100)
ambient temperature: Ambient temperature is the temperature of the surrounding cooling medium, such as gas or liquid, which comes into contact with the heated parts of the apparatus (See NEMA MG-1)
approved: Acceptable to the authority having jurisdiction.
autoignition temperature (AIT): The minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid, or gas independently of the heating or heated element.
Class B rise: Based on a maximum 40°ðC ambient, a motor stator temperature rise at 1.0 service factor of 80°ðC (measured by resistance) or 80°ðC, 85°ðC, or 90°ðC (measured by embedded detectors) in accordance with NEMA MG-1-2011, depending on the motor size, motor type, enclosure type, and voltage rating. The rise at 1.0 service factor corresponds to Class B type of insulation system in the NEMA MG-1-2011 temperature rise tables.
Common Mode Voltage: In the context of Adjustable Speed Drives, Common Mode Voltage (CMV) is the displacement of the neutral point (and each phase voltage) of the ASD output from ground due to the switching of the solid state devices in the drive. It is an alternating voltage whose magnitude and frequency components are dependent on the drive topology. All present drive topologies create CMV to some extent. CMV can also be created at the motor if phase circuit conductors, unsymmetrical with respect to the equipment grounding conductor(s) or grounded sheaths or raceways, are used between the ASD output and the motor.
continuous duty: operation at a substantially constant load for an indefinitely long time. This is also known as continuous rating in NEMA MG-1.
corona: A type of localized discharge resulting from transient gaseous ionization on an insulation system when the voltage stress exceeds a critical value. The ionization is usually localized over a portion of the distance between the electrodes of the system. (Corona activity can result in surface discharges and surface tracking on motor windings.) Corona is visible partial discharges in gases adjacent to a conductor. (See IEEE 1434 [B51])
explosionproof equipment: Equipment enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.
exposed surface: A surface that is internal to an enclosure or an external surface of an enclosure which could be exposed to the surrounding flammable atmosphere, without the benefit of an enclosure that would contain an explosion or exclude the hazardous gas. (An exposed internal surface may be the rotor, stator, or space heater surfaces of open and TEFC motors. An exposed external surface is the exterior surface, which could be exposed to the surrounding flammable atmosphere such as the exterior surface of explosionproof, pressurized, or force ventilated enclosures.)
high-inertia load: A load that has a moment of inertia that exceeds normal values, as described in NEMA MG-1, and for which the motor needs to be designed to have both thermal and mechanical capability to accelerate the load to operating speed without exceeding its limits. (See NEMA MG-1-2011 Tables 12-7, 21-6, & 20-1)
identified: (as applied to equipment). Recognizable as suitable for the specific purpose, function, use, environment, application, and so forth, where described in a particular Code requirement.
IC Code: The IC Code designates the method of electrical machine cooling as described in NEMA MG-1, Part 6.
IP Code: The IP Code designates the degree of protection provided by the enclosure of a rotating machine as described in NEMA MG-1, Part 5.
listed: Equipment, materials, or services included in a list published by an organization that is acceptable to the authority having jurisdiction and concerned with evaluation of products or services, that maintains periodic inspection of production of listed equipment or materials or periodic evaluation of services, and whose listing states that either the equipment, material, or service meets appropriate designated standards or has been tested and found suitable for a specified purpose.
Motor stator insulation Monitoring: (MSIM) A monitoring system that utilizes a specialized high sensitivity current transformer (HSCT) to perform an online (while the motor is in service and the process is running) measurement and trending of motor insulation condition and degradation, eliminating or reducing the need for offline insulation testing.
multisection motor: A motor whose construction utilizes a component block approach in the assembly of the enclosure, that is, the enclosure has a number of bolted joints which could connect together the stator frame, the ventilation hood, the motor base, the bearing supports, and enclosure covers.
NEMA frame: This refers to the NEMA MG-1 system of a standardized frame designation for AC machines including 449 frame size and smaller. NEMA MG-1-2009 (Revision 1-2010)2011, Part 4 provides critical mounting dimensions for each frame size.
normal operating condition: As applied to motors, a normal operating condition is operating at rated full-load steady state conditions.11 Locked-rotor, starting, single-phasing, and operating above base nameplate kilowatt or horsepower are not normal operating conditions.
overload: Loading in excess of normal rating of equipment. For a motor, it is considered overloaded when operated above its base nameplate kilowatt or horsepower.
partial discharge: A localized electric discharge resulting from ionization in an insulation system when the voltage stress exceeds the critical value. This discharge partially bridges the insulation in the voids internal to the motor winding insulation.
per unit torque (as applied in Annex H): Per unit torque is the test value of load torque divided by the motor rated torque at nameplate rated conditions, such as rated voltage and rated frequency.
service factor: A multiplier that, when applied to the rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor.
spark: A sudden and irreversible transition from a stable corona discharge to a stable arc discharge. It is a luminous electrical discharge of short duration between two electrodes in an insulating medium. It is generally brighter and carries more current than corona, and its color is mainly determined by the type of insulating medium. It generates radio noise of wider frequency spectrum (extending into hundreds of megahertz) and wider magnitude range than corona. A spark is not classified as corona. Sparking can also include static discharge, sparking due to mechanical contact, and capacitive discharges (ie. across bearing oil film and separating switch contacts).
Acronyms and abbreviations
AFD adjustable frequency drive (ASD is the IEEE preferred term)
AHJ authority having jurisdiction
AIT autoignition temperature
API American Petroleum Institute
ASD adjustable speed drive
ASTM American Society for Testing and Materials
CEC Canadian Electrical Code
CENELEC European Committee for Electrotechnical Standardization
CMV Common Mode Voltage
CSA Canadian Standards Association International
DCS Distributed Control System
DPFV Drip-Proof Forced Ventilated
FLC full-load current
FLT full-load torque
FPN Fine Print Note (formerly used in the National Electrical Code®)
HDO High-drop out
HRG High Resistance Ground
HSCT high sensitivity current transformer
IC IC Code
IEC International Electrotechnical Commission
IP IP Code
LCI Load Commutated Inverter
LFL lower flammable limit
LRC locked rotor current
LVPCB low-voltage power circuit breaker
MCCB molded-case circuit breaker
MESG maximum experimental safe gap
MIC minimum igniting current
MIE minimum ignition energy
MOV metal oxide varistor
MSIM Motor stator insulation Monitoring
NEC National Electrical Code
NEMA National Electrical Manufacturers Association
NFPA National Fire Protection Association
NRTL Nationally Recognized Testing Laboratory
NTC negative temperature coefficient resistors
ODE opposite drive end
ODP open dripproof
OEM original equipment manufacturer
OLPD on-line partial discharge
pu per unit
PTC positive temperature coefficient resistors
PWM pulse-width modulation
RP recommended practice
RPM revolutions per minute
RTD resistance temperature detector
SCR silicon controlled rectifier
SF service factor
T Code Temperature Code or Identification Number per 2014 NEC Table 500.8(C)
TEAAC totally enclosed air-to-air cooled
TEFC totally enclosed fan cooled
TEFV totally enclosed force-ventilated
TENV totally enclosed nonventilated
TEPV totally enclosed pipe-ventilated
TEWAC totally enclosed water-to-air cooled
TFE tetrafluoroethylene
UFL upper flammable limit
UL Underwriters Laboratories Inc.
VFD Variable Frequency Drive (ASD is the IEEE preferred term)
VSD Variable Speed Drive (ASD is the IEEE preferred term)
WPI weather protected Type I
WPII weather protected Type II
General discussion
This recommended practice applies specifically to three-phase integral horsepower motors. Many factors should be considered in choosing motor protection: motor importance, motor rating (from one to several thousand horsepower), thermal limit of rotor or stator, environment, power system source and its neutral grounding method, type of motor controller, etc. Protection for each specific motor installation should meet the requirements of the application. Power quality of the plant distribution system should be given appropriate attention, especially with regard to voltage sags and surges, harmonics, service interruptions, and operation of distribution line reclosers. Items in Clause 5 and Clause 6 should be considered as checklists when deciding upon protection for a given motor installation. After the types of protection have been selected, manufacturers bulletins should be studied to ensure proper application of the specific protection chosen.
Low-voltage systems
Low-voltage (LV) systems are nominally 1000 V or less. NEMA MG-1 lists the standard motor nameplate ratings along with the preferred size limits for several standard motor voltages. At present, a maximum of 575 V and 750 kW (1000 hp) exists for motor nameplate ratings. Table 1 below shows typical LV motor ratings for various system voltages.
Nameplate voltage ratings of standard LV induction motors
Nominal System VoltageTypical Nameplate VoltageSingle-phase Motors
120 V
240 V
115 V
230 VThree-phase Motors
208 V
240 V
480 V
600 V
200 V
230 V
460 V
575 V
Medium-voltage systems
Medium-voltage (MV) systems range above 1000 V and up to 69 kV. Industrial and commercial power systems typically operate with distribution voltages of 2.4 kV, 4.16 kV, 6.9 kV, and 13.8 kV and above. The selection of the motor voltages is described in Clause 3.4 of IEEE Std 141-1993. Table 2 below shows typical MV motor ratings for various system voltages.
Nameplate voltage ratings of standard MV induction motors
Nominal System VoltageTypical Nameplate VoltageThree-phase Motors
2400 V
4160 V
6900 V
13.8 kV
2300 V
4000 V
6600 V
13.2 kV
Factors to consider in protection of motors
The factors in 5.1 through 5.11 should be considered when selecting motor protection.
Motor characteristics
Motor characteristics include type, speed, voltage, horsepower rating, service factor, NEMA design (i.e., A, B, C, or D, which are the torque and speed characteristics for low- and medium-voltage motors as described in NEMA MG 1), application, power factor rating, type of motor enclosure, bearing lubrication types, arrangement of windings and their temperature limits, thermal capabilities of rotor and stator during starting, running, and stall conditions. See Table 3.
Typical characteristics and applications of fixed frequency medium ac squirrel-cage motors
Polyphase characteristicsLocked- rotor torque (percent rated load torque)Pull-up torque (percent rated load torque)Breakdown torque (percent rated load torque)Locked- rotor current (percent rated load current)SlipTypical applicationsRelative efficiencyDesign A Normal locked rotor torque and high locked rotor current70275a65190a175300Not defined0.55%Fans, blowers, centrifugal pumps and compressors, motor-generator sets, etc., where starting torque requirements are relatively lowMedium or highDesign B
Normal locked-rotor torque and normal locked-rotor current70275a65190a175300a6008000.55%Fans, blowers, centrifugal pumps and compressors, motor-generator sets, etc., where starting torque requirements are relatively lowMedium or highDesign C
High locked-rotor torque and normal locked-rotor current200285a140195a190225a60080015%Conveyors, crushers, stirring machines, agitators, reciprocating pumps and compressors, etc., where starting under load is requiredMediumDesign D
High locked-rotor torque and high slip275Not defined2756008005%High peak loads with or without flywheels such as punch presses, shears, elevators, extractors, winches, hoists, oil-well pumping and wire-drawing machinesMediumIEC Design H
High locked rotor torque and high locked rotor current200285a140195a190225a800100015%Conveyors, crushers, stirring machines, agitators, reciprocating pumps and compressors, etc., where starting under load is requiredMediumIEC Design N
Normal locked-rotor torque and high locked rotor current70190a60140a160200a80010000.53%Fans, blowers, centrifugal pumps and compressors, motor-generator sets, etc., where starting torque requirements are relatively lowMedium or highNOTEThese typical characteristics represent common usage of the motorsfor further details consult the specific performance standards for the complete requirements.
Reprinted from NEMA MG10-2001 by permission of the National Electrical Manufacturers Association.aHigher values are for motors having lower horsepower ratings.
Motor-starting conditions
Motor-starting conditions include across-the-line (full voltage non-reversing or reversing) or reduced voltage, adjustable speed drive (ASD), voltage drop and degree of inrush current during starting, repetitive starts, and frequency and total number of starts. See Figure 1 and Padden and Pillai [B10].
Typical motor-starting and capability curves (specific motor terminal voltage and for cold start)
Protection methods and settings are also affected by the starting methods. A comparison of different reduced-voltage starters is given in Table 4, similar to Table 10-17 in IEEE Std. 141. Various starting methods are discussed in subsections 5.2.1 through 5.2.6. In IEEE Std. 399 Clause 9, motor starting studies are described in more detail. If for example in Table 4, normal inrush is 6 times full load current and an 80% tap autotransformer start is applied, the actual inrush multiplier used for determining the appropriate motor representation in the calculations is (6 x 0.67) x full load current = (4.02) x full load current. Resistor or reactor starting limits the line starting current by the same amount as motor terminal voltage is reduced (that is, 65% of applied bus voltage gives 65% of normal line starting current). Wye (Y) -start, delta (() run starting delivers 33% of normal starting line current with full voltage at the motor terminals. The starting current at any other voltage is, correspondingly, reduced by the same amount. Part winding starting allows 60% of normal starting line current at full voltage and reduces inrush according to other voltages.
Comparison of different reduced-voltage starters
Autotransformer*Primary
resistor or reactorPart windingWye-delta50% Tap65% Tap80% Tap65% Tap80% Tap2-stepStarting current drawn from line as percentage of that which would be drawn upon full-voltage starting28%45%67%65%80%60%33 1/3 %Starting torque developed as percentage of that which would be developed on full-voltage starting25%42%64%42%64%50%33 1/3 %Increases slightly with speedIncreases greatly with speedSmoothness of accelerationSecond in order of smoothnessSmoothness of reduced-voltage types. As motor gains speed, current decreases. Voltage drop across resistor decreases and motor terminal voltage increasesFourth in order of smoothnessThird in order of smoothnessStarting current and torque adjustmentAdjustable within limits of various tapsAdjustable within limits of various tapsFixed* Closed transition
Approximate values only. Exact values can be obtained from motor manufacturer.
Full-voltage start usually draws between 500% and 600% of full-load current
Solid-state reduced-voltage motor start
As discussed in more detail in IEEE Std. 141 section 10.6.3.6, Solid-state motor starters can control the starting cycle and provide reduced voltage starting for standard ac squirrel-cage induction motors. They provide an adjustable, controlled acceleration and eliminate high power demands during starting. These starters are available in standard models for motors rated from fractional sizes to 4500 kW (6000 hp). One type of reduced-voltage starter uses six thyristors in a full-wave configuration to vary the input voltage from zero to full on, so that the motor accelerates smoothly from zero to full running speed. The thyristors are activated by an electronic control section that has an initial step voltage adjustment. This adjustment, combined with a ramped voltage and current-limit override, provides constant current (torque) to the motor until it reaches full speed.
Variations in the design of starting circuit are as follows:
The solid-state reduced voltage motor starter maintains a constant level of kilovoltamperes and reduces sudden torque surges to the motor. The current limiter, in conjunction with the acceleration ramp, holds the current constant at a preset level during the start-up period. When the start cycle is complete, the motor is running at almost full voltage with, essentially, a sine wave in each phase.
Thyristors are used only during the starting phase. At full voltage, a shorting contactor closes and the circuit operates as a conventional electromechanical starter.
A starter with linear-timed acceleration uses a closed-loop feedback system to maintain the motor acceleration at a constant rate. The required feedback signal is provided by a dc tachometer coupled to the motor.
Courtesy of Schneider-ElectrictElectric
Typical solid state reduced voltage motor starter with internal shorting relay, schematic
Courtesy of Schneider-ElectrictElectric
Typical solid state reduced voltage motor starter with optional shorting contactor, schematic
Autotransformer start
IEEE Std. 141, section 10.6.3.3 discusses in more detail. For limiting starting current and torque on polyphase induction motors to comply with power supply regulations or to avoid excessive shock to the driven machine, or to limit excessive voltage drop. Overload and undervoltage protection are provided. Equipped with mechanical interlock to assure proper starting sequence. Taps are provided on the autotransformer for adjusting starting torque and current. Since the autotransformer controller reduces the voltage by transformation, the starting torque of the motor will vary almost directly as does the line current, even though the motor current is reduced directly with the voltage impressed on the motor.
Autotransformer, magnetic, systems are the same as the manual as described above, but suitable for remote control. It has a timing relay for adjustment of time at which full voltage is applied.
To overcome the objection of the open-circuit transition associated with an auto transformer starter, a circuit known as the Korndorfer connection is in common use. This type of starter requires a two-pole and a three-pole start contactor. The two-pole contactor opens first on the transition from start to run, opening the connections to the neutral of the autotransformer. The windings of the transformer are then momentarily used as series reactors during the transfer, allowing a closed-circuit transition. Although it is somewhat more complicated, this type of starter is frequently used on high-inertial centrifugal compressors to obtain the advantages of low line-current surges and closed-circuit transition.
Primary resistor or reactor start
As discussed in IEEE Std. 141, section 10.6.3.3, automatic reduced voltage starter designed for geared or belted drive where sudden application of full-voltage torque must be avoided. Inrush current is limited by the value of the resistor or reactor; starting torque is a function of the square of the applied voltage. Therefore, if the initial voltage is reduced to 50%, the starting torque of the motor will be 25% of its full-voltage starting torque. A compromise must be made between the required starting torque and the inrush current allowed on the system. It provides both overload and undervoltage protection and is suitable for remote control. The resistor or reactor is shorted out as speed approaches rated rpm.
Part-winding start
As discussed in more detail in IEEE Std. 141, section 10.6.3.3, Used on light or low-inertia loads where the power system requires limitations on the increments of current inrush. It consists of two magnetic starters, each selected for one of the two motor windings, and a time-delay relay controlling the time at which the second winding is energized. It provides overload and undervoltage protection and is suitable for remote control.
Wye-delta start
IEEE Std. 141, section 10.6.3.3 discusses in more detail. Wye-delta is also known as star-delta. This type starter is most applicable to starting motors that drive high inertia loads with resulting long acceleration times. When the motor has accelerated on the wye (or star) connection, it is automatically reconnected by contactors for normal delta operation. This type of starter requires 6 motor leads.
In selecting the type of reduced-voltage starter, consideration should be given to the motor control transition from starting to running. In a closed-circuit transition, power to the motor is not interrupted during the starting sequence, whereas on open-circuit transition it is interrupted. Closed-circuit transition is recommended for all applications to minimize inrush voltage disturbances and torque pulses.
Courtesy of Schneider-Electric
Wye-delta open transition starter
Courtesy of Schneider-Electric
Wye-delta closed transition starter
Multi-speed motor start
IEEE Std. 141, section 10.6.3.5 discusses this in more detail. Multi-speed motor controllers are designed for the automatic control of two-, three-, or four-speed squirrel-cage motors of either the consequent-pole or separate-winding types. They are available for constant-horsepower, constant-torque, or variable-torque three-phase motors used on fans, blowers, refrigeration compressors, and similar machinery.
Caution should be taken when the motor is slowing down to not energize the slow speed winding while the motor is rotating above synchronous speed because it can cause high torque, voltage spikes, and current spikes.
Ambient conditions
Ambient conditions include maximum and minimum temperatures, altitude, adjacent heat sources, and ventilation arrangement. NEMA designed motor temperature rises are based upon a reference ambient temperature of 40°C.
Driven equipment
Load characteristics are important in the selection of the motor; otherwise, the driven equipment may lead to locked rotor, failure to reach normal speed, excessive heating during acceleration, overloading, and stalling. See Figure 26, which illustrates the relationship between the accelerating current of a motor versus the thermal damage limits of the motor during accelerating and running conditions. Present practice is to add an electronic reduced-voltage starter for motors that may have accelerating problems or to add an ASD for motors that could be operated at a reduced speed for some reasonable period of the duty cycle. The protection of motors driven by ASDs is discussed in Clause 9.0. For a detailed study of reduced-voltage starting, refer to sections in 5.2 and read Chapter 7 of IEEE Std 241-1990.
Typical time-current and thermal limit characteristic curves
Power system quality
Power system quality issues include types of system grounding, exposure to lightning and switching surges, capacitors and their controls for power factor correction, fault capacity, exposure to automatic reclosing or transfer, possibilities of single-phase supply (e.g., broken conductor, open disconnect switch or circuit breaker pole, blown fuse), and other loads that can cause voltage unbalance. Another factor is harmonics, which may cause motor overheating and affect the performance of electronic protective devices.
Motor importance
Factors that determine motor importance include motor cost, forced outage costs, amount of maintenance and operating supervision to be provided, and ease and cost of repair or replacement. A motor that is important to a plants operating continuity or process safety should include a pre-trip alarm for operator intervention as a first step. An example is to initiate an alarm when a ground fault is detected on high-resistance-grounded neutral low-voltage systems. This scheme can also be applied to medium-voltage systems below 13.8 kV; but at the 13.8 kV voltage level, use of a trip, rather than alarm, is preferred.
Load-side faults for motor controllers
Although most of this subclause concerns low-voltage applications, the principles apply to medium-voltage applications of motor controllers, as well. Calculation of available fault current in a circuit is described in Chapter 2. Fuse and circuit breaker protection for conductors in feeder and branch circuits are described in Chapter 5, Chapter 6, and Chapter 7. greater than 750 kW (1000 hp) or and motors with a neutral. Various methods of ground protection are available such as ground sensing, zero- sequence sensing and residual sensing.
Underexcitation (loss- of- field) protection (Device 40)
This Underexcitation type of protection is only applicableapplies only to synchronous motors. It is applied to The relay can monitor the rotor field winding. Loss-of-field condition affects the VAR import/export condition of a synchronous motor, and multifunction relays (Devce 11) monitor VAR flow to detect this condition.
Over excitation (volts/hertz) protection (Device 24)
This type ofOverexcitation protection (Device 24) is only applicableapplies only to synchronous motors. It is appliedThis protection monitors to the rotor field winding.
Bearing Protection Relay (Device 38)
This A bearing-protection relay monitors the bearing temperature and trips the motor when the operating temperature reaches the trip setpoint.
Mechanical Condition Protection Relay (Device 39)
This Mechanical-condition protection relates todetects the occurrence of abnormal mechanical conditions such as vibration, eccentricity, expansion, shock, tilting or and seal failure. It uses various sensors to send signals to protection relays.
Thermal and electronic overload protection (Device 49)
This protection relates to thermal protection.
Refer to sections 7.1.1, 7.3, 8.4.1, and 9.1.6
Mechanical tThermal overload relays are constructed as either melting alloy or bimetallic. Although three- phase construction- block design are the most common, single-phase elements may might sometimes be encountered. The relays are designed to operate within a current range, as follows:
Selection of the heater element should be based upon the relay manufacturers tables relating motor characteristics and ambient temperature conditions, and be based on the location of the motor relative to the relay. This method is employed because only minor adjustments need to be made in the relay itself to set a trip value to match the motor current.
After the selection ofselecting the heater, the melting alloy unit is considered non-tamperable.
Older bimetallic types may can have limited adjustment of trip setting intended to compensate for ambient temperature. Newer relays have a wider adjustment range of adjustment.
The thermal memory of bimetallic overload relays provides somewhat satisfactory protection for cyclic overloading and closely-repeated motor starts.
A manual reset feature is available and is normally trip free (manual override is not possible).
Some relays are available as ambient-temperature-compensated or and as noncompensated. Noncompensated is an advantage when the relay and motor are in the same ambient condition because the relay opening time changes with temperature in a similar manner as the motor overload capability changes with temperature.
NEMA ICS 2-2000 has standardized motor overload relays into three classes denoting time delay to trip on locked-rotor current: Class 10 for fast trip, 10 s at six times the overload rating; Class 20, for intermediate trip, for 20 s at six times the overload rating; and Class 30 for long-time trip, 30 s at six times the overload rating. In most applications, the Class 10 relay is applied for hermetic and other motors with a service factor of 1.00 or 1.05. The Class 20 relay is commonly used for higher greater-rated service factor motors, such as NEMA T frame motors. A Class 30 relay is used in applications where high-inertia loads cause the motor to have a long starting time, such as conveyor belt motors. Electronic devices, sometimes integral with the contactor, sense the current in all three phases. They These can be adjusted for Class 10, Class 20, or Class 30. Refer to Figure 6 9 for typical thermal overload curves for cold condition and warm condition, a) and b) respectively.
a) Thermal overload curves, cold b) Thermal overload curves, warm
Reprinted with permission from Schneider-Electric
Thermal overload curves, a) cold and b) warm
Multifunction relay (Device 11)
An important development has been the multifunction motor protection relay. Recognized as a powerful tool, the multifunction relay incorporates many protective functions that normally would normally be applied through the use of individual separate protective relays. With the multifunction relay , but are all protection elements are incorporated into one enclosure. For example, the multifunction relay incorporates provides short-circuit and overcurrent protection infor each phase, all phases together, - and for ground-fault protection. Depending upon the options selected, the relay could include protection against stalls, locked rotor, over-temperature alarm or and trip, current unbalance, metering, and communications. No detailed discussion of the relay is included in this subclause because the possible functions are described under other protective relays, such as Device 50 and Device 51.
The 1-linesingle-line and 3-linethree-line diagrams showing protective device functions can use the Filled Box method (as illustrated in this standard) or use the List Box method described in C37.2-2008 Figure A.2 and Figure A.3, respectively.
Device 11, MV multifunction relay applications
A multifunction relay can protect a
Mmedium-voltage motor starters consist ofwith a vacuum contactor protected by an intelligent relay. While a bimetallic thermal overload relay could be used (, as is done for many low-voltage motors,), a greater measure of protection is usually desiredrequired for medium-voltage motors due to theirbecause of the cost and the critical role of these motors in many applications.
As shown in Figures 7 and Figure 8, fused MV starters with contactors are used in some applications. The medium-voltage starter consist of a contactor (vacuum, SF6, or oil-filled) protected by a multifunction n intelligent relay and fuses. For some users, a medium-voltage circuit breaker is the preferred device for starting and stopping motors, particularly for infrequent starting and larger motors as shown in Figures 14 and 15. As with a medium-voltage starter, a medium-voltage circuit breaker consist of a contactor (vacuum, SF6, or oil-filled) protected by an intelligent relay.
Formerly, individual separate devices may have been were used for various protective functions. Several of thesem are noted earlier in this document (: instantaneous overcurrent [Device 50], inverse-time overcurrent [Device 51], and undervoltage [Device 27]). While offering adequate protection, those these devices occupied significant space, required significantconsiderable labor to mount and wire, and required individual calibration and maintenance for each relay. Whether medium-voltage starters or medium-voltage breakers are used for motor starting, the same multifunction relay may can often be used with either device to effect motor protection.
Modern multifunction relays offer protective and monitoring features impossible in older electro-mechanical devices. Waveform capture, starts-per-hour protection, fault- data logs, broken- rotor-bar detection, trending, and motor- load profiling are some of the diagnostic and monitoring functions available. Modern multifunction relays model the thermal state of the motor (see section REF _Ref395242957 \w \h 8.1), using a thermal model (Device 49). Thermal capacity (TC) calculations in the relay match the motor damage curves better than simple overcurrent protection, allowing more and safer motor loading.
Usually, mMultifunction relays usually provide operator interface panels, consisting of LCD screens with touch-screens or soft function keys with LCD screens,that are used for setup and for scrolling among through the parameters (see REF _Ref395245417 \h Fig. 1). Many Most multifunction relays offer programming configuration software. By using software a user can program the setup motor protection from a computer, upload new setups, download and save files, and monitor the relay locally and remotely. Some alsoMost have Boolean logic capability (AND, OR, NAND, NOR gates) programmable via the software. Second-generation motor relays have all Boolean functions, and configuration is by dragging symbols and interconnecting these with a computer mouse.
Fig. SEQ Figure \* ARABIC 1 Modern multifunction relay (courtesy of Basler Electric Company)
Additional input/output capacity may might be standard or optional for starter auxiliaries. Motor and bearing RTD inputs, formerly requiring a separate relay, may might be available.
< IAS magazine has motor protection, page 10 History, Nov 2012;>
Many users wish to haveinstallations benefit from communications capability incorporated into protective relays. Communications- capabilities allow interfacing with the software package for setup, troubleshooting, and monitoring. Communications also allows remote monitoring of motors for operating status, operating current, fault history, and others. Older, separate relays did not generally include communications. Communications may can be accomplished via RS-232, or USB, and Ethernet, through or fiber ports or and RS-485copper-wire connections. Protocols may can be Modbus RTU, Modbus TCP, IEC- 61850, and others. Wireless Ethernet networks are popular in industry applications. .
Device 11M is often used in MV ASD (adjustable-speed drives) applications as discussed in section 9.3.
Device 11, Low-voltageLV multifunction relay applications
Traditionally,
While protection for low-voltage motors was traditionally provided through use ofby heaters in bimetallic overload relays or from by solid-state overload relays. Today, many users now desirewant greater protection. Multifunction relays are now offeredavailable for that this purpose.
In addition to overload protection, multifunction relays are available forprovide low-current (or and low-power) protection for pumps to provide cavitation protection, jam and stall protection, frequency-of-start protection, ground -fault protection, and others. Arc- flash sensing is now being incorporated into some multifunction relays.
Multifunction relays typically provide a human-machine interface (HMI) panel through which setup and monitoring may beare accomplished. Fault-type, cause of fault, run/stop status, running Amperesamperes, and other parameters may can be read from the display. Remote HMI ports may might be offered for mounting in motor control centers and enclosed starter enclosures.
Multifunction relays may can also include as standard or optionally RTD inputs and additional analog input/output points. Input/output points may can be used to interface with external control, reversing, multi-speed, and reduced-voltage starting systems.
Some applications require cCommunications is now desired by some users. Where While traditional overload relays have not included communications, many multifunction relays either include communications in the standard feature-set or offer it optionally. Communications available include RS-232, RS-485, Modbus RTU, Modbus TCP, IEC -61850, a Ethernet, and others..
Device 11M maycan be used in LV ASD applications as discussed in section 9.2.
< [this new section needs to include current application technology for protection and communications]>
Low-voltage motor protection
Conventionally, low-voltage motors drive small process equipment and auxiliary equipment. These motors, may might operate continuously or may might be in cyclical services. These applications use motor contactors in motor control centers (MCCs) or combination starters.
For low voltage motor protection, motor coordination studies are broken into five main areas, which are shown in a simplified flow chart in Figure 4710. Data and device selections from each area flow forward and are used to achieve protection and selectivity at the next area. As each area of protection is analyzed, the protective device curves are added to the coordination plot, which is a time-current plot on a log-log graph. A typical coordination plot for a low-voltage motor application is in Figure 6912.
Reprinted with permission from Padden Engineering, LLC
Five main areas of low-voltage motor coordination studies [B10].
One-line diagrams of typical low-voltage starters for industrial applications using MCCs or combination starters are shown in View (a), View (b), View (c), View (d) and View (e) of Figure 811.
SHAPE \* MERGEFORMAT
a) Typical starter with fuses b) Typical starter with a circuit breaker
c) Typical location for power factor correction capacitors d) Typical location for control power transformer
e) Typical starter with molded-case switch and fuses
Reprinted with permission from Padden Engineering, LLC
Typical low-voltage starter one-line diagrams for industrial applications using MCCs or combination starters
Low-voltage motor overcurrent protection
Overload protection for low-voltage motors is usually provided by thermal overcurrent relays or electronic overcurrent devices. In some cases, dual-element fuses or a thermal-magnetic circuit breaker may serve as the primary overload devices, but are normally backup protection for overload relays. Short-circuit protection for low-voltage motors is usually provided by fuses, a thermal-magnetic circuit breaker, or an instantaneous trip device (or motor circuit protection) in combination with an overload relay. Ground-fault protection for low-voltage motors is usually provided by the short-circuit protection device, but ground-fault relays may be installed. (See Bradfield and Heath [B1]; Nailen [B8]; Gregory and Padden [B4] and [B3]; Smith [B12].)
Thermal and electronic overload relays
Overload relays are sized in accordance with the National Electrical Code® (NEC®) (NFPA 70-2011). NEC Section 430-32 references the motor nameplate rating. Power factor correction capacitors installed for individual motors may be connected as shown in View (c) of Figure 5811, and no current adjustment need be made to the overload devices. However, this connection is not the only method of providing individual power factor correction and has been known to cause contactor failures due to resonance with other motor capacitors (see Nailen [B8]). When capacitors are installed between the overload device and the motor, the overload relay provides circuit impedance, which generally dampens the resonance problem. However, the overload relay current rating should be adjusted to account for the reduced current flowing to the motor-capacitor combination (see NEC 460.8 and 460.9). Part 14.43.3 of NEMA MG 1- recommends a bus connection when several motors are connected to the bus to minimize the potential harmonic resonance.
Overload relays and other devices for motor overload protection that are not capable of opening short circuits shall be protected by fuses or circuit breakers with ratings or settings in accordance with NEC Article 430-52 or by a motor short-circuit protector in accordance with NEC Article 430-52 (see NEC 430.40).
Time-delay (or dual-element) fuses
Time-delay (dual element) fuses are available from 0.1 A through 600 A. Fuses for short-circuit and ground-fault protection shall be sized in accordance with NEC Article 430-52 and Table 430-52. The full-load current values used for that table are in Table 430-248, Table 430-249, and Table 430-250. The rating of a time-delay fuse shall be permitted to be increased, but in no case exceed 225% (400% for Class CC fuses) of full-load current. A one-line diagram of a typical starter with fuses is shown in View (a) of Figure 58. Also available are fuses without time delay, which can provide short-circuit and ground-fault protection, but may not provide any backup protection.
Inverse-time circuit breakers
These circuit breakers (i.e., molded case) are available from 10 A through 3000 A when constructed with thermal-magnetic trip elements, and up to 5000 A when constructed with solid state trip elements. Both types of trip devices are referred to in the NEC as inverse-time circuit breakers and shall be sized in accordance with NEC Article 430-52 and Table 430-52. The full-load current values used for that table are in Table 430-248, Table 430-249, and Table 430-250. The rating of an inverse-time circuit breaker shall be permitted to be increased, but in no case exceed,
400% for full-load currents of 100 A or less,
300% for full-load currents greater than 100 A.
A one-line diagram of a typical starter with a circuit breaker is shown in View (b) of Figure 85.
Instantaneous trip circuit breakers
Instantaneous trip circuit breakers (i.e., molded-case) are available from 3 A through 1200 A. The instantaneous setting can typically be adjusted in fixed steps to between 3 to 13 or 3 to 10 times the continuous-current rating. Instantaneous trip circuit breakers are tested under UL 489 [B13]. The trip range of the breaker should be within +30% or 20% of the set point. On the coordination plot, these devices have a broad bandwidth corresponding to these tolerances.
These breakers are referenced as instantaneous trip breakers and shall be sized in accordance with NEC Article 430-52 and Table 430-52. The full-load current values used for that table are in Table 430-248, Table 430-249, and Table 430-250. Trip settings above 800% for other than Design B energy efficient motors and above 1100% for Design B energy efficient motors shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous trip circuit breaker at 800% or 1100%. An adjustable instantaneous trip circuit breaker shall be used when it is part of a listed combination controller having coordinated motor overload, short-circuit, and ground-fault protection in each conductor and if it operates at not more than 1300% of full-load motor current for other than NEMA Design B energy efficient motors and no more than 1700% of motor full-load current for Design B energy efficient motors. A one-line diagram of a typical starter with a circuit breaker is shown in View (b) of Figure 5811
Two points should be reviewed by the engineer. First, the overload device is normally the only line of protection from overloads and high-impedance faults when using instantaneous trip circuit breakers. A failure of the overload device, the overload wiring, or the contactor can prevent the circuit from being isolated due to overload or high-impedance fault conditions. Where backup protection is desired for these abnormal conditions, an inverse-time circuit breaker or dual-element fuses should be selected.
Second, the selection of the contactor and conductor sizes depends on the setting of the instantaneous trip function. NEMA-rated magnetic contactors are tested to break up toto as many as 10 times the full-load current values given in NEC Table 430-248, Table 430-249, and Table 430-250 for the corresponding horsepower rating of the contactor. When an overload device trips, the contactor is called upon to open the circuit. Therefore, the contactor should be rated to break the circuit. Under high-impedance fault conditions, the current may be in the range of 10 to 17 times the motor full-load current. The instantaneous trip breaker may be set above the 10 times full-load current break test value of the contactor. Refer to Figure 6 9 for the time-current curves of a 480 V, 100 hp motor application with a 175 A instantaneous trip breaker, a Class 20 overload, and a NEMA size 4 magnetic contactor (i.e., 1350 A break rating). This figure illustrates a case where the instantaneous trip is set about 12 times the full-load current of 124 A (see NEC Table 430-250). The #2/0 AWG XHHW conductor is rated for 175 A at 75 °C. The contactor is not protected using the setting of about 1500 A. A lower instantaneous setting would protect the contactor, but some motors may trip the breaker on starting. Each controller shall be capable of starting and stopping the motor it controls and interrupting the locked-rotor current of the motor (see NEC 430.82). Controller ratings shall meet NEC 430.83 requirements. The disconnecting means shall have an ampere rating not less than 115% of the full-load current rating of the motor (see NEC 430.110).
Reprinted with permission from Padden Engineering, LLC
Time-current curve for a 460 V, 100 hp motor with size 4 contactor, Class 20 overloads, and an instantaneous trip circuit breaker with a setting of 12 times full-load current
In a recently published book, the authors reveal that some high-efficiency motors draw up to 2.83 times locked-rotor current during starting, and they recommended a 19.2 times full-load current on the instantaneous breaker setting, approximately 3 times locked-rotor current in one case (see Prabhakara, et al. [B11]). A typical value used in the industrial applications is 1.76 times locked-rotor current for estimating asymmetrical inrush current. To prevent false tripping of the instantaneous trip breaker on starting, several options are available:
Use an autotransformer or other means for reduced voltage starting to limit the inrush current.
Specify a contactor with a higher break rating and set the instantaneous breaker at a higher setting within the NEC limits.
Use an inverse-time circuit breaker in place of the instantaneous trip breaker so that the instantaneous setting, if available, can be set above the motor inrush current.
Low-voltage motor ground-fault protection
Many low-voltage motor applications utilize fuses or MCCBs for ground-fault protection. However, the type of protection selected is dependent upon the type of system grounding.
Solidly grounded systems
Fuses and circuit breakers normally provide adequate ground-fault protection for motors on solidly grounded systems. However, for larger motors applications, such as the 100 hp motor shown in Figure 6912, miscoordination occurs. For example, this motor is protected by an instantaneous only circuit breaker set at 1500 A trip. The main breaker ground trip is set at 1200 A, the maximum allowed by NEC Article 230-95, where a shutdown does not introduce additional hazards. Miscoordination can occur in the region between the ground trip device on the main low-voltage power circuit breaker (LVPCB) and the instantaneous trip circuit breaker protecting the motor. LVPCBs, specified with long-time and short-time functions only (i.e., no instantaneous element), can usually be coordinated selectively.
If selectivity between the individual motor protective device and the main breaker is desired for ground faults, additional protective devices should be installed for the larger motors or interlocking ground-fault devices should be installed. For solidly grounded systems, the protective devices should be wired to open the breaker, not the contactor, unless the contactors are rated high enough to interrupt the available fault current. Some breakers have integral solid-state devices that sense ground faults and open the breaker. Contactors may also have integral solid-state devices that sense ground faults, but these may open the contactor and the rating must be verified. Also, zero-sequence current transformers (CTs) and trip units can be installed to shunt-trip the circuit breakers or switch, provided that the circuit breaker or switch has a shunt trip included (shunt trips are usually special order, not standard)
Low-resistance-grounded systems
Low-resistance-grounded systems are not normally used on low-voltage applications because ground fault currents may not be high enough to operate the protective devices.
High-resistance-grounded systems
For high-resistance-grounded systems, where the fault current is usually 5- A to 10- A range, no individual separate motor ground-fault protection is generally provided. Instead, an alarm at the grounding resistor signals that a ground fault has occurred. A ground pulsed signal is used to locate the fault. The faulted circuit is then manually cleared. Caution should be used when selecting conductor insulation materials and ratings for use on high-resistance-grounded systems, particularly on smaller conductors (e.g., size 10 AWG and below).
Some modern motor controllers provide ground fault sensing in the electronic trip units. Zone selective interlock (ZSI) tripping is another type of ground-fault protection that permits the first ground fault to be alarmed only, with rapid tripping following a ground fault on a different phase. This trip rating illustrates the advantage of high-resistance grounding when operation may continue during the presence of the first ground fault. Removal of the first ground fault is important, however, to prevent escalated damage from a second ground fault on a different phase. The MCCBs used in high-resistance-grounded systems should be rated for line-to-line voltage (e.g., 480 V not 480/277 V for a 480 nominal system voltage). (Also the single-pole interrupting rating should be checked to clear the second fault on a different phase: ground as well as line-to-ground faults on two separate phases, one on each side of the breaker. See Gregory et al [B2].)
Low-voltage motor stator winding over temperature (Device 49S)
The purpose of stator winding over-temperature protection is to detect excessive stator winding temperature prior to the occurrence of motor damage. In low-voltage motors in non-critical services, the temperature sensors are normally wired to trip the motor control circuit and to open the contactor. .
Thermostat winding over-temperature devices
Thermostats are the most common type of stator temperature sensors installed in three-phase industrial service 460 V motors from 11 kW through 150 kW (15 hp through 200 hp). Many manufacturers wind the stators with the devices installed and cut off the leads if a customer does not specify the protection. Thermostat devices are bimetallic, normally closed devices (or normally open devices) that operate at one fixed temperature. They are normally wired in series with the control circuit at 120 V. These devices are normally sealed from the atmosphere, but are not rated as hermetically sealed for hazardous NEC Class 1 Division 2 areas.
Thermistor winding over-temperature devices
Thermistors are used to operate relays for either alarm or trip functions, or both. They have resistance characteristics that are nonlinear with respect to temperature and thus are not used to indicate temperature. Thermistors must match the control device. Two types of thermistors exist:
Positive temperature coefficient, PTC. The resistance of a positive temperature coefficient thermistor increases with temperature. An open circuit in this thermistor appears as a high-temperature condition and operates the relay. This arrangement is fail-safe.
Negative temperature coefficient. The resistance of a negative temperature coefficient thermistor decreases as temperature increases. An open circuit in this thermistor appears as a low-temperature condition and does not cause relay operation. This device primarily monitors temperature rather than operating a relay for protection.
Resistance temperature detector (RTD) winding over-temperature devices
RTDs may be considered in larger or critical service low-voltage motors. In those cases, the RTDs are usually connected into a device that provides an alarm and/or trip functions. The most common practice is to install six RTDs, two per phase, of the 100- © platinum elements class B to IEC 60751 for small motors and 100- &! platinum element class A to IEC 60751 for large or important motor. [BB5]More information on RTDs is contained in 8.4.1.1.
Low-voltage motor undervoltage protection
Undervoltage protection is used to protect motors from several damaging conditions: low voltage due to a voltage sag, automatic reclosing or automatic transfer, and power restoration. In a voltage sag, the motor draws more current than normal and has unusually high heating. Excessive heating can be a serious problem in hazardous areas where the motor must stay within its T marking.
When the supply voltage is switched off during automatic reclosing and transfers, the motors initially continue to rotate and retain an internal voltage. This voltage decays with motor speed and internal flux. If the system voltage is restored out of phase with a significant motor internal voltage, high inrush can occur. Such current can damage the motor windings or produce torques damaging to the shaft, foundation, drive coupling, or gears. IEEE Std C37.96-2012 discusses considerations for the probability of damage occurring for various motor and system parameters.
When power is restored after an outage, the starting sequence should be programmed so that all motors on the system are not starting simultaneously. This step is important for the generating equipment, as well as for transformers and conductors. Undervoltage devices are not normally installed on essential loads such as motors for fire pumps.
Undervoltage relays
Low-voltage undervoltage relays are typically electronic devices that monitor all three phases. These devices can be furnished with a time delay to trip, a time delay to restart, or instantaneous for trip and restart. Usually, the designer sets the device at 85% of line voltage with a time delay off and a time delay for restart. Normally, the undervoltage relays are wired into the motor control circuit to open the contactor.
Undervoltage sensors for circuit breakers
Some MCCBs have an undervoltage sensor adapter that trips the circuit breaker on a low-voltage condition. The circuit breakers are reset manually. Where automatic restart is necessary, this method should not be used. These sensors may not be as reliable as separate undervoltage relays, and this factor should be considered when designing the circuit.
Medium-voltage motor protection
< Dan Ransom lead, Jeff Hensley, David Bredhold, Ted Kloba>
CConventionally, large motors drive the main process equipment and typically, these motors operate continuously for the length of the batch process. To control stopping and starting, medium-voltage circuit breakers are often used to apply the power to the large motors. Medium-voltage motor voltage ratings are 2300 V, 4000 V, 6600 V, and 13 200 V per Part 20.5 of NEMA MG 1-. When a motor must be started frequently, it may might be necessary (even economical) to use motor contactors in a combination controller with a current-limiting fuse or circuit breaker because of the greater life of the contactors. Use care when applying fused contactors on solidly grounded neutral systems because the contactor is incapable of interrupting high large fault currents, especially ground faults. As a result, ground-fault relays and differential relays should not open the contactor unless rated for the available fault current. . The manufacturer performs short-circuit tests on the combination controller to confirm safe performance when interrupting the fault on the system to which it is rated.
In principle, the protection of protecting medium-voltage motors is similar to low-voltage motors, but the requirements are more demanding. Being closer to the utility source, medium-voltage motors are more susceptible to voltage sags and surges, reclosing, and higher greater available fault levels. Because of the higher bus voltage and load currents, instrument transformers are used to reduce these currents to lower smaller values used with protective relays. The most common instrument transformer secondary ratings for voltage transformers (VTs) are 120 V (line-to-line) in North America, while other voltages are typical for certain regions, for example 100 V (line-to-line) or and 110 V (line-to-line) in other parts of the world. The most common instrument transformer secondary ratings for current transformers (CTs) are 5 A in North America, while certain other regions use 1 A. Specific facilities may can also include several different ratings, therefore it is critical for the coordination engineer to clearly identify the VTs, CTs, and relay ratings. CTs should be selected according to requirements of the motor protection relay in order to ensure the precise protection relay performance. The Rrelay manufacturers provide such CT requirements in relay manuals; and such these requirements should be observed. CTs should provide the saturation- free time required for relay operation. CT performance for distance protection may can have very extreme requirements. CT matching (manufactured in the same batch) and saturation- free time are important for differential protection sensitivity and settings to prevent misoperation. CT saturation- free time is also important for overcurrent protection and so on, especially when direction features are used; for example, due tobecause of missing zero-crossings the directional decision might be wrong in case of CT saturation. See IEEE 3004.1 and IEEE C37.110 for additional information on CT and VT selection and performance.
Typically, tThe circuit breakers [i.e., air, sulfur hexafluoride (SF6), vacuum], instrument transformers, and protective relays are typically mounted in switchgear.
Refer to Figure 1013 and 10114 for typical 1-linesingle-line and 3-linethree-line diagrams of MV induction- motor protection using a circuit breaker. The critical service protection functions from Table 6 REF _Ref396114157 \h Table B. 1 are illustrated using a multifunction motor protection relay, Device 11M. Device 11M sends a signals to trip the circuit breaker for various conditions, including short-circuit protection.
Reprinted with permission from C&I Engineering.Basler Electric Company
MV induction motor protection 1-linesingle-line diagram, Device 11M, critical service protection functions, with circuit breaker
Reprinted with permission from C&I EngineeringD. Bredhold.
MV induction motor protection 3-linethree-line diagram, Device 11M, critical service protection functions, with circuit breaker
MV motor thermal overload protection (Device 49)
Motors generate heat while running. Using an accurate model of the motor heating (using equivalent motor current) yields benefits in using the motor to maximum effect without damage.
NOTE: Air and water flow affect motor temperature.
Thermal protection of a motor is vital to motor longevity. Older motor protection methods used overcurrent 50 and 51 elements instead of the 49 thermal model (Device 49TC) and had no backup resistance temperature detector (RTD) temperature devices (49S). Comparing these two methods of motor protection (Device 50/51 and Device 49TC), demonstrates the benefits of the Device 49TC thermal model for motor protection versus using an overcurrent method [Bxx]. Note that Ccompromises must be made when employing overcurrent protection because typical 50/51 element curves do not match motor damage curves and do not take negative-sequence current effects into account. Rule of thumbA general rule is this: : for every 10°C rise in temperature, motor insulation life is cut in half.
For cyclic overloads (for example, crushers, conveyor-belt motors) thermal model Device 49 protection is superior to overcurrent-based protection because the thermal-model protection allows more, safe work from the motor. During an overload, motor heating is a long-term effectit takes time to raise the temperature of the motor mass. The thermal model tracks this heating and allows temporary overload operation. Overcurrent protection trips too quickly, before the motor has reached a critical temperature. Once the overload cycles to a less-loaded state, the thermal model tracks the resultant motor cooling.
A motor thermal model uses an equivalent current, Ieq, calculation that best represents the actual motor flux dynamics. This equivalent motor current accounts for the heating effects of negative-sequence and other currents present in motor applications.
Using thermal overload protection may extends motor life and allows efficient usage of motor capabilities. Figure 11 1215 shows a set of typical motor operational curves for starting (accelerating) and for running (see IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction Machines, IEEE Standard 620, for an explanation of the construction of this chart). Generally, with Device 49TC protection, the motor-protection curve can be placed directly underneath the starting and running damage curves; replicating the exact shape of the damage curve. Thermal overload protection using the Device 49TC element generally provides better protection for medium-voltage motors than simple overcurrent protection.
Reprinted with permission from Basler Electric Company.
Typical Motor Curves
Motors on high-inertia loads take a much longer time to accelerate. W, and without proper protection,, this long acceleration time could lead to nuisance tripping. In addition, a low-voltage condition can also lead result into a longer acceleration time, . These situations thus requiringrequire careful selection of the type of protection relays and settings. A good practice for these motors is to request motor acceleration curves that are plotted for the cases of 100 % percent and 80 % percent of rated motor-starting voltage. In section 8.2.1, other techniques are shown described foron methods for of high-inertia-start protection. In some cases, protective relaying relays might not adequately solve the problem. A turning gear motor can be applied to start the large- inertia motor. -
Motors on high-inertia loads (pumps and large fans) may require modifications of the Device 49 thermal overload protection curve. Digital and multi-function motor protection relays generally offer this capability. Configuration of the overload protection curve should include extended starting times and currents, thus preventing nuisance trips upon starting these devices.
MV motor overcurrent protection
Figure 12316, Figure 13417, and Figure 14518 illustrate additional techniques for other overcurrent protectionapproaches for large motor sprotection. The accelerating curve is shown differently in each case to demonstrate that no single curve is accepted as a standard. The motor accelerating curve should be provided by the motor or equipment supplier before setting the relays.
In Figure 81316, a NEMA Design A or Design B motor curve is shown with protection for starting and running using a Device 51, a time-overcurrent relay element with inverse or very inverse characteristics. Within the overcurrent relay is a second element, a Device 50, which operates without delay to protect against a shortcircuit. ; where Device 50 is only recommended only for circuit breaker applications, and is not recommended for fused starters. Normally, three overcurrent elements are used, each element is supplied from a separate CT. Some designers use only two of the three relays for overcurrent protection, and set Device 51 of the third relay relatively low (110% to 120% of the full-load current) to alarm on an overcurrent condition. Codes may might not permit this practice in some cases, and redundancy is lost during relay testing. In this latter scheme, the two protective phase relays could be set for extreme overcurrent conditions at 125 percent% to 140 percent% of the full-load current.
Conventional overcurrent motor protection
In Figure 13417, protection for a high-inertia load allows for the longer accelerating time. A conventional motor reaches rated speed within 10 s to 15 s, a high-inertia load may can take 30 s or longer (for example, centrifuges can take about as much as 40 minutes to reach rated speed when starting wye/delta). As a result, little time difference exists between the accelerating current curve and the motor thermal limits. Several approaches are available, as shown in Figure 9 1417 and Figure 101518. A, and an impedance (Device 21) method is shown in Figure 9 1619 . In Figure 91316, Device 51 has long-time inverse or very-inverse characteristics set above the accelerating current. (Definite- time delay is another term to describe this element.) Device 50 is a high dropout (HDO) element that resets rapidly when the starting current drops to a magnitude of 85 percent% to 90 percent% of the set current without delay. For starting, a time delay of less than 1 s is needed to permit the Device 50 (HDO) to be set at 1.15 pu of locked-rotor current (LRC). This delay prevents false trips caused by asymmetrical starting currents, yet provides short-circuit protection after the time delay. A second Device 50 element is set at approximately two times the LRC to protect against short-circuits during starting.
Protection of high-inertia motor
Figure 14 17 illustrates a second method for protecting a high-inertia motor. This approach also uses two Device 50 elements per phase. The conventional Device 50 is set in the normal regular way to protect against short circuits. The second Device 50 is used in conjunction with a Device 51 overcurrent element to block tripping by the Device 51 for overcurrent conditions below less than the Device 50 setting. This scheme offers an overcurrent alarm, while allowing the motor to continue operating, unless the actual overcurrent exceeds a high setting. The use of this scheme is dependent upon the operating philosophy of the facility.
Large motors should be specified with RTDs or thermistors or RTDs buried in the windings for high-temperature backup detectionprotection. Generally, temperature changes detected with RTD/thermistor thermal overload (Device 49S) TC) are slower to develop than overcurrent increases and the thermal model (Device 49). Actual faults within the windings would be detected faster by the current- differential protection (Device 87) or a sensitive ground-fault current (Device 51N) protection schemes.
A ALLOWABLE HEATING CURVE
B STARTING CURRENT
C 51 LONG TIME OC
D DEFINITE TIME SET 120% FL
E INVERSE INST 50
Alternate method of protecting a high-inertia motor
Figure 15 1619 shows a scheme that relies upon the characteristics of an impedance distance relay element (Device 21) to permit tripping if the high-inertia motor does not accelerate to a certain speed within a fixed period. Upon motor circuit energization, the locked-rotor current is primarily inductive, because a blocked motor could be considered a transformer with shorted secondary windings. As the motor accelerates, the current decreases from a subtransient to a transient value, and the power factor and measured impedance increase. Also used with the Device 21 are either an overcurrent relay (Device 51) or an overvoltage relay (Device 59) that operates as a timing device in this case. This scheme guards against a stalled motor. Other schemes exist, such as zero-speed devices used with timers (Device 48). Figure 16 1720 illustrates how the locked-rotor protection functions below the safe stall time.
Protection of high-inertia motor using an impedance relay
Schematic of locked-rotor protection of Figure 1116
Fault protection
Motor current- differential element (Device 87M)
Motor current- differential protection measures the current flow into a load and compares it to the current measured on the neutral side of the motor . F; for normal operation the current going in and the current going out match and cancel. A current difference is detected as a fault. These schemes can be technically applied to any motor load, but often are applied only to large or critical motors where damage could be costly or replacement difficult. By detecting faults at a low level, damage can be confined to the windings. Generally apply differential- current protection as follows:
With all motors 750 kW (1000 hp) and larger on ungrounded systems.
With all motors 750 kW (1000 hp) and larger on grounded systems where the ground-fault protection is considered inadequate without differential protection to protect against phase-to-phase faults.
Motors 1900 kW (2500 hp) and larger.
Device 87M, Conventional phase- differential overcurrent relay
A conventional phase- differential relay senses low-level phase faults and removes the electric power quickly before extensive motor damage develops. This scheme uses six identical CTs (one pair for each phase) and three relays (one per phase). The CTs should be sized to carry full-load current continuously and to not saturate during an external or internal fault (see Figure 171821). The currents from each pair of CTs circulate through the relay-restraining windings under normal (i.e., no-fault) conditions. For a fault in the motor windings or in the cable, the CT secondary currents have different magnitudes and/or polarities, and the differential current from each CT adds to the other and operates the Device 87 to trip the motor circuit breaker. This scheme is employed for both delta-connected motors and for wye-connected motors. With the wye-connected motor, three of the CTs are normally located at the starter (or motor switchgear) and the other three in the three phases at the motor winding neutral.
Conventional phase differential protection using three (3) percentage differential relays
Device 87M, Self-balancing differential using window CTs
Three window (or toroidal) CTs are normally installed at the motor. One CT per phase is used with the motor line and neutral leads of one phase passed through the CT so that the flux from the two currents normally cancels in the CT. A winding phase-to-phase fault or a phase-to-ground fault results in an output from the CTs of the associated phases. That current operates the associated elements (see Figure 181922).
Self-balancing differential protection (one element shown)
Normally in one relay package , the Device 87 CTs and elements would be the same CTs used for zero-sequence instantaneous ground overcurrent protection (see 8.3.3.2) with the relay set between 0.25 A and 1.0 A pickup. Therefore, this differential scheme usually has a lower smaller primary pickup in amperes than the conventional differential scheme because the CT ratio is usually greater with the conventional scheme. This differential scheme has a slight advantage over the scheme in Figure 13 171821 in detecting ground faults. For motors installed on grounded systems this difference is significant because most faults begin as ground faults. The usual objective of motor-fault protection is to remove the fault before the stator iron is damaged significantly.
Application problems have occurred with this scheme when the available fault current is very high large and when the high-speed, balanced-core differential protection signals to triptripping of the motor starter before the current-limiting fuses clear the fault (thus protecting the starter). Because the starter has such a low small fault rating, some engineers have slowed the operation of the relay, by delay or a different relay type, to distinguish between a developing low-level fault and a direct short.
With the CTs located at the motor, this scheme does not detect a fault in the cables supplying power to the motor. Normally, a fault in these cables would be detected by the overcurrent protection. For large motors, coordinating the supply phase-overcurrent protection with the motor thermal overload and overcurrent protection is often a problem. The presence of motor differential protection is sometimes considered to make this coordination less essential. In this regard, the current differential is better than the self-balancing differential (Figure 141922) because the motor cables are also included in the differential protection zone. Hence coordination between the motor differential and supply phase-overcurrent relays is complete.
As with zero-sequence ground-fault overcurrent protection, testing the overall CT and relay combinations is important during commissioning. Current in a test conductor should be passed through the window of each CT. Because normally the relays do not carry current, an open circuit in a CT secondary or wiring to a relay can be discovered by overall testing.
Device 87M, split-winding current unbalance
Purpose
The purpose of the split winding current- unbalance device is to detect quickly a low-magnitude fault condition. This protection also serves as a backup to instantaneous phase-overcurrent and ground-fault overcurrent protection. Normally, this protection is applied only to motors having two (or three) winding paths in parallel per phase (see Figure 192023). This protection type should not be used on part-winding start applications because the windings are energized at different times during the start sequence.
Split-winding motor overcurrent protection used with two windings per phase (one relay shown)
Arrangement of CTs and relays
The usual application is with a motor having two or more winding paths in parallel per phase. The six line leads (i.e., two per phase) of the motor are brought out, and one CT is connected in each of the six leads. Choose the primary current rating of the CTs to carry full-load current (FLC).
The CTs can be installed at the motor. It might be convenient, however, to use six cables to connect the motor to the starter (or switchgear), and in this case the CTs can be located in the starter.
The currents from each pair of CTs, associated with the same phase, are subtracted, and their difference is fed to a short-time, inverse- time -overcurrent relay. Three of these relays are required (i.e., one per phase), and each is set at 1.0 time dial and between 0.5 A and 2.5 A. The relay should be set above the maximum current unbalance (including CT accuracy) that can occur between the two parallel windings for any motor-loading condition.
Evaluation of split-winding current unbalance protection
The following factors should be considered when evaluating split-winding current unbalance protection:
Total cost would be somewhat less than conventional current differential and more than self-balancing differential.
The primary pickup current for this protection would be about half of the primary pickup current of conventional phase differential because both schemes require the CT primaries to be rated to carry normal load currents. Self-balancing differential would usually have a lower smaller primary pickup current.
This protection has a slight time delay compared to the current- differential schemes.
When the CTs are located in the motor starter, split- winding protection has the same advantage over self-balancing differential as does current differential;, namelyspecifically, it detects a fault in the motor cables and may might facilitate coordination with the supply feeder overcurrent relays (see 8.3.1.2).
A feature of this protection is the ability to sense short-circuited winding turns. The number of turns that must be short-circuited before detection occurs depends upon the motor winding arrangement, the relay pickup, and CT ratio. An analysis of the specific motor winding would be required to determine the worthiness of this feature. Short-circuited turns could cause a ground fault, which could be detected by the self-balancing differential scheme before this split winding protection would sense the short-circuited- turns condition.
Often a split differential scheme can be effective where one CT is in one of the parallel paths and the other CT sees the total phase current.
Application of split winding protection
Device 87M split- winding protection is used rarely, but is feasible for important motors rated above 3750 kW (5000 hp) that have two or more winding paths in parallel per phase.
Ground-fault protection (Devices 50G, 51G, 50N, 51N)
Purpose
The purpose of ground-fault protection is to detect ground-fault conditions with no intentional delay and to be certain that the unbalance current represents a true ground fault (i.e., not current due to asymmetry in the primary current or current from CT saturation). Upon detecting a ground fault, the protection can trip the motor circuit, or only alarm for high- resistance grounded systems, depending upon the facility operating practice.
Device 50G, Instantaneous ground-fault protection
Using a zero-sequence (or window) CT that has been designed for instantaneous ground-fault protection and tested with a specific ground-fault relay is recommended (see Figure 16202124). For medium-voltage applications, the power system should be low- resistance -grounded or hybrid grounded. The Device 50G element should be set to operate for a primary ground-fault current in the range of 10 A to 30 A. A time delay should be added when the installation has surge protection on the motors.
Ground fault overcurrent protection using a zero-sequence CT
Device 51G, time-overcurrent ground-fault protection
Many installations have surge protection at the motor terminals, and a surge discharge through an arrester could cause an instantaneous relay element to have a false trip. To avoid this event, a Device 51G should be applied, in place of the Device 50G in Figure 202124, and set to trip within a few cycles of the fault-sensing pickup.
Installation of cable for ground-fault protection
The following precautions should be observed in applying the relay and zero-sequence CT and in installing the cables through the CT:
If the cable passes through the CT window and terminates in a pothead on the source side of the CT, the pothead should be mounted on a bracket insulated from ground. Then the pothead should be grounded by passing a ground conductor through the CT window and connecting it to the pothead.
If metal-covered cable passes through the CT window, the metal covering should be kept on the source side of the CT, insulated from ground. The terminator for the metal covering may be grounded by passing a ground conductor through the CT window and then connecting it to the terminator.
Cable shields should be grounded by passing a ground conductor through the CT window and then connecting it to the shields.
The overall CT and ground relay scheme should be tested by passing current in a test conductor through the CT window. Because normally no current exists in the relay, an open circuit in the CT secondary or wiring to the relay can be discovered by this overall test.
Device 51N, residually connected CTs and ground-fault relay
Using the residual connection from three CTs (i.e., one per phase) to supply the ground-fault relay is not ideal because high large phase currents (e.g., due from motor starting inrush or phase faults) may can cause unequal saturation of the CTs and produce a false residual current measurement. As a result, undesired tripping of the ground relay can occur. For this reason, a Device 50N is not recommended in the residual connection. A Device 51N installed in the residual connection would be more appropriate for these installations.
Selection of resistor for low-resistance system grounding
The purpose of resistance grounding is to provide current sufficient for protective relays to operate upon detection of a ground fault, but sufficiently small to limit the magnitude and resulting damage to the motor. (In mine distribution systems, the objective is to limit equipment-frame-to-earth voltages for safety reasons.) However, the ground-fault current should not be so small that the windings near the neutral end are unprotected. In the past, protection within 5 percent % to 10 percent% of the neutral has often been considered adequate. Selection of the ground resistor should also consider the number of steps in ground-fault overcurrent protection coordination (see Love [B5] and [B6]). On this basis, the ground resistor chosen for the system neutral grounding normally limits the ground-fault current within the range of 400 A to 2000 A. However, some companies prefer neutral ground-fault current limited to 200 A to 800 A; this difference emphasizes the need to coordinate the protection of a system. Usually, aA 10- s time rating is usually chosen for the resistor.
To avoid excessive transient overvoltages, the resistor should be chosen so that the following zero-sequence impedance ratio is achieved:
R0 / X0 should be equal to or greater than 2.
A more detailed discussion of the selection of the resistor can be found in Chapter 8.
Monitors
In addition to protection against failures caused by electrical abnormalities, advances in instrumentation and techniques have enabled protective methods that monitor machinery characteristics and, as a result, can detect trends of equipment failures during the incipient stage. This development has manifested monitors, sensors, and detectors that use inputs not related directly to measured electrical quantities of voltage and current.
Stator winding over temperature (Device 49S)
The purpose of stator winding over-temperature protection is to detect excessive stator winding temperatures prior to motor damage. Often, this protection is arranged just to alarm on motors operated with competent supervision. Sometimes two temperature settings are used, the lower lesser setting for alarm, the higher greater setting to trip. The trip setting depends on the type of winding insulation and on the user operating requirements. Stator over- temperature trip is usually set 5 10 °C below less than the insulation class maximum temperature rating. Motor manufacturers may might provide recommended alarm and trip settings; and for NEMA motors a maximum allowed temperature setting may might be provided.
Device 49S, RTDs
Six RTDs (two per phase) should be specified in motors rated 375 kW (500 hp) and abovegreater. These devices are installed in the winding slots when the motor is being wound. The six RTDs are spaced around the circumference of the motor core to monitor all phases. The most commonly used type is three-lead 100- © platinum. Other elements and lead configurations are available. For example a four- lead RTD is used for applications that require higher greater accuracy. The RTD device resistance increases with temperature, and a Wheatstone bridge or similar circuitry is used to provide temperature indication and output operation. . The value of the temperature trip depends on the type of winding insulation used (see Table 1) and the operating requirements of the user. The RTD type and the monitor input type must alignmatch.
For safety, RTDs should be grounded, and that ground in turn places a ground on the control module. Therefore, the control module should not be operated directly from a switchgear dc battery because these dc control schemes should normally operate ungrounded to achieve maximum reliability. However, loss of ac control voltage caused by a blown fuse could remove protection, unless the null point is near the trip setting at which time it could cause tripping.
An open RTD or an open RTD circuit appears as an infinite resistance and causes a false trip because this corresponds to a very high temperature. Some motor protection relays use RTD voting to prevent the threat of a false temperature reading (and subsequent false alarms and false tripping). To indicate an excess-temperature condition, the relay must receive sufficient high-temperature indications from multiple RTDs (the number of RTD votes is configurable). In this way damaged and open-circuit RTD inputs are ignored.
The following arrangements of RTDs are used frequently:
Monitor all six leads continuously with alarm points and time-delayed higher trip points using one monitor or a programmable logic controller.
Monitor six leads with alarm points and have use a manual trip.
Configure alarm points and trip points for selected sectors of the motor. Use a selector switch and combination indicator and alarm elements. (Precaution: An open circuit in the switch contact will cause a false trip. Bridging contacts are required.)
Use a selector switch and an indicator only.
Use one, two, or three (i.e., one per phase) alarm relays; and use one, two, or three (i.e., one per phase) trip relays set at a higher temperature.
The arrangement and monitoring of the RTDs should identify damaged RTDs. A damaged RTD or RTD connection circuit, will typically exhibit either of these two conditions:an open -circuit or a short -circuit. The protection system should be able to identify these conditions and annunciate an appropriate alarm. An open RTD or an open RTD circuit will appear as an infinite resistance to the protection system and, if not identified as an open-circuit by the protection system, will cause a false trip since because the measured infinite resistance will be identified as a very high temperature. A shorted RTD or a shorted RTD circuit will appear as a resistance lower smaller than the range for a given RTD selection. If not identified as a shorted RTD or a short -circuit by the protection system, a very low temperature will be recorded and actual RTD temperature will not be valid.
When there are two or more RTDs per phase, RTD voting provides an extra level of thermal feedback reliability, for motor thermal protection, in the event of individual RTD or wiring malfunctions. If enabled, a second (or more) healthy RTD must indicate a temperature in excess of its the trip- temperature set point, for any enabled RTD channels, before a trip command will be issued by the protection system. This feature provides for uninterrupted operation when a single damaged or open-circuit RTD input is detected. An alarm protection setting should be configured to identify the degradation of any given RTD input.
RTD voting typically applies only to RTD channels assigned for winding temperature monitoring.
If RTD voting protection is not enabled, any one winding RTD temperature input, in excess of its trip temperature setting, will initiate a motor trip if the control scheme is configured to do so.
Device 49S, Thermocouples
Thermocouples are used to indicate temperatures for alarm and trip functions, in a similar manner to RTDs. However, an open circuit in the thermocouple leads does not cause a trip because the output appears as a low-temperature condition. Where higher accuracy is required, use of a Class 1 thermocouple is advised. Use suitable thermocouple extension cable when locating the thermocouple more than 0.5 m from the transducer. (See Table 3 for temperature classification for wire insulation.) For safety reasons a thermocouple is often grounded, typically by grounding the sheath at the transducer termination.
For safety, similar to the RTDs, the thermocouple must have a grounded junction. Usually, thermocouple grounding is on the sheath at the terminal box. Thermocouple outputs are compatible with conventional temperature-monitoring and data-logging schemes, and some relays have (420 mA and 010 V) analog inputs for inputting thermocouple data via the proper interface.
Thermocouple wire must have a temperature rating for the specific application. Refer to ASTM E230 and ISA MC 96.1.[BB7] [BB6]
Temperature classifications for wire insulation
Limit or Errors ASTM E230- ANSI MC 96.1
Device 49S, Thermistors
Thermistors are used to operate relays for alarm or and trip functions, or both. These devices are not used to provide temperature indication. However, thermistors are often combined with thermocouples, which provide indication, while the thermistor operates a relatively inexpensive relay. See 7.3.2 for further details. The controller must align with the thermistors.
Device 49S, Thermostats and temperature bulbs
Thermostats and temperature bulbs are used on some motors. For instance, thermostats are bimetallic elements and are used on random- wound motors (A rapid transfer of large motors from one energized power system to another energized power system could cause very high large motor inrush currents and severe mechanical shock to the motor. The abnormal inrush currents may might be high large enough to trip circuit breakers or and blow fuses, and these currents could damage motor system components. The mechanical jolt could damage the motor, shaft, and couplings.
These effects can occur in emergency or and standby power systems when a motor is de-energized and then rapidly reconnected to another source of power that is out-of-phase with the motors regenerated voltage. Motors above greater than 37 kW (50 hp) driving high-inertia loads (e.g., crushers, shredders, fans) may require special consideration.
The problem can be eliminated if the motor circuits can be de-energized long enoughfor a period that to permits the residual voltage to decay before power is again applied to the motor. This step can be done in two ways. In one available method, auxiliary contacts or a relay on the automatic transfer switch can open the motor holding coil circuits, while the transfer is delayed ( about 3 s). This method is sometimes effective, but requires interwiring between the transfer switch and the motor starters , and depends upon the reliability of a timing device. Another method utilizes uses a transfer switch with a timed, center-off position. The switch opens, goes to the neutral or off position, is timed to stayremains there about 3 s to 10 s, and then completes the transfer. This approach eliminates any interwiring to the motors. The required time delay should be set carefully; this delay and can vary varies as the system conditions change. A third position (neutraloff) creates the danger that the transfer switch may might remain indefinitely in the off a neutral position in the event of a control circuit or contactor malfunction.
Another solution is to parallel momentarily the two power sources on transfer, connecting both sources together and then dropping one. This approach is completely effective because power to the motors is never interrupted. However, it can require new equipment. If one source is utility power, a problem can occur because some utilities do not permit paralleling another source with their systems. In obtaining permission for the paralleling from the utility, a design review can lead to additional protective relaying. An additional factor is that the combined available fault current may exceed the ratings of the connected electrical switching equipment.
In-phase transfer is another solution to the problem. An accessory on the transfer switch, known as a fast in-phase monitor or synch-check element (Device 25), measures the phase-angle difference between the two power sources. An on-site generator set would be is controlled by an automatic synchronizer, which recognizes that the two sources continually go in and out of phase. At the proper window or acceptable phase-angle difference between the sources, the synch-check element initiates transfer. The design allows for the operating time of the transfer switch so that the oncoming source is connected to the motors in phase or at a phase difference small enough to eliminate excessive inrush currents and mechanical shock. No special field adjustments or interwiring to the motors are required. For typical transfer switches with transfer times of 10 cycles (166 ms) or less, and for frequency differences between the sources of up toas much as 2 Hz, the fast synch-check element provides a safe transfer of motors.
Protection against failure to rotate
Failure to rotate
A failure to rotate occurs when the supply is single phased or if the motor or driven machine is jammed. The following protection is available:
Relays can be used to detect single phasing (see 6.3.2).
The direct means to detect failure to rotate is to use a shaft-speed sensor and timer to check whether a preset speed has been reached by the end of a short preset interval after energizing the motor. This protection is necessary for induction and brushless synchronous motors that have a permissible locked-rotor time less than normal acceleration time.
For induction and brushless synchronous motors having a permissible locked-rotor time greater than normal acceleration time, relying upon the thermal overload element (Device 49TC) is preferred. Using an inverse-time, phase-overcurrent element (Device 51) as a backup is normal (see 6.4 and 6.5).
For brush synchronous motors having a permissible locked-rotor time less than normal acceleration time, one method of protection is to use a frequency-sensitive relay connected to the field discharge resistor and a timer, because the frequency of the induced field current flowing through the discharge resistor is related to the motor speed. A frequency-sensitive, adjustable time-delay voltage relay is also available to provide this protection.
For brush synchronous motors having a permissible locked-rotor time greater than normal acceleration time, relying upon the damper-winding protection and incomplete starting sequence protection is normal.
For a large induction motor protection to start, an impedance element (Device 21) can be applied (see 8.2, Figure 111619, and Figure 121720).
Reverse rotation or loss of phase
A reversal in phase rotation or a loss of phase can be detected by a reverse-phase voltage element (Device 47) [see 6.3.4 b)] if the reversal or loss occurs in the system on the supply side of the relay. This relay element cannot detect a reversal or loss that occurs between the motor and the point at which the relay element is connected to the system. Set this element for no more than 5 percent voltage imbalance. Unbalanced voltages create large unbalanced currents (approximately six to ten times the percent voltage unbalance per NEMA MG-1-2011, 14.36.5); these negative-sequence currents can damage motor stator windings.
A backup method for preventing unbalanced power system damage is using a current-imbalance unbalance element (Device 46). Set this element to 0.2 pu negative-sequence current.
A directional speed switch mounted on the shaft and a timer can be used to detect starting with reverse rotation. Some motor loads are equipped with a ratchet arrangement to prevent reverse rotation.
Alternatively a machinery protection system may can be used with a speed monitor to detect reverse rotation (see section 8.4.4)
Protection for AC ASD applications
Motors fed from non-sinusoidal sources require careful consideration of the components of the system including the motor, cables, adjustable speed drive (ASD), power source, and protection devices and settings. Clause 5 presents factors to consider in protection of motors whereby these factors also apply to ASD applications. This section covers ASD general information, low-voltage ASD applications, and medium-voltage ASD applications.
ASD general information
This section covers some basic information for ASD applications including terminology, motor selection, ASD selection, filters and reactors, regeneration and dynamic braking, and auxiliary monitoring of ASDs.
ASD terminology
There is no single industry recognized name for adjustable speed drives (ASDs), which is the IEEE preferred terminology and will be used in this document. ASDs are ac or dc drives and are also called Variable Speed Drives (VSDs). Several terms describe ac ASDs only: Adjustable Frequency Controller (AFC), Adjustable Frequency Drive (AFD), Variable Frequency Drive (VFD) and Power conversion equipment. A Power Converter converts ac to dc; and an Inverter inverts dc to ac: ASDs discussed in this section may be current source, voltage source, or PWM.
NEMA MG-1 Part 30 and 31 inverter duty
A motor should be selected to operate at or below its nameplate rating over the entire speed range for the specific application. Motor design and manufacturing standards address issues of concern for motors operating on non-sinusoidal power.
NEMA has recognized the elevated stresses imposed on induction motors by adjustable frequency controls and has developed a performance standard for motors that are specifically identified as inverter duty or inverter rated. Part 31 of NEMA MG-1 addresses issues of particular concern to ASD-fed motors such as basis of rating over a speed range, thermal aging of insulation for operation at different loads and speeds, minimum breakaway and breakdown torque requirements, overload and overspeed capabilities, voltage spikes, and vibration, among others. Of unique pertinence to such definite-purpose motors is the ability to better withstand the repetitive voltage spikes that are characteristic of modern, fast switching devices used in adjustable frequency controls.
In addition to NEMA, other motor standards that address ASD applications include IEEE 841, API 541, API 546, and API 547.
Selecting drives
Clause 5 presents factors to consider in protection of motors whereby these factors also apply to ASD applications. Also refer to IEEE Std. 1566 for MV ASD selection and purchasing information. Some considerations are also below:
Load characteristics
Motor nameplate data
Motor speed control range, heating and performance considerations
Breakaway torque requirements
Load acceleration/deceleration requirements
Environment (temperature, altitude, humidity)
Multimotor or single motor
Power system (voltage, harmonics, short-circuit current)
Code requirements
Application considerations (motor cable lead length and configuration)
Courtesy of Schneider-Electric
Typical LV ASD
Limiting system harmonics can be a significant factor for selecting a drive. Figure 32a35a) is based on a 6 pulse drive system. Refer to Figure 36 for a more detailed example of a 6 pulse MV ASD. Refer to Figure 32b35b) showing a 12 pulse system., The protection is identical because the 12 pulse consists of two 6 pulse channels operating simultaneously, with the input source voltage shifted by 30 degrees. In many instances, however, the input isolation transformer is a three-winding transformer with one secondary winding connected in delta, and the other secondary connected in ungrounded wye to obtain the 30 degree phase shift. This is done to significantly reduce the formation of harmonics and the higher the pulse count the lower the harmonics. The transformer differential scheme would therefore include both secondary windings. The same philosophy would be used with drive systems with higher than 12 pulse designs. Drives use pulse counts that are multiples of 6, where some are 54 pulse and some may be higher.
Typcial ASD zones of protection (6 pulse) b) Typical ASD zones of protection
(12 or 18 pulse with transformer)
Reprinted with permission from Schneider-Electric.
Typical Adjustable Speed drive zones of protection
ASD bypass circuit protection
For ASD systems that utilize a bypass device to allow motor operation at rated full-load speed, motor overload protection must be provided in the bypass circuit.
Bypass circuitry is usually accomplished using bypass contactors and are often used with AC drives. There are two reasons for bypass. The most common is for maintenance purposes. If the drive is out for maintenance the bypass contactor is closed to allow the motor to run across the line (ATL). The second reason is to allow the ASD to bring one motor to full speed and then switch to another motor. This allows the user to have one drive for many motors. The motor is provided with all required motor protection while in bypass mode. To avoid damage to the ASD, the motor field should be allowed to decay for a minimum of three open-circuit AC time constants after disconnecting from line power and before connecting to the control. It also should be ensured that the ASD has the capability of restarting a coasting motor before switching from line power back to control power while the motor is still rotating.
Refer to Figures 33 36 and 34 37 for two contactor and three contactor bypass systems, respectively. In Zone 1, the LV and MV protection schemes may vary.
Reprinted with permission from Schneider-Electric.
Two-contactor bypass system
Reprinted with permission from Schneider-Electric.
Three-contactor bypass system
ASD output filters and reactors
These load filters are typically required for long cable lengths and will further reduce the reflected wave amplitude seen on the cable and increase cable life.
Reflected wave voltage spikes can damage the motor winding, especially the first turns of a winding. NEMA's "Application Guide For AC Adjustable Speed Drive Systems" section 5.2.9 addresses this phenomenon. Several factors including pulse rise time, cable length, minimum time between pulses, minimum pulse duration, transition type (single or double), and the use of multiple motors affect the peak voltage at the motor's terminals. Some ASDs allow adjustments for the minimum time between pulses and/or minimum pulse durations in order to reduce the magnitude of the reflected wave voltage spikes.
If the motor cable length is long enough to cause the reflected wave voltage spikes that could damage the motor, then an output reactor can be used to attenuate the peak voltage. A reactor with impedance of 1.5% is often sufficient to reduce the voltage spikes although some manufacturers use 3% if the voltage drop can be tolerated. For longer runs a dV/dt filter can be used. These filters have a 1.5% reactor and an RC snubber circuit to increase the rise time of the pulses, thereby reducing the dV/dt. For very long cable runs, a sinewave filter must be used. These have multiple stages of inductors and capacitors and significantly filter out the PWM leaving a sinusoidal voltage to go to the motor. The reactor filter, dV/dt filter, and sinewave filter are progressively more costly, respectively.
Overtemperature and overload protection
Overheating of motors can occur even at current levels less than a motors rated full-load current. Overheating can be the result of the shaft-mounted fan operating at less than rated nameplate RPM. For motors that utilize external forced air or liquid cooling systems, overtemperature can occur if the cooling system is not operating. In these instances, overtemperature protection using direct temperature sensing is recommended, or additional means should be provided to ensure that the cooling system is operating (flow or pressure sensing, interlocking of ASD system and cooling system, etc.)
ASD systems must protect against motor overtemperature conditions where the motor is not rated to operate at the nameplate rated current over the speed range required by the application. Such protection may be provided by:
Motor thermal protector in accordance with NEC Section 430.32
ASD system with load and speed-sensitive overload protection and thermal memory retention upon shutdown or power loss, except that thermal memory retention upon shutdown or power loss is not required for continuous duty loads.
Overtemperature protection relay utilizing thermal sensors embedded in the motor and meeting the requirements of NEC Sections 430.32(A)(2) or (B)(2).
Thermal sensor embedded in the motor whose communications are received and acted upon by an ASD system.
For multiple motor applications, individual motor overtemperature protection shall be provided as required in NEC Section 430.126(A).
When running an ASD at the rated frequency of 50 Hz or 60 Hz, a standard overload curve can be used as shown in Figure 6 by the 50 Hz line where the Class 10 overload would trip in 10 s at 6x rated current. The starting/maximum current setting for a typical ASD may be in the range of 115% to 170% of nameplate rated current, Figure 35 38 below expands the main area of interest for the ASD application.
A motor cooled by a separate blower can also be protected by a separate overload such as Class 10 bi-metallic overload relay or built-in to the software and electronics of the internal overload of the ASD.
However, when running at reduced speeds, self-cooled motors cannot generate as much air flow as they can at rated speed, and the motor may overheat at lower current levels. Reduced speed overheating is more common for constant torque applications vs. variable torque applications. All applications have minimum time limits at various speeds to maintain adequate cooling.
Some ASD internal overloads take this reduced cooling at slower speeds (lower frequency) into account and adapt the overload curves for lower speed operation.
Reprinted with permission from Schneider-Electric.
Typical ASD internal overload protection for self-cooled motors
The x-axis is the ratio of the actual current to rated current of the motor, the Full Load Current (FLC). The 50 Hz curve represents the nominal frequency of the motor, either 50 Hz or 60 Hz. The lower speed curves show the levels at which the electronic overload would trip to protect the slow-turning motor, even though the current could be less than the rated FLC of the motor.
Where the ASD is marked to indicate that motor overload protection is included, additional overload protection is not required if the overload protection is sized to protect the motor for a specific application.
If the ASD manufacturer does not provide an overload that can adapt the trip curve depending on the speed of the self-cooled motor, then direct measurement of the motor winding temperature should be accomplished by positive temperature coefficient resistors (PTC), thermocouple devices, thermal switches, or RTD sensors with a relay to trip on overtemperature. Refer to 7.3.2 for more information on PTC devices.
Regeneration and dynamic braking
Most adjustable speed drives can provide a controlled speed deceleration of the connect load. However for very fast stopping requirements, alternative braking systems may be required. In some applications, the controlled deceleration time of the motor and load combination can be excessive and can present concerns for some situations. In the event of an emergency stop, the input and/or output power from the drive are removed. The connected load may continue to rotate and coast to a stop, after the removal of power, due to the load inertia. If an immediate or emergency stop of rotation of the motor and connected load is required, a braking method must be used. The most common types of motor braking systems, available with some ASD, are Dynamic Braking and DC Injection Braking systems.
A dynamic braking system converts the energy, being regenerated from the rotating motor and connected load, into heat that is dissipated through a connection to suitably sized motor braking resistors. The braking resistor must be suitably sized to the load. Using dynamic braking stops the motor at the fastest rate allowed by the load but it cannot act as a holding brake. Dynamic braking should not be considered if the connected load typically operates at slow speeds (generally at less than 10Hz) and braking is required. The dynamic braking system or ASD braking parameters should be configured to limit the application time of the external resistors in the event of a system malfunction as excessive amounts of heat energy will be generated the event of an extended over-current condition. Alternative control methods can include braking application contactors, with control circuits configured with resistor thermal sensors and/or braking current overload protection, which, in the event of an extended over-current condition, would open the connection to the braking resistors. The protection mode in these cases is to protect the braking resistors from excessive currents or a duty cycle longer than their designed level.
A DC injection braking system applies DC current, sourced from or controlled by the drive, which is quickly applied to the stator windings of the motor. The ASD typically will include parameters for the control of the application time and braking current applied. The programmed level of the DC current and application time determines how fast the motor will decelerate and come to a stop. For slow speed loads, DC injection will be the most effective braking method as it does not rely on the regenerative energy, from the motor, to provide the braking energy. Many DC Injection Braking systems can be configured to also prevent the rotor from spinning freely, when the main power is removed from the motor. It is recommended that some level of over-temperature protection be incorporate in the motor, tied back to the DC injection system, since the application of the DC current does generate significant heat within the motor. Because of this, some systems can also be configured to provide a level of motor preheating.
Protection device monitoring by auxiliary control equipment
In some applications vibration and temperature monitoring devices may be monitored by an auxiliary control panel; e.g. compressor or pump unit control panel, DCS or station control panel. In these applications; permissive to start, alarm and shutdown signals should be coordinated with the drive to ensure the protection of the motor and the driven equipment.
Low voltage AC ASD motor protection
Refer to Figure 32 for the typical zones of protection for a LV ASD application. The LV breaker in Zone 1 protects the cable feeding the ASD and provides short-circuit protection for the ASD. The internal software and electronics of the ASD typically provide overload and overtemperature protection for the motor. In addition, the ASD has current limit features providing short-circuit protection and detection, which trips/gates off the IGBTs and isolates faults downstream. Overtemperature and overload protection is discussed in 9.1.6. It may be desirable to protect larger low voltage ac motors with protective relays.
Low voltage ASD bypass motor protection
If a bypass system is used, motor protection is shown in Figures 33 and 34.
LV ASD multiple motor applications
For multiple motor applications, individual motor overload protection shall be provided in accordance with NEC® Article 430, including NEC sections 430.130 and 430.131.
MV AC ASD motor protection
Use some of the content in: The protection technology of high voltage variable frequency drive system, Qing Xion et al, IEEE paper?
The main objective of this section is to outline the general protection philosophies related to AC Drive motor protection. Most AC drives use transformer isolated frequency conversion circuitry to drive synchronous and induction motors and, therefore appear as transformer loads on the electrical system. It should be noted that these motors operate asynchronously from the electrical power system
MV protection
Figure 36 illustrates a protection approach from an adjustable-speed drive application and can be used for guidance in the selection of adequate protection for motors used with AC Drives. It should be noted that drives can have different topologies and components therefore the selection of adequate protection needs to be based on the specific application and technologies used.
The two medium-voltage, adjustable-speed drive systems in common use are the induction motor drive and the synchronous motor drive. Both systems may include an input isolation transformer, source side converter, dc link reactor (and/or capacitors), load side inverter, and motor. The synchronous motor drive also includes an excitation system. The converters employ power electronic devices (such as thyristors) to control voltage and/or current. Other drive technologies would require similar protection considerations. These include pulse width modulated, VSI, and CSI systems.
The drive system protection can be divided into the following three zones of protection as shown in Figure 3639:
Zone 1 - input zone (input transformers are included for some systems)
Zone 2 power electronics
Zone 3 motor
Protection commonly included in ASD
The protection elements included within the ASD controls vary from manufacturer to manufacturer. As shown in Figure 3639, the protection can be broken down into three major categories, line side protection (Zone 1), system level protection (Zone 2), and load side protection (Zone 3). The following is the protection most commonly included in ASDs:
a) Line side protection (Zone 1)
Short-circuit/overcurrent some are protected with a fuse, circuit breaker, or protective relay overcurrent function
Overload overcurrent protection with time delay
Voltage unbalance loss of input phase
Ground-fault overcurrent
b) System Level Protection (Zone 2)
DC Overvoltage
DC Undervoltage loss of control power
Over temperature this includes the rectifier and inverter heat sinks as well as the enclosure temperature
c) Load side protection (Zone 3)
Ground-fault
Motor overcurrent
Motor overload I2t
Motor stall
Motor over speed
Current unbalance
Under load may indicate a process malfunction and will protect the machinery and the process in this fault condition
External Fault an external relay input
Motor Differential
Overexcitation (V/Hz)
Vibration (See section
)
Thermal overtemperatue
Switching protection (snubber circuit)
Typically ASDs offer a current limiter and torque limiter function. These functions can be programmed in order to keep the current and/or the torque at a maximum allowed limit. If the current or torque demand from the process or speed controller exceeds the current/torque limit, the actual speed is limited and the current/torque is kept below the limits. This function can be used to limit the current to the motor.
Reprinted with permission from Rockwell Automation.
MV Typical Adjustable Speed drive protection
(Figure 39 from IEEE Std PC37.96-2012 2012)
Reprinted with permission from Rockwell Automation.
Zone 1 protection with a transformer and ground fault protection; and without a transformer
Figure 40 Zone 1 protection with and without a transformer
Zone 1 protection
The feeder breaker supplying the ASD typically is equipped with overload and short circuit protection for the input transformer and / or the drive electronics as shown in Figure 3740. Typically, a phase time overcurrent element (51) is applied for overload protection and an instantaneous overcurrent element (50) short circuit protection. A 51 element that operates on the fundamental frequency (i.e., not rms) may be set with a lower pickup, as it will not respond to the harmonic components of the load current. If there is an isolation transformer, the 50 element is typically set at 140% of the transformer secondary through-fault current and above the transformer inrush current. In cases where the drive employs an active front-end the 50 element can be set lower as the drive normally limits the starting current to less than two times rated. Occasionally, a differential relay has been applied to the primary feeder to provide high speed tripping for faults up to the transformer high-side winding.
Differential protection for large isolation transformers can be considered but may not be practical. For instance, in large ASD applications, the ASD isolation transformer typically has multiple secondary windings. In those cases, it is not practical to have conventional differential protection. The feeder overcurrent relay can then be relied upon to provide high speed protection for the isolation transformer primary windings. Relays which do not respond to DC offset currents should be selected to allow for the instantaneous element (50) to be set as sensitive as possible. The feeder overcurrent (51) can provide conventional time-delayed protection. For multiple secondary winding configurations the feeder overcurrent relay (51) may not provide protection for secondary winding faults. The drive integral protection would protect for those faults. In some cases, the drive integral protection includes a power differential that compares the transformer input and drive output power.
Where isolation transformers are used which have not been specifically designed for harmonic loading, IEEE C57.110 Recommended Practice for Establishing Transformer Capability when Supplying Non-sinusoidal Load Currents [B2] may be used to apply transformer de-rating factors for each harmonic. Devices exist that will provide thermal protection based on this guide.
There may be additional protection applied for faults on the secondary side of the isolation transformer. This may include a zero-sequence voltage detection circuit if the transformer secondary is ungrounded or a residual or neutral overcurrent for a grounded wye secondary connection. Some ASD manufacturers employ fuses for transformer through-fault protection.
Zone 2 protection
The firing of the thyristors is controlled by the drive digital control system. The control system uses the input ac source voltage to determined proper firing angles and magnitudes. The control system is, therefore, sensitive to irregularities in the input voltage source. The control system monitors the drive input and output voltages typically via voltage transformers and attenuating resistors. The control system includes minimum and maximum line voltage settings that actuate alarms and/or trips. In addition, over/under voltages are also monitored. Typical alarm settings would be 90% and 110%. Trip settings will vary depending on the manufacturer; however, a typical low-voltage trip setting would be in the range of 70% for 30 seconds, and a typical high-voltage setting would be in the range of 130% for a few cycles. Some drives may also include DC link reactor overvoltage protection.
Short-circuit protection is typically provided by fuses installed ahead of the thyristors. Blown fuse detection is provided by the drive control system. Upstream protective relays should be coordinated with fuse characteristics.
Overcurrent protection is provided for the converter electronics and interconnected bus or wiring. Current levels are limited to acceptable levels by control action and the drive is tripped if current is above these levels for a preselected time. During speed changes, allowable current levels are determined by the current limit setting. During normal operation, current levels are typically limited to rated current. If currents remain above these levels for a predetermined time, the drive is tripped.
Large drive systems depend heavily on the drive cooling system. Failure of the cooling system can cause overheating of the input and output converter thyristors in a few seconds. Drives can be air cooled or liquid cooled. In either case, temperature switches should be provided to monitor temperatures in the converter sections. In air-cooled designs, air flow switches are usually provided to monitor cooling fan operation. High temperature indication or loss of cooling air flow typically would cause a drive to trip.
The dc link reactor between the input and output converters is subject to over temperature during abnormal operation or inadequate cooling. Multiple temperature switches are typically provided to monitor reactor ambient temperatures. Alarm and trip settings are based on the reactor insulation rating. Some drive systems include dc overvoltage detection. An overvoltage detection results in the suppression of gate pulses to reduce dc link voltage. The control system monitors dc link voltage via a resistive divider. The exact setting of the dc overvoltage detection should be determined by the drive system manufacturer. A typical overvoltage setting is in the range of 110% to 120%.
See Figure 38 41 for the location of protective functions in Zone 2.
Reprinted with permission from Rockwell Automation.
Table B.3 Table B. SEQ Table_B. \* ARABIC 3 Security, communication, and other protection functions*
Protective functionIEEE device function numberCommentsSecurity processing function16EC or 16SCVPN, encryption module, etc.Firewall16EFOr message filtering functionNetwork managed function16EM(e.g., configured via SNMP)Router16ERSwitch16ES or 16SSExample: Ethernet switch is 16ES,
dial-up port switch is 16SSEthernet managed switch16ESMEthernet router with firewall, VPN for secure communications16ERFCMSee IEEE C37.2-2008
Annex B, Figure B.2Serial encrypting modem16SCTSee IEEE C37.2-2008
Annex B, Figure B.1Other serial communications components16STExample: 16ST = Auto-answer modem or telephone switchClock (or timing source in IEEE Std C37.2) CLK ClockNon fault disturbance recording
Digital fault recordingDDR
DFRRDRE Basic functionality (IEC)
RADR Analogue channel (IEC)
RBDR Binary channel (IEC)Environmental data ENVFault Locator RFLO (IEC)High impedance fault detector HIZHistorian HSTHuman machine interface HMILogic, scheme LGCSubstation metering METMMTR, MMXU (IEC)Phasor data concentrator PDCPhasor measurement unit PMUPower quality monitor PQMRemote input / output device RIORemote terminal unit RTUSequence of events recorder SERTrip circuit monitor TCM*Refer to IEEE Std C37.2-2008 device function numbers
B.2 Main device letters
The main device letters denote to which the numbered device is applied or is related.The meaning of each single suffix letter, or combination of letters, should be clearly designated in the legend of the drawings or publications applying to the equipment.
A Alarm / auxillary power
AC - Alternating Current
AN Anode
B - Battery/ blower / bus
BK Brake
BL Block (valve)
BP Bypass
BT Bus tie
C Capacitor / condenser / compensator / carrier current / case / compressor
CA Cathode
CH -Check (valve)
D Discharge (valve)
DC - Direct current
E Exciter
F Feeder / field / filament/ filter /fan
G - Generator or Ground (See Note 1)
H Heater/ housing
L Line/ logic
M Motor / metering
MOC Mechanism operated contact4
N Neutral/ network (See Note 1)
The suffix N is preferred when the device is connected in the residual of a polyphase circuit, is connected across a broken delta, or is internally derived from the polyphase current or voltage quantities. The suffix G is preferred where the measured quantity is in the path to ground, or, in the case of ground fault detectors, is the current flowing to ground. See IEEE C37.2-2008, Figure C.2 in Annex C for examples.
MOC denotes a circuit breaker mechanism-operated auxiliary switch that is mounted on the stationary housing of a removable circuitBreaker
P Pump/ phase comparison
R - Reactor/ rectifier/ room/ rotor
S Secondary / stator / strainer / sump / suction (valve), synchronizing
T Transformer / thyratron
TC Trip Coil
TH Transformer (high-voltage side)
TL Transformer (low-voltage side)TM Telemeter
TOC - Truck-operated contact (See Note 3)
TT Transformer (tertiary-voltage side)
U Unit or Under
TOC denotes a circuit breaker truck-operated auxiliary switch that is mounted on the stationary housing of a removable circuit Breaker.
B.2 Other suffix letters
Other suffix letters for protective devices that may typically be used for motor protection schemes are shown below. The meaning of each single suffix letter, or combination of letters, should be clearly designated in the legend of the drawings or publications applying to the equipment.
BF - Breaker Failure
J - Jam function of a multifunction motor protection relay
LR Locked rotor
O Over
PF Power factor
Q Reactive Power
T - Test / trip / trailing
TC Torque control / thermal capacity
TD - Time delay
TDC - Time-delay closing contact
TDDO - Time delayed relay coil drop-out
TDO - Time-delay opening contact
TDPU - Time delayed relay coil pickup
THD - Total harmonic distortion
U - Up / under
V- Voltage restrained / voltage controlled
VB Vibration
W - Watts
Z - Impedance
B.3 Auxiliary Devices
These letters denote separate auxiliary devices, such as:
C -Closing relay / contactor
CL Auxiliary relay - closed (energized when main device is in closed position)
L - Lowering relay
O - Opening relay / contactor
OP - Auxiliary relay, open (energized when main device is in open position).
PB - Push button
R - Raising relay
U - Up position switch relay
V Valve
X - Auxiliary relay(See Note 1)
Y - Auxiliary relay
Z - Auxiliary relay (See Note 1)
When controlling circuit breakers with an X-Y relay control scheme, the X relay is the device whose main contacts are used to energize the closing coil or the device which in some other manner, such as by the release of stored energy, causes the breaker to close. The contacts of the Y relay provide the anti-pump feature for the circuit breaker.
B.5 Auxiliary Contact Position Definitions
The letters a and b shall be used for all auxiliary, position, and limit switch contacts for such devices and equipment as circuit breakers, contactors, valves and rheostats, and contacts of relays as follows:a Contact that is open when the main device is in the standard reference position, commonly referred to as the nonoperated or de-energized position, and that closes when the device assumes the opposite position.
b Contact that is closed when the main device is in the standard reference position, commonly referred to as the nonoperated or de-energized position, and that opens when the device assumes the opposite position.
Annex C Protection setting considerations (informative)
C.1 Typical motor protection settings
Typical recommended protection settings for motor protection devices are shown below:
Table BC.1 Typical motor protection device settings
DescriptionsAlarmTripThermal overload105%(110% = SF 1.0), (125% = SF 1.15)Unbalance voltage2%3-5%Unbalance (current)10%15% - 20%Ground fault0.1 ground CT0.15 ground CTUnder/Over Voltage±10%±15% 20%Stator winding temperature-15oC insulation max-10oC insulation macBearing temperature15 oC before damage10 oC before damageShort circuit 600% - 720%
C.2 Current unbalance and ground fault protection in HRG system
C.3 Overcurrent protection in fixed capacitor applications
Annex DC Motor Protection Examples
DC.1 LV Instantaneous breaker example
DC.2 LV ASD example,
DC.3 MV fused contactor example
DC. 4 MV breaker start example
DC.5 MV ASD example
< induction Paul Cardinal Ed Larsen>
Annex E Partial discharge (informative)
Partial discharge activity can be measured periodically or monitored online continually. The trending of PD over time is used to predict motor insulation problems, identify motors to be removed from service for repair and maintenance, thereby protecting the motor from catastrophic failure (e.g. winding short-circuit).
While partial discharge monitoring is not in its infancy, this technology is still rapidly developing and the latest technologies should be considered.
E.1 PD background
< 2012 paper>
The condition assessment of stator insulation using on-line partial discharge (OLPD) testing and monitoring has been used for over 50 years. Pioneering developments in Canada/USA (using HV coupling capacitor (HVCC) sensors, Resistive Temperature Detector (RTD) sensors and Stator Slot Couplers (SSC) sensors) and in the UK/Europe (using both HVCC and air-cored, Rogowski Coil (RC) sensors) in the 1960s showed that it was possible to make effective measurements of PD activity in the medium voltage (MV) stator windings of the machines [E1]. Over the past 50 years, the OLPD testing and monitoring of rotating machines has been discussed in [1
.]by many authors as a key tool to understanding the causes of stator insulation failure. Today, the continuous monitoring of PD activity in rotating machines is an effective method to identify sites of localized damage or degradation ahead of scheduled preventative maintenance outages.
The relevance of OLPD monitoring within rotating machines is discussed by Stone et al in [4] and [5] as an effective technique to determine the condition of the MV stator winding insulation. In the cases discussed by Renforth et al in [2012 paper] the authors used pre-installed 80 pF, HVCC sensors, one or more per phase, to monitor both phase-to-phase and phase-to-earth PD activity within the rotating machine. Interpretation of the severity of any PD activity has traditionally been carried out by considering the peak PD level (typically given in mV or pC) and also the number of PD pulses across the 50/60 Hz power cycle.
With regards to the application of the online partial discharge (OLPD) monitoring of MV motors located in Ex/ATEX hazardous gas zones, a recent study by S. Haq et al in [3] discussed minimum discharge levels required to produce a possible spark risk. This study was made on rotating machines operating within different gas groups and reports that static charge within the stator winding should be kept below 10 nC for a motor operating in gas group B (Hydrogen) to minimize the risk of gas ignition (10 nC equates to a minimum ignition energy of approximately 0.019 mJ). This provides a benchmark, maximum level for static charge in motors exposed to flammable atmospheres, emphasizing the benefit of continuous monitoring of partial discharge activity.
NOTE: PD is not a purely static charge in this sense; it is known that PD activity can lead to active charge surface tracking of 100 nC+ (100,000 pC+) in extreme cases.
E.2 PD sensor options
< 2012 paper>
There are a wide range of sensor options available for the on-line detection of PD activity in motors. These sensors include Slot Section Couplers (SSC), Resistance Temperature Detector (RTD), High Voltage Coupling Capacitors (HVCC), High Frequency Current Transformers (HFCT), Rogowski Coil (RC), and Transient Earth Voltage (TEV) sensors. The three main types of OLPD sensors used for MV motors (HVCC, HFCT and RC sensors) are shown below in Figure E.1.
It can be noted from Figure E.1 that at 10 MHz, the HVCC sensor is the most sensitive, followed by the HFCT sensor, and then the RC sensor. The most suitable sensor solution for any application will depend on the motor to be tested and the most suitable point of attachment (POA) for the sensor on the network, either in the motor terminal box or at the switchgear feeder cable enclosure.
Reprinted with permission from HVPD.
Figure E.1 - Three main types of OLPD sensors used for MV motors
As discussed in the IEEE and IEC standards [8] and [9] (in sections 11 and 5 respectively), the risk of misinterpreting PD signals always exists due to interference exhibiting similar characteristics as the PD signals. These IEEE and IEC standards discuss time and frequency domain methods of noise separation, types of interference that can be expected in OLPD testing, and the importance of distinguishing between the origins of the PD activity. The remote monitoring technique developed by the authors has been built on PD classification knowledge rules described in [6] and [7] that use time and frequency domain parameters to distinguish between PD types and interference. This new, remote PD monitoring technique with HFCT sensors located at the central switchboard can also reduce the risk of Variable Speed Drive (VSD) and inverter drive pulses at the machine being classified as noise, as such pulses are attenuated by the low-pass filtering effect of the power cable from the machines under test to the remotely connected OLPD sensors.
While the HVCC sensor is the most widely applied sensor for OLPD monitoring of MV motors, split-core, ferrite HFCT sensors have now been implemented (after over 15 years of industrial application) for the on-line PD testing of in-service MV cables and motors. The HFCT sensor works inductively to detect PD currents that propagate from the motor winding to the connected power cables. Due to the wideband frequency response of the HFCT sensor (from around 100 kHz to 30 MHz) it is suitable for permanent installation within either the motor terminal box or switchgear cable enclosure at the remote end of the cable as shown in Figure E.2.
E.3 HFCT sensor installation
< 2013 paper>
Figure E.2 shows a typical permanent OLPD sensor installation consisting of three HFCT sensors (one per phase mounted on silicon collar supports on the cable tails) and a Transient Earth Voltage (TEV) sensor inside a 10 kV switchgear cable enclosure. The photo on the left shows the sensor installation engineer measuring the clearance and creepage distances of the MV cable termination to ensure these meet the required International Electrotechnical Commission (IEC) and IEEE clearance and creepage distances maintained with the HFCT sensors installed.
Reprinted with permission from HVPD.
Figure E.2 Permanent HFCT sensor installation one per phase (inside the cable box of a 10 kV switchgear panel)
The dimensions of the MV power cable are important as they determine the inductance and capacitance per meter (or per foot) of the cable, which effects the distortion of transient PD pulses as they travel along it. For example PVC-insulated cables usually cause the most PD pulse attenuation, followed by PILC and EPR cables while XLPE cables cause the least attenuation of the travelling pulse. This is related to the different values of relative permittivity of the dielectrics ( QUOTE used in the different cable types and the geometry/dimensions of the cable. On-line PD measurements on XLPE cables (in the voltage range 3.3 - 15 kV) have been shown to have an effective measurement range for the HFCT sensors of up to 2.5 km/1.5 miles from the motor under test [1], as illustrated below in Figure E.3.
Figure E.3 Measurement range for a wideband HFCT sensor connected at the switchgear enclosure for XLPE cables.
E.4 Peak PD level and PD activity guidelines
< 2013 paper>
A review of published papers on the measurement and analysis of OLPD severity in MV motors by Renforth etc. [1] has shown that the most effective way to measure the severity of any PD activity is to use a combination of three measurements:
Peak PD level (Q) measurements (in nC for HFCT).
Number of PD pulses (N).
PD activity (Normalized Quantity Number - NQN) this is a measure of the cumulative PD activity across the power cycle (measured in nC/cycle for HFCT).
It can also be noted that, as discussed in the IEEE and IEC standards [6, 7] - sections 11 and 5 respectively, the risk of misinterpreting OLPD signals always exists due to sources of electromagnetic (E/M) noise producing similar characteristics as the PD signals. The IEEE and IEC standards [6, 7] discuss time and frequency domain methods of noise separation, the types of interference that can be expected in OLPD testing, and the importance of distinguishing between the different origins of the PD and/or noise sources. Table E.2 below gives guideline OLPD levels for MV stator winding insulation condition assessment against Peak PD levels (in nC nanoCoulombs) and cumulative PD Activity (in nC/cycle - nanoCoulombs per power cycle) on motors in the 10-15 kV voltage range.
TABLE E.2
OLPC guideline levels for MV Motors in the 10-15 kV voltage class
On-line PD Level Guidelines for MV Motors 10 15 kV * Condition
Assessment Peak OLPD Level
(nC)OLPD Activity (nC/cycle) Excellent < 2 < 50 Good 2 4 50 99 Average 4 10 100 249 Still Acceptable 10 15 250 500 Inspection Recommended 15 25 501 999 Unreliable > 25 > 1000
The peak PD level and PD activity guidelines shown in Table E.2 are based on Renforth et. al [2013 paper] and also information published in the IEEE and the International Council on Large Electric Systems (CIGRE) guides for OLPD testing of in-service MV motors [6, 8]. While these guideline PD levels provide a good basis for the initial condition assessment of a MV motors stator insulation, it should be noted that the OLPD condition assessment should not be based on PD magnitude and activity levels alone. It is the trend and variation in the OLPD activity over time which is generally considered as being more important in terms of a diagnostic tool. Therefore it is also important to closely monitor the trends in both the Peak PD level and, more importantly, PD Activity over time, using continuous OLPD monitoring systems.
REFERENCES
G. C. Stone and J. Kapler, Stator Winding Monitoring, in IEEE Cement Industry Conference, 1997.
E. Wen Shu and S. Boggs, Dispersion and PD Detection in Shielded Power Cable, Dielectrics and Electrical Insulation Society, Vol. 24, No. 1, January 2008.
S. Haq, B. Mistry and R. Omranipour, How Safe is the Insulation of Rotating Machines Operating in Gas Groups B, C & D?, IEEE-PCIC, Toronto 18-20 September 2011.
G. C. Stone and V. Warren, Objective Methods to Interpret Partial Discharge Data on Rotating-Machine Stator Windings, in IEEE Transactions on Industry Applications, Vol. 42, No. 1, January 2006.
V. Warren, G. C. Stone and M. Fenger, Advancements in Partial Discharge Analysis to Diagnose Stator Winding Problems, in IEEE International Symposium on Electrical Insulation, Anaheim, CA USA, April 2000.
R. Mackinlay and C. Walton, 2001, Diagnostics for MV cables and switchgear as a tool for effective asset management", Proceedings CIRED June 2001.
M. Seltzer-Grant, D. Denissov, R. Mackinlay, F. Petzold, L. Renforth and H. Schlapp, On-Line Continuous Monitoring for In Service Distribution Class Cables and Switchgears, Proceedings of the 21st International Conference on Electricity Distribution (CIRED), Frankfurt, 6-9 June 2011.
IEEE Std. 1434: Trial Use Guide to the Measurement of Partial Discharges in Rotating Machinery, 2000.
British Standards Institution. BS EN: 60034-27, Off-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines, London, 2011.
Y. Asiri, A. Vouk, L. Renforth, D. Clark and J. Copper, Neural Network Based Classification of Partial Discharge in HV Motors, IEEE Conference and Exhibition on Electrical Insulation, Annapolis, MD, USA, 5-8 June 2011, pp.333-339.
R. R. Mackinlay, New Methods in On-line PD Detection for HV Plant in UHVnet Conference, Cardiff, UK, January 2005.
G. C. Stone, S. R. Campbell and H. G. Sedding, Characteristics of Noise and Interphasal PD, in IEEE Electrical Insulation Conference, Annapolis, MD USA, June 2011.
REFERENCES
L. A. Renforth, P.S. Hamer, D. Clark, S. Goodfellow and R. Armstrong, A new technique for the remote partial discharge monitoring of the stator insulation of high-voltage motors located in Ex (hazardous gas) locations
IEEE-PCIC 2012-067, New Orleans, September 2012.
Motor Reliability Working Group, IEEE Transactions on Industry Applications Report of Large Motor Reliability Survey of Industrial and Commercial Installations, Part I and II, Vol. IA-21, No. 4, pp.863-872.
EPRI, Improved motors for utility applications and improved motors for utility applications industry assessment study, Vol. 1, EPRI EL-2678, 1763-1, final report, and Vol. 2, 1763-1 final report, October 1982.
M. J. Foxall, A. P. Duffy, J. Gow, M. Seltzer-Grant and L. Renforth, Development of a new high current, Hybrid Ferrite-Rogowski, high frequency current transformer for partial discharge sensing in medium and high voltage cabling, in 59th International Wire & Cable Symposium - November 7th - 10th 2010 - Rhode Island Convention Centre, Providence, RI, USA.
G. C. Stone and V. Warren, Objective Methods to Interpret Partial Discharge Data on Rotating-Machine Stator Windings, in IEEE Transactions on Industry Applications, Vol. 42, No. 1, January 2006.
IEEE Std. 1434: Trial Use Guide to the Measurement of Partial Discharges in Rotating Machinery, 2000.
British Standards Institution. BS EN: 60034-27, Off-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines, London, 2011.
CIGRE Task Force 15.11/33.03.02 Knowledge Rules for Partial Discharge Diagnosis In Service, April 2003.
E.4 Cable PD
Standard cable insulation voltage ratings are 600 volts rms (850 volts peak), 2000 volts rms, and 5000 volts rms, with 1000 volts rms available on a limited basis. Reflective wave stress of 2 per unit to 2.4 per unit on 480 volt is 1300 to 1560 volts peak while 575 volt systems result in 1620 to 1945 volts peak stress. Although peak reflected wave duration is less than 1 QUOTE second, it occurs at the carrier frequency rate, which is typically 3 to 12 kHz for drives up to 15 kW (20 hp) and 1.5 to 3 kHz for larger drives. Thus, there is a concern whether a satisfactory service life for 600 volts rms rated cable is achievable with 2 to 2.4 per unit repetitive wave stress.
The dielectric failure mechanism most likely to reduce cable life is if the insulation is susceptible to corona at the 2 to 2.4 per unit peak transient voltage. Partial discharge inception voltage (PDIV) is the minimum applied voltage at which partial discharges occur, that is, the lowest applied voltage that causes electrical breakdown of the air around the cable or in air voids. No degradation in sine wave rated cable life is expected if the measured PDIV peak voltage is higher than 2 to 2.4 per unit reflected wave peak voltage. PDIV tests performed on 600 volt XLPE and 15 mil THHN PVC insulation under wet conditions of 90 per cent relative humidity for 48 hours show the PDIV of XLPE decreased only 5 per cent, while PVC had a 50 per cent reduction in PDIV level. There is concern for 15 mil thickness PVC wire use in wet locations because the reduced PDIV level is within the level of the peak reflected wave voltage range that occur on 480 V and 575 volt applications. Therefore in wet locations 600 volt XLPE insulated cables or 600 volt PVC insulated cable of phase insulation thickness of QUOTE e" 30 mils should be specified to handle the 2 per unit reflected wave transient.
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See footnote 3.
Reprinted with permission from NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas Copyright © 2008, National Fire Protection Association, Quincy, MA. This reprinted material is not the complete and official position of the NFPA on the reference subject which is represented only by the standard in its entirety.
Information on references can be found in Clause 2.
See Footnote 3.
See Footnote 3.
See Footnote 3.
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