Ieee Std 56-2016

IEEE Guide for Insulation Maintenance of Electric Machines

IEEE Power and Energy Society

Sponsored by the Electric Machinery Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std 56™-2016

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IEEE Std 56™-2016

IEEE Guide for Insulation Maintenance of Electric Machines Sponsor

Electric Machinery Committee of the

IEEE Power and Energy Society Approved 22 September 2016

IEEE-SA Standards Board

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Abstract: This insulation maintenance guide is applicable to rotating electric machines rated from 35 kVA and higher. The procedures detailed herein may also be useful for insulation maintenance of other types of machines. Keywords: aging mechanisms, alternating, armature, brush, commutator, core, corona, current, GLVWULEXWLRQIDFWRU(/&,'HOHFWULFHSR[\¿HOGÀX[*OREDO93,JURXQGZDOO,(((ŒLQVXODWLRQ ,RQL]DWLRQLRQL]LQJUDGLDWLRQPDFKLQHVPDLQWHQDQFHPLFDSDUWLDOGLVFKDUJHSLWFKIDFWRUSRZHU factor, recurrent surge oscillography, reliability, resistance temperature detector, rotating, RSO, RTD, semiconducting stress control coating, service, stator, testing, thermal cycling, thermal deterioration

The Institute of Electrical and Electronics Engineers, Inc. 3DUN$YHQXH1HZ
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Participants At the time this guide was completed, the P56 Working Group had the following membership: Douglas J. Conley, Chair David L. McKinnon, Vice Chair James Lau, Secretary David Agnew Kevin Alewine Ray Bartnikas Kevin Becker Tyler Black Stefano Bomben Andrew Brown Donald Campbell William Chen Ian Culbert Eric David Shari¿ Emad Tim F. Emery Jeff Fenwick Shawn A. Filliben Nancy E. Frost

Paul Gaberson Tyler Gaerke Anna Gegenava Richard Gupton Gary Heuston Fon Hiew Richard Huber Claude Hudon Jeffrey Hudson Marcelo Jacob da Silva Aleksandra Jeremic Aleksandr Khazanov Amir Khosravi Thomas Klamt Laurent Lamarre Gerhard Lemesch Melanie Levesque

William McDermid Charles Millet Gavita Mugala Beant Nindra Sophie Noel Ramtin Omranipour Howard Penrose Ashfak Shaikh Jeffrey Sheaffer Reza Soltani Gregory C. Stone Remi Tremblay Roger Wicks Joe Williams Chuck Wilson Hugh Zhu

The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Martin Baur Thomas Bishop William Bloethe Stefano Bomben Andrew Brown Gustavo Brunello William Byrd Weijen Chen Douglas J. Conley Matthew Davis Gary Donner Jeff Fenwick Shawn A. Filliben Rostyslaw Fostiak Dale Fredrickson Nancy E. Frost Paul Gaberson Frank Gerleve Alexander GlaningerKatschnig J. Travis Grif¿th

Randall Groves Ajit Gwal Werner Hoelzl Yuri Khersonsky Heshmatollah Khosravi Joseph L. Koep¿nger Jim Kulchisky Lucas Kunz William Larzelere James Lau William Lockley Omar Mazzoni William McBride William McCown William McDermid David L. McKinnon Don McLaren Charles Millet Jerry Murphy Arthur Neubauer Michael Newman

Howard Penrose Christopher Petrola Alvaro Portillo Iulian Pro¿r Johannes Rickmann Nikunj Shah Ashfak Shaikh Jeffrey Sheaffer Suresh Shrimavle Charles Simmons Jeremy Smith Jerry Smith Wayne Stec Gregory C. Stone David Tepen James Timperley Remi Tremblay John Vergis Kenneth White Dean Yager Hugh Zhu

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When the IEEE-SA Standards Board approved this guide on 22 September, it had the following membership: Jean-Philippe Faure, Chair Ted Burse, Vice Chair John D. Kulick, Past Chair Konstantinos Karachalios, Secretary Chuck Adams Masayuki Ariyoshi Stephen Dukes Jianbin Fan J. Travis Grif¿th Gary Hoffman

Ronald W. Hotchkiss Michael Janezic Joseph L. Koep¿nger* Hung Ling Kevin Lu Annette D. Reilly Gary Robinson

Mehmet Ulema Yingli Wen Howard Wolfman Don Wright Yu Yuan Daidi Zhong

*Member Emeritus

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Introduction 7KLVLQWURGXFWLRQLVQRWSDUWRI,(((6WG,(((*XLGHIRU,QVXODWLRQ0DLQWHQDQFHRI(OHFWULF0DFKLQHV

This guide represents the merger of the following two standards: —

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Contents  2YHUYLHZ ...................................................................................................................................................   6FRSH ..................................................................................................................................................   3XUSRVH ...............................................................................................................................................  2. Normative references ................................................................................................................................   'H¿QLWLRQV .................................................................................................................................................  4. Safety ........................................................................................................................................................   *HQHUDO ...............................................................................................................................................  4.2 Machine rotation.................................................................................................................................  4.3 Solvents ..............................................................................................................................................  4.4 Asbestos, lead, and other hazardous materials ....................................................................................   6LJQL¿FDQFHRIPDLQWHQDQFH......................................................................................................................  6. Insulation systems in general use ..............................................................................................................   ,QVXODWLQJPDWHULDOV ............................................................................................................................  6.2 Armature winding insulation ..............................................................................................................  6.3 Wound rotor windings (three-phase induction machines)...................................................................  6.4 Field winding insulation .....................................................................................................................  6.5 Core and frame-assembly insulation .................................................................................................. 22  6HUYLFHFRQGLWLRQVDIIHFWLQJLQVXODWLRQOLIH ............................................................................................... 22  $JLQJPHFKDQLVPV ............................................................................................................................. 22  $&6WDWLRQDU\DUPDWXUHZLQGLQJDJLQJPHFKDQLVPV .......................................................................... 23  &\OLQGULFDOURXQGURWRU ¿HOGZLQGLQJDJLQJPHFKDQLVPV ................................................................   6DOLHQWSROHURWDWLQJ¿HOGZLQGLQJDJLQJPHFKDQLVPV........................................................................ 33  :RXQGURWRUZLQGLQJDJLQJPHFKDQLVPV............................................................................................ 36  '&PRWRUDQGJHQHUDWRU¿HOGZLQGLQJDJLQJPHFKDQLVPV .................................................................   '&PRWRUDQGJHQHUDWRUDUPDWXUHZLQGLQJDJLQJPHFKDQLVPV ..........................................................   '&PRWRUDQGJHQHUDWRUFRPPXWDWRUDJLQJPHFKDQLVPV ...................................................................   6WDWRUFRUHLQVXODWLRQDJLQJPHFKDQLVPV............................................................................................   9LVXDOLQVSHFWLRQPHWKRGV ......................................................................................................................... 45  9LVXDOLQVSHFWLRQVDIHW\ ...................................................................................................................... 45  $UPDWXUHZLQGLQJ ............................................................................................................................... 46  )LHOGZLQGLQJV ....................................................................................................................................   &RUHDQGIUDPHDVVHPEO\....................................................................................................................   ,QVXODWLRQPDLQWHQDQFHWHVWLQJ ..................................................................................................................   3ULQFLSOHVRIPDLQWHQDQFHWHVWLQJ .......................................................................................................   7HVWVFRQGXFWHGRQWKH¿HOGZLQGLQJ ..................................................................................................   7HVWVFRQGXFWHGRQWKHDUPDWXUHVWDWRU ............................................................................................. 53  &OHDQLQJ .................................................................................................................................................. 65  *HQHUDO ............................................................................................................................................. 65  &OHDQLQJWHFKQLTXHV ......................................................................................................................... 66 Annex A (informative) Bibliography..............................................................................................................  Annex B (informative) Thermosetting resins used in insulation systems ......................................................  $QQH[&LQIRUPDWLYH 6WDWRUFRUHLQWHUODPLQDULQVXODWLRQKLJKÀX[ WHVWSURFHGXUH ................................... 



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Annex D (informative) Stator core low energy (EL CID) test ........................................................................  Annex E (informative) Machine condition visual inspection appraisal—Checklist ...................................... 



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IEEE Guide for Insulation Maintenance of Electric Machines IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, health, or environmental protection, or ensure against interference with or from other devices or networks. Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations. 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 http://standards.ieee.org/IPR/disclaimers.html.

1. Overview 5RWDWLQJHOHFWULFPDFKLQHVDUHFRPSOH[VWUXFWXUHVWKDWDUHVXEMHFWHGWRYDULRXVOHYHOVRIVWUHVVHVDQGHQYLURQmental factors and therefore require maintenance. This guide provides an authoritative overview of insulation systems and the various tests and inspections employed for maintenance of them.

1.1 Scope 7KLVLQVXODWLRQPDLQWHQDQFHJXLGHLVDSSOLFDEOHWRURWDWLQJHOHFWULFPDFKLQHVUDWHGIURPN9$DQGKLJKHU The procedures detailed herein may also be useful for insulation maintenance of other types of machines.

1.2 Purpose The purpose of this guide is to present information necessary to permit an effective evaluation of the insulation systems of rotating electrical machines. Such an evaluation can serve as a guide to the degree of maintenance or replacement as might be deemed necessary, and also offer some indication of the future service reliability of the equipment under consideration.

2. 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.



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,(&2IIOLQHSDUWLDOGLVFKDUJHPHDVXUHPHQWVRQWKHVWDWRUZLQGLQJLQVXODWLRQRIURWDWLQJHOHFWULFDO machines. ,(((6WGŒ,(((6WDQGDUG7HFKQLTXHVIRU+LJK9ROWDJH7HVWLQJ2,3 ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU7HVWLQJ,QVXODWLRQ5HVLVWDQFHRI(OHFWULF0DFKLQHU\ ,(((6WGŒ,(((*XLGHIRU)LHOG7HVWLQJRI(OHFWULF3RZHU$SSDUDWXV,QVXODWLRQ²(OHFWULFDO0DFKLQHU\ ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU,QVXODWLRQ7HVWLQJRI/DUJH$&5RWDWLQJ0DFKLQHU\ZLWK +LJK'LUHFW9ROWDJH ,(((6WGŒ,(((6WDQGDUG7HVW3URFHGXUHIRU6\QFKURQRXV0DFKLQHV ,(((6WGŒ,(((6WDQGDUG0DVWHU7HVW&RGHIRU5HVLVWDQFH0HDVXUHPHQW:LWKGUDZQ 4 ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU*HQHUDO3ULQFLSOHVRI7HPSHUDWXUH0HDVXUHPHQWDV$Splied to Electrical Apparatus (Withdrawn). ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU0HDVXUHPHQWRI3RZHU)DFWRU7LS8SRI(OHFWULF0DFKLQery Stator Coil Insulation. ,(((6WGŒ,(((6WDQGDUGIRU4XDOLI\LQJ&RQWLQXRXV'XW\&ODVV(0RWRUVIRU1XFOHDU3RZHU*HQHUating Stations. ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU,QVXODWLRQ7HVWLQJRI/DUJH$&(OHFWULF0DFKLQHU\ZLWK +LJK9ROWDJHDW9HU\/RZ)UHTXHQF\ ,((( 6WG Œ ,((( 5HFRPPHQGHG 3UDFWLFH IRU 6DIHW\ LQ +LJK 9ROWDJH DQG +LJK 3RZHU 7HVWLQJ (Withdrawn). ,(((6WGŒ,(((*XLGHIRU7HVWLQJ7XUQWR7XUQ,QVXODWLRQRQ)RUP:RXQG6WDWRU&RLOVIRU$OWHUQDWing-Current Electric Machines. ,(((6WGŒ,(((*XLGHIRU2QOLQH0RQLWRULQJRI/DUJH6\QFKURQRXV*HQHUDWRUV09$DQG$ERYH  ,(((6WGŒ,(((*XLGHIRUWKH0HDVXUHPHQWRI3DUWLDO'LVFKDUJHVLQ$&(OHFWULF0DFKLQHU\ ,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU4XDOLW\&RQWURO7HVWLQJRI([WHUQDO'LVFKDUJHVRQ6WDWRU Coils, Bars, and Windings. ,(((6WG&Œ,(((6WDQGDUGIRU6DOLHQW3ROH+]DQG+]6\QFKURQRXV*HQHUDWRUVDQG*HQHUDWRU 0RWRUVIRU+\GUDXOLF7XUELQH$SSOLFDWLRQV5DWHG09$DQG$ERYH ,(((6WG&Œ,(((6WDQGDUGIRU&\OLQGULFDO5RWRU+]DQG+]6\QFKURQRXV*HQHUDWRUV5DWHG 09$DQG$ERYH  ,(&SXEOLFDWLRQVDUHDYDLODEOHIURPWKH6DOHV'HSDUWPHQWRIWKH,QWHUQDWLRQDO(OHFWURWHFKQLFDO&RPPLVVLRQ&DVH3RVWDOHUXHGH 9DUHPEp&+*HQqYH6ZLW]HUODQG6XLVVHhttp://www.iec.ch/). IEC publications are also available in the United States from WKH6DOHV'HSDUWPHQW$PHULFDQ1DWLRQDO6WDQGDUGV,QVWLWXWH:HVWUG6WUHHWWK)ORRU1HZ


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 'H¿QLWLRQV )RUWKHSXUSRVHVRIWKLVGRFXPHQWWKHIROORZLQJWHUPVDQGGH¿QLWLRQVDSSO\7KHIEEE Standards Dictionary: *ORVVDU\RI7HUPVDQG'H¿QLWLRQV5VKRXOGEHFRQVXOWHGIRUWHUPVQRWGH¿QHGLQWKLVFODXVH discharge detector (ionization or corona detector): An instrument that can be connected in or across an energized insulation circuit to detect current or voltage pulses produced by electric discharges within the circuit. KLJKÀX[WHVW7KHKLJKÀX[WHVWFRPPRQO\NQRZQDVWKHORRSWHVWULQJWHVWRUIXOOÀX[WHVWLVDWHVWPHWKRG developed for evaluating laminated magnetic cores to detect shorting between the laminations. The detection PHWKRGLVDFFRPSOLVKHGE\DSSO\LQJDKLJKHQHUJ\ÀX[GHQVLW\PDJQHWLF¿HOGDWDSSUR[LPDWHO\WKHUDWHGSHDN QRORDGÀX[WRWKHFRUHDQGWKHQHYDOXDWLQJLIVKRUWVKDYHFUHDWHGDORFDOWHPSHUDWXUHULVHWKDWFRXOGOHDGWR detrimental heating during operation of the machine. ionizing radiation3DUWLFOHVRUSKRWRQVRIVXI¿FLHQWHQHUJ\WRSURGXFHLRQL]DWLRQLQLQWHUDFWLRQVZLWKPDWWHU ORZHQHUJ\ÀX[WHVW7KHORZHQHUJ\ÀX[WHVWFRPPRQO\NQRZQDVWKH(/&,'WHVWLVDWHVWPHWKRGGHveloped for evaluating laminated magnetic cores to detect shorting between the laminations. The detection PHWKRGLVDFFRPSOLVKHGE\DSSO\LQJDORZÀX[GHQVLW\PDJQHWLF¿HOGWRWKHFRUHDQGWKHQHYDOXDWLQJLIVKRUWV have created a circulating current loop that could lead to detrimental heating during operation of the machine. power factor (PF): The cosine of the dielectric phase angle or the sine of the dielectric loss angle when tested under a sinusoidal voltage. The ratio of the power dissipated in the insulation, in watts, to the product of the effective voltage and current in voltamperes, when tested under a sinusoidal voltage and prescribed conditions. The insulation power factor is equal to the cosine of the phase angle between the voltage and the resulting current when both the voltage and current are sinusoidal. resistance temperature detector (RTD) (resistance thermometer resistor) (resistance thermometer detector): A resistor made of some material for which the electrical resistivity is a known function of the temperature and that is intended for use with a resistance thermometer. It is usually in such a form that it can be placed in the region where the temperature is to be determined. semiconducting slot coating: The partially conductive paint or tape layer in intimate contact with the groundwall insulation in the slot portion of the stator core. This coating ensures that there is little voltage between the surface of the coil or bar and the grounded stator core. stator bar: A unit of winding on the stator of a machine. Two single-turn stator bars make one turn in the stator winding. stator coil: A unit of a winding on the stator of a machine comprising of one or more (for multi-turn coils) turns in the winding. stator core: The stationary magnetic circuit of an electric machine. It is commonly an assembly of laminations of magnetic steel, ready for winding. stator: The portion that includes and supports the stationary active parts. The stator includes the stationary portions of the magnetic circuit and the associated winding and leads. It may, depending on the design, include a frame or shell, core, winding supports, ventilation circuits, coolers, and temperature detectors. A base, if provided, is not ordinarily considered to be part of the stator. stress control coating: The paint or tape on the outside of the groundwall insulation that extends several centimeters beyond the semiconducting slot coating in high-voltage stator bars and coils. The stress control coating 5

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RIWHQFRQWDLQVVLOLFRQFDUELGHSDUWLFOHVZKLFKWHQGWROLQHDUL]HWKHHOHFWULF¿HOGGLVWULEXWLRQDORQJWKHFRLORU bar end-turn. The stress control coating overlaps the semiconducting slot coating to provide electrical contact between them.

4. Safety 7HVWLQJDQGPDLQWHQDQFHDFWLYLWLHVVKRXOGEHFRQGXFWHGDQGVXSHUYLVHGE\TXDOL¿HGSHUVRQQHODQGDGHTXDWH safety precautions should be taken to avoid injury to personnel and damage to property. It is not the intent of this document to prescribe safety and health requirements. Before starting any work, the relevant laws, regulatory standards, manufacturer instructions, and company policies should be consulted.

4.1 General 3ULRUWRSHUIRUPLQJDQ\WHVWLQVSHFWLRQRUVHUYLFLQJRIDQHOHFWULFPDFKLQHZKHUHWKHXQH[SHFWHGHQHUJL]LQJ start up, or release of kinetic or stored energy could occur and cause injury or damage, the apparatus shall be GHHQHUJL]HGLVRODWHGEORFNHGDQGVHFXUHGWRFRQWUROKD]DUGRXVHQHUJ\3HUVRQQHODQGHTXLSPHQWVKDOOQRW be considered protected until the appropriate safety procedures have been implemented. Safety procedures may involve the use of locking devices, warning tags, physical barriers, safety tape, caution signs, and/or observers as necessary to restrict access to the equipment being maintained. There should be a meeting of all personnel involved in or affected by the maintenance activities. Test and inspection procedures should be disFXVVHGVRWKHUHLVDFOHDUXQGHUVWDQGLQJRIDOODVSHFWVRIWKHZRUNWREHSHUIRUPHG3DUWLFXODUHPSKDVLVVKRXOG be placed on personnel and equipment hazards, and the safety precautions associated with these hazards. In addition, details of the maintenance activities should be discussed to help ensure the successful completion of WKHSODQQHGWDVNV5HVSRQVLELOLWLHVIRUWKHYDULRXVGXWLHVLQYROYHGLQSHUIRUPLQJWKHZRUNVKRXOGEHDVVLJQHG DQGGRFXPHQWHG5HIHUWR

4.2 Machine rotation Some test and inspection procedures are performed with the machine rotating slowly and with cover plates, guards, and end-shields removed. In the case of hydrogenerators under test, the machine may be operated at rated speed or at over speed with its covers removed. These tests present mechanical and electrical hazards, and appropriate procedures are required to prevent injury to personnel and damage to equipment.

4.3 Solvents 3HUVRQVZKRFDUU\RXWFOHDQLQJXVLQJVROYHQWVVKDOOEHLQVWUXFWHGRQWKHVDIHVWRUDJHXVHDQGHPHUJHQF\ actions related to the solvents used. Manufacturer’s recommendations, local procedures, and safety regulaWLRQVVKRXOGEHIROORZHGWRKHOSHQVXUHWKHSURSHUXVHRISHUVRQDOSURWHFWLYHHTXLSPHQW33( DQGWKHFRUUHFW handling of waste materials. There are suitable non-toxic solvents that can be used for cleaning the windings/ machines in most cases.

4.4 Asbestos, lead, and other hazardous materials 2SHUDWRUVVKRXOGEHDZDUHWKDWROGHUPDFKLQHVPD\FRQWDLQDVEHVWRVSRO\FKORULQDWHGELSKHQ\OV3&% RU lead products that could pose a threat to worker health and safety if disturbed. Electrical apparatus that has been subjected to extreme overheating or other unusual conditions may be contaminated with dangerous or toxic substances. Chemical sampling and testing may be warranted to determine the presence of hazardous materials. Clean up or abatement activities may be necessary and workers should take special precautions, LQFOXGLQJWKHXVHRI33(



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 6LJQL¿FDQFHRIPDLQWHQDQFH Data obtained from online monitoring, regular inspection, and testing programs can provide an evaluation of the present condition of the equipment, give some indication of long-term trends, and indicate probable need for future repair or replacement. The insulation systems are the most susceptible to aging or damage due to these stresses. The service life of an electric machine will, therefore, largely depend on the serviceability of the insulation systems. The extent of a maintenance program will depend largely on the operator’s own experience and policy, but should also take into account the importance of service reliability for the equipment. Where high service reliability is required, a regular maintenance program involving periodic disassembly and knowledgeable viVXDOH[DPLQDWLRQRIWKHHTXLSPHQWWRJHWKHUZLWKWKHDSSOLFDWLRQRIHOHFWULFDOWHVWVRISURYHQVLJQL¿FDQFHLV strongly recommended. The size and age of the machine will relate to the applicability of the individual tests and inspections. It should be recognized that over-potential tests can damage insulation that is contaminated or in marginal condition. Where there is uncertainty of the condition of the insulation system, refer to &ODXVH for additional information. Consultation with the manufacturer is recommended for setting up an appropriate maintenance-testing program.

6. Insulation systems in general use Insulation is present in various machine components, but the complexity of the subject is such that only a general description can be given here.

6.1 Insulating materials Electrical insulation uses many constituent insulating materials in various combinations that result in various thermal ratings. Some of the material can be found in higher or lower thermal classes depending on the resultant composite insulation system. Some common insulating materials applied to electric machines have the IROORZLQJWKHUPDOFODVVL¿FDWLRQV a)

Class 105 (formerly Class A): Impregnated cotton, silk, cellulose-based paper, linen (cambric).

b)

Class 130 (formerly Class B):0LFDJODVV¿EHUDVEHVWRVHWF7\SLFDOERQGLQJPDWHULDOVDUHVKHOODF DVSKDOWYDUQLVKDQGVRPHSRO\HVWHUUHVLQV$OVRSRO\HVWHUWHUHSKWKDODWH3(7 ¿OPVDQGYDULRXVODPLnated papers.

c)

Class 155 (formerly Class F):0LFDJODVV¿EHU3(7DVEHVWRVHWF%RQGLQJPDWHULDOVDUHXVXDOO\HSoxy, epoxy-polyester or acrylic resins.

d)

Class 180 (formerly Class H):6LOLFRQHHODVWRPHUPLFDJODVV¿EHUDVEHVWRVHWF%RQGLQJPDWHULDOV may consist of silicone resins.

Mica is a vital component in most insulation for high-voltage electric machines because it has good dielectric strength at high temperatures and is resistant to partial discharges. Muscovite and phlogopite are the two forms of mica that are most commonly used for electrical insulation. 2ULJLQDOO\PLFDZDVXVHGLQWKHIRUPRIODUJHÀDNHVRUVSOLWWLQJV:KHQDSSOLHGLQWKLVZD\LWLVGLI¿FXOWWR exclude all voids, and some problems with delaminations may occur. Mica is also used in the form of mica paSHUZKHUHVPDOOÀDNHOHWWHVRIPLFDDUHGHSRVLWHGRQDQGERQGHGWRDEDFNLQJWDSH:LWKPLFDSDSHULWLVHDVLHU to produce an insulating layer with a very low void fraction. However, mica paper is less resistant to partial GLVFKDUJHVWKDQLVODUJHÀDNHPLFD



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Materials bonded with shellac or with asphalt varnish are termed “thermoplastic”. Materials bonded with polyester or epoxy resin are termed “thermosetting”. Further information on thermosetting resins will be found in Annex B. :KHQPLFDDQGJODVV¿EHUVDUHERQGHGWRJHWKHUZLWKYDUQLVKRUUHVLQWKH\IRUPDFRPSRVLWHLQVXODWLRQV\VWHP Such a system typically has good thermal, electrical, and mechanical properties.

WARNING Asbestos may be present in slot packing materials, armor tape and strand insulation of many older windings. 6RPHVORWSDFNLQJPDWHULDOVDQGYDUQLVKHGFDPEULFPD\FRQWDLQSRO\FKORULQDWHGELSKHQ\OV3&%V $SSURpriate workplace safety and environmental regulations should be followed when examining, disturbing or disposing of these materials.

6.2 Armature winding insulation The armature windings of ac machines are typically stationary and are therefore known as stator windings. The armature windings of dc machines are rotating. In addition, dc machine rotating elements normally require various commutator insulations, banding, and other specialized supporting materials. The armature winding with its associated leads is the main current carrying winding of the machine. The coils of the armature winding have strands, ground insulation and may have turn insulation. Wedges, blocks, and other insulated mechanical supports are a part of the armature winding assembly. The insulation systems listed in  through  have been used for armature windings. 6.2.1 Strand insulation The individual strands of armature coil conductors are usually insulated. Strand insulation can be made up of RUJDQLFUHVLQHQDPHOVSRO\PHULF¿OPVUHVLQERQGHG¿EHUVVXFKDVSDSHUFRWWRQDVEHVWRVJODVVSRO\HVWHU or combinations thereof) or resin bonded mica. ,QWKHVFRWWRQZDVFRPPRQO\XVHGDVWKHLQVXODWLRQRQLQGLYLGXDOFRSSHUVWUDQGV%\WKHVDVEHVWRV ZDVLQXVHDVVWUDQGLQVXODWLRQEHFDXVHRILWVKLJKHUWHPSHUDWXUHFODVVL¿FDWLRQ%\WKHVSRO\HVWHUJODVV ¿EHUZDVFRPPRQO\XVHGDVVWUDQGLQVXODWLRQ6RPHPDQXIDFWXUHUVDOWHUQDWHSRO\HVWHUJODVV¿EHULQVXODWHG strands with strands that have an enamel coating. 6.2.2 Turn insulation In a coil with more than one turn, groups of strands forming a single-turn (conductor) may be held together and insulated. Individual strand insulation, as described in , may also serve as turn insulation. Where dedicated turn insulation is provided for multi-turn coils it usually involves similar materials to those in the groundwall insulation in the slot section. 6.2.3 Groundwall insulation *URXQGZDOOLQVXODWLRQLVWKHPDWHULDOLQWHQGHGWRLQVXODWHWKHFXUUHQWFDUU\LQJFRPSRQHQWVHJWKHFRLOVWKH circuit rings, and connections) from one another and from the non-current-carrying components, which are usually grounded (such as the core iron, the frame, and other structural members). *URXQGZDOOLQVXODWLRQWDNHVGLIIHUHQWIRUPVGHSHQGLQJRQWKHW\SHRIPDFKLQHDQGWKHPDQXIDFWXUHU¶VSUDFWLFHV*URXQGZDOOLQVXODWLRQLVJHQHUDOO\DGU\W\SHPXOWLOD\HUHGV\VWHPFRPSULVHGRIYDULRXVERQGHGDQG



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¿OOHGLQVXODWLQJPDWHULDOV0LFDEDVHGSURGXFWVDUHJHQHUDOO\SUHIHUUHGLQKLJKYROWDJHPDFKLQHVIRUDWOHDVWD part of the groundwall insulation system. Typical insulation systems in use are as follows:

6

a)

Phenolic Resins (Class 105 Insulation):3KHQROIRUPDOGHK\GHSKHQROXUHDDQGSKHQROPHODPLQH UHVLQVDUHV\QWKHWLFSRO\PHUVRIWHQFRPELQHGZLWKFRWWRQRUJODVVIDEULFVWRSURGXFH¿EHUUHLQIRUFHG composites for use in structural and electrical insulation applications. These materials are normally XVHGIRUORZHUYROWDJHDQGORZHUWKHUPDOFODVVL¿FDWLRQPDFKLQHV

b)

Varnished Cambric (Class 105 Insulation): Due to the absence of mica, this insulation system was XVXDOO\UHVWULFWHGWRZLQGLQJVUDWHG9DQGEHORZ+HDWWUDQVIHULVUHODWLYHO\SRRUDVLVLWVUHVLVWDQFHWRPRLVWXUHDQGRLO$W\SLFDOWHPSHUDWXUHULVHUDWLQJIRUVXFKZLQGLQJVLVƒ&

c)

Shellac Micafolium (Class 130 Insulation):,QWKLVV\VWHPPLFDÀDNHVDUHERQGHGWRJHWKHUE\VKHOlac to form sheets. These sheets are wrapped and hot pressed around the slot section of the coil. The end-windings are insulated with tape, such as asphalt-mica, or sometimes only with varnished cambric. Due to the evaporation of the volatiles in the shellac this system may have a high void content and is thus susceptible to partial discharge damage as well as to reduced heat transfer.

d)

Asphalt Micafolium (Class 130 Insulation): As above, except that asphalt is substituted for shellac in the slot section.

e)

Asphalt Bonded Mica Tape (Class 130 Insulation): The entire coil is insulated with asphalt-bonded KDOIODSSHGPLFDWDSH7KHPLFDÀDNHVDUHERQGHGWRJHWKHUZLWKDVSKDOWDQGDWWDFKHGWRDSDSHUWDSH %\WKHVVRPHPDQXIDFWXUHUVDGGHGDSRO\HVWHUWHUHSKWKDODWH¿OP3(7¿OPVXFKDV0\ODUŠ6) tape to allow greater tension to be used during the taping operation. It was common to apply asphalt varnish as the coil was being taped. Some manufacturers used an autoclave in which vacuum was GUDZQWRUHPRYHYRODWLOHVIROORZHGE\ÀRRGLQJRIWKHWDQNZLWKKRWDVSKDOWDQGDSSOLFDWLRQRISUHVVXUHLQRUGHUWRFRQVROLGDWHWKHOD\HUVRIDVSKDOWPLFDWDSH3ULRUWRLQVWDOODWLRQLQWKHVWDWRUFRUHLWZDV FRPPRQWRKHDWWKHFRLOVWRUHQGHUWKHPÀH[LEOHDQGWKXVDFKLHYHDEHWWHU¿WLQWKHVORW/LIWFRLOVKDG to be heated to facilitate bending at the knuckle. Asphalt mica stator coils can be susceptible to delamLQDWLRQRU³SXI¿QJ´DVDUHVXOWRIRYHUORDGSRRUYHQWLODWLRQRUWKHXVHRIXQVXLWDEOHDVSKDOWYDUQLVK The asphalt bonded mica tape insulation system is also vulnerable to tape separation near the end of the stator core as a result of thermal cycling. This is especially true of windings in long cores such as turbo alternators.

f)

Polyester Bonded Mica Tape (VPI) (Typically Class 130 Insulation):7KLVV\VWHPZDV¿UVWLQWURGXFHG LQ1RUWK$PHULFDLQWKHHDUO\V$WWKDWWLPHLWLQYROYHGH[FOXVLYHO\WKHXVHRIODUJHÀDNHPLFD although several advances in both materials and processing technologies have improved the perforPDQFHDQGUHOLDELOLW\VLQFHLWZDVLQWURGXFHG,QWKHYDFXXPSUHVVXUHLPSUHJQDWLRQ93, SURFHVVWKH coils or bars are placed in an autoclave and subjected to a high vacuum for drying purposes. The tank is WKHQÀRRGHGZLWKSRO\HVWHUUHVLQDQGSUHVVXUHLVDSSOLHGWRDFKLHYHWKHGHVLUHGLPSUHJQDWLRQ)ROORZLQJUHPRYDOIURPWKHWDQNWKHFRLOVDUHFXUHG7KHUHDUHDGYDQFHGSRO\HVWHUV\VWHPVXSWR&ODVV

g)

Epoxy Bonded Mica Tape (VPI) (Typically Class 155 Insulation): It has become common to use epoxy LQSODFHRISRO\HVWHUUHVLQLQ93,RSHUDWLRQVLQRUGHUWRREWDLQLPSURYHGERQGLQJFKDUDFWHULVWLFVDQGD KLJKHUWHPSHUDWXUHFODVVL¿FDWLRQ7KLVLPSUHJQDWLRQSURFHVVLVXVHGIRULQGLYLGXDOVWDWRUFRLOVRUEDUV that are hot pressed in molds, to cure the groundwall bonding resin, prior to inserting them into the VWDWRUVORWV2QHLPSUHJQDWLQJHSR[\UHVLQWKDWKDVEHHQXVHGLQD93,SURFHVVLVDFDWDO\]HGEOHQGRI bisphenol-A resin with a hardener. Once the catalyst has been added the resin must be refrigerated in RUGHUWRUHGXFHUHDFWLYLW\7KHUHDUHDGYDQFHGHSR[\EDVHGV\VWHPVXSWR&ODVV

h)

Epoxy Polyester Blend or Hybrid Systems (VPI) (Typically Class 155 Insulation): Some modern insulations systems use a blend of epoxy and polyester resins to achieve a mix of the elasticity and strength attributes of both base resin systems.

Mylar is a registered trademark of Dupont Tejjin Films.



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i)

Global VPI (Class 155 up to Class 180 Insulation): The coils or bars are insulated with mica tape over which are applied the semiconducting slot armor tape and the stress control tape on the end arms. The coils or bars are installed in an unbonded state in the stator core. The entire core and winding is then SODFHGLQDQDXWRFODYHIRULPSUHJQDWLRQZLWKUHVLQIROORZLQJZKLFKWKHUHVLQLVFXUHG*OREDO93, resin choices include epoxy, polyester, polyesterimide, silicone, and combinations of these materials. To maintain the success of this process on longer machines some manufacturers utilize a slip plane between the core and coil surface to allow for the longitudinal expansion and contraction of the copper with respect to the core.

j)

Epoxy Bonded Mica Tape (Resin Rich) (Class 155 Insulation): This system involves a mica paper IRUPXODWLRQLQZKLFKWKHPLFDSDSHURUÀDNHVDUHGHSRVLWHGRQDJODVV¿EHUEDFNLQJWDSH$QXQFXUHG %VWDJH HSR[\UHVLQLVDSSOLHGGXULQJWKHPDQXIDFWXUHRIWKHWDSH$SRO\HVWHUW\SLFDOO\3(7 ¿OPRU fabric layer may be included to make it easier to handle the tape during its application to coils or bars. Once the bars or coils have been insulated, the B stage resin is cured under elevated temperature and pressure in one of the following two ways:   In the heated press method, mold angles are applied to the slot section of the coil or bar prior to insertion in the heated press. The rate of temperature and pressure increase is critical in the bonding process. Additional heaters are used with splints and shrink tape to cure the tape on the end arms. 2)

An autoclave process is used by some manufacturers. This involves the following three major steps: — High vacuum to remove moisture, volatiles, and trapped air. — (OHYDWHGWHPSHUDWXUHDWUHGXFHGSUHVVXUHWRDOORZWKH%VWDJHUHVLQWRÀRZWRDOLPLWHG extent. — High temperature and high pressure while the resin cures.

k)

Silicone Rubber (Class 180 Insulation): Silicone rubber is a material that is suitable for use at high WHPSHUDWXUHV:KHQXVHGDVWKHJURXQGZDOOLQVXODWLRQRIDVWDWRUFRLOLWXVXDOO\KDVD¿EHUJODVVEDFNLQJ,QWKHDEVHQFHRIPLFDLWLVFRPPRQO\UHVWULFWHGWRORZHUYROWDJHVHJ9DQGEHORZ $ disadvantage of silicone rubber is its vulnerability to mechanical damage.

l)

Varnish Dip and Bake (Class 130 Insulation up to Class 180 Insulation): Especially at lower voltages 9DQGEHORZ DQGIRUVPDOOVL]HGVWDWRUVVXFKDVVRPHPRWRUVDQGUDQGRPZRXQGGHVLJQVWKH varnish dip/oven bake process is common. In this case the stator coils are insulated with materials LQFOXGLQJHQDPHOLQVXODWHGFRQGXFWRUVZLWKDYDULHW\RI¿OPVDUDPLGSDSHUVPLFDWDSHVDQGODPinates as ground wall and phase insulation. Following this, the coils are installed in the stator core and connected together with all applicable lashings and bracing. The varnish or resin is then applied by dipping, often in automated equipment or by alternate methods such as trickling; or in some cases as a B-stage coating on the conductors and insulators. Common impregnants include polyesters and epoxies in a variety of evaporative formulations utilizing aromatic solvents or water as the carrier or ZLWKDFDWDO\VWDVUHDFWLYHSRO\PHULFFXUH7KHUHDUHPDQ\FXULQJVFKHPHVDVZHOOLQFOXGLQJ89OLJKW reactive materials, room temperature catalysts, induction or resistance heating, and traditional/automated ovens.

6.2.4 Semiconducting slot coating The surface of slot portions of stator coils and bars, including several centimeters of the coil beyond the core, is normally semiconducting. The semiconducting characteristic is accomplished by the application of semiconducting varnish over the armor tape (if any) or by the use of semiconducting armor tapes. These treatments DUHRIWHQUHIHUUHGWRDVFRQGXFWLYHDQGDUHJHQHUDOO\DSSOLHGWRPDFKLQHVZLWKUDWHGYROWDJHRIN9DQGDERYH



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6.2.5 Stress control coating Due to the semi-conducting slot coating, the surface of the stator coils and bars just outside of the core is at ground potential. The surface of the end turn insulation, however, is typically not at ground potential. In order WROLQHDUL]HWKHHOHFWULF¿HOGGLVWULEXWLRQDORQJWKHFRLORUEDUHQGWXUQDVWUHVVFRQWUROFRDWLQJLVDSSOLHG7KLV coating can be made from varnishes or tapes that have a non-linear resistance characteristic through the use of silicon carbide. These stress control coatings are often referred to as grading paint or tape and are generally DSSOLHGWRVWDWRUFRLOVDQGEDUVLQVXODWHGZLWKWKHUPRVHWWLQJPDWHULDOVLQPDFKLQHVZLWKUDWHGYROWDJHRIN9 and above. Such coatings are less necessary for thermoplastic insulation systems which operate at a lower electric stress. 6.2.6 Stator slot tightening systems 9DULRXVV\VWHPVDUHXVHGWRVHFXUHWKHVWDWRUZLQGLQJLQWKHVORWWRDFKLHYHHOHFWULFDOFRQWDFWDQGWKHUPDOFRQtact to the core. These are required to maintain this contact when subjected to the operational forces, thermal mechanical forces, and material shrinkage or creep. These include side packing and top packing in the form RIÀDWDQGULSSOHVSULQJPDWHULDOV2WKHUV\VWHPVDOVRH[LVW:LWKJOREDOO\YDFXXPSUHVVXUHLPSUHJQDWHGLQVXlation systems, the coils are integrally sealed in the core and may require slip layers for control of unmatched thermal expansion of various slot contents. If the stator coils or bars have semiconducting slot coatings, the slot side packing should also be semiconducting. 6.2.7 Support insulation Supports may be nonmetallic or metallic in design. Nonmetallic supports include blocks, spacers, ties, slot ZHGJHVVORW¿OOHUVHWF1RQPHWDOOLFVXSSRUWVDUHPDGHRIYDULRXVLQVXODWLQJPDWHULDOVLQFOXGLQJZRRGPROGHGSDUWVFRPSUHVVHGODPLQDWHVRIFRWWRQDVEHVWRVJODVVRUV\QWKHWLF¿EHUVDQGIHOWSDGVLPSUHJQDWHGZLWK various types of bonding agents including phenolic, polyester, and epoxy resins. These materials have a range RIWKHUPDOFODVVL¿FDWLRQVDVZHOODVGLIIHUHQWSK\VLFDODQGHOHFWULFDOSURSHUWLHV0HWDOOLFVXSSRUWVVXFKDVWKH surge (bull) ring and its supports are insulated where necessary. 6.2.8 Circuit ring (also known as parallel ring) insulation 7KHZLQGLQJVRIDPDFKLQHDVVRFLDWHGZLWKHDFKSKDVHDQGSROHVWDUWDQG¿QLVKDWSK\VLFDOORFDWLRQVDURXQG the machine which are determined based on the physical location of the phase with respect to other phases in WKHFLUFXPIHUHQFHRIWKHPDFKLQH9DULRXVZLQGLQJGHVLJQVFDQDIIHFWWKHVHVWDUWDQG¿QLVKORFDWLRQV7KH main leads of the machine are located generally together at one location; therefore, connections must be provided between phase groups and from phase groups to the leads. These rings are typically circumferential and are typically located behind the winding, usually on one end of the machine. These rings complete the electrical circuits of the phases and often provide multiple parallels between phase groups. The rings are usually physically parallel to one another; therefore, they are often referred to as circuit rings, parallel rings, or connection rings. The rings are typically made of highly conductive copper either in bar, wire, or tubular form. The current carrying capability depends on the machine size, rating and the arrangement of the phase groups into various parallel circuits. The voltage difference between adjacent rings and to ground also varies depending on size, rating and circuit arrangements. In many cases some adjacent rings in the machine will have a voltage between them that equals the line-to-line operating voltage of the machine. There are various approaches taken to insulate between the circuit rings. Typically the insulation is the same materials as used to insulate the armature windings. In some cases the circuit rings have a semiconducting outHUFRURQDSURWHFWLRQ2&3 OD\HUDWWKHLUVXUIDFHZLWKJUDGLHQWVQHDUWHUPLQDWLRQSRLQWV²WKLVDOORZVWKHULQJV WREHSODFHGFORVHUWRJHWKHU,QPDQ\FDVHVWKHULQJVGRQRWKDYHDQ\2&3OD\HUWKHUHIRUHWKHFLUFXLWULQJV VXSSRUWPHFKDQLVPVPXVWEHFDUHIXOO\FRQ¿JXUHGZLWKPDWHULDOVDQGVSDFHVSURYLGHGWRPLQLPL]HVXUIDFH partial discharge.



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Design considerations for circuit rings include addressing the voltages between rings as well as how they are FRROHGDQGKRZWKH\UHDFWWRHGG\FXUUHQWVGXHWRFXUUHQWÀRZZLWKLQWKHFRQGXFWRUDVZHOODVSUR[LPLW\HIfects of other rings. Whether circuit rings consist of insulated wires or copper bars, maintenance inspection should include visual inspection to look for indications of overheating, dusting from looseness, deterioration of the insulation, and deterioration of surface coatings. Typically the circuit rings are tested for appropriate insulation condition as part of the winding high potential tests. 6.2.9 Commutator insulation A commutator is a cylindrical assembly of wedge-shaped copper segments separated from each other and ground by insulation that is usually mica-based. This structure is mechanically locked together by various WHFKQLTXHVLQFOXGLQJ9JURRYHVFRQHVDQGVXSSRUWULQJVDWFRPPXWDWRUEDUHQGVVWHHOVKULQNULQJVRYHUULQJ LQVXODWLRQRQWKHFRPPXWDWRUVXUIDFHDQGKLJKO\WHQVLRQHG¿EHUJODVVEDQGVDSSOLHGLQWRJURRYHVLQWKHFRPmutator surface. Small-size units are often compression molded with a high-strength molding compound.

6.3 Wound rotor windings (three-phase induction machines) The rotating secondary windings of wound rotor ac induction machines are similar to armature windings. The three-phase winding, with its associated leads and collector rings is the secondary current carrying winding RIWKHPDFKLQHEXWWKHFXUUHQWPD\EHKLJKHUWKDQWKHVWDWRUFXUUHQW7KHYROWDJHLVXVXDOO\OHVVWKDQ9 but there is no real limit. The coils have ground insulation and may have turn insulation. Wedges, blocks, and other insulated mechanical supports are a part of the wound rotor winding assembly. They also have banding or retaining rings because of the centripetal forces. The coils of the winding may be constructed of insulated rectangular copper conductors wound with the long edge radial in partially closed slots or may be constructed with strands similar to an armature winding in an open slot. 6.3.1 Partially closed slots—strap or bar windings (generally used on high-speed machines) The straps may be half straps or open ended full coils. There are usually two to four straps inserted radially in each partially closed slot. Each strap is insulated with insulation appropriate to the secondary voltage, usually mica. This insulation may also serve as the ground insulation. 6.3.2 Open or semi-enclosed slots Coils made of copper wire(s) wound in loops or bars. 6.3.3 Strand (wire) insulation In wound rotors, the conductors can be wound in loops as full coils or in bars (as half-coils). Strand insulation FDQEHPDGHXSRIHQDPHOVSRO\PHULF¿OPVUHVLQERQGHG¿EHUVVXFKDVSDSHUFRWWRQDVEHVWRVJODVVSRO\ester, or combinations thereof) or resin bonded mica. Aramid paper can be used in high temperature machines. The turn voltages are generally not high enough to require mica insulation or dedicated turn insulation. 6.3.4 Groundwall insulation Sometimes wound-rotor groundwall insulation is similar to those listed in 6.2.3. However, usually slot liners are used to provide ground insulation between the rotor winding and core slots. Common slot liner materials DUHDUDPLGSDSHUVXFKDV1RPH[Š DQGSRO\HVWHUJODVVSRO\HVWHURUSRO\HVWHUJODVVODPLQDWHVXVLQJ¿OPV VXFKDV0\ODUŠ 

 

1RPH[LVDUHJLVWHUHGWUDGHPDUNRI(,GX3RQWGH1HPRXUVDQG&RPSDQ\ Equivalent products may be used if they can be shown to lead to the same results.



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6.3.5 Winding support wedges Since there are high centrifugal forces imposed on the rotor winding during operation, substantial slot wedges are required to retain the winding in rotating cores. For this purpose, polyester or epoxy glass composite wedges are used in modern wound-rotor induction motors, while phenolic cotton composite wedges were commonly used in older motors. 6.3.6 Collector rings There are three collector rings mounted on a steel hub with insulation to separate the rings from the hub. This LQVXODWLRQPD\EHPLFDRURWKHUPDWHULDOVDSSOLHGRYHUVL]HWKHQWXUQHGGRZQWRVL]HWRDOORZIRUDVKULQN¿WRI the rings. The three rings are separated from one another by a distance dependent on the secondary three-phase voltage. A track-resistant compound is usually applied over all insulating surfaces of the collector.

6.4 Field winding insulation 6.4.1 Field windings 7KH¿HOGZLQGLQJVRIDFPDFKLQHVDUHQRUPDOO\URWDWLQJDQGFDQEHHLWKHUVDOLHQWSROHRUF\OLQGULFDOW\SH7KH ¿HOGFRLOVRIGFPDFKLQHVDUHVWDWLRQDU\DQGDUHFRQVWUXFWHGLQDVLPLODUIDVKLRQWRDFVDOLHQWSROHURWDWLQJ¿HOG coils, except that they need not be built to withstand the effect of rotational forces. Field coils for dc machines DUHXVXDOO\DFRPSOH[DVVHPEO\RIH[FLWLQJDQGFRPPXWDWLQJFRLOVHDFK¿WWHGRYHUDQGLQVXODWHGIURPDSROH piece. Some coils contain multiple windings. ,QDOOFDVHV¿HOGZLQGLQJVKDYHWXUQDQGJURXQGLQVXODWLRQLQVXODWHGPHFKDQLFDOVXSSRUWVDQGOHDGLQVXlation. In addition, rotors for ac machines may have collector ring insulation, retaining ring insulation, and banding insulation. 6.4.2 Turn (conductor) insulation 7XUQLQVXODWLRQRQZLUHZRXQG¿HOGFRLOVXVXDOO\LQFRUSRUDWHVDWKLQLQVXODWLQJOD\HURQWKHVWUDQGLWVHOI9DULRXVPDWHULDOVVXFKDVDVEHVWRVFRWWRQ¿EHUJODVVSDSHUVPLFDVDQGV\QWKHWLFPDWHULDOVKDYHEHHQXVHG Turn insulation on strap-wound coils usually incorporates various forms of tape or strip material with resin bonding. 6.4.3 Ground insulation $YDULHW\RIRUJDQLFDQGLQRUJDQLFPDWHULDOVDUHXVHGIRUJURXQGLQVXODWLRQRQWKH¿HOGFRLOVRIURWDWLQJPDchines. Some early machines may have used wood and cambric materials, which may have been replaced in later machines with phenolic laminates and sheet mica. Mica tapes were also used. Today common materials are mica laminates, glass laminates, and aramid paper. 6.4.4 Collector insulation The insulation used on collector rings and leads must be adequate both for support and electrical creepage to WKHJURXQGHGVKDIW7KHLQVXODWLRQXVXDOO\FRQVLVWVRIODPLQDWHG¿EHUVRUPLFDVXLWDEO\ERQGHGRULPSUHJQDWHG 6.4.5 Brush rigging insulation The insulated components on brush riggings are generally made from molding compounds, laminated boards, RUWXEHVPDGHIURPSDSHUFRWWRQRUJODVV¿EHUVVXLWDEO\ERQGHGDQGLPSUHJQDWHG0RLVWXUHUHVLVWDQWVXUIDFHV are very important for these components.



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6.5 Core and frame-assembly insulation 6.5.1 Stator core interlaminar insulation (core plate insulation) Stator cores are built up with thin steel laminations insulated from each other to reduce core losses. A variety of thin insulating coatings, such as varnish, silicates (water-glass), oxides, ceramics and coatings that are combinations of organic and inorganic materials. On very small machines or machines where the volts per layer in the stator core are low, iron oxides produced by appropriate annealing processes are often used. On large PDFKLQHVZLWKKLJKHUÀX[ZKHUHWKHYROWVSHUOD\HUDUHKLJKHUWKHVWDQGDUGSUDFWLFHLVWRGHEXUUDQGUHFRDW the laminations. 6.5.2 Insulation punchings In some designs, where deemed necessary, pieces of thin insulation sheet are cut to the shape of the punchings and are used to supplement the interlaminar insulation. These layers are also sometimes used as backing for YHQWLODWLRQOD\HUVZKHUHWKHUHDUHVSRWZHOGHGRUULYHWHGYHQW¿QJHUV7\SLFDOO\WKHPDWHULDOVXVHGDUHJODVV HSR[\ODPLQDWHVRUDUDPLG¿EHUSDSHUVGXHWRWKHFRPSUHVVLYHFUHHSUHTXLUHPHQWV2OGHUPDFKLQHVZLWKORZHU WKHUPDOFODVVL¿FDWLRQKDYHXVHGFHOOXORVHRUFRWWRQSDSHULQVXODWLRQSXQFKLQJV 6.5.3 Core tightening through bolt insulation The core tightening through bolts are insulated from ground along their length with insulating materials suitably bonded. Bolt-end hardware, such as nuts and washers, must also be insulated from ground. Key bars or bolts, used at the outer diameter of the core for tightening, usually require no insulation. Typically the materiDOVXVHGDUHJODVVHSR[\FRPSRVLWHVRUDUDPLG¿EHUFRPSRVLWHVIRUWKHVHLQVXODWLQJFRPSRQHQWVKRZHYHUHSoxy mica is sometimes used. In some newer designs, some rods, bolts, nuts and other mechanical components are fully manufactured of non-conductive glass composite materials. 6.5.4 Other insulating parts Insulation is sometimes used on bearings or bearing brackets to eliminate shaft currents. Insulation is also used to isolate temperature-measuring devices such as thermocouples (TCs), resistance temperature detectors 57'V DQGWKHUPLVWRUV

7. Service conditions affecting insulation life Electric machines and their insulation systems are subjected to mechanical, electrical, thermal and environPHQWDOVWUHVVHVWKDWJLYHULVHWRPDQ\GHWHULRUDWLQJLQÀXHQFHV7KHGHJUDGDWLRQUDWHRIHOHFWULFDOLQVXODWLRQ systems is substantially greater when these stresses act simultaneously than when they are sequentially applied (Bartnikas and Morin >%@). A thermal stress applied independently prior to application of electrical stress is appreciably less deleterious than when the two stresses act simultaneously (Bartnikas and Morin >%@). The V\QHUJLVWLFHIIHFWVRIHQYLURQPHQWDOVWUHVVHVDUHPRUHGLI¿FXOWWRDVFHUWDLQ$QLQVXODWLRQV\VWHPRSHUDWLQJLQ the presence of nuclear radiation may, in some cases, have a prolonged life expectancy. For example, polymeric based insulating systems operating in the presence of ionizing radiation may exhibit initially a prolonged life due to radiation induced cross linking of the polymers (Campbell, Bartnikas, and Eichhorn >%@).

7.1 Aging mechanisms No machine insulation system that is economically produced is expected to last forever. The thermal, mechanical, electrical, and environmental stresses will gradually reduce the electrical and mechanical strength of the LQVXODWLQJPDWHULDOV$WVRPHSRLQWWKHPDWHULDOVZLOOKDYHDJHGVLJQL¿FDQWO\,QVXFKDFDVHWKHLQVXODWLRQ breaks down or cracks under the normal operating voltages or as a result of a transient electrical or mechanical situation (e.g., from lightning or switching voltage surges, motor switch-on in-rush current or current transients from faults in the power system that cause large electromagnetic impulses or rapid load changes). If the

22

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insulation breakdown occurs in the stator groundwall or turn insulation, this will rapidly lead to high-power frequency fault currents and circuit breaker operation. Failure of the strand insulation in stators, or the turn insulation (and to a limited degree, the ground insulation) in rotors will not result in motor or generator failXUH+RZHYHUSHUIRUPDQFHZLOOEHDGYHUVHO\DIIHFWHGVLQFHWKHPDJQHWLF¿HOGLQWHQVLWLHVZLOOEHZHDNHUDQG QRQV\PPHWULFDOOHDGLQJWRYLEUDWLRQRUWKHHI¿FLHQF\RIWKHPDFKLQHZLOOEHUHGXFHGGXHWRFLUFXODWLQJ currents. The circulating currents will cause additional heating that will accelerate insulation aging processes. Additional failure processes can occur due to on-off cycling of motors or load cycling of generators. Such cycling leads to large and sometimes rapid swings in winding temperatures, and to a lesser extent core temperatures. Such temperature swings can lead to different thermally induced growth among the different winding components, developing shear stresses between the components. 7KHVLQJOHDQGPXOWLVWUHVVLQWHUDFWLRQVWRJHWKHUZLWKORDGF\FOLQJ\LHOGDERXWGLIIHUHQWLGHQWL¿DEOHIDLOXUH SURFHVVHVLQVWDWRUZLQGLQJVDQGDERXWPHFKDQLVPVLQURWRUZLQGLQJV:KLFKSURFHVVZLOORFFXULQDVSHFL¿FPDFKLQHDQGKRZTXLFNO\WKHIDLOXUHZLOORFFXUZLOOGHSHQGRQWKHIROORZLQJ a)

The design stress levels (operating temperatures, mechanical stress, etc.), the machine designer employed, and how close these levels are to the insulation material capabilities.

b)

How well the windings were manufactured and assembled.

c)

The operating environment the user provides. For instance, is the machine run at constant load or cycled? Is it over-loaded? Are oil, moisture, or abrasive particles present?

d)

How well the windings are maintained; is the user keeping them clean, and is the user keeping them tight to prevent vibration, etc.?

Knowing which deterioration processes are occurring is important, since any machine maintenance to extend winding life should directly address the processes.

7.2 AC Stationary armature winding aging mechanisms 7.2.1 Thermal deterioration Thermal stresses induce thermal aging of the insulation system whose rate is increased with the service temperature to which it is subjected (Bartnikas and Morin >%@). The deterioration rate approximately doubles IRUHYHU\ƒ&ULVHLQWHPSHUDWXUH7KHDJLQJUDWHLVLQÀXHQFHGE\XQXVXDOO\KLJKWHPSHUDWXUHVRIRSHUDWLRQ caused by conditions such as overload, high ambient temperature, loss of or restricted ventilation, or loss of cooling medium, or foreign materials deposited on windings. Long-term operation of a winding at high temperature leads to embrittlement of the insulation bonding resins and delamination. 7.2.2 Thermal cycling Thermal cycling processes can occur due to on-off cycling of motors, or load cycling of generators. Such cycling leads to large and sometimes rapid swings in component temperatures with different thermally induced growth among the different winding components, developing shear stresses between them. Such temperature swings result in accelerated insulation aging. For example, when a large generator goes from no load to full load in a few minutes, the stator winding copper temperature goes from a low temperature to a high temperature, and the copper grows axially along the slot. Immediately after the load increase, the insulation temperature remains relatively low. The result is that the groundwall insulation experiences a much smaller axial growth than the copper, which expands more than the JURXQGZDOOWRFUHDWHDVKHDUVWUHVVEHWZHHQWKHWZR:LWKDVXI¿FLHQWQXPEHURIORDGF\FOHVWKHJURXQGZDOO may separate from the conductors, creating an air gap between the two that can cause the following:

23

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a)

,QVXODWLRQGHJUDGDWLRQIURPSDUWLDOGLVFKDUJH3' DWWDFN

b)

Abrasion of turn or strand insulation from relative movement

c)

Abrasion of slot ground insulation from radial movement in the slot

d)

Failure from contaminant ingress

e)

Failure from high end-winding vibration

Inadequate impregnation of the groundwall insulation near the conductor stack is more likely in vacuum presVXUHLPSUHJQDWHGVWDWRUZLQGLQJVWKDQLQUHVLQULFKV\VWHPV)RUYROWDJHUDWLQJVRIN9DQGKLJKHUIDLOXUHV from the aging mechanisms noted previously can occur in as short a period as 2 years. 7.2.3 Internal water leaks This problem relates to large hydro and steam turbine generators with direct water-cooled stator windings. 6XFKPDFKLQHVJHQHUDOO\KDYHUDWLQJVJUHDWHUWKDQ09$,IWKHJHQHUDWRULVDSUHVVXUL]HGK\GURJHQGHVLJQ with hydrogen pressure greater than the water pressure, the possibility of water leaking into the stator winding insulation is reduced, but not eliminated. The most likely causes of water leaks are improper re-assembly of EDUWR7HÀRQŠ hose connections during maintenance, and crevice corrosion cracking of brazed joint between the bar nozzle and bar copper strands. Other possible causes are porosity of the brazing between bar nozzles and strands and bar strand cracks. Small water leaks can have the following three effects on the stator winding insulation: —

5HGXFWLRQRIWKHJURXQGLQVXODWLRQGLHOHFWULFVWUHQJWKZKLFKZLOOPDNHWKHZLQGLQJPRUHSURQHWRIDLOure if an overvoltage occurs due to switchyard faults, or ac/dc high potential testing.

—

6ORW3'LIJURXQGZDOOLQVXODWLRQGHODPLQDWLRQGXHWRZDWHULQJUHVVSURJUHVVHVWRWKHVORWUHJLRQVRI line end bars this can lead to winding failure.

—

5HGXFWLRQRIWKHPHFKDQLFDOVWUHQJWKRIWKHJURXQGZDOOLQVXODWLRQ7KLVPDNHVWKHZLQGLQJVXVFHSWLble to failure from high electromechanical forces induced by current surges from the power system, or out-of-phase synchronization.

7.2.4 Poor impregnation 5DQGRPZLQGLQJVFDQEHYDUQLVKRUUHVLQWUHDWHGE\YDULRXVPHWKRGVLQFOXGLQJWULFNOHGLSRUYDFXXPSUHVsure impregnation. Larger form wound machines may utilize resin-rich mica tapes or be vacuum pressure impregnated and cured before insertion into the stator. These machines may also be built utilizing a global vacuXPSUHVVXUHLPSUHJQDWLRQSURFHVV*93, DIWHUWKHFRLOVDUHZRXQGLQWRWKHVWDWRU:KDWHYHUWKHGHVLJQWKHVH treatments are utilized to seal the windings against moisture and contaminants, improve thermal conductivity, FRQVROLGDWHWKHFRLOVDQGLQIRUPZRXQGPDFKLQHVPLQLPL]HYRLGVL]HDQGFRQWHQWWRFRQWUROWKHOHYHORI3' activity. If the windings are poorly impregnated, several aging mechanisms can result including destructive levels of partial discharge, loose conductors or coils resulting in abrasion and premature electrical failure, poor mechanical support of the coil overhangs as well as allowing contamination of the windings. 7.2.5 Loose coils or bars in the slot 7KLVSUREOHPQRUPDOO\RFFXUVLQFRLOVRUEDUVZLWKWKHUPRVHWUHVLQULFKDQGLQGLYLGXDO93,LQVXODWLRQV\Vtems, which have cured and consolidated slot sections, if the slot radial and lateral support system is aged, improperly implemented, operated outside of design, or improperly designed.

7HIORQŠLVDUHJLVWHUHGWUDGHPDUNDQGDEUDQGQDPHRZQHGE\&KHPRXUV



24

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,IDFRLORUEDULVQRWWLJKWO\KHOGLQLWVFRUHVORWLWZLOOYLEUDWHSULPDULO\LQWKHUDGLDOGLUHFWLRQXQGHUWKHLQÀXence of magnetically induced mechanical forces with a frequency of twice the power supply frequency, i.e., +]IRU+]SRZHU7KHPDJQLWXGHRIWKHVHIRUFHVLVSURSRUWLRQDOWRWKHVTXDUHRIWKHFXUUHQWSDVVLQJ through the conductors and so they vary with machine output. The resulting relative movement between the winding ground insulation outer surface and the core, which is somewhat serrated, abrades the semiconducting coating (if applied) and then the groundwall insulation. If not detected soon enough this insulation degradation mechanism can result in a winding ground fault. 7.2.6 Semiconducting coating degradation This aging process deals with deterioration of the semiconducting coating on the slot section coil or bar in the DEVHQFHRIYLEUDWLRQGXHWRORRVHQHVV1RUPDOO\IRUPZRXQGVWDWRUZLQGLQJVUDWHGN9DQGDERYH KDYH such coatings in the form of carbon-loaded tapes, or paints. Semiconducting coatings may be used in lower voltage windings if supplied from variable voltage and frequency drives. 3RRUTXDOLW\PDWHULDOVDQGWKHUPDODJLQJZLWKVHUYLFHDUHWKHPRVWOLNHO\FDXVHVRIVHPLFRQGXFWLQJFRDWLQJ degradation. Both of these can lead to the material becoming non-conductive (in localized areas) by oxidation RIWKHFDUERQSDUWLFOHV,IWKLVRFFXUVRQKLJKYROWDJHFRLOV3'DFWLYLW\EHWZHHQWKHEDUFRLODQGFRUHZLOOVWDUW to develop in the affected areas. Air-cooled stator windings are more susceptible to this problem since the SUHVVXUL]HGJDVLQK\GURJHQFRROHGPDFKLQHVVXSSUHVVHVWKHUHVXOWLQJ3'DFWLYLW\,IVORW3'IURPWKLVDJLQJ mechanism develops in an air-cooled machine it will create ozone, which is a very chemically reactive gas that combines with other gasses in the air to create nitric acid. This nitric acid can attack insulation and packing materials in the slot. This can lead to coil/bar looseness, which if not detected early enough, can cause degradation of insulation from abrasion, as described in . Experience indicates that coils/bars with paint-based coatings are more susceptible to this problem. This aging mechanism will take many decades to cause a failure LIWKHZLQGLQJLVNHSWWLJKWLQWKHVORWVLQFHPLFDEDVHGLQVXODWLRQKDVDKLJK3'UHVLVWDQFH 7.2.7 Electrical/mechanical (contact) erosion There are several erosion mechanisms that occur on coil surfaces within the slot that contribute to aging of the semiconducting coatings and the main groundwall insulation. Over time, these effects can increase in intensity DQGHYHQFRPELQHZKLFKW\SLFDOO\OHDGVWRIXUWKHUDQGIDVWHUHURVLRQ5HJDUGOHVVRIWKHFDXVHEXWGHSHQGing on the energy available, these erosion mechanisms contribute to aging of the semiconducting coatings. Some of these mechanisms can occur with very high energy and lead to more rapid aging of the semiconducting coating as well as the groundwall insulation leading to premature failure. One widely recognized aging mechanism is a result of electrical/mechanical erosion, which occurs when the electromechanical forces of operation cause the core-to-coil semiconducting coatings contact to be compromised. This phenomenon could occur anywhere in the winding. Under normal operation for most machines, electromagnetic force will be induced, pressing the coil downZDUGLQWKHVORWWRZDUGWKH\RNHDVWKHFRLOFXUUHQWLQWHUDFWVZLWKWKHÀX[WKDWFURVVHVWKHVORW7KLVHOHFWURmagnetic force alternates between a full downward force and zero twice per cycle. A small tangential force DOVRRFFXUVGXHWRFRLOFXUUHQWLQWHUDFWLQJZLWKUDGLDOOHDNDJHÀX[LQWKHVORW0DFKLQHVZLWKORQJWKLQWHHWK KDYHDQDGGLWLRQDOWDQJHQWLDOIRUFHDVWKHWHHWKWKHPVHOYHVPRYHZLWKWKHURWDWLRQRIWKHURWRUÀX[1RWHWKDW some coil pitches and some fault conditions can lead to forces in the direction to eject the coil from the slot. Large machines are built with various stator slot-tightening systems as a means of restraining the coils because they are subjected to these forces as described in Clause 6:KHQWKHVHWLJKWHQLQJV\VWHPVDUHQRWVXI¿FLHQW the coil-to-core contact can be compromised. Two sources of current exist at the coil-to-core contact in the slots of the machine. One is a current, which is associated with the capacitive charging and discharging of the insulation surface. Higher coil voltages create KLJKHUFDSDFLWLYHFXUUHQWV7KHRWKHUFXUUHQWLVLQGXFHGD[LDOO\DORQJWKHFRLOGXHWRWKHÀX[LQWKHFRUH%\ GHVLJQWKHUHVLVWLYLW\RIWKHVHPLFRQGXFWLQJFRDWLQJLVNHSWVXI¿FLHQWO\KLJKWROLPLWD[LDOO\LQGXFHGFXUUHQW DQGDVVRFLDWHGKHDWLQJ$WWKHVDPHWLPHWKHUHVLVWLYLW\RIWKHVHPLFRQGXFWLQJFRDWLQJPXVWEHNHSWVXI¿FLHQW-

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O\ORZWRNHHSHOHFWURVWDWLFEXLOGXSDORQJWKHFRLOVXUIDFHVXI¿FLHQWO\ORZ%RWKRIWKHVHFRLOVXUIDFHFXUUHQWV depend on good contact between the coil semiconducting coatings and core surfaces. Electrical contact between these surfaces is affected by the contact pressure as more pressure will create a larger area of contact, and less pressure will create lower areas of contact. Forces, as well as motion of the core teeth themselves, can change the contact pressure and can result in motion of the coil in the slot which can lead to arcing. When the area of contact becomes smaller than the capability of the remaining material to carry the current, an arc occurs. As the semiconducting coatings are eroded, the electrostatic arcing becomes more prevalent on higher voltage coils and the combination can lead to higher rates of deterioration. Once the conductive FRDWLQJVKDYHEHHQVXI¿FLHQWO\HURGHGIURPWKHFRLODQGLWEHFRPHVORRVHLQWKHVORWWKHPRYHPHQWRIWKHFRLO may also create erosion of the insulation strictly by mechanical abrasion of the insulation with the core. There are various terms used to describe these phenomena, and some of these terms do not precisely describe DVSHFL¿FPHFKDQLVP)RULQVWDQFHslot discharge is often used to describe any discharging in the slot; however, often this term is intended only to describe high-intensity partial discharge that is the breakdown of a gaseous void. Also, spark erosion may refer to any erosion created by sparking, but it is often used with the intent of describing only vibration-induced sparking. The term slot poundingLVDOVRXVHGEXWLVOHVVVSHFL¿F as it does not account for the electrical aspects of the phenomena. 7.2.8 Semiconducting/stress control coating interface failure This problem is associated with the semiconducting coating deterioration process described in  and only occurs in form windings that have both slot semiconducting and an overlapping stress control tape or paint coating (normally silicon carbide) just outside the slot. The stress control coating has a characteristic of having a resistance that is high in areas of low electrical stress and low in areas of high electrical stress. Since the end-winding surfaces are at the same potential as the conductors, its purpose is to make the electrical stress DWWKHHQGRIWKHVHPLFRQGXFWLQJFRDWLQJPRUHXQLIRUP7KLVSUREOHPLVQRUPDOO\FRQ¿QHGWRZLQGLQJVUDWHG N9DQGDERYHEXWFDQRFFXULQORZHUYROWDJHZLQGLQJVVXSSOLHGIURPSXOVHZLGWKPRGXODWHG3:0 YROWDJH VRXUFHYDULDEOHIUHTXHQF\GULYHV9)'V  If the overlapping electrical connection between the semiconducting and stress control coatings degrades to EHFRPHQRQFRQGXFWLYHWKHVWUHVVFRQWUROFRDWLQJ³ÀRDWV´DQGLWVYROWDJHZLOOULVHWRWKHYROWDJHRIWKHFRSSHU conductors due to capacitive coupling. If this happens, a very high voltage separated by a small gap, develops between the grading and semiconducting coatings on bars/coils near the line ends. This air gap breaks down, resulting in discharges over the surface of the coil/bar between the two different coatings. Since this discharge mechanism is parallel to the insulation surface, degradation from it is a very slow process. However, if discharges are present, they may be an indication that the more serious problem of slot semiconducting coating degradation is also occurring. 7.2.9 Electrical stresses The service life of an insulation system varies inversely with the voltage, VíQ, where the exponent n is deSHQGHQWXSRQWKHVSHFL¿FYROWDJHGHSHQGHQWDJLQJPHFKDQLVP$EQRUPDOYROWDJHVH[FHHGLQJWKHVSHFL¿HG service rating, such as those caused by switching, lightning surges, or drive systems further accelerates the DJLQJSURFHVV3DUWLDOGLVFKDUJHVDWKLJKHURSHUDWLQJYROWDJHVPD\SURGXFHVHYHUDOXQGHVLUDEOHHIIHFWVVXFK as chemical degradation, localized heating, ionic bombardment, and physical erosion. These effects tend to increase the aging rate. (OHFWULFDOLQVXODWLRQWKDWRSHUDWHVLQDKLJKVWUHVV¿HOGLVVXEMHFWWRGHWHULRUDWLQJLQÀXHQFHVQRWSUHVHQWDW ORZHUVWUHVVOHYHOV3DUWLDOGLVFKDUJHFRURQD FDQFDXVHGHJUDGDWLRQRILQVXODWLRQE\FKHPLFDOHIIHFWVDQGE\ ion bombardment. This occurs when the voltage gradient on gas molecules in void spaces in the insulation exceeds a certain value, depending on the nature of the gas and its pressure and temperature. Ozone and nitrogen oxides that can attack organic materials in the insulation may be formed. Ozone, nitrogen oxides, and oxalic acid crystals may be formed when polymeric materials are exposed to discharges. The symptoms of this type

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of degradation are mostly whitish deposits. The effects of corona are not manifested to a noticeable degree in hydrogen cooled machines, since the oxygen content is small and degradation is principally physical pitting and erosion. 3RVLWLYHDQGQHJDWLYHLRQVDVZHOODVHOHFWURQV JHQHUDWHGGXULQJHDFKSDUWLDOGLVFKDUJHHYHQWDUHDFFHOHUDWHG LQWKHGLUHFWLRQRIWKHHOHFWULFDO¿HOG7KLVVXEMHFWVWKHERXQGDULHVRIWKHRFFOXGHGFDYLWLHVWRUHJXODUO\RFcurring bombardment by the charged particles in each half cycle of voltage. The resulting deterioration may manifest itself in the formation of electrical trees propagating into the dielectric from the void boundaries and tracking along the inside walls. 7.2.10 Electrical tracking due to contamination ,IHOHFWULFDOWUDFNLQJGXHWRFRQGXFWLYHFRDWLQJFRQWDPLQDWLRQGRHVRFFXUFXUUHQWZLOOÀRZDFURVVWKHVXUfaces of the winding, especially in the end-winding regions. Such contamination can result from the ingress of oil from bearings, hydrogen seals in combination with moisture, or carbon particles (such as from carbon brushes) from the atmosphere. Open enclosure, air cooled machines are most susceptible to this degradation mechanism. If the insulation surfaces and blocking between adjacent high-voltage sections of end-winding, or circuit ring EXVHVLQGLIIHUHQWSKDVHVEHFRPHFRQWDPLQDWHGZLWKFRQGXFWLYHPDWHULDOVFXUUHQWVZLOOÀRZEHWZHHQWKHP This happens because the surfaces of these sections of winding will rise to the potential of their conductors, between which voltages approaching the phase-to-phase value are present. If the contamination resistance was uniform, little deterioration would likely result from these currents. However, dry areas where the resistance is much higher are commonly present, and as a result, the whole voltage can appear across these high resistances to cause electrical breakdown of the adjacent air or hydrogen. This discharge degrades and may carbonize the underlying organic resin and tape. This area then becomes very conductive and the high electrical stress then transfers to another dry area. In the longer term, an electrical track can develop between phases and can start eating into the groundwall insulation, potentially leading to a phase-to-phase failure. This mechanism is usualO\YHU\VORZDQGFDQWDNHPRUHWKDQ\HDUVDIWHULQLWLDWLRQWRFDXVHDIDLOXUH 7.2.11 Voltage surges 9ROWDJHVXUJHVLQVWDWRUZLQGLQJLQVXODWLRQV\VWHPVLQPRWRUVDQGJHQHUDWRUVDUHWUDQVLHQWEXUVWVRIUHODWLYHO\ high voltage that increase the electrical stress beyond which normally occurs in service. Such voltages can occur from the following: a)

Lightning strikes

b)

3RZHUV\VWHPJURXQGIDXOWV

c)

Out-of-phase generator breaker closing

d)

Motor circuit breaker closing and opening

e)

9ROWDJHVRXUFHPRWRUFRQYHUWHUGULYHV

,WLVXQOLNHO\WKDWWKH¿UVWIRXUVXUJHVRXUFHVZLOODJHWKHZLQGLQJLQVXODWLRQEXWWKH\ZLOOFDXVHLWWRIDLOLIWKH dielectric strength is inadequate, or has been reduced by some other aging mechanism. This is not the case with converter drives since they can continuously impose high voltages with fast rise times that can cause insulation aging. 9ROWDJHVXUJHVIURPWKH¿UVWIRXUVRXUFHVDUHPRVWOLNHO\WRDIIHFWVWDWRUZLQGLQJVLQPRWRUVRUJHQHUDWRUVZLWK multi-turn stator coils. When a high frequency voltage is applied, the voltage distribution is non-linear with DPXFKJUHDWHUSHUFHQWDJHRIWKHYROWDJHDSSHDULQJDFURVVWKH¿UVWFRLOFRQQHFWHGWRWKHSKDVHWHUPLQDO7KLV non-uniform voltage distribution occurs because the series inductive impedance of the winding is relatively large compared to the capacitive impedance to ground at this high frequency. In addition, it has been shown



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WKDWWKLVYROWDJHGRHVQRWHYHQO\GLVWULEXWHDFURVVWKH¿UVWFRLO7KHFRQVHTXHQFHRIWKLVXQHYHQGLVWULEXWLRQLV WKDWLQWHUWXUQYROWDJHVZKLFKFDQEHRUGHUVRIPDJQLWXGHKLJKHUWKDQWKH+]RU+]VWHDG\VWDWHYDOXH  can be induced in the line-end coils. These surges can cause line end coil turn insulation failures if the turn LQVXODWLRQGLHOHFWULFVWUHQJWKLVWRRORZRUKDVEHHQZHDNHQHGE\LQVXODWLRQDJLQJ²HJWKHUPDODJLQJRU3' activity at ground insulation-to-conductor interface. Such failures can also occur in motors fed from voltage source inverters, but most manufacturers appreciate the need to strengthen turn insulation by using mica to avoid failures from these repetitive voltages. 7KHYROWDJHZDYHIRUPIURPD3:0YROWDJHVRXUFHFRQYHUWHUFDQOHDGWRLQFUHDVHGJURXQGZDOOLQVXODWLRQ heating, which can increase the winding temperature and thus accelerate the normal thermal aging processes described previously. 7KH3'PD\EHODUJHUDQGPRUHIUHTXHQWZLWKDFRQYHUWHUEHFDXVHWKHSHDNYROWDJHVDUHXVXDOO\KLJKHUWKDQ WKHSHDNYROWDJHIURPDVLQXVRLGDOVXSSO\7KHSHDNWRSHDNYROWDJHFDQEHKLJKHUWKDQWKDWIURPD+]RU Hz supply due to the transmission line effects that may cause the step voltage changes that occur with converters to possibly double. The partially conductive coatings that normally cover the coil insulation in the stator slot and the silicon carELGHPDWHULDODWWKHVORWH[LWVDUHLQWHQGHGWRVXSSUHVVWKHSUREDELOLW\RI3'RFFXUULQJRQWKHFRLOVXUIDFHLQ WKHVORWDQGMXVWRXWVLGHRILW6HYHUDOVWXGLHVKDYHVKRZQWKDWXQGHU3:0YROWDJHWKHVHFRDWLQJVZLOORSHUDWH at higher temperatures and thus increase the rate of thermal aging, if they are not properly designed (Stone, et al. >%@6KDUL¿HWDO>%@, Boggs >%@ 6LQFH3:0YROWDJHZDYHIRUPVFRQWDLQYROWDJHVDWKLJKIUHTXHQFLHVIURPWKHULVHWLPHRIWKHYROWDJHVWHSVDQGWKH3:0VZLWFKLQJUDWH KLJKHUFDSDFLWLYHFXUUHQWVÀRZ WKURXJKWKHJURXQGZDOODQGWKHQWKURXJKWKH3'VXSSUHVVLRQFRDWLQJV7KHVHKLJKHUFXUUHQWVFUHDWHKLJKHUI2R ORVVHVLQWKHFRDWLQJVWKDQZRXOGRFFXUXQGHU+]RU+]RSHUDWLRQLQFUHDVLQJWKHRSHUDWLQJWHPSHUDWXUH of the coatings. The effect is exacerbated because the higher frequencies also cause the silicon carbide materials to be less effective in linearizing the voltage along the surface of the coils—which tends to concentrate the heating to a shorter area. 7.2.12 Environmental factors In addition to the electrical tracking due to contamination discussed in  there are other environmental factors that can cause winding insulation to age and fail. These include the following: a)

Chemical attack

b)

Abrasive particles

c)

Ionizing radiation

d)

Magnetic material (termites)

7.2.12.1 Chemical attack Most types of older insulation systems are prone to chemically induced degradation due to the presence of solvents, oil, water, or other chemicals and gasses. For example magnet wire insulation materials such as polyesWHUFDQVRIWHQDQGVZHOOIURPH[SRVXUHWRPRLVWXUH*URXQGZDOOLQVXODWLRQXVLQJDVSKDOWYDUQLVKDQGVRPHRI the earlier polyester bonding agents are prone to softening and swelling and loss of mechanical strength when exposed to moisture or chemicals. 6RIWHQLQJRILQVXODWLRQPDNHVLWVXVFHSWLEOHWRFROGÀRZ²LHWKHLQVXODWLRQJUDGXDOO\EHFRPHVWKLQQHULQ areas where mechanical pressure is applied. If the thickness of the insulation is reduced to such an extent that it can no longer tolerate normal or higher transient voltages, then it will fail.



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Modern stator winding insulation systems are more resistant to most types of chemical attack, but can still be affected by some chemicals and gasses. However, if epoxy is exposed to oil and water for many years it will eventually start to degrade. Also, if it is known that a winding will be exposed to a humid environment it can be designed to be sealed against moisture—e.g., nuclear plant motors that have to survive under main steam line break conditions. 7.2.12.2 Abrasive particles Abrasive particles in the cooling gas stream can grind away stator winding insulation to the extent that it fails under normal or transient voltage conditions. This is most likely to occur in machines with open-air ventilation operating environments where sand, iron ore dust, etc. are present in the operating environment. If abrasive particles enter the machine enclosure, they will be blown though the stator windings at high velociW\E\WKHFRROLQJDLU)XUWKHUPRUHVXI¿FLHQWTXDQWLWLHVRIWKHVHPDWHULDOVDWDKLJKHQRXJKYHORFLW\ZLOODEUDGH WKHLQVXODWLRQVXUIDFHVSDUDOOHOWRWKHLUÀRZWRUHGXFHWKHLUWKLFNQHVV6XFKDEUDVLRQFDQHYHQWXDOO\H[SRVHWKH winding copper, which leads to failures. The most likely areas to be affected by abrasion are the end-windings and the sections of winding bridging radial core cooling ducts. 7.2.12.3 Ionizing radiation 3RO\PHUVDUHSDUWLFXODUO\VXVFHSWLEOHWRLRQL]LQJUDGLDWLRQZKLFKLVIRXQGWROHDGWRWKHSURGXFWLRQRIHOHFtronically excited states of molecules and the formation of radicals (Bhimani >%@) The extent of the resulting polymer degradation depends upon the absorbed radiation dose and dose rate (Bawart >%@, Bhimani >%@). 7KHUDGLDWLRQLQGXFHGGHJUDGDWLRQUDWHRIWKHLQVXODWLQJPDWHULDOLVIXUWKHUPRUHLQÀXHQFHGE\WKHHOHFWULFDO thermal and mechanical stresses to which the insulation may be simultaneously subjected. As the mechanical properties of the insulating material deteriorate, electrical breakdown ensues when the material reaches its brittle fracture state. In cases where the insulating material is only exposed to short intermittent periods of ionizing radiation, its electrical properties such as conductivity and dissipation factor will exhibit an increase during the irradiation period as well as for a short period following the removal of the radiation source. The increase in the conducWLYLW\DQGGLHOHFWULFORVVHVLVDGLUHFWUHVXOWRIWKHHOHFWURQVEHLQJH[FLWHGLQWRWKHFRQGXFWLRQEDQG7KH¿QLWH conductivity—remaining after the removal of radiation—is caused by the still mobile electrons which had fallen into shallow traps. However, these conduction electrons eventually fall into deep traps and become immobilized. Only when the next radiation exposure takes place can they be emitted. The copious supply of free electrons during the irradiation periods will also lower the partial discharge inception and extinction voltages; this abundant availability of free electrons may facilitate the occurrence of pseudo-glow discharges. There are a number of high-voltage motor applications; for example, reactor coolant pump motors within the reactor containment area of a nuclear generating station, where radiation levels can be high. If acceptable insulation life is to be achieved, the insulation systems in these machines must contain materials with a high radiation resistance to prevent rapid deterioration of mechanical properties of the binders and backing materials. Materials such as ceramics, mica, glass and epoxy resins are known to be only slightly affected by the radiation levels seen in these applications. Organic backing, bracing, and bonding materials, on the other hand, are strongly affected by ionizing radiation while polymers with aromatic rings will tolerate larger doses without deterioration. Therefore, due to the susceptibility of organic insulations to radiation damage, great care is required in selecting the proper backing, insulation bonding, and bracing materials for the winding of machines to be used in a radiation environment. 7KHW\SHVRIUDGLDWLRQIURPQXFOHDUUHDFWRUVDUHSULPDULO\ĮDQGȕSDUWLFOHVJDPPDUD\VDQGQHXWURQV6LQFH ĮDQGȕSDUWLFOHVGRQRWSHQHWUDWHWKHUHDFWRUVKLHOGWKH\DUHQRWDVLJQL¿FDQWIDFWRU2QWKHRWKHUKDQGJDPPD rays and neutrons do penetrate this shield, reacting with insulating materials to produce electrons that can be responsible for radiation damage.



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The two molecular changes that may be produced by radiation in an organic insulation are cross-linking of molecular chains and bond scission, or cutting of polymer chains. Cross-linking builds up the molecular structure, initially increasing tensile strength, but then reduces elongation and eventually results in the loss of impact strength. This changes rubbery or plastic materials into hard, brittle solids. Scission breaks down molecular size, reduces tensile strength, and usually adversely affects other properties. 0RVWPRGHUQKLJKYROWDJHLQVXODWLRQV\VWHPVFRQWDLQPLFDZLWKEDFNLQJPDWHULDOVVXFKDVJODVVRU'DFURQŠ JODVV¿EHUVDOORIZKLFKKDYHIDLUO\KLJKUDGLDWLRQUHVLVWDQFH7KHVXVFHSWLEOHSDUWRIWKHLQVXODWLRQV\VWHPVLV WKHRUJDQLFLPSUHJQDWLQJUHVLQXVHGWRELQGWKHVHPDWHULDOVWRJHWKHU5HVLQVXVHGPXVWWKHUHIRUHEHFDUHIXOO\ selected to help ensure good insulation system radiation resistance. (Bawart >%@, Bhimani >%@). Materials susceptible to radiation aging lose their mechanical properties. This makes them susceptible to mechanical failure under the stresses imposed, e.g., if the bonding resin becomes brittle, it will crack and delamination may occur, and the failure mechanism associated with this will cause electrical breakdown of the insulation. It is for this reason that insulation systems used in motors that operate in high radiation areas are HQYLURQPHQWDOO\TXDOL¿HGE\VLPXODWHGDJLQJDQGWHVWLQJ,(((6WGJLYHVJXLGHOLQHVRQKRZVXFKTXDOL¿FDWLRQWHVWLQJ²ZKLFKLQFOXGHVUDGLDWLRQDJLQJ²VKRXOGEHSHUIRUPHG+RZHYHUWKLVLQIRUPDWLRQPD\QRW apply if the radiation environment is very harsh. 7.2.12.4 Magnetic material (termites) 6PDOOPDJQHWLFSDUWLFOHVPD\YLEUDWHLQWKHDOWHUQDWLQJHOHFWURPDJQHWLF¿HOGV7KHVHFDQZHDULQWRWKHLQVXlation. This can then lead to insulation faults. These moving particles are commonly referred to as magnetic termites. 7.2.13 End-winding vibration If a stator end-winding is not adequately braced the coils/bars will vibrate relative to their support structure GXHWRWKHHOHFWURPDJQHWLFIRUFHVFUHDWHGE\WKH+]RU+]FXUUHQWÀRZLQJWKURXJKWKHVWDWRUFRLOVDQG bars and rotational speed vibration. This relative movement can cause abrasion, cracking and eventual failure of the insulation. In addition, end-winding vibration can lead to cracking of conductor strands as a result of high cycle fatigue. Such problems are most likely to occur on form-wound two-pole and four-pole generators and motors, since such machines have long end-windings, which may have resonant frequencies close to the frequency of the magnetic forces and rotational speed frequency. End-winding vibration is one of the most common failure mechanisms of large two-pole steam and gas turbine generators rated at several hundred megawatts and above. Electromagnetic forces can also cause end-winding vibration problems in hydrogenerators with relatively long overhangs compared to short-core lengths. If the end-windings are not adequately supported, any form-wound stator can fail due to this problem. Inadequately supported long end-windings in form-wound machines will result in vibration in the radial and circumferential directions. Assuming the coils/bars are tight in their slots, this will cause them to pivot at the stator slot exits to cause fatigue cracks in the groundwall insulation just outside of the stator slot, leading to a phase-to-ground fault. If the stator end-winding blocking and bracing are loose, then coils and bars can also vibrate relative to one another and their support structure. This coils/bar movement will cause them to rub against blocking, surge rings, support cones (large generators), and/or other end-winding support structures. This rubbing will cause insulation abrasion and thinning. Fiberglass roving, which is hard, is very effective in cutting through the groundwall insulation. Such insulation abrasion, if not corrected, can cause failure of the phase-to-ground insulation. ,QGLUHFWZDWHUFRROHGVWDWRUV+]RU+]HOHFWURPDJQHWLFHQGZLQGLQJYLEUDWLRQFDQFDXVHIDWLJXH cracking of the brazed connections between top and bottom bars and/or the water nozzles. This can allow water to leak into the insulation (see ). Also, if hydrogen becomes entrained in the stator cooling water this 

'DFURQLVDUHJLVWHUHGWUDGHPDUNRI(,GX3RQWGH1HPRXUVDQG&RPSDQ\



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can lead to winding failure. This happens because the hydrogen bubbles combined with excessive vibration can lead to cavitation erosion of the copper conductor strands. The resulting copper thinning together with the vibration eventually causes the strands to crack from fatigue, allowing large amounts of hydrogen into the FRROLQJZDWHU,IVXI¿FLHQWK\GURJHQHQWHUVWKHZDWHUV\VWHPWKLVFDQOHDGWREORFNLQJRIWKHZDWHUFRROLQJ circuit leading to local overheating. End-winding vibration in direct hydrogen cooled stator winding has led to broken copper strands from high cycle fatigue. This leads to arcing between strands at the break, causing localized overheating of the insulation. The heating from this type of failure eventually causes the insulation to melt leading to a ground fault. This type of winding may have resistors installed between the conductors and hollow hydrogen tubes within WKHVWDWRUEDUVWRJLYHD¿[HGSRWHQWLDOEHWZHHQWKHWZR+LJKHQGZLQGLQJYLEUDWLRQFDQFDXVHWKHVHUHVLVWRU connections to fail.

 &\OLQGULFDOURXQGURWRU ¿HOGZLQGLQJDJLQJPHFKDQLVPV This subclause deals with the common aging mechanisms found in cylindrical rotor windings that are also called round rotor windings that are used mainly in medium to large turbine generators, but are also starting to appear in large 2-pole motors supplied from variable speed drives. 7.3.1 Thermal aging 2OGHUF\OLQGULFDO¿HOGZLQGLQJVRIWKLVW\SHFRQWDLQRUJDQLFFRPSRQHQWVZKLFKZKHQWKHUPDOO\DJHGLQVHUvice, shrink and allow the inorganic components to be displaced by the cyclic mechanical forces experienced in operation. This can lead to cracks and gaps in the groundwall and turn insulation resulting in electrical failure. 6WDUWLQJLQWKHVPRGHUQWKHUPRVHWWLQJUHVLQVDQGJODVVIDEULFVEHJDQWRUHSODFHWKHROGHURUJDQLFPDWHrials. The end-winding blocking was changed from phenolic-bonded asbestos cloth laminates to glass cloth with either polyester or epoxy laminating resins. Similar changes were made in the slot cell insulation. These FKDQJHVUDLVHGWKHWHPSHUDWXUHFODVVRIWKHLQVXODWLRQWR&ODVV) DQGVLJQL¿FDQWO\UHGXFHGWKHLQFLGHQFH RIWKHUPDODJLQJ6PDOOHU¿HOGVDUHQRZRIWHQLQVXODWHGZLWKQRQZRYHQDUDPLGVKHHWVDQGWDSHVZKHUHDV ODUJHU¿HOGVRIWHQXVHQRQZRYHQJODVVODPLQDWHVIRULQVXODWLRQDQGEORFNLQJ *ODVVODPLQDWHVERQGHGZLWKHSR[\RUSRO\HVWHUUHVLQVDUHFRPPRQO\XVHGIRUERWKWKHWXUQDQGWKHJURXQG insulation in direct-gas-cooled rotor windings. For some lower ratings, aramid paper insulation is often used for both the turn and ground insulation. Thermal degradation of these materials may be treated as a chemical rate phenomenon (described by the Arrhenius relationship) and includes loss of volatiles, oxidation, depolymerization, shrinkage, surface cracking, and embrittlement. The higher the temperature, the faster the chemical reaction, resulting in shortened life of the insulation under thermal degradation. 0RGHUQZLQGLQJVXVHJODVVODPLQDWHLQVXODWLRQV\VWHPVW\SLFDOO\PDGHIURP&ODVV) LQVXODWLRQPDWHULDOVIRURSHUDWLRQDW&ODVV% WHPSHUDWXUHV6LQFHWKHDYHUDJHURWRUZLQGLQJRSHUDWLQJWHPSHUDWXUHLVLQWKH UDQJHRIƒ&WRƒ&WKHUHZRXOGDSSHDUWREHDQDGHTXDWHWKHUPDOPDUJLQ+RZHYHUWKHPDUJLQLVUHGXFHG at hot spots in the winding, which are not normally measured directly since some machines have brushless exFLWHUVGLUHFWO\FRQQHFWHGWRWKHURWRU¿HOGZLQGLQJIRUWKRVHZLWKVOLSULQJVRQO\DQDYHUDJHURWRUWHPSHUDWXUH FDQEHGHULYHGIURPURWRUDPSVDQGYROWV'HSHQGLQJRQWKHW\SHRIWKHFRROLQJJDVÀRZV\VWHPWKHHVWLPDWHG KRWVSRWWHPSHUDWXUHFRXOGH[FHHGƒ&,IWKLVLVWKHFDVHWKHUPDODJLQJEHFRPHVDIDFWRUSDUWLFXODUO\ ZKHUHVRPH&ODVVPDWHULDOVKDYHEHHQXVHG7KHWKHUPDOGHJUDGDWLRQLVOHVVOLNHO\RQK\GURJHQFRROHG rotors because of the lack of oxygen, which accelerates chemical aging, and because of the operating temperature margin usually available in hydrogen-cooled rotors.



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7.3.2 Thermal cycling Operating temperatures have a direct aging effect on the insulation materials, as seen in the previous section. However, when the operating temperature varies due to load changes, start-stops, etc., additional stresses are set up that accelerate the thermal aging process. The temperature changes cause the expansion and contraction of the winding copper relative to the insulation, giving rise to insulation aging from abrasion. Unless the rotor winding design can accommodate this movement of the copper, additional built-up stresses can damage not only the insulation, but also the winding copper. During operation, copper losses from the winding and to a lesser extent windage and stray losses from the rotor forging, cause an increase in temperature of the rotor components. The physical location of the components in the rotor is altered due to the axial thermal expansion caused by the increase in temperature. When the unit is shut down the rotor cools down and the components contract to their original position, provided that the copper has not been stretched beyond its elastic limit and there is no restriction to their movement. Depending on the duty, this expansion-contraction cycle may be repeated hundreds of times during the life of the unit. The axial movement of the copper tends to abrade the ground insulation, especially toward the end of the rotor slots. Wear imposed on the winding insulation and other components due to this repeated back and forth movement results in mechanical aging due to thermal cycling. 3HDNLQJDQGWZRVKLIWHGPDFKLQHVDUHPRUHVXVFHSWLEOHWRWKHUPDOF\FOLQJGDPDJHFRPSDUHGWREDVHORDGHG units because of the higher number of start-stop cycles. Longer rotors are likely to experience a higher level of damage due to the larger amount of expansion and relative movement. Similarly, modern air-cooled units, which generally operate at a higher temperature, will be affected more than hydrogen-cooled units. 7.3.3 Abrasion due to imbalance or turning gear operation 7ZRDQGIRXUSROHURWRUVLQODUJHJHQHUDWRUVFDQZHLJKDVPXFKDVWRQV0RUHRYHUWKH\KDYHFRPSRnents, such as the windings, that can move independently in the axial, radial, and transverse directions. To help ensure long-term reliable performance the rotor must run smoothly within acceptable vibration limits under all operating conditions. High vibration can lead to relative movement of the rotor winding components, which, in turn, can lead to insulation and copper abrasion. Considerable effort is required to ensure a mechanically balanced rotor system; starting with the design stage, to factory assembly and testing, site assembly, and setup, to operating practices and monitoring. Nevertheless, this balance is upset at times due to a number of factors that result in an increase in rotor vibration, which can cause further insulation damage and even lead to shutdown of the generator. Although rotor vibration is a mechanical phenomenon, its origin can be electrical or thermal in nature. Because of electrical and thermal stresses that build up during operation, additional forces are superimposed on the distributed weight of the rotor. Initially, these forces may be small; however, the underlying mechanisms being progressive in nature, the forces can become large enough over time to affect the weight distribution of WKHURWRU5RWRUG\QDPLFSHUIRUPDQFHLVVHQVLWLYHWRFKDQJHVLQWKHZHLJKWGLVWULEXWLRQSDUWLFXODUO\IRUWZR pole designs in which the longer and thinner rotor forging is more susceptible to bending forces. The increased vibration due to rotor unbalance can cause damage to the rotor components, leading to further imbalance. Ultimately, if relative movement occurs between the copper and the insulation or the insulation and the rotor body, causes abrasion that can lead to turn shorts or ground faults. Large turbine generator rotors have to be operated on turning gear at very low speeds (a few rpm) to prevent catenary “sets” in the shaft during unit shutdown. Operation at turning gear speed with low radial rotational forces on the windings can cause copper-dusting abrasion, particularly when there are two or more conductor sub-strands that are not insulated from one another and the slot side packing is not tight enough to prevent sideways strand movement. The copper particles can lead to turn or ground shorts.

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7.3.4 Electrical tracking from contamination ,QVXODWLRQLVXVHGWRHOHFWULFDOO\VHSDUDWHWKHFRSSHUZLQGLQJWKDWRSHUDWHVDWYROWDJHVXSWRDERXW9UHODtive to grounded rotor body. The insulation takes many forms and shapes due to the complex nature of the rotor winding, which must allow for movement due to expansion, and the geometry of the rotor forging, end rings, balance rings, support rings, slip rings, and connection hardware. During winding assembly, care is taken to ensure that the isolation between the live copper winding turns and between copper and ground is maintained at these numerous interfaces. Normally, the creepage paths between turns and to ground are more than adequate. However during service, contamination at these critical interfaces can reduce the creepage path to such an extent that turn-to-turn and ground shorts could develop. Hundreds of interfaces exist in a rotor where insulation separates the live parts from the grounded components such as the forging, wedges, retaining rings, and balance rings. Intermittent surface discharge between turns or from the winding components to grounded parts occurs when these insulation interfaces are compromised due to surface contamination. The discharge results in a chemical reaction of the components involved, producing carbon and other chemicals. The products of the reaction lodge themselves across the interface, creating a path of reduced resistance along which subsequent discharges occur. 7.3.5 Repetitive voltage surges ,QJHQHUDWRUURWRUVRIWKLVW\SHWKHDSSOLHGYROWDJHLVDURXQG9GFZKLFKLVZHOOZLWKLQWKHUDWLQJRILQVXlating materials used, particularly in modern designs. This voltage further divides between the turns to result LQDVOLWWOHDV9EHWZHHQWKHPZKLFKLVDZHOOZLWKLQWKHWXUQLQVXODWLRQYROWDJHUDWLQJ7KHWXUQLQVXODWLRQ ZRXOGODVWLQGH¿QLWHO\XQGHUWKHVHYHU\ORZHOHFWULFDOVWUHVVHV+RZHYHUWUDQVLHQWRYHUYROWDJHVIURPWKHH[FLtation supply or system transients, can be orders of magnitude higher, and may lead to insulation degradation. Events internal or external to the excitation system can induce large transient voltages in the rotor windings. The occasional spike may not be harmful, but continuous repetitive spikes from an excitation system can cause gradual deterioration from partial discharges. This aging mechanism is similar to the stator winding aging process caused by converter drives. Insulation that has already been weakened by other aging mechanisms LVSDUWLFXODUO\YXOQHUDEOHWRUHSHWLWLYHYROWDJHVXUJHV9ROWDJHVXUJHVDUHPRVWOLNHO\WRFDXVHWXUQWRWXUQ faults since the insulation between turns is the thinnest and is subject to high levels of mechanical stresses. 7.3.6 Rotational force At operating speed, rotor winding components are subjected to high mechanical compressive stresses from URWDWLRQDOIRUFHV,QODUJHWXUELQHJHQHUDWRUVWKHVHIRUFHVFDQH[FHHGWRQVDWWKHZHGJHVDQGWRQV DWHDFKUHWDLQLQJULQJ6LJQL¿FDQWWDQJHQWLDOIRUFHVDUHDOVRSUHVHQWSDUWLFXODUO\GXULQJVWDUWXSDQGVKXWGRZQ of the generator. The contribution of the copper conductors to the total stress on the insulation materials is an important factor. Insulation materials made from quality materials and with adequate design margins can endure these compressive forces over long-term operation. However, where the materials are weakened due to inadequate quality control or other aging mechanisms such as thermal aging, the insulation can bend, buckOHDQGFUDFNXQGHUWKHLQÀXHQFHRIWKHODUJHURWDWLRQDOIRUFHV7KLVFDQOHDGWRWXUQWRWXUQVKRUWVRUJURXQG faults. The insulation materials involved include slot liners, turn insulation, slot packing and pads, bracing materials, and connection insulation. The effects of rotational forces are a function of the design of the winding slot wedging and end-winding bracing systems, the properties of the materials used, and the frequency of start-stop cycles.

 6DOLHQWSROHURWDWLQJ¿HOGZLQGLQJDJLQJPHFKDQLVPV There are two types of windings used in salient pole rotors. These are the “strip-on-edge” and “multi-layer ZLUHZRXQG´W\SHV7KHVWULSRQHGJHZLQGLQJFRQVLVWVRIUHFWDQJXODUÀDWFRSSHUVWULSVEHQWHGJHZLVHWRIRUP a coil. The type used depends on the rating and speed of the machine. The coil insulating materials and their ar-

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rangement are dependent on the winding construction, in that, while the insulation between the coils and poles can be the same for both types, the turn insulation is quite different. 7.4.1 Thermal aging All insulating and nonmetallic bracing materials deteriorate with time due to the heat from the windings. The rate at which component materials deteriorate is a function of their thermal properties and the temperatures to which they are subjected. If the thermal ratings of component materials have been properly selected, the thermal aging and associated deterioration will occur gradually over an acceptable service life. &ODVVZLQGLQJPDWHULDOVVXFKDVRUJDQLFYDUQLVKHVVKHOODFVDVEHVWRVERDUGVRUSKHQROLFERQGHGJODVV ¿EHUVXVHGIRUJURXQGWXUQDQGSROHZDVKHULQVXODWLRQDUHPRUHVXVFHSWLEOHWRVKULQNLQJDQGFUDFNLQJXQGHU WKHLQÀXHQFHRIWKHUPDODJLQJ 0RGHUQVDOLHQWSROHZLQGLQJGHVLJQVW\SLFDOO\XVHDUDPLGSDSHURUUHVLQERQGHG¿EHUJODVVJURXQGDQGDUDPLG paper turn insulation in strip-on-edge windings, glass laminate pole washers, Dacron-glass-covered high-temperature enamel turn insulation in wire wound poles, and thermosetting bonding resins to provide insulation V\VWHPVWKDWKDYHDWKHUPDOUDWLQJRIDWOHDVW&ODVV) ,IWKHVHPDWHULDOVDUHRSHUDWHGDW&ODVV%  temperatures, they should have a more than adequate thermal life. The materials most susceptible to thermal degradation are organic bonding and backing materials; whereas, inorganic components such as mica, glass, and asbestos are unaffected at the normal operating temperatures of electrical machines. 7KHWKHUPDOOLIHRILQVXODWLRQDWKRWVSRWVLQZLQGLQJVLVVLJQL¿FDQWO\UHGXFHGVLQFHWKHPDUJLQEHWZHHQRSHUDWLQJWHPSHUDWXUHDQGWKHUPDOUDWLQJLVPXFKOHVV7KLVHIIHFWLVPRUHFULWLFDOLQROGHU&ODVVLQVXODWLRQ V\VWHPVDQGWKHSUHVHQFHRIVXFKKRWVSRWVLVYHU\GLI¿FXOWWRGHWHFW The following are the most common causes of thermal aging in salient pole windings: a)

Overloading or high air temperatures leading to operating temperatures well above design values.

b)

,QDGHTXDWHFRROLQJZKLFKFDQEHJHQHUDO²HJLQVXI¿FLHQWFRROLQJDLURUFRROLQJZDWHURUORFDOGHDG spots in the cooling circuit due to poor design, manufacturing, or maintenance procedures.

c)

The use of materials that have inadequate thermal properties and consequently deteriorate at an unacceptable rate when operated within design temperature limits.

d)

Over excitation of rotor windings for long periods of time.

e)

Negative sequence currents due to system voltage imbalance, etc., which leads to circulating currents on the rotor winding.

7.4.2 Thermal cycling Insulation aging from thermal cycling occurs mainly in synchronous motors and hydrogenerators that are started and stopped frequently. There are two heat sources within a rotor when a synchronous motor is started. One mainly applies to motors WKDWDUHVWDUWHGGLUHFWO\RQOLQHFDXVLQJKHDWLQJGXHWRFXUUHQWVÀRZLQJLQWKHSROHWLSVRIVROLGSROHURWRUVRU the damper winding in those with laminated poles. The other is the I2R losses (heat) generated in the windings once excitation is applied. Frequent starts and stops cause winding expansion and contraction as a result of the SUHVHQFHRUORVVRIWKHVHZLQGLQJKHDWVRXUFHV5HODWLYHPRYHPHQWRIZLQGLQJDQGLQVXODWLRQGXHWRWKHGLIIHUHQWFRHI¿FLHQWVRIWKHUPDOH[SDQVLRQLQWKHYDULRXVFRPSRQHQWVOHDGVWRLQVXODWLRQDEUDVLRQ The thermal cycling resulting from frequent starts and stops leads to the cracking of the resin or varnish bonding the insulation system components together. This causes loosening and relative movement between these components, which increases looseness and abrasion. Also, if the windings are restrained from returning to

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IEEE Std 56-2016 IEEE Guide for Insulation Maintenance of Electric Machines

their cold position, they may become distorted. Poor design or too-rapid/too-frequent load cycles for the design are the root causes. 7.4.3 Pollution (tracking and moisture absorption) Salient pole rotor windings—especially strip-on-edge types—are generally susceptible to failure from contamination by conducting materials because they rely on adequate creepage distances between bare copper conductors to prevent shorts. Such problems are not con¿ned to machines with open-type enclosures, since oil leaking from bearings, moisture from condensation, leaking air coolers, and dust from hydrogenerator brakes can contaminate windings. Such problems can be avoided in wire-wound types by encapsulating the pole windings and connections to keep contaminants out. When contaminants such as moisture, coal dust, and oil-dust mixtures cover the surfaces of salient pole windings, they can produce conducting paths between turns and to ground. This can lead to turn-to-turn failures (especially in strip-on-edge types) and ground faults. Certain chemicals can also attack insulating materials, thereby causing them to degrade. Earlier insulation systems containing materials such as asbestos, cotton ¿bers, paper, etc., bonded by organic varnishes are much more susceptible to failure from moisture absorption. 7.4.4 Abrasive particles As with stator windings [see 7.2.11 b)], rotor windings operated in environments containing abrasive dusts can also experience insulation failures from dust impingement. Abrasive dust from the surrounding atmosphere carried into the interior of a motor or generator by cooling air will abrade the rotor winding insulation surfaces. This may eventually expose the conductors in multilayer wire-wound poles, resulting in turn shorts. Also, the ground insulation in both types of salient pole windings and their interconnections may be eroded to cause ground faults. 7.4.5 Rotational force Among the most common causes of failure in salient pole rotor windings are the continuous forces imposed on them by rotation and the cyclic rotational forces induced by starting and stopping. The radial and tangential rotational forces imposed on rotor winding insulation system components tend to distort the coil conductors and inter-coil connections, and crack the coil insulation if they are not adequately braced. If the pole winding bracing is inadequate or becomes loose, the resulting coil vibration and movement of the coils on the poles will cause abrasion of the conductor and ground insulation. Inadequate inter-pole bracing in large, high-speed machines will lead to coil distortion; whereas erosion from loose windings will occur mainly during starts and stops. Winding looseness can also lead to pole washer and inter-coil connection cracking from fatigue. Mechanical winding stresses will become excessive and cause serious winding damage if the rotor is made to run over speed. Inadvertent over speed of the rotor can result from slow response of the wicket gates in a hydrogenerator after opening its breaker and faulty valves in pumps that allow a head of water to drive the pump set at high speed in the reverse direction. 7.4.6 Repetitive voltage surges The normal dc voltage applied to rotor windings does not cause rotor insulation aging. Also, normal voltage levels in a rotor winding are usually so low that they will not induce insulation aging even in weakened materials. Hence, normal operating electrical stress is not an important cause of aging. However transient overvoltages induced by fault conditions on the stator side, or faulty synchronization can cause aged rotor winding insulation to puncture.

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High transient overvoltages may be induced into rotor windings by phase-to-phase stator winding short circuits, faulty synchronization, asynchronous operation, or static excitation systems (see ). Such transient voltages, in conjunction with weak insulation or insulation that has been degraded by thermal or mechanical aging, can cause failures that are predominantly turn-to-turn. These overvoltages are most severe in salient SROHZLQGLQJVGXHWRWKHLUGHVLJQFRQ¿JXUDWLRQ

7.5 Wound rotor winding aging mechanisms 2QWKHEDVLVRIWKLVVWDQGDUG¶VPLQLPXPPDFKLQHUDWLQJRIN9$LWLVDVVXPHGWKDWSULPDULO\WKUHHSKDVH bar-lap/wave-wound rotor windings need be considered. Operating voltages for this type of winding are genHUDOO\OLPLWHGWRDERXW9RUOHVVVRWKHUHLVQRQHHGIRUWKHXVHRILQVXODWLRQPDWHULDOVZLWKDKLJKGLelectric strength. As a result of the low operating voltage, continuous electrical aging is not a factor. Modern ZLQGLQJVRIWKLVW\SHW\SLFDOO\KDYHUHVLQERQGHGJODVV¿EHUWDSHDSSOLHGDVFRQGXFWRULQVXODWLRQDQGDUDPLG SDSHURUSRO\HVWHUSRO\HWK\OHQHWHUHSKWKDODWH3(7 ¿OPSRO\HVWHUVORWOLQHUV7KHIROORZLQJDUHHOHFWULFDO DQGPHFKDQLFDOIDLOXUHPHFKDQLVPVWKDWDUHVSHFL¿FWRZRXQGURWRUZLQGLQJVDUHGLVFXVVHGLQ through . Other features of this type of winding are brazed joints to make connections between bars, end-winding banding to control mechanical stresses from rotational force, and slip rings to connect each phase to an external source for the control of rotor current. 7.5.1 Thermal aging The thermal aging and its effects in this type of winding is similar to that discussed in  for round rotor windings. The thermal aging effects are similar since the materials used in both windings are similar, and both DUHVXEMHFWHGWRVLJQL¿FDQWPHFKDQLFDOVWUHVVHVUHVXOWLQJIURPWKHURWDWLRQDOIRUFHV7KHUHIRUHQRGLVFXVVLRQ on this topic is included under wound rotor windings. 7.5.2 Transient overvoltages In a wound-rotor induction motor, there is a transformer effect between the stator and rotor windings. Consequently, power-system surge voltages imposed on the stator winding will induce overvoltages in the rotor winding. This overvoltage may puncture the turn or ground insulation. 3URYLGHGWKHUHLVDGHTXDWHWXUQDQGJURXQGLQVXODWLRQRQWKHURWRUZLQGLQJVXFKYROWDJHVVKRXOGQRWFDXVH electrical aging; that is, partial discharge is unlikely. Transients will, however, accelerate the failure of insulation that is initially weak, or that has been degraded by thermal or mechanical aging. 7.5.3 Unbalanced stator voltages Unbalanced stator winding power supply voltages will induce negative sequence voltages and currents in the rotor winding. These negative sequence currents increase rotor winding heating in all phases and, therefore, induce accelerated thermal aging of both the turn and ground insulation. 7.5.4 High resistance connections If a joint between two conductors has been poorly soldered or brazed, it will present a high resistance to the FXUUHQWÀRZLQJWKURXJKLWXQGHUORDGDQGWKLVZLOOSURGXFHRYHUKHDWLQJRIWKHMRLQWLQVXODWLRQ7KHH[FHVVLYH amount of heat produced by high-resistance bar-to-bar connections induces rapid thermal aging of the insulation around the connection and on adjacent connections until a turn-to-turn, phase-to-phase, or ground fault GHYHORSV,QPDQ\FDVHVWKHKHDWJHQHUDWHGLVVXI¿FLHQWWRPHOWWKHVROGHURUEUD]LQJPDWHULDOLQWKHMRLQW$ secondary effect could be thermal damage and failure of the end-winding banding discussed in . 7.5.5 End-winding banding failures Application of banding over the rotor end-windings is required to brace them against the high rotational forces LPSRVHGRQWKHPGXULQJRSHUDWLRQ8SXQWLOWKHHDUO\VHQGZLQGLQJEDQGLQJFRQVLVWHGRIDQXPEHURI

36

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turns of round steel wire applied tightly over an insulating layer, which was required to give mechanical and electrical separation from the conductor insulation. The round wires were bonded together with a low meltLQJSRLQWVROGHU7KHGHYHORSPHQWRISUHVWUHVVHGUHVLQFRDWHG¿EHUPDWHULDOSURPSWHGPRWRUPDQXIDFWXUHUV to start using this material because of its superior mechanical and thermal capabilities, as well as its elasticity. ,IWKHVWHHOZLUHRUUHVLQFRDWHG¿EHUPDWHULDOIDLOVIURPRYHUKHDWLQJRYHUVWUHVVLQJRUSRRUPDQXIDFWXULQJWKH HQGZLQGLQJVÀ\RXWZDUGXQGHUWKHLQÀXHQFHRIURWDWLRQDOIRUFHVDQGSLHFHVEUHDNRII7KLVUHVXOWVLQDURWRU winding ground fault, and frequently, a consequential stator winding failure. 7.5.6 Slip ring insulation shorting and grounding The three slip rings in a wound-rotor motor must be separated from the shaft by a layer of insulation applied EHWZHHQWKHWZR7KHVSDFLQJEHWZHHQWKHULQJVPXVWEHVXI¿FLHQWWRSURYLGHDQDGHTXDWHHOHFWULFDOFUHHSDJH distance and barriers are sometimes used to increase this. Also, the two outer rings are usually connected to the winding leads via studs that pass through the other rings. These studs must be electrically isolated from the slip ULQJVDQGWKLVLVQRUPDOO\GRQHE\¿WWLQJLQVXODWLQJWXEHVRYHUWKHP7KHIDLOXUHPHFKDQLVPVLQWKLVVHFWLRQ DOVRDSSO\WRURXQGURWRUDQGVDOLHQWSROHPDFKLQHVZLWKVOLSULQJFRQQHFWLRQVWRWKHLU¿HOGZLQGLQJV If the slip ring enclosure is contaminated with oil, dust from brushes, moisture, or a combination of these, then shorting between rings and the shaft and/or between the rings can occur. If this happens, serious damage can occur to the shaft, the rotor windings, and the slip rings. Also, if the shaft or stud insulation fails due to thermal aging or mechanical stresses, these types of failures will also occur. 7.5.7 Pollution (tracking and moisture absorption) All windings are susceptible to aging and failure from this cause, especially if they are not well sealed. Even though the operating voltages of wound rotors are much lower than those of stator windings, the absorption of moisture and surface contamination can lead to ground faults if the winding is not sealed. Cracked insulation or impregnating resin is more likely to occur in this type of winding since it is subjected to high mechanical stresses.

 '&PRWRUDQGJHQHUDWRU¿HOGZLQGLQJDJLQJPHFKDQLVPV 7KHFRQVWUXFWLRQRIGFPDFKLQHVHULHVVKXQWDQGLQWHUSROH¿HOGZLQGLQJVLVVLPLODUWRWKDWRIWKH³PXOWLOD\er wire wound” salient pole rotor type discussed in . As such, many of the aging mechanisms are similar. However, from the descriptions below it can be seen that there are some differences in the failure mechanisms. 7.6.1 Thermal aging All insulating and nonmetallic bracing materials deteriorate with time due to the heat from the windings. The rate at which component materials deteriorate is a function of their thermal properties and the temperatures to which they are subjected. If the thermal ratings of component materials have been properly selected, the thermal aging and associated deterioration will occur gradually over an acceptable service life. ,QROGHU&ODVVZLQGLQJVPDWHULDOVVXFKDVDVEHVWRVEDFNHGPLFDVSOLWWLQJVERQGHGZLWKRUJDQLFYDUQLVKHV VKHOODFIRUH[DPSOH ZHUHXVHGIRUSROHJURXQGLQVXODWLRQDQGDVEHVWRVERDUGRUSKHQROLFERQGHGJODVV¿bers were used for pole washers. Also, in early insulation systems organic materials were used to insulate conGXFWRUV7KHVHPDWHULDOVDUHPRUHVXVFHSWLEOHWRVKULQNLQJDQGFUDFNLQJXQGHUWKHLQÀXHQFHRIWKHUPDODJLQJ 0RGHUQGF¿HOGSROHZLQGLQJGHVLJQVW\SLFDOO\XVHDUDPLGSDSHURUUHVLQERQGHG¿EHUJODVVJURXQGDQGDUamid paper turn insulation in strip-on-edge windings, glass laminate pole washers, Dacron-glass-covered high-temperature enamel turn insulation, and thermosetting bonding resins to provide insulation systems that KDYHDWKHUPDOUDWLQJRIDWOHDVW&ODVV) ,IWKHVHPDWHULDOVDUHRSHUDWHGDW&ODVV% WHPSHUDWXUHV they should have a more than adequate thermal life. The materials most susceptible to thermal degradation are



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IEEE Std 56-2016 IEEE Guide for Insulation Maintenance of Electric Machines

organic bonding and backing materials, whereas inorganic components such as mica, glass, and asbestos are unaffected at the normal operating temperatures of electrical machines. The thermal life of insulation at hot spots in windings is signi¿cantly reduced since the margin between operating temperature and thermal rating is much less. This effect is more critical in older Class 130 insulation systems, and the presence of such hot spots is very dif¿cult to detect. The following are the most common causes of thermal aging in salient pole windings: a)

Overloading or high air temperatures leading to operating temperatures well above design values.

b)

Inadequate cooling, which can be general—e.g., insuf¿cient cooling air or cooling water, or local dead spots in the cooling circuit due to poor design, manufacturing, or maintenance procedures.

c)

The use of materials that have inadequate thermal properties and consequently deteriorate at an unacceptable rate when operated within design temperature limits.

d)

Weakening of both turn and ground insulation from this type of aging can cause winding failures from both ground and inter-turn shorts both of which will make the machine inoperable.

7.6.2 Thermal cycling Insulation aging from thermal cycling occurs in motors and generators that are load cycled. I2R losses (heat) generated in the windings causes the ¿eld winding conductors to expand. Frequent variations of the ¿eld current causes expansion and contraction as a result of the variations in the I2R losses. Relative movement of winding and insulation due to the different coef¿cients of thermal expansion in the various components will result from the cyclic heating. Thermal cycling leads to the cracking of the resin or varnish bonding the insulation system components together. This causes loosening and relative movement between these components, which increases looseness and abrasion. Also, if the windings are restrained from returning to their cold position, they may become distorted. Poor design or too-rapid or too-frequent load cycles for the design are the root causes. As with thermal aging, thermal cycling can lead to winding ground faults and inter-turn shorts. 7.6.3 Abrasive particles As with stator windings [see 7.2.11 b)], ¿eld windings operated in environments containing abrasive dusts can also experience insulation failures from dust impingement. Abrasive dust from the surrounding atmosphere carried into the interior of a motor or generator by cooling air will abrade the rotor winding insulation surfaces. This may eventually expose the conductors in the multi-layer, wire-wound poles, resulting in turn shorts. Also, the ground insulation in ¿eld pole windings and their interconnections may be eroded to cause ground faults. 7.6.4 Pollution (tracking and moisture adsorption) All windings are susceptible to aging and failure from this cause, especially if they are not well sealed. Even though the operating voltages of dc machine ¿eld windings are much lower than those of stator windings, the absorption of moisture and surface contamination can lead to ground faults if the winding is not sealed.

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7.7 DC motor and generator armature winding aging mechanisms The following descriptions are based on the assumption that only bar-type armature windings are used for the size of machine covered by this standard. It should also be noted that this type of winding is similar to that for the wound rotor type discussed in , so many of the aging mechanisms will be the same. 7.7.1 Thermal aging The thermal aging and its effects in this type of winding are similar to that discussed under  for round URWRUZLQGLQJV7KLVLVVRVLQFHWKHPDWHULDOVXVHGDUHVLPLODUDQGERWKDUHVXEMHFWHGWRVLJQL¿FDQWPHFKDQLFDO stresses resulting from rotational forces. Therefore no discussion on this topic is included under dc machine armature windings. 7.7.2 High resistance connections If a joint between two conductors or between a conductor and the commutator riser has been poorly soldered RUEUD]HGLWZLOOSUHVHQWDKLJKUHVLVWDQFHWRWKHFXUUHQWÀRZLQJWKURXJKLWXQGHUORDGDQGWKLVZLOOSURGXFH overheating of the joint insulation. The excessive amount of heat produced by high-resistance, bar-to-bar connections induces rapid thermal aging of the insulation around the connection and on adjacent connections XQWLODWXUQWRWXUQSKDVHWRSKDVHRUJURXQGIDXOWGHYHORSV,QPDQ\FDVHVWKHKHDWJHQHUDWHGLVVXI¿FLHQWWR melt the solder or brazing material in the joint. A secondary effect could be thermal damage and failure of the end-winding banding discussed in . 7.7.3 End-winding banding failures The causes of these are essentially the same as for wound rotor windings discussed in , so no detailed descriptions are provided in this clause. 7.7.4 Pollution (tracking and moisture adsorption) The aging and failure mechanisms relating to this are essentially the same as those discussed for wound rotors windings in , so no detailed description is given in this clause.

7.8 DC motor and generator commutator aging mechanisms Common aging mechanisms found in direct current machine commutators are covered in  through . 7.8.1 Glass band contamination The glass band tape on glass-banded commutators can fail if contamination such as carbon dust gets underneath the band. This banding is usually protected against contamination by covering it with a material such as DÀXRURSRO\PHUHODVWRPHU6ROYHQWVVKRXOGQRWEHXVHGWRFOHDQFRPPXWDWRUVEHFDXVHFRQWDPLQDWLRQFDQEH washed under the bands. The purpose of the glass band tape is to continually apply tension on the copper and insulated segment pack of the commutator during service to maintain stability at high speeds. 7.8.2 Electrical tracking A buildup of carbon dust behind the commutator risers on the steel shell causes problems with arcing. If carERQGXVWEXLOGVXSRQWKHPLFDSODWHWKDWLQVXODWHVWKHEDUVDQGWKLVMXQFWLRQLVEULGJHGDÀDVKRYHUFDQRFFXU 7.8.3 Commutator wear $SURSHU¿OPRQWKHFRPPXWDWRUVHUYLFHLVQHFHVVDU\LQRUGHUWRSURYLGHSURSHUFRPPXWDWLRQ7HPSHUDWXUH DWPRVSKHUHDQGEUXVKJUDGHDIIHFWWKH¿OPDQGLIWKH¿OPLVFKDQJHGHOHFWULFDOO\RUPHFKDQLFDOO\FRPPXWDWRU wear will be accelerated. Chemical contamination, abrasive dust, and oil vapor will wear away or change the FRPPXWDWRU¿OPWRDQRQFRQGXFWLYH¿OPHYHQWXDOO\FDXVLQJWKUHDGLQJ7KUHDGLQJLVFLUFXPIHUHQWLDOJURRYHV



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on the commutator caused by an abrasive or electro-chemical action of the brushes, light electrical loading, light brush pressure, or porous brushes. Commutator bar surface etching caused by arcing between the brushes and commutator typically looks like SLWWLQJHURGHGRUEXUQWEDUV,IHWFKLQJLVQRWFRUUHFWHGÀDWVSRWVZLOOGHYHORSRQWKHFRPPXWDWRUVHUYLFH)ODW spots can also be caused by vibration. Another problem that can occur is “copper drag”. Copper is dragged over the trailing edges of the commutator EDUVDQGLWORRNVOLNHVPDOOÀDNHVRUIHDWKHUV,WLVFDXVHGE\DFRQWDPLQDWHGDWPRVSKHUHH[FHVVLYHYLEUDWLRQ low current density of the brush or the wrong brush grade, or copper imbedded into the brush. As the commutator surface gradually wears, the undercut portion of the commutator (mica insulation between copper bars) will eventually protrude above the copper surface. If this is not corrected, excessive commutator wear and brush wear will develop. ,IWKHSUREOHPVZLWKFRPPXWDWRUZHDUDUHQRWFRUUHFWHGDÀDVKRYHUZLOOHYHQWXDOO\RFFXU 7.8.4 Commutator eccentricity If the commutator runs off center, the brushes will ride up and down within their holders on every rotation. As the speed increases, the brushes lose contact with the surface causing burning on the commutator. This eccentricity can be caused by distortion due to wide temperature changes and high speed, a bent shaft, bearings that are not running true, an offset coupling, and whether the commutator was machined off center. 7.8.5 Commutator brush wear The wear of commutator brushes may be accelerated by the rough surface of a commutator. A commutator with excessive runout is considered damaging to the brushes with which it is in contact. Acceptable amount of runout depends on machine parameters; consult the original equipment manufacturer (OEM) for acceptable values and limits. Silicone vapor or abrasive dust contaminants in the air such as welding fumes or grinding of metals in the powerhouse will accelerate brush wear. Excessive sparking caused by incorrect brush pressure and/or an incorrect brush grade will also cause accelerated brush wear.

7.9 Stator core insulation aging mechanisms This subclause discusses the most common causes of stator core insulation failures in both induction and synchronous machines. Core lamination insulation shorting and mechanical damage can occur from a variety of aging and failure mechanisms that can be thermal, electrical, mechanical, design, or manufacturing related. Stator cores used in large turbine generators, hydrogenerators, and motors, which have segmented stator cores, are most susceptible to failures from these causes. Some of these failure mechanisms will only occur in specific types of machines, whereas others are applicable to all types. 7.9.1 Thermal aging Degradation of the core condition due to the effects of thermal aging can occur in all rotating machine laminated cores. Core overheating will cause accelerated aging of the core insulation if its thermal rating is exceeded for an extended period of time. This is most likely to occur if an organic varnish is used, since this will dry out due to loss of solvents by evaporation of low-molecular-weight components. Once this occurs, the varnish becomes brittle, cracks, and eventually breaks down. As a consequence, interlamination shorts will develop, eddy currents will increase, and this will eventually lead to core melting due to even higher temperature operation. In large hydrogen-cooled turbo generators, condition monitors may be installed to provide early detection of core insulation overheating. These monitors detect the presence of materials driven off the core by excessive heating in the hydrogen cooling gas.



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a)

General Core Overheating. The most common causes of general core overheating are as follows:   Loss of cooling water in totally enclosed machines.

b)

2)

High ambient air temperatures for open-ventilated, air-cooled machines.

3)

Blockage of air inlets in open-ventilated air-cooled machines due to pollution or debris.

4)

Complete or partial blockage of cooling-air passageways due to the accumulation of oil, dirt, etc.

5)

Turbine generator operation at reduced hydrogen pressure

Local Core Overheating. The most common causes of this are as follows:   Inadequate cooling of certain areas of the core due to poor design or blockage by debris (e.g., FRROLQJDLURUK\GURJHQÀRZLVWRRORZRUQRQH[LVWHQWLQWKHVHDUHDV 2)

Manufacturing errors (e.g., some cooling-medium, passages have been blocked due to missing holes or cut-outs in the core support structure)

Overheating can, of course, also result from core insulation degradation initiated by electrical or mechanical aging mechanisms, as described in  and . 7.9.2 Electrical aging (OHFWULFDODJLQJRFFXUVZKHQWKHYROWDJHDFURVVWKHODPLQDWLRQLQVXODWLRQLQGXFHGE\PDJQHWLFÀX[HVHOHFtromagnetic forces, or high ground-fault currents causes deterioration. Although direct current and transient voltages may cause aging, it is normally ac-voltage induced effects that cause the most severe damage. Degradation from electrical aging can occur in all types of stator and rotor laminated cores. The root causes of electrically induced core insulation degradation can be subdivided into the categories of overheating due to over or under excitation, winding ground faults, and stator-to-rotor rubs due to unbalanced magnetic pull effects. These are discussed in . 7.9.2.1 Stator core end overheating due to under excitation 7KHPDLQDLUJDSÀX[LQV\QFKURQRXVPDFKLQHVLVLQWKHUDGLDOGLUHFWLRQ7KLVÀX[LVUHVSRQVLEOHIRUJHQHUDWLQJ WKHYROWDJHLQWKHVWDWRUZLQGLQJ,QDGGLWLRQV\QFKURQRXVPDFKLQHVKDYHVLJQL¿FDQWOHDNDJHÀX[HVLQWKH HQGUHJLRQHVSHFLDOO\ZKHQWKHURWRUZLQGLQJLVXQGHUH[FLWHG7KHVHIULQJLQJ¿HOGVDUHSURGXFHGE\FXUUHQWV in the stator and rotor end-windings and by the discontinuities at the stator and rotor core surfaces. The axial FRPSRQHQWRIWKLV¿HOGJHQHUDWHVFLUFXODWLQJFXUUHQWVZLWKLQWKHVHJPHQWVRIWKHHQGUHJLRQVWDWRUODPLQDtions, generating some additional electrical losses and thus, heat. Some design features that help to reduce FRUHHQGKHDWLQJDUHVWHSSHGFRUHHQGVVOLWWHHWKDQGÀX[VKXQWVRUÀX[VKLHOGV7KHHGG\FXUUHQWVGXHWRWKH D[LDOPDJQHWLF¿HOGFDXVHVWUD\ORVVHVLQWKHHQGUHJLRQV7KHD[LDOPDJQHWLF¿HOGLVVHQVLWLYHWRFKDQJHVLQ ORDGDQGSRZHUIDFWRU'XULQJOHDGLQJSRZHUIDFWRUXQGHUH[FLWDWLRQ RSHUDWLRQWKLV¿HOGFDQEHTXLWHKLJKLQ large machines, especially if they have long end-windings like in large high-speed synchronous machines or direct-water-cooled windings. This can degrade the interlaminar insulation as follows: a)

Higher temperatures occur, which may reduce the dielectric strength of the interlaminar insulation over time and also give rise to other stresses due to expansion and relative motion between components. Modern inorganic core insulation such as aluminum orthophosphate is capable of withstanding WHPSHUDWXUHVDVKLJKDVƒ&+RZHYHULQVRPHPDFKLQHV²HVSHFLDOO\LIWKH\ZHUHEXLOWEHIRUH ²WKLVOLPLWLVPXFKORZHU,QDGGLWLRQWKHORQJWHUPHIIHFWRIRSHUDWLQJQHDUWKHFRUHLQVXODWLRQ rated temperature is that this could reduce its life.

b)

The circulating currents in the laminations can result in relatively high voltages being developed between adjacent core laminations. Under extreme conditions this voltage may be an order of magnitude



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higher than normal. It has been shown that minor defects in the interlaminar insulation may provide a path for circulating currents, causing further, perhaps serious, local deterioration. The combined effect of the two previously noted mechanisms in conjunction with other existing stresses can damage interlaminar insulation in the stator core end regions near the bore. This increases circulating currents between laminations, causing temperature rise, local weakening, and tooth vibration and breakage from high cycle fatigue. Some manufacturers install thermocouples in this area of the core during manufacturing to monitor for insulation degradation from such aging mechanisms. A rising trend or a sudden increase in temperature recorded by these sensors can provide an early warning of the problem. 7.9.2.2 Overheating of back-of-stator core due to over excitation In order to keep the physical size of large, two- and four-pole synchronous machines within reasonable limits, LWLVQHFHVVDU\WRH[FLWHWKHVWDWRUFRUHDWDIDLUO\KLJKPDJQHWLFÀX[GHQVLW\/DPLQDWHGVWHHODVZHOODVWKH lamination insulation, is therefore selected to avoid high core losses. The lamination insulation is chosen for LWVORZGLHOHFWULFSHUPLWWLYLW\DQGJRRGLQVXODWLRQSURSHUWLHVXQGHUKLJKVWUHVV2YHUH[FLWLQJWKH¿HOGZLQGLQJ FDQSURGXFHKLJKHUWKDQQRUPDOPDJQHWLFÀX[GHQVLWLHVDQGUHVXOWLQKLJKWHPSHUDWXUHVGXHWRLQFUHDVHGFRUH losses. 7RUHGXFHWKHVL]HRIVRPHPDFKLQHVWKH\RNHRIWKHFRUHPD\EHUHGXFHGLQVL]HVXFKWKDWWKHÀX[GLVWULEXWLRQ may be higher in the yoke than in the teeth. This makes these machines more prone to overheating in the core behind the slot due to over excitation. In general, the core behind the slot has relatively less ventilation than the teeth; therefore, the temperature increase is particularly steep as the iron begins to saturate. Once the temperature has been elevated, the chances of lamination insulation breakdown are increased. Such a breakdown would give rise to interlaminar shorts and increased eddy currents, which can cause even higher temperatures. The higher temperature can also cause mechanical stresses, resulting in distortion and vibration. $QRWKHUFRQWULEXWLQJHIIHFWGXHWRRYHUH[FLWDWLRQLVWKDWDVWKHFRUHVDWXUDWHVPRUHOHDNDJHÀX[ZLOORFFXUEHKLQGWKHFRUHLQWKHIUDPHDQGVXSSRUWLQJFRPSRQHQWV7KLVFDQOHDGWRKLJKHUFXUUHQWÀRZDQGDUFLQJDWDUHDV of contact between the core and structural components. When combined, these effects can eventually lead to fusing of laminations, melting of iron, and core failure. 7.9.2.3 Stator winding ground faults in core slots 7KHHQHUJ\DQGKHDWSURGXFHGE\VWDWRUJURXQGIDXOWVLQRUMXVWRXWVLGHWKHVORWUHJLRQDUHRIWHQVXI¿FLHQWWR melt and fuse the core laminations at the core surface. If this core damage is not repaired when the failed coil or bar is replaced, the new coil could also fail to ground as a result of the heat generated by the shorted laminations. It is, therefore, important to perform tests to check the condition of core insulation in the vicinity of ground fault damage before installing a new bar, coil, or winding. 7.9.2.4 Stator core faults from through-bolt insulation damage In some medium to large motor and generator designs, the stator core pressure is maintained by bolts that pass though axial holes in the stator core laminations and endplates and have nuts, steel washers, and insulating washers installed on either end. Core pressure is maintained by keeping these nuts tight. These through bolts have to be electrically isolated from the core with tube insulation to prevent core insulation shorting; if retaining nuts become loose or the bolts stretch, the bolt insulation can fail from insulation abrasion resulting from core lamination movement. Should this happen, core lamination shorting and core burning may occur. 7.9.3 Mechanical aging The most common causes of mechanical degradation in cores are inadequate core pressure applied in manufacture, core pressure reduction in service due to relaxation of the core support structure, core vibration, backof-core looseness, and mechanical damage causing smearing of the core surface at the bore. Degradation due

42

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to core looseness is predominantly found in large generators and motors with a segmented core construction. Core insulation damage due to vibration is most commonly found in large two-pole turbine generators. Mechanical damage to the core bore due to foreign body impact can occur in any type of machine. 7.9.3.1 Core looseness :KHQWKHODPLQDWLRQVLQDVWDWRUFRUHEHFRPHORRVHWKH\FDQPRYHUHODWLYHWRRQHDQRWKHUXQGHUWKHLQÀXHQFH of mechanical vibration and/or electromagnetic forces, and the insulation on them degrades due to abrasion. If not detected in time, the lamination insulation in the areas of core looseness is removed and lamination shortLQJRFFXUV7KHHGG\FXUUHQWVWKDWÀRZDVDUHVXOWRIWKLVVKRUWLQJFUHDWHH[FHVVLYHKHDWWKDWFDQHYHQWXDOO\OHDG to core melting and lamination fracture. Core looseness can also result from vibration caused by the natural IUHTXHQF\RIWKHFRUHDQGIUDPHEHLQJWRRFORVHWRWKHPDLQWZLFHSRZHUIUHTXHQF\+]RU+] HOHFtromagnetic core excitation frequency that occurs in all ac machines. The stator cores in large machines with segmented laminations are built on axial key bars (often two per lamLQDWLRQ ZHOGHGWRWKHIUDPHRUFRQQHFWHGWKURXJKDVSULQJVWUXFWXUHWRWKHIRXQGDWLRQ$GRYHWDLO¿WEHWZHHQ HDFKFRUHODPLQDWLRQVHJPHQWDQGWKHNH\EDUSURYLGHVUDGLDOVXSSRUWIRUWKHFRUH,IWKH¿WEHWZHHQWKHFRUH laminations and key bars is, or becomes loose, then arcing between the two and shorting of the core laminations at the back of the stator core will occur. 7.9.3.2 Stator core relaxation, fretting, and failure—turbine generators Core design and manufacturing problems contribute to this type of core deterioration. The following gives some general information, which may not be applicable to particular situations. Excessive use of resilient materials during manufacture may contribute to relaxation during service. Core pressure is another important factor to consider during manufacture. This becomes more critical as the length of the core increases with the rating. If the core support structure relaxes in service, then the core laminations become loose. The most common location of such looseness is at the stator bore since this is farthest from where the core pressure is applied. If the laminations at the core bore are loose at the end of the core, the following sequence of degradation occurs if this problem is not detected and quickly corrected: a)

7KHFRUHLQVXODWLRQLVDEUDGHGGXHWRODPLQDWLRQUHODWLYHPRYHPHQWXQGHUWKHLQÀXHQFHRID[LDOHOHFWURPDJQHWLFIRUFHVIURPHQGOHDNDJHÀX[HV

b)

Core lamination shorting and overheating starts to occur when the core insulation is removed by abrasion.

c)

Eventually, pieces of the lamination teeth will break off due to fatigue failure, and vent spacers may also break free. Such debris can cause core insulation damage in other locations.

7.9.3.3 Stator core vibration—turbine generators Some of the causes of high stator vibration in service are as follows: a)

Inadequate support of the core in the stator frame, creating an assembly resonant frequency close to WZLFHWKHSRZHUVXSSO\IUHTXHQF\+]RU+] 

b)

Unbalanced phase loading.

c)

,QDGHTXDWHVWDWRUHQGZLQGLQJVXSSRUWFDXVLQJYLEUDWLRQVWKDWDUHUHÀHFWHGEDFNWRWKHFRUH 

43

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(YHQZLWKRXWWKHVHIDFWRUVDFHUWDLQDPRXQWRIFRUHYLEUDWLRQH[LVWVGXHWRWKH+]RU+]ovalizing IRUFHFDXVHGE\WKHPDJQHWLF¿HOG7KHGLVSODFHPHQWRIDORRVHFRUHFDXVHGE\WKHVHIRUFHVPD\UHVXOWLQUHODtive motion between laminations and fretting of the lamination insulation to the point of breakdown. 7.9.3.4 Stator core fretting, relaxation, and failure—hydrogenerators 6LQFHDK\GURJHQHUDWRUFRUHKDVDODUJHGLDPHWHUDQGVKRUWGHSWKEHKLQGWKHZLQGLQJVORWLWLVUHODWLYHO\ÀH[ible and the frame is the main support. The magnetic forces between the rotor poles and stator core and the thermal expansion of the stator core will, therefore, tend to produce displacements in the core. In general, the displacements and resulting vibration due to magnetic forces between the rotor poles and the stator core are small in multi-pole generators. When the number of slots per pole per phase is an integer, the traveling wave produced by the magnetic force has a number of nodes equal to twice the number of poles; this results in smaller displacement. In the case of fractional slot windings, that is, windings that have a non-integer number of slots per pole per phase, larger displacement is possible due to wavelengths that can be longer than the pole pitch. These mechanisms will cause corrosion fretting at the junction of the core to the key bars and will accelerate the degradation of the core. However, the bore section is particularly susceptible. It is physically weaker than the section behind the slot DQGFDUULHVDKLJKHUÀX[GHQVLW\:KHQVWDWRUFRUHEXFNOLQJRFFXUVGXHWRODUJHUWKHUPDOH[SDQVLRQRIWKHFRUH than the stator frame, the core pressure relaxes in some areas along the stator bore, and the lamination teeth can vibrate. As these mechanisms weaken, the pressure between the laminations, the teeth are likely to chatter and break off. The vibration could produce powder as the lamination insulation and lamination wears away. 6HFRQGDU\GDPDJHWRWKHVWDWRUDQGWKHURWRUFDQRFFXULIWKHGHEULV¿QGVLWVZD\LQWRWKHDLUJDS 7.9.3.5 Back-of-stator core overheating and burning Overheating and burning of the back of a stator core can be caused by a loose connection between the core laminations and the stator frame. Axial key bars welded to the frame are used for piling the core laminations. $GRYHWDLO¿WEHWZHHQHDFKFRUHSODWHVHJPHQWDQGWKHNH\EDUSURYLGHVUDGLDOVXSSRUWIRUWKHFRUH7RDOORZ DVVHPEO\RUVWDFNLQJRQWKHNH\EDUVDFOHDUDQFHLVQHFHVVDU\LQWKH³GRYHWDLO´¿W,IH[FHVVLYHWKLVFOHDUDQFH will permit relative motion, intermittent contact, circulating currents and overheating during operation. /HDNDJHÀX[DWWKHEDFNRIWKHFRUHLQGXFHVFXUUHQWVLQWKHNH\EDUV7KHVHFXUUHQWVÀRZWRJURXQGWKURXJK WKHVWDWRUIUDPHZLWKRXWFDXVLQJDQ\KDUPSURYLGHGWKHUHLVQRSDWKIRUWKHPWRÀRZWKURXJKWKHFRUH7KLVLV the case if the punchings are in good contact (positive grounding) with the key bars or if they are completely isolated electrically (insulated key bars). However, should the clearance become excessive at some point, any core vibration could cause intermittent contact at that point. This can cause cracking of key bar insulation, if used, or core-to-key bar arcing if uninsulated key bars are used. In either case arcing between cores and key EDUVFDQUHVXOWOHDGLQJWRRYHUKHDWLQJDQGFRUHPHOWLQJLQWKHORFDODUHD6PDOODPRXQWVRIPHOWLQJDUHGLI¿FXOW to detect due to inaccessibility and the impracticability of monitoring a large area with thermocouples. An early indication of this problem is an upward trend in frame vibration due to the increase in core-to-key bar clearances. Should a number of intermittent contacts develop on the key bars, the possibility of current transfer between key bars would increase. The currents would begin to circulate through the low-resistance path offered by the laminations. The resultant overheating could escalate into failure of interlaminar insulation and an increase in circulating currents and temperature, leading to fusing of laminations, melting, and ultimately, core failure due to widespread melting of the laminations. This problem can be greatly reduced by interconnecting all the key bars at each end of the core by means of ZHOGHGFRSSHUVWUDSVWRIRUPDULQJIRUFDUU\LQJWKHFLUFXODWLQJFXUUHQWV%HFDXVHUHWUR¿WWLQJFDQEHDPDMRU task, the best time to do this is during manufacturing.

44

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7.9.3.6 Stator-to-rotor rubs These types of failures mostly occur in machines that have relatively small air gaps, such as induction motors, salient pole synchronous motors, or generators. Bearing failures, excessive unbalanced magnetic pull due to poor air gap eccentricity or migration of stator frame, can allow the rotor and the stator to rub surfaces. If this occurs, smearing of both the stator core at its bore and the rotor core outside diameter will occur. In addition, WKHURWRUWRVWDWRUUXEELQJPD\JHQHUDWHVXI¿FLHQWKHDWWRFDXVHUDSLGWKHUPDODJLQJRIWKHVWDWRUZLQGLQJLQsulation and a consequent ground fault. Such rubs can cause shorting of the insulation on both the stator and laminated rotor. 7.9.3.7 Loose metal components entering the air gap Bolts and other metallic components that break free from machine internal components can enter the air gap. Such objects can be projected into the air gap. If this occurs, stator core gouging or smearing and consequent insulation surface shorting can occur, often at multiple locations. Since large machines have much larger air gaps and are often more complex in design, they are more susceptible to such damage (Duke et al. >%@(35, 7XUELQH*HQHUDWRU8VHUV*URXS:RUNVKRS>%@).

8. Visual inspection methods 9LVXDOLQVSHFWLRQVRIVWDWRUZLQGLQJVDUHXVXDOO\PDGHDWFRQYHQLHQWLQWHUYDOVLQWKHUDQJHRIWR\HDUV Machine availability, operating hours, and maintenance history should be considered when selecting the frequency of inspections. Depending on the machine design and physical size, a limited visual inspection can be conducted with the rotor in place. Stator bar looseness in the end-windings or at the core edge may be visible. Step iron deterioration may also be detected and the general level of winding contamination can be determined with the rotor in place. The bushing box area should also be inspected to evaluate the circuit rings and tightness of the end-winding structure. 5RERWLFLQVSHFWLRQWHFKQLTXHVDUHDYDLODEOHIURPVHYHUDOVRXUFHV7KHVHFDQSURYLGHPRUHGHWDLOVDORQJWKH airgap than visual inspection from the stator ends. Both stator and rotor surface can be inspected and a visual record can be produced for future reference. $URWRURXWLQVSHFWLRQVKRXOGEHFRQVLGHUHGHYHU\WR\HDUVEDVHGRQWKHUHVXOWVRISDVWOLPLWHGLQVSHFWLRQV and operating information. A rotor out inspection permits detailed rotor and stator condition assessment. Some testing, such as corona probe and core iron testing, must be performed with the rotor or several poles of a salient pole machine removed. Major stator repairs such as re-wedging can only be performed with the rotor removed. This is a major activity and should not be scheduled without good reason. To achieve maximum effectiveness, a visual inspection program should be directed initially to those areas that have been shown by previous experience to be most prone to the forms of damage or degradation caused by the LQÀXHQFHVOLVWHGLQWKLVJXLGH A suggested condition appraisal, summarizing areas prone to deterioration or damage, is shown in Annex E.

8.1 Visual inspection safety *HQHUDWRULQVSHFWLRQVPXVWEHFRQGXFWHGZLWKWKHSURSHUVDIHW\SUHFDXWLRQV6RPHRIWKHUHTXLUHPHQWVPD\ EHVLWHVSHFL¿FVLQFHDJHQHUDWRULVRIWHQGHVLJQDWHGD³FRQ¿QHGVSDFH´7KHPDFKLQHVWDWRUZLQGLQJVPXVWEH grounded with a visible opening in each phase circuit to prevent energizing the machine during an inspection. Spool pieces in the carbon dioxide line and hydrogen line are to be removed to prevent gas from entering during the inspection. Oxygen levels are to be monitored during the inspection. The proper turbine generator or hydrogenerator penstock clearances must be signed prior to the generator inspection. Hydrogenerators have other additional safety requirements such as the following:

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—

Locking in place the servomotors for the guide vanes or needles of the machine inlet valve or intake gate.

—

Brakes engaged and/or rotor jacked to prevent water getting into the turbine or rotation while conducting the inspection.

8.2 Armature winding 8.2.1 Thermal aging ([DPLQDWLRQRIFRLOVPD\UHYHDOJHQHUDOSXI¿QHVVVZHOOLQJLQWRYHQWLODWLRQGXFWVRUDODFNRI¿UPQHVVRIWKH insulation. These symptoms suggest a loss of bond and separation between insulation layers or between the insulation and the conductors. (The winding insulation will have a hollow sound when tapped.) The insulation may also be brittle. Shrinkage of the insulation may lead to loosening of the coil in the slot or of the bracing. The subsequent vibration can result in groundwall abrasion and loss of coil semiconducting slot stress control. 8.2.2 Cracking Cracking of the insulation or in the surface paint may result from prolonged or abnormal mechanical stresses. A common cause of cracking in armature windings is looseness of the bracing structure. 8.2.3 Girth cracking &LUFXPIHUHQWLDOFUDFNLQJRIWKHJURXQGZDOOLGHQWL¿HVJLUWKFUDFNLQJ*LUWKFUDFNLQJFDQRFFXURQDVSKDOWLF ZLQGLQJVSDUWLFXODUO\LQPDFKLQHVZLWKFRUHOHQJWKVJUHDWHUWKDQDERXWPIW 3DUWLFXODUDWWHQWLRQVKRXOG be paid to the areas immediately adjacent to the ends of the slots. Where considerable cracking is observed, it is recommended that the wedges at the ends of the slot be removed for inspection, as dangerous cracks may also have occurred just within the slots. 8.2.4 Contamination Surface contamination adversely affects insulation strength. The most common contaminants are carbon and other conducting substances such as oil and moisture. 3DUWLFXODUO\GDPDJLQJDUHPDJQHWLFSDUWLFOHVLURQWHUPLWHV WKDWYLEUDWHZLWKWKHHIIHFWVRIWKHPDJQHWLF¿HOG in the machine. 8.2.5 Carbon deposits Carbon accumulation over insulation surfaces can provide paths for leakage currents. For example, the risers (the connection straps between commutator bars and coils) may collect carbon deposits that can initiate electrical tracking, with resultant burning, and subsequent failure. 8.2.6 Abrasion Insulation surfaces may be damaged by contact with abrasive substances. Abrasion-resistant coatings are often used to extend the life of windings operating in abrasive environments.  /RRVHVORWZHGJHVRUVORW¿OOHUV This condition may result in abrasion of the insulation. It can also reduce the effectiveness of coil bracing against short circuit and other abnormal mechanical forces. The semiconducting stress control system can be damaged, resulting in slot discharge.

46

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8.2.8 Erosion Foreign substances impinging against insulation surfaces may cause erosion. Even the high-velocity cooling air in salient pole machines can, over time, wear down the winding protective paint coating. 8.2.9 Corrosion/chemical attack Corrosive atmospheric conditions commonly found in chemical plants, rubber mills, and paper manufacturing facilities can chemically attack some insulation materials. Solvents can swell and reduce the mechanical properties of insulating materials. 8.2.10 Erosion by partial discharge Insulation of higher-voltage rated windings can be deteriorated by partial discharges in the slot section and in the end-windings. Evidence of partial discharge is found on the surface of the coils and is typically white, gray, or red deposits in areas where the insulation is subject to high electrical stresses. Some experience is required to distinguish these effects from powdering, which can occur as a result of movement between surfaces such as in loose end-winding structures. 8.2.11 Rotational forces The effects of over speed may be observed on dc armatures by distortion of the windings, commutator risers, looseness or cracking of the banding, or movement of slot wedges. 8.2.12 Commutator condition Commutators should be checked for uneven discoloration, which can result from short-circuiting due to breakGRZQRILQVXODWLRQEHWZHHQEDUV7KH\VKRXOGDOVREHFKHFNHGIRUSLQKROHVDQGEXUUVFDXVHGE\ÀDVKRYHU Commutator groove bands should be carefully inspected for the following: —

Dryness or darkening, which may be an indication of loss in strength due to over-temperature

—

Circumferential cracks within the band or at the groove walls, which may admit conductive contaminants.

Separation of the band at the groove wall may be indicative of internal damage. 5HVWUDLQLQJEDQGVRU³VWULQJEDQGV´WKDWVHFXUHWKHH[SRVHGVXUIDFHRIWKHFRPPXWDWRUFRQHVVKRXOGEHLQspected for separation from the segment surface. Separation at this point is an area for the entry of contaminants. The area immediately behind the commutator can also be a repository for carbon.

8.3 Field windings In addition to insulation degradation from causes similar to those listed in , close attention should be directHGWRWKH¿HOGZLQGLQJVOLVWHGLQ through . 8.3.1 Coil distortion 'LVWRUWLRQRI¿HOGFRLOVPD\EHFDXVHGE\DEQRUPDOPHFKDQLFDOHOHFWULFDORUWKHUPDOIRUFHV6XFKGLVWRUWLRQ may cause looseness that can lead to failure of turn or ground insulation.



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8.3.2 Loose collars or coils )RUDQROGHU¿HOGSROHDVVHPEO\XWLOL]LQJZRRGRUQRQHSR[\PDWHULDOIRUFROODUVVKULQNDJHRYHUWLPHPD\ FDXVHFROODUORRVHQHVV7KLVORRVHQHVVJUHDWO\GHSHQGVRQKRZWKH¿HOGSROHZDVDVVHPEOHGDQGWKHWROHUDQFHV VSHFL¿HG7KHSXUSRVHRIWKHFROODURQD¿HOGSROHLVWRLQFUHDVHWKHFUHHSDJHSDWKLQVXODWLRQVXUIDFHSDWK  to ground. While the machine is in service, centrifugal forces will keep the collar tight up against the coil and VKRXOGQRWFDXVHDQ\GLI¿FXOW\:KHQWKHPDFKLQHFRPHVWRUHVWDVPDOOPRYHPHQWRIWKHFROODULQWKHUDGLDO and tangential direction may occur. The degree to which it moves radially and tangentially will determine how quickly, if at all, it degrades the groundwall insulation. :KHQLQVSHFWLQJDFROODU¿UPKDQGSUHVVXUHVKRXOGEHXVHGWRWU\DQGPRYHWKHFROODULQWKHUDGLDOGLUHFWLRQ 2QDWLJKW¿HOGFROODUQRPRYHPHQWVKRXOGEHQRWLFHG2QDORRVHFROODUVRPHPRYHPHQWLQWKHRUGHURIOHVV WKDQPPLQ UDGLDOPD\RFFXU&RQVXOWWKH2(0LIPRYHPHQWLVGHWHFWHGIRUSRVVLEOHUHPHGLDODFWLRQ )RUPRGHUQGD\¿HOGSROHDVVHPEOLHVRUUHLQVXODWHGDVVHPEOLHVHSR[\DQG¿EHUJODVVDUHXVHGIRUWKHFROODUV so shrinkage and looseness should not be an issue. Furthermore, the collars may be sealed against the groundwall insulation using silicone, which will assist in the prevention of contamination ingress, as well as preventing motion of the collar. On a horizontal unit during startup and shutdown, the radial motion of a loose collar is a greater issue. 7KH¿HOGFRLOLVDVVHPEOHGRQWRWKHSROHERG\ZLWKWKHJURXQGZDOOLQVXODWLRQDOUHDG\LQSODFHRUZLWKWKHFRLO encased with the groundwall insulation and placed over the pole body. In either case, the coil itself is positioned onto the pole body using shims held in place with some kind of binder (shellac or varnish). The coil may have been shimmed along its length additionally to make sure it was secure, though this would depend on the PDQXIDFWXUHU,WLVSRVVLEOHZLWKWLPHWKDWWKHELQGLQJDJHQWXVHGRQWKHVKLPVGHWHULRUDWHVDQGWKH¿HOGFRLO LVORRVHZLWKUHVSHFWWRWKHSROHERG\,QPRVWFDVHVLWLVQRWSRVVLEOHWRPRYHWKH¿HOGFRLOE\KDQGEHFDXVHRI LWVZHLJKW$ORRVH¿HOGFRLOWKDWKDVDEUDGHGWKHJURXQGZDOOLQVXODWLRQZLOOUHVXOWLQSRRUUHDGLQJVGXULQJWKH LQVXODWLRQUHVLVWDQFHWHVW,WZLOOEHQHFHVVDU\WRLVRODWHWKHSROHDQGGLVDVVHPEOHWRFRQ¿UPWKHFDXVH 8.3.3 Rotor coil tightness In cylindrical rotors (also known as “round rotor” in ,(((6WDQGDUGV'LFWLRQDU\*ORVVDU\RI7HUPVDQG'H¿nitions), evidence of heating of wedges at their contact with the retaining ring body, and “half-mooning” or cracks on the retaining rings, can be caused by high circulating currents. These currents may be due to unbalanced operation, excessive loads, or sustained single-phase faults close to the generator, such as in the leads or generator bus. The condition and tightness of end-winding blocking, end-winding distortion, signs of deterioration or movement of the retaining ring insulating liner due to the previously noted effects, and any other looseness should be noted. 3RZGHUHGLQVXODWLRQRQVXUIDFHVRULQDLUGXFWVLVHYLGHQFHRIFRLOPRYHPHQW5HGR[LGHDWPHWDOOLFMRLQWVLV evidence of fretting (relative movement of metal parts). 7KH LQWHJULW\ RI ¿HOG OHDG FRQQHFWLRQV DQG FRQGLWLRQ RI FROOHFWRU DQG FROOHFWRUOHDG LQVXODWLRQ VKRXOG EH checked on a regular basis, since failure of a joint during operation will lead to very serious consequences. 8.3.4 Brush rigging ,QVXODWLRQVXSSRUWLQJWKHEUXVKULJJLQJVKRXOGEHFKHFNHGIRUHYLGHQFHRIÀDVKRYHURUFDUERQL]HGOHDNDJH paths and cleaned from contamination on a regular basis. Spring pressures and brush alignment should be inspected. Abnormal wear should also be documented and remedied as the cause is normally an environmental issue.



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8.3.5 Collector $YLVXDOLQVSHFWLRQRIWKHULQJVXUIDFHWKDWORRNVIRUDQ\SLWWLQJGLVFRORUDWLRQVXUIDFH¿QLVKDQGRUXQHYHQ wear should be performed. Collector rings often use helical grooves to allow for expulsion of dust and debris, as well as to allow the brush proper contact by preventing the brush from riding on a cushion of air. Check these grooves for contaminants and measure the groove depth to assure that they are within the manufacturer’s VSHFL¿FDWLRQ Inspect for any contamination in the collector housing area, which could destroy the collector contact surface ¿OP&RPPRQFRQWDPLQDQWVDUHDVIROORZVÀXRULQHFKORULQHEURPLQHLRGLQHVLOLFRQHDOFRKROVNHWRQHV and esters among others. (Note that silicon from common silicone sealants used anywhere near the collector has often been found to be a very detrimental contaminant.) A visual inspection for cracks in the collector insulation and banding should be performed, and any signs of excessive of uneven heating of arc pitting in the surface should be noted.

8.4 Core and frame assembly The following items listed in  through DUHFRQVLGHUHGWREHWKHPRVWVLJQL¿FDQWLQLQVSHFWLQJWKH core and frame assembly. 8.4.1 Stator (armature) core A close examination should be made at the core surface for evidence of damage. Failure of interlaminar insulation may occur and is usually precipitated by external causes. Among these causes are the following: —

Operation in the far under excited region (round rotor machine)

—

Over-excitation

—

Mechanical damage due to foreign objects

—

Surface rubs

—

9LEUDWLRQ

—

Excessive heating due to power arcs created by winding failure

—

([FHVVLYHORVVHVLQWKH¿QJHUSODWHVRIODUJHPDFKLQHV

Failure may also result from rewind processing when excessive heat is applied to insulated laminated cores for coil removal. Such damage can initiate winding insulation faults and equipment failure. Careful inspection of the core condition is therefore mandatory whenever the machine in question is out of service for maintenance purposes. If distress is observed, a loop test is recommended. The loop tests are described in Annex C and Annex D. Inspect for looseness of core laminations. Loose core laminations at the air-gap side of the core (teeth)— particularly at core ends—will vibrate, abrade interlaminar insulation (and ground insulation), short circuit laminations, and cause heating. Also, vibrating laminations may fatigue, crack, break off, and contaminate the machine with iron particles. Iron oxide powder deposits (evidence of fretting) are an indication of loose core iron or loose wedges. Inspect ventilation ducts for loose or broken ventilation duct separators (I-beam or other spacers). These can cause core looseness or they can break off, resulting in mechanical damage to coil insulation and to the core interlaminar insulation. 2YHUKHDWLQJRIWKHHQG¿QJHUSODWHVLVHYLGHQWE\GLVFRORUDWLRQRIWKHSDLQWRUFRPSRQHQWVLQWKHDUHDVDIIHFWed. Abnormal overheating can lead to thermal degradation of the interlaminar insulation.



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(QGÀX[VKLHOGRYHUKHDWLQJZKHQSUHVHQW LVHYLGHQFHGE\GLVFRORUDWLRQRIWKHSDLQWRUFRPSRQHQWVLQWKH areas affected. When these shields are insulated, abnormal heating can lead to thermal degradation of the insulation. 8.4.2 Core insulated through bolts :KHUHWKHVHDUH¿WWHGH[DPLQDWLRQRIWKHLQVXODWHGZDVKHUDQGDVVRFLDWHGSLHFHVLVLQGLFDWHGWRJHWKHUZLWK YHUL¿FDWLRQRIWLJKWQHVVDQGORFNLQJRIWKHQXWV7KHLQVXODWLRQUHVLVWDQFHRIWKURXJKEROWVWRWKHFRUHVKRXOG also be checked. 8.4.3 Bearing, hydrogen seal, and other insulation Whenever bearings and other mechanical parts are disassembled, inspect their insulation for signs of deterioUDWLRQ3LWWLQJLQWKHEHDULQJPDWHULDOPD\EHHYLGHQFHRIEHDULQJLQVXODWLRQIDLOXUH5HIHUWR,(((6WGIRU electrical test procedure.

9. Insulation maintenance testing 9.1 Principles of maintenance testing A list of electrical tests designed to detect particular areas of weakness is included in this maintenance guide. Additional information is contained in IEEE Std 62.2. It should be noted that all tests are not applicable to all machines. The tests listed below have been used generally, either to establish the long-time trends in parts of WKHLQVXODWLRQVWUXFWXUHRUWRGHWHFWVSHFL¿FW\SHVRIÀDZVWKDWPD\GHYHORSLQSRUWLRQVRIWKHLQVXODWLRQ:LWK many maintenance tests, the trends measured over a period of years are normally more important than absolute PHDVXUHGYDOXHVGHWHUPLQHGDWDVSHFL¿FLQVSHFWLRQSHULRG$VXGGHQFKDQJHLQWKHYDOXHVIRUDJLYHQPDFKLQH should be investigated and the cause determined. Some electrical tests may be potentially damaging to the insulation. The risks with such tests should also be recognized. For example, when a winding that has been providing good service is tested, but has a fault near ground or neutral connections, the fault may be worsened to a potentially nonfunctional condition. Where there is uncertainty about insulation condition, it is recommended that the manufacturer be consulted; if the manufacturer is no longer available, an insulation specialist should be consulted. Insulation maintenance tests in this guide have been grouped as follows: a)

7HVWVFRQGXFWHGRQWKH¿HOG

b)

Tests conducted on the armature

7KHLUFODVVL¿FDWLRQDQGLQFOXVLRQLQWKLVPDQQHULVIRUFRQYHQLHQFHDQGWKHVHOHFWLRQRIPDLQWHQDQFHWHVWVZLOO depend on the user’s own philosophy, performance records, production, and economics. The user is encourDJHGWRGLVFXVV¿QGLQJVZLWKWKHPDQXIDFWXUHURULQVXODWLRQVSHFLDOLVWIRULQWHUSUHWDWLRQDQGWUHQGV The tests are given in synopsis form in this guide; however, further details are provided in Annex C, Annex D, and in the appropriate IEEE standards to which reference is made.

 7HVWVFRQGXFWHGRQWKH¿HOGZLQGLQJ $OO¿HOGZLQGLQJVKDYHWZRW\SHVRILQVXODWLRQJURXQGLQVXODWLRQDQGWXUQLQVXODWLRQ7KHLQVXODWLRQUHVLVtance of the ground insulation can be measured according to IEEE Std 43. In addition, the suitability for service can be determined using an over potential test. The insulation resistance or condition of the turn insulation can be determined by the tests described in the following subclauses.



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There are times when continued operation of a machine in a fault mode may be essential to avoid an outage. In generators, shorted turns in a rotor can be of a minor nature—unlike shorted turns in the stator—and they may not necessarily require immediate re-insulation. Although not recommended, rotors have been known to operate for years with a few random short circuits between successive turns in the rotor winding. However, if subsequent periodic impedance testing shows the shorting to be progressive in nature, re-insulation will likely be necessary to ensure reliable operation. Continued operation of large salient pole motors known to have shorted turns is not recommended, since further damage can occur due to high currents in the shorted turns, particularly during start operations. 9.2.1 Insulation resistance Insulation resistance testing is based on determining the current through the insulation and across the surface when a direct voltage is applied. The current is dependent on the voltage and time of application, the area and thickness of the insulation, and on temperature and humidity conditions during the test. 6LQFHLQVXODWHGURWRUZLQGLQJYROWDJHVDUHJHQHUDOO\OHVVWKDQ9WKLVWHVWLVQRUPDOO\SHUIRUPHGZLWKD 9LQVXODWLRQUHVLVWDQFHWHVWHU)RU¿HOGZLQGLQJVZLWKVOLSULQJVWKLVWHVWFDQEHSHUIRUPHGE\FRQQHFWLQJ the insulation resistance tester between one slip ring and the rotor shaft, with the brushes lifted. ,IDV\QFKURQRXVPDFKLQH¿HOGZLQGLQJLVH[FLWHGE\DEUXVKOHVVH[FLWHUWKHQWKLVKDVWREHGLVFRQQHFWHGIURP this device to allow this test to be performed. IEEE Std 43 outlines a recommended practice for insulation resistance testing and the corrections to be made for temperature and humidity conditions. IEEE Std 43 also provides recommended values for minimum insulation resistance for safe operation. The insulation resistance test is used to determine the insulation condition prior to application of an overvoltage test, but in the case of rotors this is seldom done. 9.2.2 Dielectric absorption In addition to the insulation resistance test, the dielectric absorption or polarization index may be measured RQ¿HOGZLQGLQJLQVXODWLRQ,QDGGLWLRQWRWKHFRQVLGHUDWLRQVSURYLGHGLQ, IEEE Std 43 outlines a recommended practice for measuring the dielectric absorption. )RUWKLVWHVWWKHSRWHQWLDOLVXVXDOO\9GF 9.2.3 Winding resistance $UHGXFWLRQLQUHVLVWDQFHRID¿HOGZLQGLQJRUFRLOPD\LQGLFDWHVKRUWLQJRIFRQGXFWRUVSRVVLEO\FDXVHGE\D deterioration of the insulation between them. For complete windings or coils with many turns, this method may not be accurate enough to detect shorted turns. The rotor winding should be at room temperature before the cold resistance measurement is made and the temSHUDWXUHRIWKHZLQGLQJFDUHIXOO\GHWHUPLQHG)RUV\QFKURQRXVPDFKLQHVLWLVQHFHVVDU\WKDW¿HOGUHVLVWDQFH and the corresponding temperatures be accurately measured using several thermometers, thermocouples, or 57'VORFDWHGDWVXLWDEOHSRLQWVVLQFHWKHWHPSHUDWXUHULVHRIWKH¿HOGZLQGLQJGXULQJRSHUDWLRQLVFRPPRQO\ GHWHUPLQHGIURPWKHFKDQJHLQUHVLVWDQFH7KHPHDVXUHG¿HOGZLQGLQJUHVLVWDQFHVKRXOGEHFRPSDUHGWRIDFWRU\WHVWYDOXHDIWHUWHPSHUDWXUHFRUUHFWLRQ DQGDQ\VLJQL¿FDQWGLIIHUHQFHVVKRXOGEHLQYHVWLJDWHG In measuring the rotor resistance by the voltage-drop method, it is essential that voltage contacts for the voltmeter be placed directly on the collector rings or exposed leads of the rotor winding. 9.2.4 Field winding voltage drop tests This test, commonly known as a “voltage drop” test, is sometimes made in the factory and can also be used as DPDLQWHQDQFHWHVW7KH¿HOGZLQGLQJLVHQHUJL]HGZLWKORZSRWHQWLDODOWHUQDWLQJYROWDJHVXFKDV9RUDW



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OHDVWWKHHTXLYDOHQWRI9SHUSROH DWFRQYHQWLRQDOSRZHUIUHTXHQF\:LWKWKH¿HOGFRLOVFRQQHFWHGLQVHULHV similar coils should have a comparable voltage drop (on salient pole rotors with springs behind the coils, an ac WHVWPD\RYHUKHDWWKHVSULQJVLIWKHWHVWFRQWLQXHVIRUPRUHWKDQPLQ  7KLVWHVWLVVXLWDEOHIRUF\OLQGULFDOURWRUZLQGLQJVRIWXUELQHJHQHUDWRUVDQGIRUZLQGLQJVRIPXOWLSOH¿HOGVRI other machines. When a coil with short-circuited turns has been discovered, the test may be expanded to measure the voltage drops across individual coil turns. 9.2.5 Impedance test The presence of short-circuited turns in the windings of cylindrical rotors of turbine generators or individual ¿HOGFRLOVRIVDOLHQWSROHJHQHUDWRUVRUPRWRUVPD\EHGHWHFWHGE\LPSHGDQFHPHDVXUHPHQWVRQLQGLYLGXDO coils. These measurements are usually obtained by applying a known current or voltage across the coil (a power frequency source is acceptable but higher frequency sources are preferred). Other parameters such as watts, power factor, and volts-ampere are then measured. Similar coils should have a comparable impedance. A coil with a shorted turn will have substantially different values for watts, impedance, and power factor. The overall ohmic value of winding impedance obtained from the impedance test is useful if an initial reading with no short-circuited turns is available for comparison. When ohmic values are used for comparison purposes, test results should have been obtained at approximately the same frequency and voltage or current (contingent upon which is the dependent and independent quantity) for the two tests being compared. ,QRSHUDWLRQWKH¿UVWVLJQVRIVKRUWFLUFXLWHGURWRUWXUQVPD\EHLQFUHDVHGURWRUYLEUDWLRQRULQFUHDVHVLQH[FLtation requirements. 7KHHIIHFWVRIVKRUWFLUFXLWHGWXUQVRQURWRUYLEUDWLRQPD\EHGXHWRHOHFWURPDJQHWLFRUWKHUPDOLQÀXHQFHV (OHFWURPDJQHWLFHIIHFWVZRXOGEHLQKHUHQWO\PRUHSURQRXQFHGRQURWRUVZLWKIRXURUPRUHSROHV5HPRYDORI excitation will often indicate whether the effects are electromagnetic, thermal, or both. If short-circuited turns cause thermal unbalance, the vibration will vary with temperature and hence will lag any increase in excitation by the length of time required for heating to occur. If variations from the cold to the hot condition are not too great, weight adjustments can sometimes be made to keep the vibration amplitude entirely within a satisfactory range for all temperatures. Otherwise, either thermal balancing or re-insulation of the short-circuited turns is necessary. If the primary effect of short-circuited rotor turns is an increase in excitation requirements, re-insulating would EHGHSHQGHQWRQWKHDELOLW\WRVXSSO\VXI¿FLHQW¿HOGH[FLWDWLRQ²XQGHUQRUPDOUHDFWLYHORDGFRQGLWLRQV² without exceeding exciter or rotor recommended operating temperature limits. 3DVWH[SHULHQFHZLWKJHQHUDWRUVKDVSURYHQWKDWVKRUWFLUFXLWHGURWRUWXUQVDUHQRWXVXDOO\SURJUHVVLYHLQQDture. Due to the increased excitation current requirements as a result of the short-circuited turn, the average rotor temperature will likely increase. Changes in excitation requirements may be detected by comparison of a recent no-load saturation curve with the original curve. If the rotor has a temperature recorder, the chart should be examined for indications of a sudden drop in rotor resistance at the time vibration appeared. If the rotor has brushless excitation, the manufacturer's instructions should be reviewed carefully before making impedance tests. 9.2.6 Flux distribution tests In addition to the impedance measurements referred to in  of this guide, several other tests are available by which short circuits between turns of cylindrical rotors can often be detected. Among these are the followLQJPHWKRGVIRUPHDVXULQJWKHÀX[GLVWULEXWLRQ

52

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—

7KHÀX[GLVWULEXWLRQRYHUWKHURWRUERG\VXUIDFHLVGHWHUPLQHGE\PHDVXULQJWKHYROWDJHRQDWHVWFRLO ZKLOHDSSO\LQJDQDFYROWDJHRIW\SLFDOO\9DW+]RU+]WRWKHURWRUZLQGLQJ7KHWHVWFRLO is arranged to span adjacent rotor teeth and an iron-cored coil is typically used on the rotor body or an air-cored coil can be used in the end-windings. The magnitude and sign of the voltage induced in the FRLOIRUHDFKSDLURIWHHWKLVSORWWHG7KHÀX[SDWWHUQVKRZVDVLJQL¿FDQWFKDQJHLQPDJQLWXGHDQGVLJQ whenever a short circuit exists in the slot being tested.

—

,QDSRSXODURQOLQHPHWKRGDSUREHLVSHUPDQHQWO\DI¿[HGWRWKHVWDWRUFRUHWRPHDVXUHWKHÀX[GXULQJ RSHUDWLRQ7KHSUREHLVVHQVLWLYHWRWKHWLPHUDWHRIFKDQJHRIWKHUDGLDOÀX[LQWKHDLUJDS7KHPDJQLWXGHRIWKLVÀX[LVSURSRUWLRQDOWRWKHFXUUHQWÀRZLQJWKURXJKWKHWXUQVIRXQGLQHDFKVORW7KLVÀX[ distribution curve evaluates the active turns in each slot by comparing slot peak magnitudes between SROHV8VLQJWKLVÀX[GLVWULEXWLRQPHDVXUHPHQWWKHQXPEHUDQGORFDWLRQRIVKRUWHGWXUQVFDQWKHUHIRUH be calculated for each pole in the rotor with the generator online.

—

7RIXOO\FKDUDFWHUL]HDF\OLQGULFDOURWRUDVHULHVRIÀX[GLVWULEXWLRQVDUHFROOHFWHGDWGLIIHUHQWZDWW DQGYDURSHUDWLQJFRQGLWLRQV,GHDOO\WKHYDULDWLRQVZLOODOORZWKHÀX[GHQVLW\FXUYH]HURFURVVLQJV )'=& WRDOLJQZLWKHDFKRIWKHOHDGLQJFRLOVORWSHDNVLQWKHÀX[GLVWULEXWLRQZDYHIRUP:KHQWKH FDZC value aligns to a particular rotor slot, the maximum sensitivity for detecting shorted turns in that slot occurs. The coils that are in the slots further away from the FDZC position will have decreased VHQVLWLYLW\GXHWRDFRPELQDWLRQRIPDJQHWLFÀX[VDWXUDWLRQRIWKHURWRULURQDQGPRGXODWLRQHIIHFWV

9.2.7 Recurrent surge oscillography (RSO) test 7KHUHFXUUHQWVXUJHRVFLOORJUDSK\562 WHVWFDQGHWHFWVKRUWHGWXUQVLQURXQGURWRU¿HOGZLQGLQJV7KLVLV GRQHE\LQMHFWLQJLGHQWLFDOVLJQDOVRQERWKZLQGLQJHQGVDQGWKHQUHFRUGLQJDQGFRPSDULQJWKHUHÀHFWHGZDYHIRUPVIURPHDFKVLGHRIWKHZLQGLQJ7KHSXOVHVXVHGLQWKH562WHVWDUHORZYROWDJHW\SLFDOO\DERXW9 $ V\PPHWULFDOURWRUZLQGLQJVKRXOGSURGXFHLGHQWLFDOUHÀHFWHGZDYHIRUPV'LIIHUHQFHVLQWKHWZRZDYHIRUPV FDQEHLGHQWL¿HGE\SORWWLQJWKHWZRZDYHIRUPVQH[WWRHDFKRWKHURUE\VXEWUDFWLQJWKHWZRZDYHIRUPV)RU example, a turn short in a coil may be represented by a peak in the subtracted waveform, with a time delay proportional to the distance of the turn short from the collector. This test can also identify the location of ground IDXOWVLQJURXQGURWRU¿HOGZLQGLQJVDQGEHXVHGDVDURWDWLQJWHVWLIWKH¿HOGZLQGLQJLVFRQQHFWHGWRVOLSULQJV

9.3 Tests conducted on the armature (stator) 9.3.1 Insulation resistance test at low voltage Insulation resistance testing is based on determining the current through the insulation and across the surface when a direct voltage is applied. The current is dependent on the voltage and time of application, the area and thickness of the insulation, and on temperature and humidity conditions during the test. This test is usually made on all or parts of an armature to ground. The test can be performed with or without a guard electrode. The test primarily indicates the degree of contamination of the insulating surfaces or solid insulation and will not usually reveal complete or uncontaminated ruptures. IEEE Std 43 outlines a recommended practice for insulation resistance testing and the corrections to be made for temperature and humidity conditions. IEEE Std 43 also provides recommended values for minimum insulation resistance for safe operation. The insulation resistance test is used to determine the insulation condition prior to application of an overvoltage test. 9.3.2 Dielectric absorption test Dielectric absorption testing is the determination of insulation resistance as a function of time. This test, like the insulation resistance test, is made on all or parts of an armature circuit to ground.

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IEEE Std 43 outlines the test procedures and equipment used for the standard method of performing the test, ZKLFKLVXVXDOO\PDGHDWDWHVWSRWHQWLDORI9WR9GF7HVWVDWKLJKHUSRWHQWLDOVDUHFRPPRQO\PDGH XVLQJWKHYROWPHWHUDPPHWHUPHWKRGRIUHVLVWDQFHGHWHUPLQDWLRQDVRXWOLQHGLQ,(((6WG 'XULQJWKLVWHVWWKHSRWHQWLDOLVKHOGXQWLOWKHLQVXODWLRQUHVLVWDQFHVWDELOL]HVRUIRUDSHULRGRIPLQ7KH slope of the time resistance characteristic gives information on the relative condition of the insulation with reVSHFWWRPRLVWXUHDQGRWKHUFRQWDPLQDQWV7KHUDWLRRIWKHPLQYDOXHWRWKHPLQYDOXHRILQVXODWLRQUHVLVWDQFHLVWHUPHGWKH³SRODUL]DWLRQLQGH[´RU³3,´7KLV3,YDOXHLVXVHIXOLQFRPSDULQJWKHUHVXOWVRISUHYLRXV tests on the same machine. Further details of these tests and suggested acceptable polarization indices for certain insulation systems are contained in IEEE Std 43. 9.3.3 Over voltage tests Overvoltage tests, also referred to as “high-potential” or “hi-pot” tests, are used to ensure minimum dielectric strength of the insulation. Such tests are made on all or parts of the circuit-to-ground insulation of the armature RU¿HOGZLQGLQJ Many users of large rotating machines apply overvoltage tests periodically and generally at the beginning of a machine overhaul or the overhaul of related equipment. This allows the detection and possible repair of insulation weaknesses during the scheduled outage. Overvoltage tests should be applied when possible to each phase in sequence, with the remaining two phases QRWXQGHUWHVWDQGGHYLFHVVXFKDV57'VWKHUPRFRXSOHVDQGÀX[SUREHVJURXQGHG,QWKLVPDQQHUWKHLQVXODtion between phases is also tested. This is only practical, however, where both ends of each phase are brought out to separate terminals, as is usually the case in generators. Except for the larger horsepower ratings, most motors have either three or four leads brought out, precluding a test between phases. The level of over potential that should be applied will depend to a very large extent on the type of machine involved, the degree of exposure to overvoltages, and the level of serviceability required from the machine in TXHVWLRQ6XFKWHVWVVKRXOGEHVXI¿FLHQWO\VHDUFKLQJWRGLVFHUQDQ\ZHDNQHVVRULQFLSLHQWZHDNQHVVLQWKHLQsulation structure that might lead to service failure. The test voltage should not, however, be so high as to cause an unnecessary breakdown and the user should be aware that overvoltage test can be destructive. Overvoltage tests may be performed either by alternating or direct voltage methods. The values of test voltages usually are selected as follows: a)

)RU+]RU+]WHVWVWKHRYHUYROWDJHPD\EHUHODWHGWRWKHUDWHGPDFKLQHYROWDJHDQGWHVWVLQWKH UDQJHRIWRRIWKHOLQHWROLQHYROWDJHDUHQRUPDO)RUUHFRPPHQGHGWHVWOHYHOVUHIHUWR ,(((6WG&DQG,(((6WG&2YHUYROWDJHWHVWVDUHW\SLFDOO\FRQGXFWHGIRUV)RUWHVW procedures, refer to IEEE Std 4.

b)

)RUWHVWVDWYHU\ORZIUHTXHQF\9/) RI+]WKHWHVWYROWDJHLVUHODWHGWRWKHPDFKLQHWHUPLQDO YROWDJHDQGSHUIRUPHGLQWKHUDQJHRIWRRIWKHUDWHGOLQHWROLQHUPVYROWDJHPXOWLSOLHGE\ IRUWKHFUHVWYDOXHRIWKH+]WHVWYROWDJH(TXLSPHQWIRUPDNLQJRYHUYROWDJHWHVWVDWYHU\ORZ IUHTXHQF\+] LVDYDLODEOH6XFKHTXLSPHQWLVW\SLFDOO\OHVVLQZHLJKWDQGVPDOOHULQVL]HWKDQWKH HTXLYDOHQW+]RU+]HTXLSPHQW,WLVSRVVLEOHGXULQJ9/)RYHUYROWDJHWHVWVWRPRQLWRURWKHUSDrameters, such as partial discharge and dissipation factor, which provides the operator with additional data as discussed in (Bawart >%@). For additional information, see IEEE Std 433.

c)

For dc tests, the recommended test voltage is a function of the rated machine voltage multiplied by a factor to represent the ratio between direct (test) voltage and alternating (rms) voltage. The recomPHQGHGYDOXHLVIURPWRRIWKHUDWHGOLQHWROLQHYROWDJHPXOWLSOLHGE\(TXLSPHQWIRU

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PDNLQJGFYROWDJHWHVWVLVW\SLFDOO\OHVVLQZHLJKWDQGVPDOOHULQVL]HWKDQWKHHTXLYDOHQW+]RU +]WHVWHTXLSPHQW)RUWHVWSURFHGXUHVUHIHUWR,(((6WG It should be recognized that if the windings are clean and dry, overvoltage tests may not detect defects that are in the end turns or in leads remote from the stator core. 9.3.4 Controlled overvoltage test (dc) A controlled overvoltage test is one in which the increase of applied direct voltage is controlled, and measured currents are continuously observed for abnormalities, with the intention of stopping the test before breakdown occurs. This test is often referred to as a “direct-current leakage,” a “step voltage,” or a “ramped voltage” test. The most common methods are the step voltage or a ramped voltage test. Methods of conducting the test and LQWHUSUHWDWLRQRIWKHUHVXOWVDUHGHWDLOHGLQ,(((6WGZKLFKZDVGHYHORSHGWRSURYLGHXQLIRUPSURFHGXUHV for the following: a)

3HUIRUPLQJKLJKGLUHFWYROWDJHDFFHSWDQFHWHVWVDQGURXWLQHPDLQWHQDQFHWHVWVRQWKHPDLQJURXQG insulation.

b)

Analyzing the variations in measured current so that any possible relationship of the components of these variations to the condition of the insulation can be more effectively studied.

Many machine operators have found this test to be a useful maintenance tool, although there is some controversy over the interpretation of the test results, and breakdown sometimes occurs without prior indication. The RSHUDWRULVXUJHGWRVWXG\,(((6WGWRGHULYHVLJQL¿FDQWEHQH¿WIURPFRQWUROOHGRYHUYROWDJHWHVWLQJ 9.3.5 Alternative method of controlled overvoltage test An alternative test method that has been adopted by some users is the “graded time test,” as detailed in AnQH[$RI,(((6WG In this test, an attempt is made to provide a linear relationship of the time-dependent absorption current with WKHWRWDOOHDNDJHFXUUHQWDQGWKHUHIRUHREWDLQDPRUHVLJQL¿FDQWLPSUHVVLRQRIWKHEHKDYLRURIWKHLQVXODWLRQ when subjected to increased voltage steps. This phenomenon is discussed in Schlief and Engvall >%@. $OWHUQDWHJUDGHGWLPHVWHSWHVWSURFHGXUHVDUHIRXQGLQ$RI,(((6WGZKLFKLQFOXGHVWKHHODSVHGWLPH schedule of the voltage steps in tabular form. 9.3.6 Other overvoltage methods Other specialized procedures for controlled dc overvoltage testing have been developed for certain applicaWLRQV7KHUHTXLUHPHQWVRIWKHDSSOLFDWLRQDQGWKHVSHFL¿FLQIRUPDWLRQGHVLUHGIURPWKHWHVWZLOOGLFWDWHZKHWKer these methods should be considered. In addition to the tests outlined previously, there are a number of other special tests that may be useful. Some of the more frequently used tests and a summary of their performance follow. 9.3.7 Insulation power-factor test or dissipation factor test 7KHVHWHVWVDUHPDLQO\XVHIXORQWKHODUJHUKLJKYROWDJH9RUKLJKHU PDFKLQHV ,(((6WGGHWDLOVWKHUHFRPPHQGHGSUDFWLFHIRUWKHSRZHUIDFWRUWLSXSWHVW 3RZHUIDFWRUYDOXHVRQFRPSOHWHZLQGLQJVDUHDQDYHUDJHRIWKHLQVXODWLRQRIDOOFRLOV7KHSRZHUIDFWRURIWKH stator insulation will be affected by the test voltage, the type of insulation, temperature of the insulation and

55

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PRLVWXUHDQGYRLGVLQWKHLQVXODWLRQ5HVXOWVDUHDOVRDIIHFWHGE\FRQGLWLRQVH[WHUQDOWRWKHPDLQLQVXODWLRQ such as the condition of the outer wrapper or slot liner, and the type of stress control coating used. Increasing power factor on the same machine over a period of time is believed to denote a general deterioration RIWKHLQVXODWLRQ*HQHUDOO\SRZHUIDFWRULQFUHDVHZLWKDJHLVXVXDOO\VPDOOIRUPDFKLQHVWKDWKDYHFRURQD control treatment on the slot portion, whereas the increase is usually much greater on machines that have coils with slot liners constructed of organic materials. The change in power factor of the stator insulation as the test voltage is raised from some low value to a higher voltage may be indicative of the amount of ionization loss in, or adjacent to the insulation. It is believed that an increase in ionization loss over a period of years indicates an increase in the size and number of voids and, hence, is an indication of deterioration within the insulation. When the coils in a machine can be individually tested, power factor can be used to compare the amount of deterioration among coils that have been operating at different voltages (e.g., between line coils and neutral coils). 9.3.8 Slot discharge and corona probe tests The slot discharge test is conducted for the purpose of checking the adequacy of the ground connection between the surfaces of the coil and the core. This test is usually applicable to machines with operating voltages LQH[FHVVRI97HVWVDUHPDGHZLWKWKHZLQGLQJHQHUJL]HGDWDSSUR[LPDWHO\WKHRSHUDWLQJVWUHVVWRJURXQG Loss of this electrical contact results in a relatively high-energy discharge between the conducting-coil surface and the core. The energy is that which results from the discharge of a substantial portion of the coil-side capacitance. Since greatly accelerated deterioration of the major ground insulation is produced by slot discharge, early detection and correction of this condition is important. Slot-discharge analyzers utilize detection circuits resonant in the frequency range where energy from surface GLVFKDUJLQJLVKLJKDSSUR[LPDWHO\+] ZKLOHEORFNLQJWKH+]RU+]YROWDJHE\PHDQVRIDKLJK SDVV¿OWHU Detection is accomplished by connecting the slot-discharge analyzer to the machine terminals, one phase at a WLPH:KHQDGLVFKDUJHH[LVWVKLJKIUHTXHQF\UHÀHFWLRQVDUHUHDGLO\REVHUYDEOHRQDQRVFLOORVFRSHFRQQHFWHG WRWKHVORWGLVFKDUJHDQDO\]HURXWSXW/RFDWLRQRIVSHFL¿FFRLOVVXIIHULQJVORWGLVFKDUJHLVDFFRPSOLVKHGE\ a probe test. The probe test utilizes the slot-discharge analyzer in conjunction with a probe that successively contacts the conducting surfaces of individual stator coils. The corona probe test is intended to be an indicator and locator of unusual ionization within the insulation structure. The ability of this test to discriminate between harmful and acceptable levels of general ionization SKHQRPHQDWKDWQRUPDOO\RFFXULQKLJKYROWDJHZLQGLQJVLVVXEMHFWWRLQWHUSUHWDWLRQ5HIHUWR,(((6WG for guidelines on corona probe acceptance levels. This test is sensitive to end-winding surface corona, as well as internal-cavity ionization in the insulation structure. Compared to slot discharge, the discharge energies involved in surface corona or internal-cavity ionization may be of a much lower order of magnitude and at higher frequencies. The corona probe is tuned to a frequency of 5 MHz for detection of these discharges. The energy in the discharge varies as the square of the voltage across the gap and directly as the effective capacitance at the point of breakdown. 3DUWLDOGLVFKDUJHFRURQD KDVVHYHUDOXQGHVLUDEOHHIIHFWVVXFKDVFKHPLFDODFWLRQSURGXFWLRQRIKHDWDQG ionic bombardment. The deteriorating effects of corona are dependent on its intensity and repetition rate as well as the design of the insulation system involved. Inorganic insulation components such as mica and glass are not affected seriously by partial discharges. Charring or decomposition of organic materials will occur in the vicinity of continued partial discharge activity.

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+RZHYHUVXUIDFHHIIHFWVPD\EHOLPLWHGE\LQVXODWLQJ¿QLVKWUHDWPHQWVLQFRUSRUDWLQJSLJPHQWDWLRQWRUHVLVW attack from the weak acid deposits formed by surface discharges in the presence of oxygen and moisture. Corona-probe test equipment consists of the following three basic units: a)

Equipment capable of energizing the stator winding at its normal operating line-to-neutral voltage at rated frequency.

b)

An antenna or corona probe. For end-winding corona measurement, the antenna is usually about PPLQ ORQJVXUURXQGHGE\DQLQVXODWLRQKRXVLQJDQGPRXQWHGRQWKHHQGRIDORQJLQVXODWLQJ handle. For internal-cavity-discharge (corona) measurements, a coil that is wound on a ferrite rod DSSUR[LPDWHO\PPLQ ORQJE\PPLQ GLDPHWHUDQGPRXQWHGRQWKHHQGRIDQLQVXODWLQJ handle is used. Measurements are made by placing the ferrite rod over the teeth enclosing the coil being tested.

c)

$QDPSOL¿HUDQGLQGLFDWRUIRUFRQQHFWLRQWRWKHDQWHQQD RUDSHDNSXOVHPHWHUIRUFRQQHFWLRQWR WKHIHUULWHFRURQDSUREH 7KHDPSOL¿HULVRQHRIWKHXVXDOW\SHIRUDXGLRIUHTXHQFLHVDQGPXVWUHMHFW +]+]DQGUDGLRIUHTXHQF\VLJQDOV7KHLQGLFDWRUPD\EHHDUSKRQHVDQRXWSXWPHWHURUDFDWKode-ray oscilloscope.

The peak-pulse meter is a broadband instrument and can be calibrated in various units. The most common unit used in the industry today is milliamps peak pulse. Measurements may be obtained from the meter itself or by connecting the meter output to an oscilloscope or chart recorder. Some instruments measure the pulses from WKHVWDWRUZLQGLQJLQ³PLOOLDPSVSHDNSXOVH´DQGWHVWDFFHSWDQFHOHYHOVDUHJLYHQLQ,(((6WG The use of the corona-probe test and the evaluation of test data obtained from it have been around since the VDQGWKLVWHVWLVVRPHWLPHVUHIHUUHGWRDVWKH79$FRURQDSUREHWHVWVLQFHWKH7HQQHVVHH9DOOH\$XWKRULW\ helped to develop and implement it. The ability of the test to distinguish varying intensities of external corona activity and internal cavity corona has been established. However, the evaluation of data to permit discrimination between harmful and acceptable levels has not yet reached the stage where industry standards are established. It should be noted that when performing this test personnel may encroach on recommended limits of approach to energized equipment. For this reason, this test should only be carried out by experienced personnel and recommended minimum limits of approach maintained at all times. Further details of these tests are given in Dakin, >%@ and >%@. 9.3.9 Partial-discharge tests 7KHRIIOLQHSDUWLDOGLVFKDUJH3' WHVWLVXVHGWRKHOSGHWHUPLQHWKHFRQGLWLRQRIWKHJURXQGLQVXODWLRQLQWKH VORWVHFWLRQVRIDVWDWRUZLQGLQJ$OVRLQZLQGLQJVUDWHG9DQGDERYHDQGORZHUYROWDJHPRWRUVVXSSOLHG from voltage source converter drives, it can determine the condition of semiconducting voltage stress control coating in the slot regions. It can also identify degradation of the interfaces between the semiconducting and stress control coatings in high-voltage windings. During an off-line test the machine is stationary, de-energized from the system and energized by an ac test source, so it will be exposed to different stresses from those present in operational service. This happens beFDXVHZKHQWKHZLQGLQJLVHQHUJL]HGLQVHUYLFHDWUDWHGOLQHWROLQHYROWDJH9// WKHSKDVHWRJURXQGYROWage varies from about VLL 3 at the line end of each phase to virtually zero at the neutral end. On the other KDQGIRUDQRIIOLQH3'WHVWWKHYROWDJHWKURXJKRXWWKHZKROHSKDVHLVDWWKHDSSOLHGSKDVHWRJURXQGWHVW level. Typically during an off-line test:



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a)

There are higher groundwall voltages toward the neutral end of the winding as it is energized to the same voltage potential as the line end and no interphasal voltages are present.

b)

The winding is at a lower temperature, so voids in the ground insulation are larger.

c)

There are no mechanical forces, vibration, etc.

In all cases it is neither possible nor practical to directly compare off-line results with online results because of the differences in electrical, mechanical, and thermal stresses between the two test conditions. 6RPHJXLGHOLQHVRQKRZWRSHUIRUPWKLVWHVWDUHJLYHQLQ,(((6WGDQG,(&)RUWKLVWHVWWRHQsure that it is only the stator winding that is being tested, it should be disconnected from all external bus work, and auxiliary equipment such as transformers, surge arrestors, surge capacitors, etc. Also, if possible the three winding phases should be disconnected from one another.

Figure 1—PD summary values This test is designed to measure partial discharge activity in a winding as described in  to allow analysis of this to identify winding insulation system degradation. 7KHNH\PHDVXUHPHQWLQD3'WHVWLVWKHSHDN3'PDJQLWXGH4P²LHWKHPDJQLWXGHRIWKHKLJKHVW3'SXOVH *HQHUDOO\WKLVYDOXHLVGHWHUPLQHGIRUDVSHFL¿FSXOVHUHSHWLWLRQUDWH)LJXUH gives an example for a pulse UHSHWLWLRQUDWHRISXOVHVSHUVHFRQG336 DQGIRU4PYDOXHVPHDVXUHGLQWHUPVRIP9S&P$RUȝ97KH SKDVHDQJOHRIWKH3'DFWLYLW\JLYHVDQLQGLFDWLRQRIWKHVRXUFHRIWKHPHDVXUHGDFWLYLW\ZKLOHWKHPDJQLWXGH RI4PLQGLFDWHVWKHVHYHULW\RIWKHLQVXODWLRQGHJUDGDWLRQ)RUH[DPSOHIRUJURXQGLQVXODWLRQGHODPLQDWLRQ due to thermal aging of the bonding resin, as described in ZKDWLVNQRZQDV³FODVVLF´3'LVSUHVHQWDQG LVLQGLFDWHGE\FOXPSVRIDFWLYLW\FHQWHUHGDURXQGƒDQGƒUHODWLYHWRDF\FOHRISKDVHWRJURXQGYROWDJH (see Figure 2 7KHPDJQLWXGHVRIWKHSRVLWLYHDQGQHJDWLYH4PYDOXHVJLYHDQLQGLFDWLRQRIWKHVHYHULW\RI VXFKGHJUDGDWLRQ7KHUHODWLYHPDJQLWXGHVRIWKHSRVLWLYHDQGQHJDWLYH4PYDOXHVJLYHDQLQGLFDWLRQRIWKH type of slot insulation degradation—e.g., if they are approximately equal, the ground insulation degradation is



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Figure 2—Classic PD patterns GLVWULEXWHGWKURXJKRXWWKHLQVXODWLRQWKLFNQHVV,I4PLVPXFKODUJHUWKDQ4PíWKHQWKLVLQGLFDWHVVHPLFRQducting coating degradation as described in &RQYHUVHO\LI4PíLVPXFKJUHDWHUWKDQ4PWKHUHLVOLNHO\ separation between the groundwall insulation and conductor stack from thermal cycling as described in . )XUWKHUGHWDLOVRIWKLVWHVWDUHJLYHQLQ,(((6WGDQG,(& 9.3.9.1 Test instrumentation 7\SLFDOO\WKHIROORZLQJWHVWHTXLSPHQWLVUHTXLUHGIRURIIOLQH3'WHVWV a)

+LJKYROWDJHFDSDFLWLYHFRXSOHUVWRIDFLOLWDWH3'PHDVXUHPHQWVZKHQFRQQHFWHGWRDQDSSURSULDWHWHVW instrument.

b)

A variable voltage ac supply capable of energizing at least one phase of the winding and preferably all three phases to the stator winding rated phase-to-ground voltage. This power supply should preferably EH3'IUHH

c)

7HVWLQVWUXPHQWWRFRQQHFWWRWKHFDSDFLWLYHFRXSOHUVWRJHWKHUZLWKDODSWRSFRPSXWHUWRDFTXLUH3' measurements.

d)

6KRUWFRQQHFWRUVWKDWDUH3'IUHHZKHQHQHUJL]HGWRFRQQHFWWKHWHVWLQVWUXPHQWWRWKHKLJKYROWDJH +9 FDSDFLWLYHFRXSOHUV

9.3.9.2 Noise reduction 7KHWZRPRVWFRPPRQPHWKRGVRIHQVXULQJQRLVHGRHVQRWDIIHFWWKHPHDVXUHG3'YDOXHVDUHWRXVHD3'IUHH SRZHUVRXUFHFRQQHFWHGWRWKHVDPHSRLQWDVWKH3'FRXSOHUV RULI3'WHVWLQJLQWKH0+]UDQJHLVEHLQJ GRQHDQGWKHZLQGLQJSKDVHVFDQEHVHSDUDWHGHQHUJL]HWKHZLQGLQJIURPRQHHQGDQGWDNH3'PHDVXUHPHQWV from the other end. The latter procedure uses the stator winding to attenuate and disperse noise so that it is not PLVWDNHQIRU3'



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9.3.9.3 Test procedure for individual phases with other two grounded The test procedure for individual phases with the other phases grounded is as follows: a)

$QLQVXODWLRQUHVLVWDQFH,5 WHVWDVGHVFULEHGLQ should be conducted prior to the partial discharge tests to determine the suitability of the winding for further testing. The insulation resistances for the stator winding should indicate the stator winding to be clean and dry and therefore acceptable IRURIIOLQH3'WHVWLQJ

b)

&RQQHFWWKHSRZHUVXSSO\DQG3'FRXSOHUVWRWKHVWDWRUZLQGLQJSKDVHWREHWHVWHG

c)

&RQQHFWWKHWHVWLQVWUXPHQWDWLRQWRWKH3'FRXSOHUV

d)

Ensure that the machine stator frame and the other two phases are solidly connected to a ground termination.

e)

5DLVHWKHWHVWYROWDJHVORZO\WRWKHPD[LPXPWHVWYROWDJHDQGPDLQWDLQWKLVYROWDJHXQWLOWKH3'VWDELOL]HVXSWRPLQWRPLQ DQGUHFRUG3'GDWD1RWHWKH3'WHQGVWRGHFUHDVHRYHUWKLVWLPHSHULRG 3'QRUPDOO\UHGXFHVGXHWRVSDFHFKDUJHHIIHFWVDQGDEXLOGXSRISUHVVXUHZLWKYRLGVLQWKHLQVXODWLRQ$OVRDWWKLVWLPHDQXOWUDYLROHWFDPHUDFDQEHXVHGWRORRNIRUVXUIDFH3'DFWLYLW\LQWKHDUHDVRI the semiconducting/stress control coating interfaces.

f)

6ORZO\ORZHUWKHDSSOLHGYROWDJHXQWLOQR3'LVGHWHFWHGDQGUHFRUGWKLVYROWDJHZKLFKLVNQRZQDVWKH 3'([WLQFWLRQ9ROWDJH3'(9 

g)

5HGXFHWKHDSSOLHGYROWDJHIXUWKHUDQGWKHQVORZO\LQFUHDVHLWXQWLO3'LVGHWHFWHGDQGUHFRUGWKHYDOXH DWZKLFKWKLVRFFXUV7KLVYROWDJHLVNQRZQDVWKH3',QFHSWLRQ9ROWDJH3',9 

h)

5HSHDWWKHWHVWIRUWKHRWKHUWZRSKDVHV

9.3.9.4 Test procedure of individual phases with all energized The test procedure of individual phases with all energized is as follows: a)

3HUIRUP,5WHVW

b)

Connect all three phases together and the power supply to a common connection point.

c)

&RQQHFW3'FRXSOHUWRSKDVHWREHWHVWHGRURQHWRHDFKSKDVH

d)

Ensure stator frame is grounded.

e)

3HUIRUPUHPDLQGHURIWHVWVSHU step e) through step h).

9.3.9.5 Test interpretation Test interpretation is as follows: a)

(YDOXDWH3'OHYHOVDQGFKDUDFWHULVWLFVDWWKHPD[LPXPSKDVHWRJURXQGWHVWYROWDJHWRHYDOXDWHWKH types of insulation degradation present and the severity (thermal degradation, semiconducting coating degradation, etc.).

b)

(YDOXDWHWKH¿QGLQJVIURPXOWUDYLROHWFDPHUDVFDQV

c)

Comparison of the results from the tests described in  and  may indicate some phase-toJURXQG3'DFWLYLW\GXHWRFRQWDPLQDWLRQLQDGHTXDWHVSDFLQJEHWZHHQHQGZLQGLQJVRUFLUFXLWULQJ bus connections in different phases. This is done by comparing the results for each phase to see if the 3'OHYHOVIRUWHVWF DUHKLJKHUWKDQIRUWHVWG ,IWKLVLVVRLQWHUSKDVHDFWLYLW\LVLQGLFDWHG



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d)

)RUDZHOOFRQVROLGDWHGZLQGLQJWKHUDWLRRIWKH3'(9RU3',9WRDWUDWHGSKDVHWRJURXQGYROWDJH VKRXOGEHRUKLJKHU6WRQHHWDO>%@ ,IWKH3'(9DQG3',9YDOXHVDUHORZWKLVLVIXUWKHULQGLFDWLRQRIVLJQL¿FDQWLQVXODWLRQV\VWHPGHJUDGDWLRQ

9.3.10 Turn-to-turn insulation test In cases where the integrity of the insulation between adjacent turns in a coil is subject of concern, tests should be made to establish that a desired level of insulation strength is present. Test equipment, employed in the application of turn insulation tests, is usually the type in which a capacitor is alternately charged and then discharged into the coil under test (or into an inducing coil that has been placed in the stator bore, over the coil under test). Since the insulation between turns of stator coils varies greatly in types of insulating materials, types of construction, and spacing, test values are usually determined after consultation with the coil or machine manufacturer. Any test value selected to verify the adequacy of inter-turn insulation should be based on the deVLJQSK\VLFDOVSDFLQJDQGHOHFWULFDOVWUHQJWKRIWKHLQVXODWLQJV\VWHP5HIHUWR,(((6WGIRUWXUQWRWXUQ testing. 9.3.11 Coil-to-core contact resistance It is essential that the corona suppression coatings applied to the surface of coils in high-voltage windings be adequately grounded. A low-resistance grounding path is usually provided by direct-contact resistance with the stator core. Measurement of the coil-to-core contact resistance may provide a useful indication of the condition of the semiconducting slot coating system. The acceptance criterion varies by machine depending on the design and size, and may change with operating hours and temperature. Consult the manufacturer for VSHFL¿FUHTXLUHPHQWV 9.3.12 Resistance temperature detectors (RTDs) 5HVLVWDQFHWHPSHUDWXUHGHWHFWRUVDUHUHVLVWDQFHFRLOVFRQVWUXFWHGWRDOORZWKHWHPSHUDWXUHWREHPHDVXUHGE\ a change in resistance. 5HVLVWDQFHWHPSHUDWXUHGHWHFWRUVDUHPDGHZLWKDUHVLVWDQFHHOHPHQWFRQVWUXFWHGXVLQJDPDWHULDOIRUZKLFK the electrical resistivity is a known function of the temperature, so as to allow the temperature to be measured by a change in resistance. 0HDVXUHPHQWVDUHXVXDOO\PDGHLQRUGHUWRYHULI\WKDW57'VDUHSURSHUO\FRQQHFWHGDQGWKDWWKH\DUHIUHHRI undesired ground contacts or open circuits. Measurements are comprised of comparisons of readings from HDFK57'ZLWKDOORWKHUVDQGVKRXOGEHPDGHDWURRPWHPSHUDWXUH5HIHUWR,(((6WGDQG,(((6WG )RUWKHPHDVXUHPHQWVDUHVLVWDQFHEULGJHLVQRUPDOO\XVHG$VSHFLDO57'PHWHUIRUGLUHFWO\UHDGLQJWHPSHUDWXUHVRIGHWHFWRUVFDQDOVREHXVHG5HIHUULQJWRFigure 3DOOWKUHHOHDGVRIDJLYHQ57'PXVWEHRIHTXDOOHQJWK and wire size so that the three lead resistances all have the same value. By subtracting the resistance measured between terminals A and B from the resistance measured between terminals A and C (or B and C), the resistance of the temperature sensing element alone can be accurately determined. The temperature element is XVXDOO\PDGHRISODWLQXPZLUHDQGLVDSSURSULDWHO\VL]HGVRWKDWLWVUHVLVWDQFHDWƒ&LVȍ)URPWKHPHDsured change in resistance the temperature of the element may be calculated. After proper meter corrections DUHDSSOLHGWHPSHUDWXUHUHDGLQJVRIHDFK57'DQGWKHWKHUPRPHWHUUHDGLQJVVKRXOGDJUHHWRZLWKLQ“ƒ& 9.3.13 Insulation resistance test of embedded temperature detectors Stator winding embedded temperature detectors (resistance or thermocouple) are connected by cable to a terminal board on the frame of the unit. Oftentimes one lead of each of the detectors is connected to a common ground strip at the terminal board. If a detector located in the slot portion should become grounded, circulating currents could occur between that ground and the terminal board ground. To guard against this possibility,



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Figure 3—RTD (3 wire) and Wheatstone Bridge Circuit insulation resistance measurement should be made on the detectors at convenient intervals. Tests are usually PDGHRQDOOGHWHFWRUVVLPXOWDQHRXVO\DW9GFDIWHUWKHWHUPLQDOERDUGFRPPRQKDVEHHQLVRODWHGIURP JURXQG$FRPPRQO\XVHGPLQLPXPYDOXHIRUWKHGHWHFWRULQVXODWLRQUHVLVWDQFHLV0ȍDW9GFUHFRUGing equipment, connected externally to the terminal board, should be isolated from the test potential). 9.3.14 Insulation resistance test of insulated stator through bolts Integrity of stator through bolt insulation is critical to the successful operation of some cores; therefore, insulation resistance to ground of stator through-bolts should be measured. Consult the manufacturer regarding the minimum insulation resistance level and the recommended test voltage level. If a recommended value is not DYDLODEOHDWDPLQLPXPWKHLQVXODWLRQUHVLVWDQFHVKRXOGEHJUHDWHUWKDQ0ȍDW9GF 9.3.15 Winding resistance A reduction in winding resistance may indicate shorting of conductors, and an increase in winding resistance may indicate poor connection. 5HVLVWDQFH RI WKH VWDWRU ZLQGLQJ LV XVXDOO\ PHDVXUHG ZLWK D ORZUHVLVWDQFH .HOYLQ  EULGJH RU E\ WKH GURSLQSRWHQWLDOPHWKRG5HIHUWR,(((6WG7KHPHDVXUHPHQWLVQRUPDOO\PDGHIRUHDFKSKDVHVHSDUDWHly. The stator winding should be at room temperature when the cold resistance measurement is made, and the WHPSHUDWXUHRIWKHZLQGLQJFDUHIXOO\GHWHUPLQHG5HIHUWR,(((6WG The resistance-temperature characteristic of copper in the range of temperatures usually encountered is a VWUDLJKWOLQHWKDWLIH[WUDSRODWHGLQWHUVHFWVWKH]HURRQWKHUHVLVWDQFHD[LVDWíƒ&%DVHGRQWKLVFKDUacteristic, the temperature corresponding to any resistance of a copper winding may be determined from (TXDWLRQ :

t2 =

R2 (234.5 + t1 ) − 234.5 R1



where t and t2 R and R2

DUHWHPSHUDWXUHVPHDVXUHGLQƒ& are winding resistances measured in ohms at t and t2 respectively

The balance of the three winding phase resistances should be checked by doing one of the following:

62

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a)

Comparing the values for each phase if they can be separated, or

b)

Comparing the terminal-to-terminal resistances if they cannot be separated.

The resistance balance can then be checked using the Equation (2):

% Unbalanced

(R Max Dev. from Average ) - (R Average ) R Average

(2)

7KHXQEDODQFHVKRXOGQRWH[FHHGWKHIROORZLQJ —

IRUIRUPZLQGLQJV

—

IRUWZROD\HUUDQGRPZLQGLQJV

—

IRUVLQJOHOD\HUFRQFHQWULFZLQGLQJV

 6WDWRUFRUHLQWHUODPLQDULQVXODWLRQ KLJKÀX[WHVW $QHIIHFWLYHWHVWRIWKHLQWHUODPLQDULQVXODWLRQFDQEHPDGHE\LQGXFLQJLQWKHVWDWRUFRUHDÀX[DWUDWHGIUHTXHQF\DWDSSUR[LPDWHO\WKHÀX[GHQVLW\LQWKHFRUHFRUUHVSRQGLQJWRWRRIUDWHGYROWDJH7KHWHVW level should be agreed to by the customer and the machine manufacturer. This test is also known as the ring ÀX[RUORRSWHVWDQGFDQEHSHUIRUPHGE\SDVVLQJDWHPSRUDU\FRLOWKURXJKWKHVWDWRUERUHDQGWKHQDURXQGRQH side of the frame. This coil should be insulated from the core and frame and be braced securely in position. A single-turn test coil (search coil) is similarly wrapped around the core and connected to a voltmeter for moniWRULQJWKHÀX[DFKLHYHG)RUPRWRUVDQGJHQHUDWRUVVHQWWRDVHUYLFHVKRSIRUUHSDLUWKLVWHVWPD\EHSHUIRUPHG using a commercial core tester with a single turn excitation winding. This test produces fault currents similar LQOHYHOWRWKRVHWKDWÀRZZKHQWKHJHQHUDWRULVLQRSHUDWLRQKRZHYHUFDXWLRQQHHGVWREHWDNHQDVWKHVH currents can produce temperature rises that can cause further damage because the cooling system is disabled during testing. Also, the conditions to detect various core shorts are not the same as in operation due to absence RID[LDOOHDNDJHÀX[$VDUHVXOWWKHUHLVORZHUVWUHVVRQWKHFRUHVWHSLURQGXULQJWKLVWHVWYHUVXVRSHUDWLRQ The manufacturer may have recommendations regarding desired bulk temperature rises since the cooling system is not in place during the test. ,QFDVHVZKHUHVLJQL¿FDQWGDPDJHLVYLVXDOO\HYLGHQWWKHLQWHUODPLQDULQVXODWLRQPXVWEHUHHVWDEOLVKHGEHIRUH the application of the test. Surface indications of shorting often appear immediately after energizing the loop; therefore, thermographic imaging at the beginning of the test over the whole surface of the core is recommended to allow stopping the test to resolve issues. Otherwise, additional overheating and burning damage may occur during the test. Methods of calculating the test-coil voltage and ampere-turn requirement are given in Annex C3ULRUWRFRQducting this test it is recommended that the manufacturer be consulted. The typical criteria for a temperature ULVHRIDKRWVSRWRYHULWVDYHUDJHVXUURXQGLQJLURQWKDWLVUHJDUGHGDVVLJQL¿FDQWLVDPD[LPXPRIƒ&IRUDQ XQZRXQGFRUHDQGƒ&IRUDZRXQGFRUH7KLVFULWHULDRUDKRWVSRWWKDWLVFRQWLQXLQJWRLQFUHDVHZLWKWLPH DWWKHFRPSOHWLRQRIWKHWHVWWLPHGXUDWLRQWKDWLVW\SLFDOO\UHJDUGHGDVVLJQL¿FDQWUHTXLUHVIXUWKHULQYHVWLJDtion—e.g., visual inspection. The heat caused by circulating currents in the core will increase approximately DVWKHVTXDUHRIWKHÀX[$VDUHVXOWVRPHDGMXVWPHQWRIWKHWHPSHUDWXUHULVHFULWHULDPD\EHFRQVLGHUHGZKHQ XVLQJDOWHUQDWHÀX[OHYHOV+RZHYHUFRQVLGHUDWLRQPXVWDOVREHWDNHQIRUXQFHUWDLQWLHVRIWKHPHDVXUHPHQW with respect to issues with temperature measurements, as well as issues with bands of higher temperatures that often occur in cores. To enable subsurface faults to be revealed by thermal imaging inspection, minimum test time durations of one hour to six hours may be required depending on the type, size, and construction of the machine. Some large WXUELQHJHQHUDWRUVPD\UHTXLUHWHVWWLPHVRIWKHRUGHURIVL[KRXUVWRHQVXUHWKDWWKHVWDWRUFRUHH[SDQGVVXI¿-

63

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ciently against the building bars to make adequate electrical contact so as to complete possible core lamination fault circuits. Some machine manufacturers may also recommend a minimum average core iron temperature rise over ambient for an adequate test, as well as maximum temperatures that should not be exceeded during the test. Thermographic cameras are often used to measure core temperatures; however, there are limitations associated with these techniques due to variations in core surface emissivity, incident angles of viewing, and interferHQFHZLWKWKHUPRJUDSKLFFDPHUDVGXHWRWKHKLJKÀX[WKDWPD\SUHYHQWWKHFDPHUDIURPDFKLHYLQJDFFXUDWH readings. Note that these cameras will typically report readings with very high precision, but accuracy is highly dependent on techniques of using the camera. In some cases thermocouples are used in combination with the thermographic camera readings to help select the appropriate emissivity values. Additionally, surIDFHPRXQWHGWKHUPRFRXSOHVPD\EHSODFHGLQUHJLRQVRILQWHUHVWLGHQWL¿HGE\YLVXDOLQVSHFWLRQVRUWKHORZ energy test described in ) along with reference locations to determine the temperature hotspot over the average surrounding iron. It is recommended that the original equipment manufacturer be consulted for guidance on appropriate hotspot criteria, time duration, temperature, and testing condition recommendations for the particular machine. 7KHWHVWSURFHGXUHVIRUWKHVWDWRUFRUHKLJKÀX[WHVWFDQEHIRXQGLQWKHELEOLRJUDSK\OLVWHGLQAnnex A. Test FDOFXODWLRQSURFHGXUHVIRUWKHKLJKÀX[WHVWVDUHJLYHQLQAnnex C.  6DIHW\FRQVLGHUDWLRQVIRUKLJKÀX[FRUHLQVXODWLRQWHVW Since considerable hazard may exist in connection with this test, all test personnel involved should be familiar with the necessary safety precautions. The following is a list of some of the major safety items related to this test: a)

Shielded cable should never be used for the magnetizing coil, as applied voltage will also be induced into the cable shield.

b)

Do not go near the magnetizing coil or the stator core when the test setup is energized.

c)

The stator frame is to be safely grounded.

d)

All electrical connections should be checked before a trial application of power is made.

e)

$SSURSULDWH¿UHSURWHFWLRQVKRXOGEHPDGHDYDLODEOHGXULQJWKHWHVW

f)

Liquid cooling system, if present, should be drained and remain empty.

g)

0DFKLQHWHUPLQDOVVKRXOGEHRSHQHGDQGVDIHO\FRYHUHGDQGÀDJJHG

h)

6WDWRU57'VDQGWKHLUUHFRUGHUFDQXVXDOO\UHPDLQLQVHUYLFHGXULQJWHVWV+RZHYHUWKHPDFKLQHPDQXIDFWXUHUVKRXOGEHFRQVXOWHGIRUVSHFL¿FUHFRPPHQGDWLRQV

i)

Adequate phone and other communication systems should be established among various points for proper test control.

j)

If thermocouples are used for temperature measurements, a considerable personnel hazard may exist since up to full search coil voltage can be induced in the thermocouple lead. Also, care should be exercised to avoid short circuiting laminations with the thermocouple lead.

k)

On large machines (for example, steam-turbine generators) cables should be secured against motion during energizing.

l)

At high energy levels the core and frame, etc., may produce high noise levels and vibration due to magnetostriction and natural frequencies of the structure—this will require proper ear protection during energizing. In some cases a different frequency may be used for the test to avoid resonances.

64

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m)

Care must be taken that no metallic objects are in contact with the air-gap side of the core laminations. Also, extraneous, metallic structural objects; metal ladders; crane cables; etc., which might form a conducting circuit around the core, should not be left in the machine during test.

 *HQHUDOFRQVLGHUDWLRQVIRUKLJKÀX[FRUHLQVXODWLRQWHVW ,WLVXQGHVLUDEOHWRKDYHDQ\FLUFXODWLQJFXUUHQWVLQGXFHGLQWRWKHZLQGLQJV57'VRURWKHULQVWUXPHQWDWLRQ from loop testing. Therefore, the items should have one of their leads open while their other lead is grounded. *URXQGHLWKHUWKHQHXWUDOOHDGVRUWKHPDLQOHDGVEXWQRWERWKDWWKHVDPHWLPHWRSUHYHQWFLUFXODWLQJFXUUHQWV LQWKHZLQGLQJV5RSHRIIDQGSODFHKLJKYROWDJHVLJQVQHDUWKHRSHQPDLQVQHXWUDOVDVWKH\ZLOOKDYHKLJK voltage on them during the loop test. If the neutral shunts have not been removed, then ground only the neutral leads and open the main leads. 9.3.17 Stator core low energy test $QRWKHUWHFKQLTXHGHYHORSHGLVWKHORZHQHUJ\ÀX[WHVW7KLVLVDOVRNQRZQDVWKH(/&,'WHVW(OHFWURPDJQHWLF&RUH,PSHUIHFWLRQ'HWHFWLRQ 7KLVWHVWRIIHUVDSURFHGXUHZLWKDORZHUSRZHUN9$ UHTXLUHPHQWDQG SURYLGHVDPHDQVIRUFRUHIDXOWGHWHFWLRQ7KHWHVWLVFRQGXFWHGDWW\SLFDOO\IRXUSHUFHQWRIUDWHGÀX[DQGWKH fault current is detected by an electromagnetic means. An electromagnetic core imperfection detector is a deYLFHIRUGHWHFWLRQRIFXUUHQWÀRZEHWZHHQFRUHODPLQDWLRQVZLWKH[FLWDWLRQ+RZHYHULQVRPHFDVHVVXFKDVLQ DFRUHWKDWKDVVSOLWVWKHUHVXOWVRIWKHORZHQHUJ\DQGKLJKÀX[WHVWVGRQRWDOZD\VDJUHH7KHKLJKÀX[ORRS test is the preferred method, but in some long horizontal steam units it is not performed due to safety considerDWLRQV7KUHHFRQFHUQVLQYROYLQJWKHORZÀX[WHVWDUHWKDWLWGRHVQRWJHQHUDWHRSHUDWLRQDOW\SHFRUHYLEUDWLRQV the core plate voltages are only a small fraction of what is seen in normal operation, and there is very little core heating. Thus, conditions to detect various core shorts are not the same as in operation. Low-energy testing is often performed due to the ease of implementation, including the fact that the test can be done with the rotor LQVWDOOHG7KHWHVWLVDOVROHVVOLNHO\WRFDXVHGDPDJHDWVXVSHFWORFDWLRQVWKDQWKHKLJKÀX[WHVW2IWHQLIDQLQGLFDWLRQLVREVHUYHGGXULQJDORZHQHUJ\ÀX[WHVWUHSDLULVDWWHPSWHGSULRUWRDKLJKÀX[WHVW+LJKÀX[WHVWLQJ LVFRQVLGHUHGPRUHGH¿QLWLYHWKDQORZÀX[WHVWLQJEHFDXVHLWLVPRUHUHSUHVHQWDWLYHRIRSHUDWLRQ 7KHWHVWSURFHGXUHIRUWKHVWDWRUFRUHORZHQHUJ\ÀX[WHVWFDQEHIRXQGLQWKHELEOLRJUDSK\OLVWHGLQAnnex A. 7HVWFDOFXODWLRQSURFHGXUHVIRUWKHORZHQHUJ\ÀX[WHVWVDUHJLYHQLQAnnex D. 9.3.18 Stator core testing with alternate frequency Both the low energy and high energy tests can be implemented using frequencies other than the normal power IUHTXHQF\RIWKHPDFKLQH3DWHQWVH[LVWRQWKHVHWHVWPHWKRGV6XWWRQ>%@). The main aspect of this method LVWKDWWKHYROWDJHEHWZHHQODPLQDWLRQVFDQEHUDLVHGZLWKLQFUHDVHGIUHTXHQF\ZKLOHWKHÀX[GHQVLW\FDQEH lowered.

10. Cleaning 10.1 General Care and good judgment must be used in any electric machinery cleaning program. Excessive or harsh cleaning procedures can damage an otherwise good machine and may result in expensive repairs or replacement. However cleaning is sometimes necessary, such as when surface contamination degrades electrical insulation performance or reduces the heat transfer capability of the machine. Electric machines, or components thereof, PD\XQGHUJRFOHDQLQJRQVLWHRUDWD¿HOGVHUYLFHVKRS The need for cleaning may be indicated from the following: a)

Operation and maintenance history

65

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b)

Equipment application (e.g., location in a polluted environment)

c)

9LVXDOLQVSHFWLRQ

d)

Low insulation resistance measurements

e)

Overheating

Once the need for cleaning has been established, the cleaning method should be tailored to the type of contamLQDWLRQDQGWKHH[WHQWRIWKHFRQWDPLQDWLRQEXLOGXS3ULRUWRDQ\FOHDQLQJLWPD\EHSUXGHQWWRWHVWIRUOHDGDQG DVEHVWRVDVWKHSUHVHQFHRIWKHVHVXEVWDQFHVZLOODIIHFWWKHFKRLFHRI33(VXFKDVUHVSLUDWRUVJRJJOHVDQG rubber gloves), cleanup methods, and waste disposal procedures. After cleaning (and drying, if necessary), the machine’s surface condition should be checked for cracks, porosLW\RURWKHUGDPDJHFDXVHGE\KDUVKFOHDQLQJPHWKRGV7KHGHVLUHGVXUIDFH¿QLVKVKRXOGEHUHHVWDEOLVKHGE\ the application of suitable varnishes, paints, or resins.

10.2 Cleaning techniques The preferred method of cleaning depends on the component and type of machine to be cleaned, as well as on the type and severity of the contamination to be removed. Whenever possible, consult with the equipment manufacturer to select cleaning materials and methods that are safe for workers and not damaging to the equipment. It may be necessary to provide forced ventilation (via fans, ventilation tubes, an air hood, etc.), particuODUO\ZKHQZRUNLVEHLQJSHUIRUPHGLQWKHERWWRPRIWKHVWDWRUSLWRULQRWKHUDUHDVZKHUHDLUÀRZLVUHVWULFWHG Machines without sealed windings may not be suitable for submerging or spraying with steam or water. The following cleaning methods, , are listed in increasing order of severity and potential harm to workers and the equipment being cleaned. 10.2.1 Vacuum cleaning /RRVHGLUWGHSRVLWVVXFKDVFDUERQGXVWFRDOGXVWDQGÀ\DVKFDQEHUHPRYHGE\YDFXXPFOHDQLQJZLWKDQ industrial-type vacuum cleaner and long hose. Nozzle shapes may be varied to facilitate cleaning hidden or GLI¿FXOWWRUHDFKDUHDV&RQWDPLQDQWVFDQEHGLVORGJHGIRUYDFXXPSLFNXSE\WKHIROORZLQJ a)

5XEELQJZLWKGU\FORWKV

b)

Brushing with a plastic or natural bristle brush (with the bristles cut short if a stiff brush is needed)

c)

6FUDSLQJZLWKVRIWZRRGRU¿EHUVFUDSHUV

Note that wire brushes or metal scrapers should not be used to loosen surface dirt because of possible damage to the surface being cleaned and the dangerous possibility of introducing magnetic or other metallic particles into the stator winding or core assembly. 10.2.2 Air lance cleaning Clean, dry compressed air may be used to blow out air vents or to dislodge trapped contaminants. It is recRPPHQGHGWKDWWKHDLUVXSSO\SUHVVXUHQRWH[FHHGNLORSDVFDOSVL DQGZLWKDQ26+$FRPSOLDQWDLU QR]]OHWRUHVWULFWQR]]OHSUHVVXUHWROHVVWKDQNLORSDVFDOSVL WRDYRLGGDPDJLQJWKHLQVXODWLRQRURWKHU fragile components. The air at the nozzle (nozzle pressure) or opening of a gun, pipe, cleaning lance, etc., used IRUFOHDQLQJSXUSRVHVVKRXOGUHPDLQDWDSUHVVXUHOHYHOEHORZSVLIRUDOOVWDWLFFRQGLWLRQV$VHFRQGURXQGRI vacuum cleaning may be necessary to remove materials that were dislodged by the compressed air.

66

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10.2.3 Solvent cleaning To prevent harming workers and damaging equipment, extra care must be exercised when using liquid cleaning solvents. Avoid the use of excess solvent, which can wash dirt deposits into cracks, crevices, and other LQDFFHVVLEOHDUHDV7U\WKHFOHDQLQJÀXLGRQDVPDOODUHD¿UVWDQGWKHQFKHFNIRUGDPDJHDQGHIIHFWLYHQHVV'R QRWVPRNHRUHDWDURXQGVROYHQWV,IWKHVROYHQWLVÀDPPDEOHNHHSLJQLWLRQVRXUFHVRXWRIWKHFOHDQLQJDUHD DQGKDYH¿UHH[WLQJXLVKHUVDYDLODEOHIRUXVH,WPD\EHDGYLVDEOHWROLPLWZRUNHUH[SRVXUHWRFOHDQLQJVROYHQWV and vapors by having more people working on the cleanup for shorter periods of time. Mild detergents and diluted alcohol are often effective in cleaning electrical equipment, and their use should be considered before applying harsher chemicals. 3HWUROHXPVROYHQWVPD\EHXVHGVSDULQJO\IRUUHPRYLQJRLO\DQGJUHDV\FRQWDPLQDQWVIURPPDFKLQHFRPSRQHQWVLQFOXGLQJDVSKDOWLFRUV\QWKHWLFUHVLQW\SHVRILQVXODWLRQ4XLWHRIWHQDOLQWIUHHFORWKOLJKWO\GDPSHQHG with solvent is effective for surface cleaning. Avoid saturating asphaltic-type insulations, which could lead WRVRIWHQLQJRIWKHLQVXODWLQJPDWHULDOV*DVROLQHQDSKWKDDQGVLPLODUOLTXLGVDUHQRWWREHXVHGIRUFOHDQLQJ EHFDXVHRI¿UHDQGH[SORVLRQKD]DUGV If a stronger or faster-drying solvent is required, a chlorinated safety solvent can be used on asphaltic and V\QWKHWLFUHVLQW\SHVRILQVXODWLRQ$JDLQVROYHQWGDPSHQHGFORWKVDUHRIWHQVXI¿FLHQWIRUZLSLQJRIIFRQWDPLQDQWV&KORULQDWHGVROYHQWVPXVWQRWEHXVHGRQVWDLQOHVVVWHHOFRPSRQHQWVZLWKRXW¿UVWFRQVXOWLQJZLWK the equipment manufacturer because of the possibility of stress corrosion caused by the chlorides. Chlorinated solvents must not be used on aluminum or copper components because of chloride attack. Mixtures of petroleum solvents and chlorinated solvents can provide better cleaning capability than the peWUROHXPVROYHQWVDORQH6XFKPL[WXUHVPXVWEHFRQVLGHUHGÀDPPDEOHHYHQWKRXJKLQVRPHSURSRUWLRQVWKH\ PLJKWQRWEH+RZHYHUGLIIHUHQFHVLQHYDSRUDWLRQUDWHVFDQFKDQJHWKHÀDPPDELOLW\FKDUDFWHULVWLFVRIWKH blend over time. Neither petroleum solvents nor chlorinated solvents should be used on silicone insulated windings because of the degrading effect on this type of insulation. Carbon tetrachloride and benzene are highly toxic solvents and are not to be used for cleaning. Solvent cleaning of cylindrical rotors should be avoided. Cleaning of cylindrical rotors should be limited to vacuuming, blowing with dry compressed air, wiping with dry or solvent dampened cloth, or combinations of these three methods. The need for more extensive cleaning may involve retaining ring removal to provide access to areas where contaminants are trapped. Carbon brushes should not be allowed to absorb solvents, particularly the chlorinated types. 10.2.4 Abrasive blasting Abrasive blasting is used to remove paint, oil, dirt, grime, and other contaminants from hard surfaces such DVVWDWRUFRUHV*URXQGFRUQFREVSXOYHUL]HGZDOQXWVKHOOVRURWKHUDEUDVLYHPDWHULDOVDUHGLVFKDUJHGIURP DSUHVVXUL]HGEODVWLQJPDFKLQHWKURXJKDQR]]OHDWWDFKHGWRDÀH[LEOHKRVH7KHDEUDVLYHSDUWLFXODWHVLPSDFW the surface and knock off the unwanted coating or contaminant. Crushed corncobs are especially effective in removing oily contaminants; whereas highly abrasive walnut shells rapidly remove unwanted surface paint. 5HJDUGOHVVRIWKHDEUDVLYHPHGLDEHLQJXVHGWKHDLUDEUDVLYHEODVWPXVWQRWEHKHOGWRRORQJRQDQ\RQHDUHDRU the component being cleaned could be damaged by abrasion. In addition, care must be taken to avoid blowing the abrasive material into inaccessible areas where it cannot be completely removed and may block ventilating passages or cause mechanical imbalance during operation.



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10.2.5 CO2 blasting CO2 (carbon dioxide) cleaning uses conventional blasting technology in combination with dry ice pellets. 9DSRUL]HGOLTXLGQLWURJHQLVXVHGWRSURSHOWKHGU\LFHSDUWLFOHVWRZDUGWKHVXUIDFHWREHFOHDQHG8QOLNHRWKHU blast-cleaning technologies that strictly rely on abrasive media impacting the surface to be cleaned, dry ice blasting also creates thermal differentials between the contaminant and the surface (due to different rates of VKULQNDJHFDXVHGE\GLIIHULQJWKHUPDOFRHI¿FLHQWVRIH[SDQVLRQ 7KHVHWKHUPDOGLIIHUHQWLDOVORRVHQWKHERQGV EHWZHHQWKHFRQWDPLQDQWDQGWKHVXUIDFHDQGLPSURYHWKHHIIHFWLYHQHVVRIWKHEODVWLQJSURFHVV5HYHUVHIUDFturing further aids in the cleaning action when molecules of the vaporized nitrogen and vaporized CO2 enter the pores of the contaminants. As the gas molecules warm and expand, they help break the bonds between the surface and the contaminants. When used properly, CO2 cleaning is a totally dry process and produces no secondary waste. It works best removing loose, non-oily surface contamination on hard, non-porous surfaces. As ZLWKRWKHUEODVWLQJWHFKQLTXHVVRPHDUHDVPD\EHGLI¿FXOWWRUHDFKGHSHQGLQJRQWKHJHRPHWU\RIWKHFRPSRnent being cleaned. Furthermore, poorly adhering surface paint may be knocked off and more serious damage PD\RFFXULIWKHSURFHGXUHLVQRWSHUIRUPHGFDUHIXOO\6SHFL¿FDOO\&22 cleaning can be very aggressive and lead to insulation damage if the CO2 pellet size is too large and/or air pressure is too high. 10.2.6 Steam cleaning Steam cleaning utilizes a high velocity jet of steam and water containing a mild nonconductive detergent. The detergent spray is followed by multiple clean water rinses. The steam cleaning method is effective on heavily FRQWDPLQDWHGZLQGLQJVDQGZLQGLQJVVXEMHFWHGWRÀRRGLQJRUVDOWFRQWDPLQDWLRQ7KHVWHDPFOHDQLQJPHWKRG usually can be used on silicone-insulated windings. Steam cleaning may not be suitable for some machines; for instance, steam cleaning is not recommended for old asphaltic-mica insulated machines, and machines whose insulation is sensitive to moisture ingress. Consult the OEM for advice on applicability of steam cleaning for a particular machine. 3ULRUWRUHWXUQLQJDVWHDPFOHDQHGPDFKLQHWRVHUYLFHLWPXVWEHGULHGRUEDNHGWRUHPRYHDOOPRLVWXUHIURP the windings and to obtain an acceptable insulation resistance value. If voltage is applied before all moisture KDVEHHQUHPRYHGWKHUHLVDULVNRILQVXODWLRQIDLOXUH5HJDUGOHVVRIWKHPHWKRGXVHGIRUGU\LQJWKHLQVXODWLRQ V\VWHPGU\RXWWHPSHUDWXUHVVKRXOGQRWH[FHHGƒ&WRƒ&DQGWKHUDWHRIWHPSHUDWXUHULVHVKRXOGEH OLPLWHGWRƒ&SHUKRXU,QH[FHSWLRQDOFDVHVZKHUHLQVXODWLRQUHVLVWDQFHKDVQRWUHDFKHGDFFHSWDEOHOHYHOV DIWHUKRUPRUHRIGU\LQJFRQVLGHUDWLRQPD\EHJLYHQWRLQFUHDVLQJWKHPD[LPXPWHPSHUDWXUHWRƒ&WR ƒ&+RZHYHUDWWHPSHUDWXUHVRIƒ&DQGDERYHWKHSRVVLELOLW\RILQVXODWLRQGDPDJHLQFUHDVHVDVJDVHV and vapors generated within the insulation by high temperature develop pressure and are forced through the insulation. This can break the continuity of the layers and cause delamination, or actually rupture the material. 9HQWLODWLRQLVUHTXLUHGWRUHPRYHWKHZDWHUYDSRUGXULQJWKHKHDWLQJF\FOH 10.2.7 Cleaning by water immersion or water hose Many of the machines covered by this guide are too large for immersion, although heavily contaminated or ÀRRGHGPDFKLQHVFDQEHZDVKHGZLWKDKRVH:DWHULPPHUVLRQFOHDQLQJPD\QRWEHVXLWDEOHIRUPDQ\PDchines. Baking and drying precautions noted under steam cleaning would also apply for water immersion or water hose cleaning. Silicone-insulated windings can be generally cleaned using the water hose method with a non-ionic, non-foaming detergent. 10.2.8 Drying and treatment considerations after cleaning $IWHUFOHDQLQJLWPD\EHQHFHVVDU\WRGU\RXWWKHZLQGLQJEHIRUHLWJRHVEDFNLQWRVHUYLFH7KLVFDQEHYHUL¿HG E\SHUIRUPLQJDQLQVXODWLRQUHVLVWDQFH,5 WHVWDOOZLQGLQJV DQGDSRODUL]DWLRQLQGH[3, WHVWIRUPZLQGLQJV ,IORZ,5DQG3,YDOXHVDUHREWDLQHGWKHQWKHZLQGLQJQHHGVWREHGULHGRXWLQDQRYHQVPDOOPDFKLQHV in a service shop), or for large machines, at site by hot air blowers or by passing a direct current through the



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ZLQGLQJ$IWHUGU\LQJLWLVRIWHQQHFHVVDU\WRVHDOWKHZLQGLQJE\UHVLQGLS93,VPDOOPDFKLQHVLQVHUYLFH shop), or by spraying end-windings with air-dry varnish/resin.



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Annex A (informative)

Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be unGHUVWRRGRUXVHGWRLPSOHPHQWWKLVVWDQGDUG5HIHUHQFHWRWKHVHUHVRXUFHVLVPDGHIRULQIRUPDWLRQDOXVHRQO\ >%@³$QDO\VLVRI(/&,'²+LJK)OX[7HVW)LHOG&RUUHODWLRQ´(35,7XUELQH*HQHUDWRU8VHUV*URXS:RUNVKRS%DUFHORQD±$SULO >%@$6700HWKRG'±5HFRPPHQGHG3UDFWLFHIRU([SRVXUHRI3RO\PHULF0DWHULDOVWR+LJK(QHUJ\ 5DGLDWLRQ:LWKGUDZQ  >%@$670'6WDQGDUG7HVW0HWKRGIRU'HWHFWLRQDQG0HDVXUHPHQWRI3DUWLDO'LVFKDUJH&RURQD  3XOVHVLQ(YDOXDWLRQRI,QVXODWLRQ6\VWHPV >%@$670'6WDQGDUG7HVW0HWKRGIRU9ROWDJH(QGXUDQFHRI6ROLG(OHFWULFDO,QVXODWLQJ0DWHULDOV6XEMHFWHGWR3DUWLDO'LVFKDUJHV&RURQD RQWKH6XUIDFH >%@$6700HWKRG'±&ODVVL¿FDWLRQ6\VWHPIRU3RO\PHULF0DWHULDOVIRU6HUYLFHLQ,RQL]LQJ5DGLDtion (Withdrawn). >%@$670'6WDQGDUG7HVW0HWKRGVIRU0HDVXUHPHQWRI(QHUJ\DQG,QWHJUDWHG&KDUJH7UDQVIHU'XH WR3DUWLDO'LVFKDUJHV&RURQD 8VLQJ%ULGJH7HFKQLTXHV >%@$670)6WDQGDUG6SHFL¿FDWLRQVIRU7HPSRUDU\3URWHFWLYH*URXQGVWR%H8VHGRQ'HHQHUJL]HG (OHFWULF3RZHU/LQHVDQG(TXLSPHQW >%@$670673(QJLQHHULQJ'LHOHFWULFV9RO,,$(OHFWULFDO3URSHUWLHVRI6ROLG,QVXODWLQJ0DWHULDOV 0ROHFXODU6WUXFWXUHDQG(OHFWULFDO%HKDYLRU%DUWQLNDVDQG(LFKKRUQ(GLWRUV $6703KLODGHOSKLD:HVW &RQVKRKRFNHQ3$SSíDQG >%@%DUWQLNDV5DQG50RULQ³$QDO\VLVRIPXOWLVWUHVVDFFHOHUDWHGDJHGVWDWRUEDUVXVLQJDWKUHHSKDVH WHVW DUUDQJHPHQW´ ,((( 7UDQVDFWLRQV RQ (QHUJ\ &RQYHUVLRQ YRO  SS ±  KWWSG[GRL RUJ7(& >%@%DUWQLNDV5DQG50RULQ³0XOWLVWUHVVDJLQJRIVWDWRUEDUVZLWKHOHFWULFDOWKHUPDODQGPHFKDQLFDO VWUHVVHVDVVLPXOWDQHRXVDFFHOHUDWLRQIDFWRUV´,(((7UDQVDFWLRQVRQ(QHUJ\&RQYHUVLRQYROSS± KWWSG[GRLRUJ7(& >%@%DZDUW0³,PSURYLQJFDEOHV\VWHPUHOLDELOLW\E\PRQLWRUHGZLWKVWDQGGLDJQRVWLFV²)HDWXULQJKLJK HI¿FLHQF\DWUHGXFHGWHVWWLPH´-,&$%/(WK,QWHUQDWLRQDO&RQIHUHQFHRQ,QVXODWHG3RZHU&DEOHV -XQH3DULV9HUVDLOOHV)UDQFH >%@%KLPDQL%:³9HU\ORZIUHTXHQF\KLJKSRWHQWLDOWHVWLQJ´$,((7UDQVDFWLRQV3DSHU± KWWSG[GRLRUJ$,((3$6 >%@%RJJV6$$.XPDGDDQG7




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>%@ 'DNLQ7: ³&RURQD 3XOVH 'HWHFWLRQ &LUFXLWV DQG7KHLU &DOLEUDWLRQ´$,((7HFKQLFDO 3DSHU 1R íSUHVHQWHGDWWKH$,((:LQWHU*HQHUDO0HHWLQJ1HZ%@'DNLQ7:&1:RUNVDQG-6-RKQVRQ³$Q(OHFWURPDJQHWLF3UREHIRU'HWHFWLQJDQG/RFDWLQJ 'LVFKDUJHVLQ/DUJH5RWDWLQJ0DFKLQH6WDWRUV´,(((7UDQVDFWLRQVRQ3RZHU$SSDUDWXVDQG6\VWHPVYRO 0DUSS±KWWSG[GRLRUJ73$6 >%@'LRQQH'DQG%RPEHQ6³%ODVWLQJ$ZD\WKH'LUW´+\GUR5HYLHZ-XQHSSí >%@'XNH&$6PLWK/(5REHUWV&$DQG&DPHURQ$::³,QYHVWLJDWLRQRIPDLQWHQDQFHWHVWV IRUJHQHUDWRULQVXODWLRQ´$,((7UDQVDFWLRQVRQ3RZHU$SSDUDWXVDQG6\VWHPVSW,,,YROSSí $XJKWWSG[GRLRUJ$,((3$6 >%@)LQGOD\'$%UHDUOH\5*DQG/RXWWLW&&³(YDOXDWLRQRIWKH,QWHUQDO,QVXODWLRQRI*HQHUDWRU &RLOV%DVHGRQ3RZHU)DFWRU0HDVXUHPHQWV´$,((7UDQVDFWLRQVYROSDUW,,,$-XQSSí KWWSG[GRLRUJ$,((3$6 >%@+DUUROG57(0)RUWDQG7$*RRGZLQ³7KH,QWHUSUHWDWLRQRI&RURQDDQG'LHOHFWULF0HDVXUHPHQWVRQWKH0LFD$VSKDOW,QVXODWLRQRID%@+HUPDQQ3.50DUKWDQG++'RRQ³'HWHFWLQJDQG/RFDWLQJ,QWHUWXUQ6KRUW&LUFXLWVRQ7XUELQH *HQHUDWRU5RWRUV´,(((7UDQVDFWLRQVRQ3RZHU$SSDUDWXVDQG6\VWHPVYRO2FWSS± KWWSG[GRLRUJ73$6 >%@+LOO*/³7HVWLQJHOHFWULFDOLQVXODWLRQRQURWDWLQJPDFKLQHU\ZLWKKLJKYROWDJHGF´$,((7UDQVDFWLRQV$,((7HFKQLFDO3DSHU± >%@,(((6WGŒ,(((*XLGHIRU2SHUDWLRQDQG0DLQWHQDQFHRI7XUELQH*HQHUDWRUV >%@,(((6WGŒ,(((6WDQGDUG7HVW3URFHGXUHIRU3RO\SKDVH,QGXFWLRQ0RWRUVDQG*HQHUDWRUV >%@,(((6WGŒ,(((*XLGHIRU2SHUDWLRQDQG0DLQWHQDQFHRI+\GUR*HQHUDWRUV >%@,(((6WGŒ5HDII 5HFRPPHQGHG3UDFWLFHIRUWKH'HWHFWLRQDQG0HDVXUHPHQWRI 3DUWLDO'LVFKDUJHV&RURQD 'XULQJ'LHOHFWULF7HVWV:LWKGUDZQ  >%@,(((6WGŒ,(((*XLGHIRU2QOLQH0RQLWRULQJRI/DUJH6\QFKURQRXV*HQHUDWRUV09$DQG Above). >%@,(((6WGŒ,(((5HFRPPHQGHG3UDFWLFHIRU4XDOLW\&RQWURO7HVWLQJRI([WHUQDO'LVFKDUJHVRQ Stator Coils, Bars, and Windings. >%@³,QVXODWLRQ7HVWLQJE\'&0HWKRGV´-DPHV*%LGGOH&RPSDQ\7HFKQLFDO3XEOLFDWLRQ7 >%@-RKQVRQ-6³,QVSHFWLRQ$QG0DLQWHQDQFH7HVWLQJRI+LJK9ROWDJH*HQHUDWRU:LQGLQJ´7UDQVDFWLRQVRIWKH$PHULFDQ,QVWLWXWHRI(OHFWULFDO(QJLQHHUVYROSW, >%@-RKQVRQ-6³3UHYHQWLYH0DLQWHQDQFH,QVSHFWLRQDQG7HVWLQJRI0RWRUVDQG*HQHUDWRUV´:HVWLQJKRXVH0DLQWHQDQFH1HZV:HVWLQJKRXVH(OHFWULF&RUSRUDWLRQ(DVW3LWWVEXUJK3$ YROQRYRO QRV,DQGYROQR



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Annex B (informative)

Thermosetting resins used in insulation systems B.1 Description of thermosetting resins 3RO\HVWHUV DUH WKH SRO\FRQGHQVDWLRQ SURGXFWV RI GLFDUER[\OLF DFLGV ZLWK GLK\GUR[\ DOFRKROV 8QVDWXUDWHG polyester resins usually contain three essential components: the polyester (usually a very viscous liquid), the monomer, and the inhibitor. Common monomers include styrene and vinyl toluene (low viscosity liquids), as well as others. During the cure, the monomer reacts with the unsaturated acid in the polyester chains to produce a cross-linked structure. As there is little volatile product, relatively void free structures can be produced. The inhibitor increases the shelf life but does not interfere with the subsequent polymerization when the mixture is heated. The term epoxy implies a ring containing one oxygen and two carbon atoms. Four common types of epoxy resins are shown in 7DEOH%: Table B.1—Common types of epoxy Resin

Description

Bisphenol-A

Most widely used.

Epoxy-novolacs

High temperature applications

Cycloaliphatics

*RRGPHFKDQLFDODQGGLHOHFWULFSURSHUWLHVDWHOHYDWHGWHPSHUDWXUHJRRGZHDWKHUDELOLW\

Aliphatic

Flexibilizing resin.

Epoxies are cured in one of the following two ways: a)

In catalytic curing, the epoxy molecules react with each other, initiated by a catalyst.

b)

In hardener initiated curing, the hardener reacts with the epoxy and becomes part of the cured material.



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Annex C (informative)

6WDWRUFRUHLQWHUODPLQDULQVXODWLRQKLJKÀX[ WHVWSURFHGXUH ,QFOXGHGLQWKLVDQQH[DUHVRPHKHOSIXOFRQVLGHUDWLRQVWRDLGHLQWKHSHUIRUPDQFHRIWKHKLJKÀX[WHVWLQJRIWKH laminar insulation in stator cores.

C.1 Design of the magnetizing coil ,QRUGHUWRWHVWWKHVWDWRUFRUHLQDKLJKÀX[WHVWLWLVQHFHVVDU\WRPDJQHWL]HWKHFRUHZLWKRXWURWRU DWDSSUR[LPDWHO\LWVQRUPDORSHUDWLQJSHDNÀX[RUDW7HVODOHYHOXVLQJDPDJQHWL]LQJFRLO7KHIROORZLQJLVSURYLGHG as guidance to set up this test: —

The turns of the magnetizing coil should encircle the stator through the main bore (after rotor is removed) and is normally routed around the outer frame. A preferable return route, if available, is near the outside diameter of the core within the frame. On large-diameter machines (such as waterwheel generators), the magnetizing coil should be distributed around the periphery of the stator to ensure XQLIRUPÀX[GLVWULEXWLRQDURXQGWKHHQWLUHFRUH$FOHDUDQFHRIFPWRFPVKRXOGEHPDLQWDLQHG EHWZHHQWKHPDJQHWL]LQJFRLOFRQGXFWRUDQGVROLGPHWDOPHWDOÀRRUIUDPHHWF WRUHGXFHVWUD\HGG\ currents.

C.2 Search coil $VLQJOHWXUQRI$:*WR$:*ZLUHV²LQVXODWHGDGHTXDWHO\IRUWKHYROWVSHUWXUQDSSOLHG²VKRXOGEH SODFHGDURXQGWKHFRUHSUHIHUDEO\GLDPHWULFDOO\RSSRVLWHIURPWKHPDJQHWL]LQJFRLO7KHDFWXDOFRUHÀX[GHQsity can be measured by placing the search coil so that it encircles only the core and does not include the frame PHPEHUV2QVRPHPDFKLQHVWKLVLVQRWSRVVLEOHDQGWKHHUURULQPHDVXUHGÀX[GHQVLW\PD\RUPD\QRWEH acceptable. An alternative is to route the search coil leads through air vents and adjust the voltage reading for the percent of laminations not included in the search coil loop. A voltmeter connected to the search coil will read approximately the volts per turn value calculated in C.3.

C.3 Calculation of the search coil voltage 7KHIROORZLQJFDOFXODWLRQVDUHSHUIRUPHGLQGHVLJQLQJWKHKLJKÀX[WHVW7KHFRQWUROOLQJWHVWSDUDPHWHUWKDW VHWVWKHFRUHÀX[LVWKHYROWVSHUWXUQDSSOLHG7KHYROWVSHUWXUQYDOXHIRUWKHVHDUFKFRLOLVFDOFXODWHGE\ (TXDWLRQ& :

Volts per Turn =

2π f φ

&

where 9ROWVSHU7XUQ LVWKHURRWPHDQVTXDUHYROWDJHRIDVLQJOHWXUQHQFRPSDVVLQJWKHFRUHLQYROWV f is the operating frequency in hertz I  LVWKHSHDNFRUHÀX[LQZHEHUV 2π is 4.443



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7KHRSHUDWLQJSHDNFRUHÀX[LQZHEHUVLVGHWHUPLQHGXVLQJEquation (C.2):

( D1 − D2 )

φ = B

2

(C.2)

Leffective

where B LVWKHSHDNVWDWRUFRUHLURQÀX[GHQVLW\LQWHVOD Leffective is the effective length of core in meters per Equation (C.3) D is the outside diameter of core in meters D2 is the inside diameter measured from the bottom of the stator slots in meters The effective length of core, Leffective should be obtained from the manufacturer. If that is not possible, the effective core length is determined using Equation (C.3): Leffective =

(L − N v bv ) SF

(C.3)

where Leffective is the effective length of core in meters L is the overall core length in meters Nv is the number of ventilation ducts bv is the width of a ventilation duct in meters SF LVWKHFRUHVWDFNLQJIDFWRUIURPPDQXIDFWXUHURUXVHWKHW\SLFDOYDOXHRI 7KHSHDNÀX[GHQVLW\LQWKHFRUH%VKRXOGEHREWDLQHGIURPWKHPDQXIDFWXUHU7KHWHVWOHYHOVKRXOGEHDJUHHG WRE\WKHFXVWRPHUDQGWKHPDFKLQHPDQXIDFWXUHU$ÀX[OHYHOIRUH[FLWDWLRQRIDSSUR[LPDWHO\RIWKHIXOO UDWHGYROWDJHDQGUDWHGIUHTXHQF\LVRIWHQXVHGZKLOH7HVODPD\EHXVHGIRUODUJHFRUHVGXHWRH[FLWDWLRQ limitations with other values possible if agreed upon by the customer and the machine manufacturer. If the SHDNÀX[GHQVLW\LVQRWSURYLGHGE\WKHPDQXIDFWXUHUWKHYROWVSHUWXUQHTXLYDOHQWWRWKHRSHUDWLQJÀX[DW of rated voltage and rated frequency can be determined by the calculation shown in Equation (C.4). Volts per Turn at 85% of Rated Flux

0.85 Vphase 2 K d K p (Turns per Phase)

(C.4)

where is the operating voltage on a phase in volts Vphase Kd is the Distribution Factor or Breadth Factor Kp LVWKH3LWFK)DFWRURU&KRUGLQJ)DFWRU Turns per Phase is the number of turns in series per one phase per parallel in the stator winding )RUDWKUHHSKDVHZ\H< FRQQHFWHGPDFKLQH Vphase

Vline-line

(C.5)

3

where Vphase Vline-line

is the operating voltage on a phase in volts is the line-to-line operating voltage in volts



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7KH3LWFK)DFWRURU&KRUGLQJ)DFWRUKp, is determined by Equation (C.6): ⎛ Coil Pitch × Rotor Poles π ⎞ K p = sin ⎜ × ⎟ Slots 2⎠ ⎝

(C.6)

where Kp

LVWKH3LWFK)DFWRURU&KRUGLQJ)DFWRU

Core Pitch Rotor Poles Slots π

is the stator winding pitch, the number of slots spanned by a single coil of the winding is the number of poles in the rotor is the number of slots in the stator LV

7KH3LWFK)DFWRUIRUDSLWFKZLQGLQJDFRPPRQSLWFK LV The Distribution Factor or Breath Factor, Kd, is determined by (TXDWLRQ& : ⎛ ⎛ ⎞ ⎞ π ⎜ sin ⎜ ⎟ ⎟ ⎝ 2 Phases ⎠ ⎟ Kd = ⎜ ⎜ ⎛ ⎞⎟ π ⎜ N sin ⎜ ⎟ ⎟⎟ ⎜ ⎝ 2 N Phases ⎠ ⎠ ⎝

&

where Kd Phases N π

is the Distribution Factor or Breadth Factor is the number of phases in the stator winding LVWKHQXPHUDWRURI(TXDWLRQ& LV

For fractional slot windings, the numerator N must be determined where N and the denominator, D, in the following ratio has no common divisor: N Slots = D ( Rotor Poles )( Phases )

&

where N is the numerator where N and D have no common divisor D is the denominator where N and D have no common divisor Phases is the number of phases in the stator winding Rotor Poles is the number of poles in the rotor Slots is the number of slots in the stator In the particular case of an integer slot winding, the value of DLV7KHW\SLFDO'LVWULEXWLRQ)DFWRUIRUDFRPPRQZLQGLQJZLWKGHJUHHSKDVHEHOWVLV7KHWXUQVSHUSKDVHVKRXOGEHREWDLQHGIURPWKHPDQXIDFturer, or it can be determined by (TXDWLRQ& : Turns per Phase

(Slots) (Turns per Coil) (Parallels) (Phases)

&



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where Turns per Phase is the number of turns in series per one phase belt in the stator winding Slots is the number of slots in the stator Turns per Coil is the number of turns in a coil Parallels is the number of parallel windings in a phase Phases is the number of phases in the stator winding

C.4 Calculation of the search coil voltage in CGS units 7KHFDOFXODWLRQVLQ&*6DUHLQFOXGHGIRUKLVWRULFDOSXUSRVHVEHFDXVHPDQ\PDFKLQHVWKDWDUHVWLOOLQRSHUDWLRQKDYHEHHQGHVLJQHGXVLQJWKHVH,Q&*6XQLWV(TXDWLRQ& is changed to the following: Volts per Turn =

2π f φ 10-8

&

where Volts per Turn is the root mean square voltage of a single turn encompassing the core in volts f is the operating frequency in hertz I LVWKHSHDNFRUHIOX[LQPD[ZHOOV 2π is 4.443 7KHRSHUDWLQJSHDNFRUHÀX[LQPD[ZHOOVLVGHWHUPLQHGXVLQJ(TXDWLRQ& :

φ = B

( D1 − D2 ) 2

&

Leffective

where

I  LVWKHSHDNFRUHÀX[LQOLQHV B  LVWKHSHDNFRUHÀX[GHQVLW\LQOLQHVSHUVTXDUHLQFKIURPWKHPDQXIDFWXUHU D is the outside diameter of core in inches D2 is the diameter to bottom of stator slots in inches Leffective LVWKHHIIHFWLYHOHQJWKRIFRUHLQLQFKHVSHU(TXDWLRQ& The effective length of core should be obtained from the manufacturer. If that is not possible, the effective core length is determined using (TXDWLRQ& :

Leffective = (L − N v bv ) SF

&

where Leffective is the effective core length in inches L is the overall core length in inches N v is the number of ventilation ducts bv is the width of a ventilation duct in inches SF LVWKHFRUHVWDFNLQJIDFWRUIURPPDQXIDFWXUHURUXVHWKHYDOXHRI To convert between the two systems of units the following should be used: 7HVOD OLQHVLQ2



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PHWHU LQ

C.5 Calculation of the magnetizing coil turns and amperage From the known supply of voltage and the volts-per-turn value previously noted, the number of turns for the magnetizing coil can be determined by direct division. The result should be rounded to the next higher integer. 7KLVQXPEHURIWXUQVVKRXOGEHXVHGLQWKH¿UVWWULDOWHVW In order to determine the size of the cable necessary for the magnetizing coil, data on ampere-turns per meter RIPHDQEDFNLURQSHULSKHU\FRUUHVSRQGLQJWRWKHFRUHÀX[GHQVLWLHVZLOOEHUHTXLUHG7KHVHGDWDVKRXOGEH obtained from the manufacturer. The magnetizing-coil current requirement is given by (TXDWLRQ& :

It =

ATM ⎛ D1 + D2 ⎞ ⎜ ⎟π Nt ⎝ 2 ⎠

&

where I t is magnetizing coil current in amperes ATM is ampere-turns per meter obtained from manufacturer N t is number of turns D is outside diameter of core in meters D2 is diameter to bottom of stator slots in meters

π LV This is the magnetizing current. For a more accurate estimation of current requirements, the watts loss current VKRXOGEHGHWHUPLQHGDVZHOO7KHVHWZRFXUUHQWVFDQWKHQEHDGGHGDVYHFWRUVZLWKGHJUHHSKDVHDQJOH between them by (TXDWLRQ& : I exc =

I t2 + I w2

&

where I t is magnetizing coil current in amperes I w is watts loss current in amperes Using the results from (TXDWLRQ& , the approximate minimum conductor area can be determined. Additional safety factors should be considered for suitable cable sizing to avoid overheating of cables and risk of damages.

C.6 Temperature measurements The magnetizing coil should be located remote from the areas suspected as damaged in order to facilitate temSHUDWXUHPHDVXUHPHQW7KLQVKDYLQJVRISDUDI¿QWKHUPRPHWHUVDI¿[HGZLWKVXLWDEOHSXWW\WKHUPRFRXSOHV SRUWDEOHS\URPHWHUVRULQIUDUHGFDPHUDVFDQEHXVHGWRGHWHFWKRWVSRWV7KHVHVKRXOGEHGHWHFWDEOHLQVWR VLIWKHORZLQWHUODPLQDUUHVLVWDQFHLVORFDWHGDWRUQHDUWKHH[SRVHGVXUIDFH,IWKHORZLQWHUODPLQDUUHVLVWDQFHLVUDGLDOO\RXWZDUGIURPWKHWRRWKVXUIDFHRULQFRUHDUHDVEHORZWKHERWWRPRIVWDWRUVORWVWRRU



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more minutes of excitation may elapse before the heat becomes evident at the exposed surfaces. If repairs are UHTXLUHGD¿QDOKHDWUXQVKRXOGEHPDGHDIWHUDOOUHSDLUVDUHFRPSOHWHG6HH for considerations of the test conditions and duration. It is recommended that the original equipment manufacturer be consulted for guidance on appropriate hotspot criteria, time duration, temperature, and testing condition recommendations for the particular machine.



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Annex D (informative)

Stator core low energy (EL CID) test The condition of the interlaminar resistance between stator laminations (punchings) of a machine core is often best evaluated by means of magnetic excitation of the core. This procedure describes a method of accomplishLQJWKLVXVLQJORZÀX[GHQVLWLHVORZSRZHUUHTXLUHPHQWVDQGVKRUWVHWXSWLPH7KHSURFHGXUHKDVWKHDGGLWLRQDOEHQH¿WRISURGXFLQJDSHUPDQHQWUHFRUGRIWKHFRQGLWLRQRIWKHLQWHUODPLQDUFRUHLQVXODWLRQ 7KHSULQFLSOHXQGHUO\LQJWKLVPHWKRGLVWKDWPHDVXUDEOHFXUUHQWVZLOOÀRZWKURXJKIDLOHGRUVHYHUHO\GHWHULRUDWHGLQWHUODPLQDULQVXODWLRQZKHQDÀX[RIRQO\DIHZSHUFHQWRIWKHUDWHGYDOXHLVLQGXFHGLQWKHFRUH

D.1 Discussion $ZHDNPDJQHWLF¿HOGWRRIQRPLQDOÀX[DUHW\SLFDOO\XVHG LVLQGXFHGLQWKHFRUHXVLQJDQH[FLWDWLRQ ORRSFRQVLVWLQJRIDIHZWXUQVRIVPDOOORZYROWDJHFDEOH7KHPDJQHWLFH[FLWDWLRQ¿HOGLVLQDFLUFXPIHUHQWLDO pattern around the stator bore, and is to be the datum phase to which all other quantities are referenced. This H[FLWDWLRQ¿HOGLQGXFHVFXUUHQWVWRÀRZEHWZHHQODPLQDWLRQVZLWKZHDNHQHGLQVXODWLRQ7KHVHUHVXOWDQWHGG\ FXUUHQWVGXHWRWKHLQWHUODPLQDULQVXODWLRQGHIHFWVDUHGHWHFWHGXVLQJD&KDWWRFNRU5RJRZVNLW\SHSLFNXS coil, which is also known as Maxwell’s worm. When such a coil is placed across two core teeth, the voltage induced by the fault current is approximately SURSRUWLRQDOWRWKHOLQHLQWHJUDODORQJLWVOHQJWK,IWKH¿HOGLQWKHFRUHLVLJQRUHGWKHYROWDJHRXWSXWRIWKHFRLO LVSURSRUWLRQDOWRWKHHGG\FXUUHQWÀRZLQJLQWKHDUHDHQFRPSDVVHGE\WKHSLFNXSFRLOWKHWZRWHHWKLWVSDQV DQGWKHFRUHEHKLQGWKHVH8QIRUWXQDWHO\GXHWRWKHFLUFXPIHUHQWLDOPDJQHWLF¿HOGFRPSRQHQWUHVXOWLQJIURP WKHH[FLWDWLRQFRLOWKHRXWSXWRIWKH5RJRZVNLW\SHFRLOFDQQRWEHXVHGGLUHFWO\WRLQGLFDWHWKHFRQGLWLRQRI WKHFRUHLQVXODWLRQ+RZHYHUWKHHGG\FXUUHQWVGXHWRWKHIDXOWVUHVXOWLQÀX[HVZKLFKDUHSKDVHVKLIWHGZLWK UHVSHFWWRWKHUHIHUHQFHÀX[&RQVHTXHQWO\WKHFRPSRQHQWRIWKHH[FLWDWLRQÀX[PHDVXUHGE\WKH5RJRZVki-type coil can be eliminated to produce a voltage that is proportional to the axial component of the eddy FXUUHQW7KHUHIHUHQFHSKDVHDQJOHLVGH¿QHGE\WKHH[FLWDWLRQFXUUHQWSKDVHDQJOH7KHTXDGUDWXUHFXUUHQWLV WKHQGH¿QHGDVDFXUUHQWWKDWLVGHJUHHVIURPWKHH[FLWDWLRQFXUUHQW 7KHRXWSXWVIURPWKH5RJRZVNLW\SHDQGUHIHUHQFHFRLOVDUHIHGWRDVLJQDOSURFHVVLQJXQLWZKLFKSHUIRUPV the excitation voltage component elimination and provides an output of the axial eddy currents detected in TXDGUDWXUHWRWKHH[FLWDWLRQFXUUHQWE\WKH&KDWWRFN5RJRZVNLW\SHFRLOLQPLOOLDPSHUHV,IWKHVWDWRUFRUH LQVXODWLRQKDVEHHQGDPDJHGUHODWLYHO\KLJKTXDGUDWXUHFXUUHQWUHDGLQJV!P$ ZLOOUHVXOW

D.2 Test setup and procedure An excitation loop should be pulled through the bore and around the outside of the stator frame. One advantage of this test over a traditional high power test is that the cable used for the excitation loop is low voltage and typically about 2 mm to 4 mm in diameter. The wires constituting the loop should be installed along the central axis of the bore, rather than letting the wires be in contact with the stator core. The core of the machine XQGHUWHVWLVH[FLWHGZLWKDZHDNPDJQHWLFÀX[LQUDQJHRIWRRIQRPLQDOÀX[7\SLFDOO\RI QRPLQDOÀX[LVXVHGIRUFRQVLVWHQF\RIHYDOXDWLRQ,IDQDOWHUQDWHYDOXHLVXVHGWKHUHVXOWVDUHVFDOHGOLQHDUO\ WRWKHOHYHO&RQVHTXHQWO\WKHH[FLWLQJFRLOSDUDPHWHUVQXPEHURIWXUQVDQGFURVVVHFWLRQ KDYHWREH calculated based upon the size of the core. Furthermore, it is customary to install a separate single turn coil to PHDVXUHWKHDFWXDOLQGXFHGÀX[DFKLHYHG



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3ULRUWRFRPPHQFHPHQWRIWKHWHVWWKHVWDWRUVKRXOGEHLQVSHFWHGIRUDQ\FRQGXFWLYHPDWHULDOZKLFKZRXOG VKRUWWKHODPLQDWLRQVWRJHWKHUDQGWKH&KDWWRFN5RJRZVNLW\SHFRLOVKRXOGEHDGMXVWHGWRULGHVPRRWKO\DQG IUHHO\RQWZRWHHWKEXWVKRXOGEHSUHYHQWHGIURPZREEOLQJRUELQGLQJ5HFRPPHQGHGSUDFWLFHDOVRLQFOXGHV numbering the core teeth to provide an easy means of referencing any faults located. 7KH5RJRZVNLW\SHFRLOLVFRQVWUXFWHGIURPPDQ\WXUQVRI¿QHZLUHZRXQGRQDÀH[LEOH8VKDSHGPDJQHWLF core. The number of turns per unit length and cross-sectional area of the core are kept constant so that a calibrated output from the coil can be obtained. For this reason, during operation the tips of the coil should be maintained uniformly close to the core iron and at the same distance as when calibrated. This is critical to the proper interpretation of the results. Care must be taken when measurements are made over any steps in the core, such as vents and the step iron region. 2QFHDOORIWKHVHUHTXLUHPHQWVKDYHEHHQPHWWKH5RJRZVNLW\SHFRLOFDQEHVHWRYHUWKHVORWDQGWKHFRPSOHWH slot is scanned with the current readings being observed or recorded. This procedure is repeated with each slot LQWXUQXQWLOWKHHQWLUHFRUHRUDVHOHFWHGSRUWLRQRILWKDVEHHQWHVWHG7KH¿UVWRQHWRWKUHHVORWVVFDQQHGPD\ EHUHWHVWHGIRUYHUL¿FDWLRQSXUSRVHV

D.3 Search coil voltage calculation There are several methods commonly used for determining the search coil voltage that is required to achieve WKHGHVLUHGÀX[OHYHOIRUWKH(/&,'WHVW7KHGLIIHUHQFHVDUHLQWKHVLPSOL¿FDWLRQVXVHGWRDSSUR[LPDWH the search coil value. 7KHVHDUFKFRLOYROWDJHFDQEHFDOFXODWHGDVRIWKHKLJKÀX[YROWDJHFDOFXODWHGE\RQHRIWKHPHWKRGVLQ Annex C. VSearch

'

K test Volts per Turn

where Volts per Turn LVWKHKLJKÀX[WHVWYROWVSHUWXUQDVGHWHUPLQHGLQ$QQH[& K test LVWKHW\SLFDO(/&,'WHVWOHYHORI Alternatively, the following can be used for a 5/6 pitch, three-phase, wye winding to calculate the search coil voltage: ⎛ 0.313 (Vline-line ) (Parallels)(Phases) ⎞ VSearch = K test ⎜ ⎟ (Slots) (Turns per Coil) ⎝ ⎠

(D.2)

where is the single turn search coil voltage in volts VSearch K test LVWKHW\SLFDO(/&,'WHVWOHYHORI Vline-line is the line to line operating voltage in volts Parallels is the number of parallel windings in a phase Phases is the number of phases in the stator winding Slots is the number of slots in the stator Turns per Coil is the number of turns in a coil The constant in Equation (D.2)LVGHWHUPLQHGIURPWKHIROORZLQJ



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0.313 #

1

(D.3)

2 3 Kd K p

where K d is the Distribution Factor or Breadth Factor as determined in Annex C K p LVWKH3LWFK)DFWRURU&KRUGLQJ)DFWRUDVGHWHUPLQHGLQ$QQH[& $VDJXLGH9PLVDW\SLFDOYDOXHIRUPRVWWXUERJHQHUDWRUPDFKLQHV)RUK\GURJHQHUDWRUV9WR9FDQ EHW\SLFDO7KHXVHRIDVWDQGDUGYROWDJH9P UHGXFHVWKHGLI¿FXOW\RIGHWHUPLQLQJWKHPDFKLQHSDUDPHWHUV DQGFDQPDNHFRPSDULVRQVIURPXQLWWRXQLWVWUDLJKWIRUZDUGKRZHYHULWFDQUHVXOWLQDÀX[GLIIHUHQWWKDQWKH GHVLUHGGHSHQGLQJRQWKHPDFKLQHGHVLJQ)RUWKHPRVWDFFXUDWHDSSOLFDWLRQRIWKHWHVWWKHÀX[OHYHO VKRXOGEHRIWKHPDFKLQHUDWHGÀX[GHQVLW\DWWKHEDFNRIWKHFRUHDWQRORDG,IWKHDSSOLHGÀX[GHQVLW\ GHYLDWHVWKHQWKHUHVXOWVVKRXOGEHQRUPDOL]HGWRWKHOHYHO7KH9PRIFRUHOHQJWKPD\RYHUHVWLPDWH WKHÀX[OHYHOIRUVPDOORUVKRUWPDFKLQHVDQGPD\XQGHUHVWLPDWHLWIRUVRPHPDFKLQHV

D.4 Interpretation This test has high sensitivity; hence it can detect magnetic disturbances which may not prejudice the reliability RIWKHVWDWRU&RQVHTXHQWO\LQWHUSUHWDWLRQRIWKHUHVXOWVLVQRWVLPSOHDQGWKHUHPD\EHVRPHGLI¿FXOW\LQGHWHUmining an appropriate level of response that warrants further investigation and/or repair. In general, responses RIJUHDWHUWKDQP$DWRIUDWHGÀX[VKRXOGEHUHJDUGHGDVVLJQL¿FDQWIDXOWVH[SHFWHGWHPSHUDWXUHULVH IRUHDFKP$RIIDXOWFXUUHQWPHDVXUHGLVƒ&WRƒ& DQGVKRXOGEHIXUWKHULQYHVWLJDWHG The correct detection of a fault signature requires consideration of the polarity of the excitation phase and the maintenance of a consistent orientation of the Chattock coil in the core. The polarity of the excitation phase VLJQDOUHODWLQJWRWKHWRURLGDOH[FLWDWLRQÀX[ LVVHWRULQYHUWHGE\RULHQWDWLRQRIH[FLWDWLRQZLQGLQJVRULHQWDWLRQRIUHIHUHQFHFRLODQGWKHRULHQWDWLRQRIWKH&KDWWRFNFRLOLQWKHFRUH5HYHUVDORIDQ\RQHRIWKHVHZLOO invert the excitation phase signal. There is no requirement to achieve any particular excitation phase polarity, though a convenient convention is to always set the excitation and Chattock coil orientation to give a positive excitation phase signal. The polarity of the quadrature signal indicating an interlamination insulation fault is strictly dependent on the direction/polarity of the excitation phase. For a fault that is within the span of the Chattock coil, the quadrature signal must be the opposite polarity of the excitation phase current. Typically, when the fault is just outside the span of the Chattock coil the signal will be in the same polarity as the excitation phase current. Indications are a result of losses from eddy currents in the circuit, false indications of shorts can be attributed to material differences in the electrical steel. It should be recognized that no reading will be obtained at a fault location if the electrical circuit is not completed elsewhere—i.e., no electrical contact between laminations and building bars. It should be noted that such a fault will not create a hot spot in normal operation if the electrical circuit is not completed. Some machines are insulated at the back of the core so that the circuit will not be completed. However, it also should EHUHFRJQL]HGWKDWWKHH[FLWDWLRQGXULQJWKLVWHVWLVVLJQL¿FDQWO\GLIIHUHQWWKDQWKDWRIQRUPDORSHUDWLRQGXHWR thermal and mechanical duty on the core during operation, and this could lead to shorting that is not seen in this low energy test.



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Annex E (informative)

Machine condition visual inspection appraisal—Checklist Component

Status

Condition

Deterioration or Damage

NONE/ SLIGHT/ MAJOR

Armature windings (stationary ac, rotating dc) A

3XII\FRLOV

B

Soft insulations

C

*LUWKFUDFNLQJ

D

Separation in groundwall

E

Bond cracks at slot ends

F

Bond cracks into slots (wedge removed for inspection)

*

Contamination of coil or connection surfaces (carbon dust, dirt, oil)

H

Abrasion damage from chemicals, abrasives, or foreign materials

I

Cracks/abrasion from mechanical forces, coil movement

-

Loose bracing structure

K

Corona damage (white, gray, or red deposits)

L

/RRVHZHGJHVRUVORW¿OOHUV

M

Distorted windings, coils, or commutator risers

N

Loose restraint bands

O

Cracked restraint bands

3

Uneven color commutator bars

4

(YLGHQFHRURFFXUUHQFHRIÀDVKRYHU

5

Evidence of bar faults or band burning at walls of glass-band grooves

Field windings (rotating ac, stationary dc) A

3XII\FRLOV

B

Soft insulations

C

Contamination of coil, collector, banding, or connection surfaces (carbon dust, dirt, oil)

D

Abrasion damage from chemicals, abrasives, or foreign materials

E

Cracks/abrasion from mechanical forces, coil movement

F

Loose wedges

*

Distortion of coils

H

Shrinkage or looseness of coils, washers, or pads from poles

I

Loose connections

-

Heating of wedges

K

Cracks in retaining rings

L

Loose end-winding blocking

M

3RZGHUHGLQVXODWLRQVLQDLUGXFWV

N

5HGR[LGHDWPHWDOOLFMRLQWV



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Field windings (rotating ac, stationary dc) O

Loose collector or collector leads

Brush rigging A

(YLGHQFHRURFFXUUHQFHRIÀDVKRYHU

B

Carbonized leakage paths

C

Loose parts

D

Carbon dust accumulation

Cores A

Evidence or occurrence of rub or impact damage (rotor rub or objects in air gap)

B

Burned punchings at bore surface

C

Heating of adjacent punchings

D

Loose or broken vent duct separators

E

Core looseness

F

+HDWLQJRIHQG¿QJHUSODWHV

*

/RRVHRUEURNHQODPLQDWLRQVDWFODPSLQJÀDQJHV

H

/RRVHRUEURNHQHQG¿QJHUSODWHV

I

Core buckling

Insulated through bolts A

Contamination

B

Looseness

C

Broken or cracked insulating washers

Bearing insulation A

Cracks

B

Distortion, evidence of excessive heating

C

Oxidized or corroded conductors/strands

D

Loose connections



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