ieee std 1799-2012 -ieee recommended practice for quality control testing of external discharges on...

5/24/2018 IEEE Std 1799-2012 -IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Windings

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IEEE Recommended Practice forQuality Control Testing of ExternalDischarges on Stator Coils Bars andWindings

Sponsored by the

Electric Machinery Committee

IEEE3 Park AvenueNew York, NY 10016-5997USA

30 November 2012

IEEE Power and Energy Society

IEEE Std 1799-2012


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IEEE Std 1799-2012

IEEE Recommended Practice forQuality Control Testing of ExternalDischarges on Stator Coils, Bars, andWindings

Sponsor

Electric Machinery Committeeof the

IEEE Power and Energy Society

Approved 19 October 2012

IEEE-SA Standards Board


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Abstract:The procedure for quality control testing of external discharges on stator coils, barsand windings of large air-cooled ac electric machines is described in this recommended practice.Keywords: ac, corona-imaging instrument, discharge inception voltage, electrical insulation,external discharges, IEEE 1799, stator winding, ultraviolet radiation

The Institute of Electrical and Electronics Engineers, Inc.3 Park Avenue, New York, NY 10016-5997, USA

Copyright 2012 by The Institute of Electrical and Electronics Engineers, Inc.All rights reserved. Published 30 November 2012. Printed in the United States of America.

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PDF: ISBN 978-0-7381-7593-5 STD97305Print: ISBN978-0-7381-7670-3 STDPD97305

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Copyright 2012 IEEE. All rights reserved.iv

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Copyright 2012 IEEE. All rights reserved.v

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Copyright 2012 IEEE. All rights reserved.vi

Participants

At the time this IEEE recommended practice was completed, the P1799 Working Group had the following

membership:

Remi Tremblay,ChairClaude Hudon, Secretary

David AgnewKevin AlewineRaymond Bartnikas

Kevin BackerStefano BombenAndy BrownDonald CampbellWilliam Chen

Doug ConleyIan CulbertJeffrey Fenwick

Shawn FillibenNancy FrostPaul Gaberson

Michel GagnBal GuptaGary Heuston

Richard HuberMarcelo JacobAleksandra JeremicAleksandr KhazanoyAmir Khosravi

Thomas KlamtInna KremzaLaurent Lamarre

Gerhard LemeschRimma MalamudWilliam McDermid

David McKinnon

Charles MilletGlenn MottersheadBeant Nindra

Sophie NoelRamtin OmranipourHoward PenroseHelene ProvencherEmad Sharifi

John SchmidtJeffrey SheafferReza Soltani

Gregory StoneChuck WilsonHugh Zhu

The following members of the individual balloting committee voted on this recommended practice.

Balloters may have voted for approval, disapproval, or abstention.

Michael AdamsDavid AgnewMartin Baur

Thomas BishopStefano BombenSteven BrockschinkChris Brooks

Donald CampbellWeijen ChenIan CulbertMatthew Davis

Ray DavisGary DonnerGary Engmann

Jeffrey FenwickJorge Fernandez DaherSudath Fernando

Rostyslaw FostiakPaul Gaberson

Michel Gagn

Randall GrovesBal GuptaWerner Hoelzl

Claude HudonInnocent KamwaJim KulchiskyChung-Yiu Lam

Benjamin LanzWilliam LockleyGreg LuriRimma Malamud

William McBrideWilliam McCownWilliam McDermid

David McKinnonDon McLarenJames Michalec

G. Harold MillerCharles Millet

Jerry Murphy

Arthur NeubauerMichael S. NewmanWilliam Newman

Sophie NoelLorraine PaddenChristopher PetrolaAlvaro Portillo

Iulian ProfirBartien SayogoJohn SchmidtJeffrey Sheaffer

Gil ShultzReza SoltaniGary Stoedter

Gregory StoneJames TimperleyRemi Tremblay

John VergisKenneth White

Hugh Zhu


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Copyright 2012 IEEE. All rights reserved.vii

When the IEEE-SA Standards Board approved this recommended practice on 19 October 2012, it had the

following membership:

Richard H. Hulett, ChairJohn Kulick, Vice Chair

Robert M. Grow,Past Chair

Konstantinos Karachalios,Secretary

Satish Aggarwal

Masayuki AriyoshiPeter BalmaWilliam Bartley

Ted BurseClint ChaplinWael DiabJean-Philippe Faure

Alexander Gelman

Paul HouzJim HughesYoung Kyun Kim

Joseph L. Koepfinger*John KulickDavid J. LawThomas Lee

Hung Ling

Oleg Logvinov

Ted OlsenGary RobinsonJon Walter Rosdahl

Mike SeaveyYatin TrivediPhil WinstonYu Yuan

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Richard DeBlasio,DOE RepresentativeMichael Janezic,NIST Representative

Julie AlessiIEEE Standards Program Manager, Document Development

Malia Zaman

IEEE Standards Program Manager, Technical Program Development


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Copyright 2012 IEEE. All rights reserved.viii

Introduction

This introduction is not part of IEEE Std 1799-2012, IEEE Recommended Practice for Quality Control Testing ofExternal Discharges on Stator Coils, Bars, and Windings.

External discharges in the end-windings are caused by inadequate workmanship for globally vacuum-pressure impregnated (VPI) stators or problems on stators assembled on-site. Poorly finished lashes with aninsufficient gap between bars produces coil-to-coil or bar-to-bar discharges. Misalignment between

adjacent coils or bars may also reduce the gap distance and generate a high electric stress larger than the air

breakdown strength. Sometimes misplaced resistance temperature detector (RTD) or air gap monitor leads

have been seen to cause partial discharges (PDs) with high-voltage bars or coils. External discharges for the

individual coil/bar could also be a result of improper design, improper material, or improper workmanship.

After many years, the deterioration induces surface degradation that may lead, in the long run, to a phase-to-ground fault and reduce the overall reliability of the system. More detail on the theory of external

discharges and their effects is given in Annex A. Some utilities have seen deterioration of the junctionbetween the stress control coating and semiconducting slot coating of stator windings after only a few years

of operation. Other secondary effects, such as the production of a large quantity of ozone, which may be

deleterious to the equipment and dangerous to personnel, is also of concern. In addition, over the years, the

ground-wall insulation thickness of stator coils and bars has been reduced to improve heat transfer throughthe ground-wall insulation. This optimization does, however, increase the dielectric stress on the insulation

and on the end-winding stress grading system making them more susceptible to developing electrical

discharges.

In the current recommended practice, the term semiconducting slot coating is preferred to

semiconductive slot coating often used in the industry. These coatings, composed of resin, varnish,

enamels, or other compounds, are filled with carbon black powder, graphite, or other filler and should haveelectrical resistivity per unit of surface of 1 102 5 105Ohms per square. The semiconducting slot

coating applied on the insulation surface of the slot parts of winding must have uniform tight contacts with

the grounded walls of the stator slot. This coating provides minimum voltage between the surface of the

coil or bar and the grounded stator core.

A stress control coating must be applied on the end turns of high-voltage stator winding and overlap thesemiconducting slot coating to provide electrical contact between them. The stress control coating has a

non-linear resistance with voltage.

This recommended practice presents two methods for evaluating the quality of materials and design,factory workmanship, and on-site workmanship. The first one, the blackout test, has been used for many

years. The second one, the corona-imaging inspection, is more recent and presents several advantages.Each method has its advantages and disadvantages.

IEEE Std 1434 mentions these two inspection methods but with very little detail. The current recommended

practice includes a more elaborate description of sample preparation, bench tests, test conditions, andacceptance criteria in the factory and on-site.


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Copyright 2012 IEEE. All rights reserved.ix

Contents

1. Overview.................................................................................................................................................... 11.1 Scope ................................................................................................................................................... 21.2 Purpose ................................................................................................................................................ 2

2. Normative references.................................................................................................................................. 23. Definitions.................................................................................................................................................. 34. Test preparation and safety......................................................................................................................... 55. Test equipment and connections................................................................................................................. 5

5.1 Sensitivity of the corona-imaging instruments .................................................................................... 66. Quality control test of external discharges with corona-imaging instrument or blackout test .................... 7

6.1 Factory test on coils and bars............................................................................................................... 86.2 Stator model test ................................................................................................................................ 106.3 Test on fully assembled stator windings............................................................................................ 14

7. Data records.............................................................................................................................................. 207.1 How to fill the data logging tables..................................................................................................... 20

Annex A (informative) Theory of optical emissions from external discharges............................................ 24Annex B (informative) Variability of discharge inception and extinction voltages ..................................... 27Annex C (informative) Example of determination of the maximum voltage for a specific

winding diagram........................................................................................................................................... 28Annex D (informative) Example of correction factor to apply to the test voltage of a stator modeland VPI stator for a machine which will operate at altitudes of more than 1000 m.....................................31Annex E (informative) Example of operating-voltage table and bar/coil identification table used

during test..................................................................................................................................................... 33Annex F (informative) Bibliography............................................................................................................ 35


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Copyright 2012 IEEE. All rights reserved.1

IEEE Recommended Practice forQuality Control Testing of ExternalDischarges on Stator Coils, Bars, andWindings

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

This quality control test is used to confirm that the insulation system of the stator winding of generator and

motor operating in air, including the semiconducting slot and stress control coatings, are free of external

discharges. Quality control of the semiconducting slot coating, stress control coating, and manufacturingprocess is best done in the factory. For stators assembled on-site, such as those for large hydro-generators,

additional tests can be performed on the fully assembled generator in order to control the quality of the

assembly and workmanship. This control includes:

a) evaluation of the spacing between end-arms and with the phase circuit rings or connections to themain phase terminals

b) confirming proper alignment of the ground plane made by the semiconducting slot coating on thestraight portion of the bar/coil with regard to the core pressure finger

c) the positioning of all cables (RTD, air gap monitor) with respect to high voltage and

d) inspection of imperfections that may have been introduced during assembly (presence of foreignobjects, misplaced slot center filler, chips and scratches to bars or coils coating)

In the case of machines assembled in the factory, such as VPI machines, the complete quality control test

can be done in the factory. However, special care should be taken so that no change in the machines
http://standards.ieee.org/IPR/disclaimers.htmlhttp://standards.ieee.org/IPR/disclaimers.html


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IEEE Std 1799-2012IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings


conditions occur during transportation (contamination by water and dust, or damage to end-arms during

movement). The use of this recommended practice may eliminate the need for users to specify minimum

clearances between bars/coils in the end-winding to avoid surface discharge activity.

1.1 Scope

This recommended practice provides a procedure to detect external discharges in form-wound bars and

coils and complete stator windings of rotating machines operating in air with a rated line-to-line voltage

greater than 4200 V at power frequency. The recommended practice is applicable to bars, coils, and

complete stator windings. The recommended practice covers two inspection methods: the visual blackouttest, and the use of corona imaging instruments.

1.2 Purpose

The purpose of this recommended practice is to suggest specimen preparation, test parameters, andprocedures for detecting external discharges associated with bars, coils, and complete stator windings using

the above mentioned methods. It also recommends acceptance criteria and a procedure for retest in theevent of a test failure.

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 ofthe referenced document (including any amendments or corrigenda) applies.

IEC 60204-1, Safety of machineryElectrical equipment of machinesPart 1: General requirements.

IEC 61508, Functional safety of electrical/electronic/programmable electronic safety-related systems.

IEEE Std 4, IEEE Standard Techniques for High-Voltage Testing.1, 2

IEEE Std 4a, Amendment to IEEE Standard Techniques for High-Voltage Testing.

IEEE Std 510-1983 (Withdrawn), Recommended Practice for Safety in High-Voltage and High-Power

Testing.3

ISO 14121-1, Safety of MachineryRisk AssessmentPart 1: Principles.

ISO/TR 14121-2, Safety of MachineryRisk AssessmentPart 2: Practical Guidance and Examples of

Methods 2.

1The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc.2IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,

USA (http://standards.ieee.org/

).3IEEE Std 510-1983 has been withdrawn; however, copies can be obtained from The Institute of Electrical and Electronics Engineers,

445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/).


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3. Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE Standards

Dictionary Onlineshould be consulted for terms not defined in this clause.4

blackout test: A test performed after eliminating all ambient light, specifically on energized electricalequipment, to detect external or surface discharges visible with the human eye (naked eye) after at least 15

min of acclimatization.

conducting materials:Composition materials which usually have a dielectric binder and conductive filler

(e.g., electrical insulation coating or compound filled with copper, silver powder, etc.).

conductive materials:Solid materials which have a large number of free electrons that can easily be put

into motion to create an electric current (e.g., metal [as steel, copper] sheet, copper foil, copper, silver

powder, etc.).

corona (air): A luminous discharge due to ionization of the air surrounding a conductor or insulated

conductor caused by a voltage gradient exceeding a certain critical value.

corona imaging instrument: An instrument used for visual detection of corona or external surfacedischarges on energized test objects in ambient light, frequently using ultraviolet radiation emitted by the

discharge source.

discharge extinction voltage (rotating machinery) DEV(ionization or corona-extinction voltage):Thevoltage at which discharge pulses that have been observed in an insulation system, using a discharge

detector of specified sensitivity, cease to be detectable as the voltage applied to the system is decreased.

discharge inception voltage (rotating machinery) DIV(ionization or corona inception voltage):The

voltage at which discharge pulses in an insulation system become observable with a discharge detector of

specified sensitivity as the voltage applied to the system is raised.

external discharge:In rotating machines, external discharges may occur on the surface of bars/coils or in

any air gap present between the bar/coil surface and the stator core, or in the end-winding of the stator.

groundwall insulation:The main high-voltage electrical insulation that separates the copper conductors

from the grounded stator core in motor and generator stator windings.

high-potential test (power operations):A test that consists of the application of a voltage higher than therated voltage for a specified time for the purpose of determining the adequacy against breakdown of high

voltage insulation system and spacing under normal conditions. Syn:high pot; hipot.

NOTEThe test is used as a proof test of new apparatus, a maintenance test on older equipment, or as one method ofevaluating developmental insulation systems.5

ionization: (A) A breakdown that occurs in parts of a dielectric when the electric stress in those parts

exceeds a critical value without initiating a complete breakdown of the insulation system. (B)The processby which an atom or molecule receives enough energy (by collision with electrons, photons, etc.) to split

into one or more free electrons and a positive ion. Ionization is a special case of charging.

NOTEIonization can occur on both internal and external parts of a device. It is a source of radio noise and candamage insulation.

4IEEE Standards Dictionary Onlinesubscription is available at:

http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html.5Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement

this standard.


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noise:Unwanted disturbances superimposed on a useful signal that tend to obscure the signals information

content.

off-line testing (test, measurement, and diagnostic equipment):Testing of the unit under test removed

from its operational environment or its operational equipment. Shop testing.

ohms per square:A unit of surface resistivity used to characterize the resistance of a thin film materialmeasured between two opposite sides of a square and is independent of the size of the square or its

dimensional units. Surface resistivity can also be measured in a concentric ring fixture.

partial discharge (PD): An electric discharge which only partially bridges the insulation between

conductors and which may or may not occur adjacent to a conductor.

NOTEPartial discharges occur when the local electric-field intensity exceeds the dielectric strength of the dielectricinvolved, resulting in local ionization and breakdown. Depending on intensity, partial discharges are oftenaccompanied by emission of light, heat, sound, radio influence voltage (with a wide frequency range) and oxidation ifPD occurs in the presence of oxygen. Corona has also been used to describe partial discharges. This is a non-

preferred term since it has other unrelated meanings.

semiconducting materials: Composition materials which usually have dielectric binder and

semiconductive filler (e.g., electrical insulation coating or compound filled with graphite, carbon blackpowder, SiC grains, etc.).

semiconducting slot coating (rotating machinery):A coating, applied on the insulation surface of the slot

parts of winding. The semiconducting coating, compound, or tape in which the powder filler or portion of

powder filler is a semiconductive material and the electrical surface resistivity of this coating in such that,when converted into a semiconducting solid layer, is in the range of 1 102 5 105 Ohms per square.

This semiconducting slot coating must have uniform tight contacts with the grounded walls of the stator

slot. This coating provides minimum voltage between the surface of the coil or bar and the grounded stator

core. (adapted from Younsi, K., Mnard, P., and Pellerin, J. [B29])6

NOTEsemiconductive slot coating: This alternative terminology, as well as Slot Corona Protection andConductive Armor of the above definition is also used in the industry but will not be used in this document.

semiconductive materials: Solid materials which have limited free electrons and main conduction iscarried by electron-hole conductivity (n-p transition) (e.g., graphite, carbon black powder, SiC grains, etc.).

stress control coating:Coating used for external discharge suppression in the end turn parts of a winding.

The semiconducting coatings, compounds, or tapes are often filled with semiconductive material such assilicon carbide grains. The resistivity of this composite is a non linear function of electric field, which

modifies the surface resistance and consequently controls the surface potential gradient to a level that is

lower than the breakdown strength of the surrounding media, or of the air, in air cooled machines. Anoverlap between the stress control coating and the semiconducting slot coating is made to provide electrical

contact between the two coatings.

NOTEOther terms for this coating are end grading system and stress grading protection but are not used in thisdocument.

6The numbers in brackets correspond to those of the bibliography in Annex F.


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ultraviolet radiation:In general, any radiant energy within the wavelength 10 nm to 380 nm (nanometers)

is considered ultraviolet radiation. For power engineering purposes, the band of interest is the one of the

emission spectrum of electrical discharges in air. The emission bands of nitrogen dominate the optical

spectrum of discharges in air. Ninety percent of the total energy of the emitted optical spectrum of PD is inthe ultraviolet region (280 nm405 nm). The main part of the emission is invisible to the human eye. A

relatively weak emission around 400 nm can be observed under conditions of absolute darkness.

UN:Line-to-line voltage

Uo:Line-to-ground voltage

4. Test preparation and safety

WARNING

The test voltages employed for the tests herein can cause personal injury, loss of life, or property damage.

Accordingly, appropriate safety precautions are necessary to reduce the risk of such losses.

The testing described in this document shall be carried out according to the safety procedures described byany relevant regulatory agencies and the safety procedures of the organization having control over activities

at the test site.

Preparation for the test should include, but not be limited to, the installation of warning signs and safety

barriers around the test equipment and the machine to be tested. Grounds shall be installed as required by

any relevant regulatory agencies and the local controlling authority. Other safety measures can be found inIEEE Std 510-1983, ISO 14121-1, ISO/TR 14121-2, IEC 60204-1, and IEC 61508.7

All personnel involved in the test shall be thoroughly familiar with the test, the test equipment, the machine

to be tested, and the hazards involved. When equipment is energized, no personnel shall infringe upon the

minimum limits of approach described by any relevant regulatory agency or the local controlling authority.

5. Test equipment and connections

Care must be taken in the selection of the ac power supply. The duration of the test, the test voltage, and thecapacitance of the winding under test are the major factors to consider in the selection. Requirements for

factory tests on bars or coils will be different with respect to the test object load and test time (duty cycle of

the supply). The test should be done at 50 Hz or 60 Hz.

It is hard to predict in advance the duration of quality control testing of external discharges on a stator

winding. The duration of a particular test, if the three phases of the winding are tested separately, will

typically take several minutes to an hour but could be longer when the number of discharge sites is large. Itis then important to stay within the current limits and duty cycle of the power supply, especially when the

load represented by the winding under test is close to the load rating of the power supply. In this condition,

the power supply risks overheating rapidly and being seriously damaged if not ventilated properly.

It is recommended that the test voltage be in the upper range of the output voltage range of the power

supply.

7Information on references can be found in Clause 2.


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When using a resonant test set, a measurement of the capacitance of the test object should be made at the

test frequency before starting the test in order to adjust the test set properly. Do not depend on the hipot set

metering; but rather use a calibrated voltage divider to accurately measure voltage during the test.

Since the load of the winding under test is predominantly capacitive, it can be calculated using Equation

(1):

Pr= 2 f C V2 (1)

where:

Pr= Reactive power in VAf = Test frequency in hertzC = Total test load capacitance in faradsV = Test voltage in voltsIt is recommended to use a power supply providing reactive compensation to cancel out most of the

capacitive load presented by the winding under test. A resonant or primary-compensated power supply

should be used.

It is recommended to connect both ends of the phase, line, and neutral ends to avoid surge voltages in the

event of a sudden voltage interruption.

5.1 Sensitivity of the corona-imaging instruments

The use of a corona-imaging instrument to enhance detection of UV radiation by external discharges may

simplify and accelerate the test. Portability and the option to take a picture or video are features of interest.

However, the main feature is the sensitivity, and not all corona-imaging instruments are equal with respect

to UV detection. Moreover, specification sheets, which use different units (e.g., lux, watt/cm2,

picocoulomb) not always related to the phenomenon of interest here, make it difficult to compare different

instruments. In order to select a corona-imaging instrument, a simple test can be done to determine whether

it answers the need for a quality control test. Instead of running a complex UV spectrum test that needs tobe compared with the emission spectrum of the discharge activity and requires a specialized spectrometer,

which is not available to most people in the electrotechnical industry, a comparison of the response of the

corona-imaging instrument with the naked eye can be used. For many years, the naked eye was the

reference for external-discharge detection during a blackout test. The sensitivity of the eye after severalminutes in complete darkness is good and can make out the faint light of discharges extending to the lower

visible wavelengths (typically more than 20 min to reach a good sensitivity. In the rest of the document 15

min is used for convenience, but longer time will lead to improved eye sensitivity. Here, since the goal is to

qualify corona imaging equipment, a slightly longer time is used. Thus, any corona-imaging instrument

performing equally well as, or better than, the eye in these conditions will be considered acceptable. Thistest, described below, can be used to qualify corona-imaging instruments before they are used in the field or

in the factory.

Standardization of the test configuration provides reproducibility; however, a corona-imaging instrumentqualification test should also be easy without necessarily requiring an elaborate test facility such as a

climatic room for atmospheric pressure, temperature, and humidity control. A simple way to check thesensitivity of the corona-imaging instrument is to create a non-uniform field with a needle plane

configuration as shown in Figure 1and to detect the discharge inception voltage (DIV) at the tip of the

needle first with the naked eye in the dark (after 20 min). Then, test again with the instrument underevaluation. Since observation with the eye has been used with satisfaction in the blackout test for years, it

can be used as a reference for the DIV of the setup. Thus, if the setup or ambient condition varies slightly

from one user to the next, the minimum sensitivity requirement will always be determined with reference tothe eye under the same conditions.


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Since there is inherent variability while performing such DIV tests for different consecutive trials, a margin

of 1.0 kV can be tolerated. The final acceptance criterion is that the DIV detected with the corona-imaging

instrument should be within 1 kV of the one observed with the naked eye after being in complete darkness

for at least 20 min. The test with the corona-imaging instrument can be performed under normal room

lighting, but reduced lighting can somewhat improve the sensitivity of PD light versus ambient light ratio.The use of incandescent light instead of UV emitting fluorescent light would also help. If the corona-

imaging instrument does not respect this criterion, it is considered not sensitive enough in the spectrum ofinterest to be used for external-discharge detection.

The dimensions in Figure 1are given as guidelines, but other similar dimensions could be used, bearing in

mind that this test is a comparative evaluation of the corona-imaging instrument against the sensitivity of

the eye.

Figure 1 Needle plane electrode configuration to create discharge activity in air

It should be noted that a non-uniform electrical field is used in order to have a significant difference

between the DIV and the breakdown voltage of the test gap. With the dimensions in Figure 1, at an

atmospheric pressure of 101.3 kPa (1 atm), a temperature of 22 C (71.6 F) and relative humidity of 60%,the DIV is about 6.3 kV, and the discharge extinction voltage (DEV) is 5.8 kV. The typical intrinsic

variability of the DIV and DEV from one trial to the next is presented in Table B.1. Under the same

conditions, the breakdown voltage of this gap is 14.5 kV. Thus, if the voltage is raised slowly to the DIV,the risk of dielectric breakdown of the air gap while performing observation of the discharge activity at or

close to the DIV is reduced.

Alternatively, a single electrode using a needle sticking up in the air could be used to perform a similarcomparative test between the eye and the corona-imaging instrument.

6. Quality control test of external discharges with corona-imaginginstrument or blackout test

The major advantage of using a corona-imaging instrument to observe the light emission from external-

discharge activity is that it extends the observation spectrum down to the UV range where the dischargespectrum is the most intense. Thus, the observation can be made without the need of a dark environment.

Normal lighting in the factory or in the plant is an acceptable condition for performing the UV test with the

corona-imaging instrument as long as a strong UV emitting lamp, such as a mercury vapor lamp, is not

D 3.1 mm

(0.122 in)

4.4 mm(0.173 in) R 0.22 mm(0.008 in)

11.2 mm(0.441 in)

25.4mm(1.0 in)

R 5.0 mm(0.197 in)

R 5.0 mm(0.197 in)

25.4mm(1.0 in)

38.8


20/48



used as room lighting. In some cases, especially in the factory, reduced lighting can enhance the UV/visible

ratio. Locating the discharge sites will be easier with a corona-imaging instrument than with the blackout

test because most instruments are also sensitive, to different extents, to visible light and thus both light

emission from external discharges and the test object are visible at once. This will also accelerate the

testing because the eye requires no time to acclimatize in obscurity as it does for the blackout test.

In addition, working under lit conditions increases the safety of the personnel working in proximity to highvoltages. For instance, during on-site testing in a power plant, the operator of the high-voltage source will

have a direct view of the people doing the discharge observation.

Instead of using a corona-imaging instrument for observation of external discharges, a blackout test can be

performed on bars, coils, or entire machines. The first requirement for this test is to be able to achieve

complete darkness in the room where the test is performed or have sufficient shielding against surrounding

light to be able to observe external discharges with the naked eye. In many cases, especially in a powerplant, just shutting off the lights would not be enough to let the eye become sufficiently sensitive to observe

the smallest discharges. In some powerhouses, it will be possible to perform blackout tests only at

nighttime to prevent daylight compromising the test. However, for safetys sake, it is recommended to limit

use of the blackout test to testing in the factory on bars and coils and on VPI machines where completedarkness is easier to achieve and where safety measures are easier to respect. For safety reasons, the use of

a corona-imaging instrument is recommended for tests carried out on-site, on fully assembled stators. In

addition, it is believed that inspection with the imaging instrument is better since the observer can see the

stator and identify bars with respect to slot and know what portion of the stator has already been observedand which portion of the winding is not yet inspected.

6.1 Factory test on coils and bars

The purpose of this test is to validate that the bars/coils produced are not subject to external discharges.

This test is to be done at the factory which manufactures the bars/coils in the presence of a users

representative. Either the blackout test or visualization using a corona-imaging instrument may be used.

Individual bars and coils are tested by subjecting them to voltage while resting on support with the

semiconducting slot coating grounded carefully for the test. Additionally, in some cases, it may be

desirable to test a group of bars in a stator model to reproduce their physical arrangement in the machineand thus control minimal spacing between end-arms and the core tightening system. This test is discussed

in 6.2.

6.1.1 Sample size

The specimens tested should be selected by the users representative and represent 5% of all bars/coils

produced for a specific stator. The bars/coils chosen for this test should have successfully passed the

routine dielectric tests and the final inspection. Particular attention should be paid to the cleanliness of the

bars/coils.

6.1.1.1 Sample coils for globally VPI stator windings

For globally VPI stator windings, sample coils must be VPI-treated and examined for external discharges.The number of sample coils may be limited to two to five coils as the complete stator winding can also be

examined after the VPI treatment. The sample coils must have slot-simulating platens attached to them and

receive a VPI treatment that resembles the VPI process used for treating the stator winding. It isrecommended to process the sample coils in advance of the stator winding operation. This will allow for

necessary remedial work on the stator coils prior to the winding operation and VPI treatment.


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6.1.2 Test methods

The test conditions should be selected from Table 1.

Table 1 Test parameters

Test # Environment Observation with1 Half-light Corona-imaging instrument

2 Darkness Naked eye

The end-user and manufacturer should agree upon the test method: test #1 or test #2.

To perform test #1, half-light is needed to distinguish samples under test clearly with the corona-imaging

instrument. Excessive light can create reflections on the sample surface that could be interpreted as corona

activity. However, external discharges are usually intermittent whereas reflections tend to show constantlight emission. In addition, unlike external discharges, reflection will be observed directly by the naked eye.

If there is still any doubt, reduce the voltage until extinction of the corona. If there is no extinction at or

close to zero voltage, it is not corona.

To perform test #2, the test setup must be installed in a dark room. All sources of light should be turned off

or masked, especially lights that could be within the field of vision during observation.

6.1.3 Test voltage on individual bars and coils

The voltage chosen for a factory test should be selected so that no external discharges will occur in

operation at the surface of the bars or coils, and the stress control coating at U 0, and at operating

temperature. Since the temperature is lower during factory testing than during operation, the temperaturedifference is often compensated by increasing the test voltage in the factory [B9]. It should also be pointed

out that the maximum sustained voltage of the stator could be 5% to 10% above the nominal voltage rating.

The increase in the test voltage is not to compensate for the aging of materials since each material agesdifferently and because the voltage distribution along the stress control coating depends on the voltage.

A survey of industrial practices has shown that a range of test voltages is currently in use for this test.

Based on experience, it is recommended that the factory test be performed at a voltage level within therange presented in Table 2. Note that this test is intended for bars and coils in their completed stage and

should not be applied to VPI coils before impregnation (green coils). The exact voltage level at which to dothe test must be determined by the user and supplier before starting the test. Voltages close to the minimum

in Table 2(this minimum is equal to 1.25 Un/3) are closer to the normal line-to-ground voltage but will

compensate less for a temperature difference with operating conditions than the maximum proposed in the

table.

Table 2 Recommended test voltage range for factory testing

Minimum test voltage (xUN) Maximum test voltage (xUN)

0.72 1.15

NOTEThese voltage values are based on nominal voltage. They do not intend to take into account transientovervoltage during a fault or disturbances or overvoltage due to an ungrounded neutral.

6.1.4 Test procedure for individual bars and coils

Bars/coils to be tested should be installed on supports, and the semiconducting slot coating should begrounded.

For bars, the high voltage should be applied to the bare copper, usually at one end of the bar

For coils, the high voltage is generally applied to both bare copper leads with the individual strandsconnected together


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Multiple bars/coils can be tested at the same time. The test setup should permit inspection of the four sides

of the bars/coils. If necessary, the bars/coils can be examined in various positions; in which case, the

voltage should be turned off and bars/coils shall be safely grounded before re-positioning.

The ambient temperature, relative humidity, and atmospheric pressure should be recorded at the beginning

of the test.

Test #1 starts by applying to the bars/coils the test voltage selected from the range specified in Table 2.

Observations may start immediately after reaching the test voltage and are initially focused on the end-

winding area (end-turns or end-arms) where the stress control coating is applied, particularly at the bend,non-straight portions, and on both sides of the bar/coil. The semiconducting slot coating is then inspected

for signs of external discharges.

Test#2 is commenced by applying to the bars/coils the test voltage selected from Table 2. After at least 15

min in complete darkness, observation with the naked eye can be focused on the end-winding area where

the stress control coating is applied to see any signs of external discharges. The semiconducting slot coatingshould also be inspected for signs of external discharges.

6.1.5 Acceptance criteria

If none of the selected bars/coils exhibit external discharges during the test, then the production set for the

stator is deemed to have met the requirements.

If the users representative or the manufacturer has a doubt about any bars/coils during the selected test, the

doubtful specimens could be re-inspected using the alternative test method described in this document.

6.1.6 Remedial actions and retest

If one bar/coil exhibits external discharges during the selected test, this specimen should be repaired by the

manufacturer. As a second verification, the test should be performed again on the repaired bar/coil and on a

second batch of specimens representing 5% of all bars/coils produced for the machine.

If one or more bars/coils present external discharges during the test on the second batch, then all the

bars/coils of the machine should be tested, repaired if necessary, and retested.

6.2 Stator model test

The purpose of this test is to validate that the clearance between one bar/coil and another or between

bars/coils to ground when installed in a stator model (mock-up core) representing the stator, is not subjectto external discharges. This test is optional but, if it is done, it should be carried out prior to manufacture of

the complete winding or core because failures may require changes in the machine design or in the winding

design as outlined in 6.2.6. This test is designed to test the assembly in the factory before the stator is fullyassembled and provide a better line of view than on the fully assembled machine.

Successfully passing the test on the complete stator is sufficient, but in the case of general clearance

issue(s), testing on the stator model may facilitate necessary remedial action. This test is to be performed at

the factory manufacturing the bars/coils in the presence of a users representative. Either the blackout test

or visualization using a corona-imaging instrument may be used.

It should be pointed out that if the test is performed in the factory for a machine to be installed at an altitudehigher than 1000 m (3281 ft), standard spacing during the test will not ensure absence of discharges on site.

Reduced pressure of the air at higher altitude will give a lower inception voltage than at sea level. For

machines operating above 1000 m (3281 ft), correction of the factory test voltage will have to be agreed


23/48



between user and manufacturer. An example of correction factors is proposed in Annex Dand based on

IEEE Std 4 and IEEE Std 4a.

6.2.1 Sample size

The stator model should be large enough to accommodate a sufficient number of coils or half-coils in sucha way that it represents one complete winding pitch. For two-pole and four-pole machines, half-coils can beused as long as the spacing between coil leads and coil knuckles can be tested. The sample size would

remain the same for lap or wave windings, but additional support should be added to the end winding.

6.2.2 Test methods

Test methods described in 6.1.2for the factory test performed on individual coils or bars may be used on

the stator model or mock-up core. The model could be made with wood. Conductive material such as steel

or aluminum plates or wood covered with conductive foil should be used to simulate the core-tighteningcomponents at both ends of the stator model. If conductive foil is used, points or abnormal sharp edges

should be avoided. Alternatively, a conducting coating can be used instead of foil to cover the wood. All

conductive and conducting materials added to the model should be grounded. The minimum spacingbetween coils or bars in the model should be the same as in the stator.

6.2.3 Test voltage

Testing on the stator model is mainly used to confirm that no external discharge will occur between

coils/bars in the end-winding area before the assembly stage. This test will confirm that the spacing is

sufficient to eliminate external discharges between coils/bars up to the maximum phase-to-phase voltageand at operating temperature. It could also be used to confirm that no external discharge activity occurs

between the coils or the bars and the tightening system of the stator core laminations. Typically the

temperature is lower in the factory than during operation, so the temperature difference should be

compensated by increasing the test voltage in the factory. The increase in voltage is not intended to

compensate for the aging of materials, as each material ages differently and because the voltage distribution

along the stress control coating depends on voltage. It should be pointed out that not all the spacings aresubjected to the full line-to-line voltage during machine operation. Moreover, several locations in the stator

winding are only exposed to line-to-ground voltage, such as the junction between the semiconducting slot

and the stress control coatings. It is thus recommended to perform this test at two voltage levels: the firstone to test all ground clearance, and the second at higher voltage to test bar-to-bar or coil-to-coil

clearances. It is recommended to test ground clearance to the voltage indicated in the left column in Table

3. This value corresponds to 115% (15% increase compensates both for the temperature difference betweenfactory and operating conditions and the maximum allowable continuous voltage) of the maximum phase-

to-ground voltage (1.15 Uo= 0.66 UN). Similarly, it is recommended to use a test voltage for bar-to-bar

(or coil-to-coil) clearances based on the actual maximum voltage that appears at the clearances. For eachlocation, the manufacturer will determine from the winding diagram the actual operating voltage (including

the crossover region between top and bottom planes). This second test value is 115% of the maximum

voltage found in the machine, as shown in the right-hand column of Table 3.

Table 3Recommended voltage range for model test

Test voltage of ground clearances Test voltage of bar-to-bar or coil-to-coil clearances

0.66 UN Maximum voltage based on winding diagram +15%

NOTEThese voltage values are based on nominal voltage. They do not intend to take into account transient

overvoltage during a fault, disturbance or overvoltage due to an ungrounded neutral.

It should be noted that for refurbished machines, the existing clearance from ground and between

connections can be very different from one machine to another. When rewinding a stator with the existing


24/48



clearances, it may not be possible to make the test at, or even close to, the maximum voltage given in Table

3. In such a case, the exact voltage level at which to do the test must be determined by the user and supplier

before starting the test.

For new machines, the required test voltage may have an impact on the design choice; for example, deeper

slots leading to a bigger stator, longer bars leading to a higher stator winding resistance, higher losses and

so on.

6.2.4 Test procedure for a group of bars or coils in a stator model

For the test in a stator model, coils or bars should be placed in the mock-up stator core having the samebore radius, the same slot size, and same core length as the actual stator core. When coils and bars are

installed in the mock-up stator core, it should be possible to see if external-discharge activity occurs:

Between top and bottom coil legs or bars in the same slot at the junction of the semiconducting slotcoating and stress control coatings

In the end-winding between adjacent top and bottom coils/bars

In the end-winding between top coils/bars and bottom coils/bars at crossovers

Between each of the coil knuckles and the lead of the adjacent coil

Between any coils or bars and the tightening system of the stator core laminations

The semiconducting slot coating of the coils or bars should be grounded. Coils and bars should be installed

in the mock-up core slots using the same thickness of slot packing material and center slot filler that will be

used in the stator slots. Some fiber blocks could be temporarily tied on the coil or bar end-turns to simulatethe thickness of winding blocking. The mock-up stator core can be made as described in 6.2.2.

The test voltage should be applied on only one coil/bar at a time with all the other coils/bars grounded.Each coil or bar installed in the mock-up core should be individually tested in relation to all the others. The

test voltage to apply to individual coils/ bars should comply with the test voltage of bar-to-bar clearances

defined in Table 3.

When the clearance between coils or bars to ground is to be verified, all coils or bars in the mock-up core

should be energized at the test voltage defined in Table 3for test voltages of ground clearances.

As PD could occur if steel elements are too close to the winding, the stator mock-up should also considersimulation of the fingers, the tightening plates, and the tightening studs.

6.2.5 Acceptance criteria

No visible discharge should be observed at the various locations described in 6.2.4at voltages up to and

including the test voltage of bar-to-bar clearances defined in Table 3for coil-to-coil or bar-to-bar clearance

verifications and up to the test voltage of ground clearances defined in Table 3for coil- or bar-to-ground

clearance verifications.

6.2.6 Remedial actions and retest

If visible discharges are found between coils or bars, different possible remedial actions could be

implemented depending on the location where these discharges are found. The remedial action should be

agreed upon by the manufacturer and the user. For some of these corrective actions, new coils or bars may

have to be manufactured. Some of the characteristics of the following bullets are depicted in Figure 2.


25/48



a) Visible discharges between top and bottom coil legs or bars in the same slot at the junction of thesemiconducting slot and stress control coatings: Applying another coat of the stress control coating

and/or improving the contact with the semiconducting slot coating could eliminate the discharges.

Another solution is to increase the thickness of the center slot filler and/or modify the shape of the

coils/bars outside of the core in order to provide more clearance at the junction of thesemiconducting slot and stress control coatings between top and bottom coil legs/bars in the same

slot. For the latter corrective action, new coils or bars will have to be manufactured. The test shouldbe repeated to demonstrate the effectiveness of the implemented corrective action.

b) Visible discharges in the end-winding between top and bottom coils or bars:Upon agreement between the manufacturer and the user, coils or bars could be placed in the mock-

up stator core in a different order from the original order in such a way that no visible discharges

are found between coils or bars. If this remedial action is selected, all coils or bars will have to be

placed in the mock-up core and tested to determine the order in which the coils or bars will have tobe installed in the stator core. Alternatively, the shape of the coils or bars should be modified in the

end-winding to provide more clearance between adjacent top and bottom coils or bars. For

example, for coils, the developed length of the bottom end-winding of the coils between the coreand the coil knuckle may have to be extended beyond its original length. For bars, the drop-back

angle at the outside of the core on the bottom bars may have to be increased. For any one of these

corrective actions, new coils or bars will have to be manufactured and the test should be repeated to

demonstrate their effectiveness.

c) Visible discharges in the end-winding between adjacent top coils/bars or between adjacent bottomcoils/bars: Upon agreement between the manufacturer and the user, coils or bars could be placed inthe mock-up stator core in a different order from the original order in such a way that no visible

discharges are found between coils or bars. If this remedial action is selected, all coils or bars will

have to be placed in the mock-up core and tested to determine the order in which the coils or barswill have to be installed in the stator core.

Alternatively, the design of the end-winding of coils/bars could be changed to provide more

clearance between adjacent top coils/bars and between adjacent bottom coils/bars. For this

corrective action, new coils/bars will have to be manufactured and the test should be repeated in

order to demonstrate their efficiency. To accomplish this, the length of the coil/bar end-winding

may have to be increased.

d) Visible discharges between each coil knuckle and the lead of the adjacent coil:Upon agreement between the manufacturer and the user, coils could be placed in the mock-up

stator core in a different order from the original order in such a way that no visible discharges are

found between each coil knuckle and the lead of the adjacent coil. If this remedial action is

selected, all coils will have to be placed in the mock-up core and tested to determine the order inwhich the coils will have to be installed in the stator core.

Alternatively, the shape of the coil lead leaving the coils between coil knuckles could be modified

to provide more clearance at this location. For this corrective action, new coils will have to be

manufactured and the test should be repeated in order to demonstrate their efficiency.Another alternative may consist in increasing the drop-back of the coil knuckles. The length of the

coil end-winding may have to be increased for that. For this corrective action, new coils will have

to be manufactured and the test should be repeated in order to demonstrate their efficiency.

Ultimately, the width of the slot and/or the number of slots in the stator core could be revised. This

corrective action requires a complete redesign of the stator core and its winding. Many generator

parameters could be altered by the redesign, so consideration must be given to review all

contractual requirements.

e) Visible discharges between any coils/bars and the tightening system of the stator core laminations: The shape of the coils/bars should be modified in the end-winding to provide more clearance

between coils or bars and the tightening system of the stator core laminations. For example, if the

discharges are with the finger plates or with the tightening plates, the length of the straight part of


26/48



the coils or bars out of the stator core may have to be increased. To avoid discharge between top

and bottom bars/coil legs, a thicker center filler could be used and/or the length of the slot corona

protection can be increased. As another example, if the discharges are with the upper or lower air

baffles or with the stator upper or lower brackets, the length of the coil/bar end-windings may have

to be reduced. Other remedial actions could be agreed upon between the manufacturer and the user.

Figure 2 Description of some of the material used as part of the insulating system of barsand coils and general location of some of the discharge sites

6.3 Test on fully assembled stator windings

This is an off-line test where the stator winding is energized with an external voltage supply. The purpose

of this test is to validate that fully assembled stator windings are not subject to external discharges in theend winding area. Discharges should be eliminated both at ground clearance and bar-to-bar or coil-to-coil

clearance. The procedure and setup will depend on the type of test used: blackout or corona-imaging

instrument. The use of a corona-imaging instrument is strongly recommended for tests performed on statorsassembled on-site for safety reasons.

This test should confirm that no external discharges will occur between bars or coils in the end-winding

area due to insufficient spacing and will also indicate the quality of the assembly and the stress controljunctions on all coils/bars of the stator winding. It should be pointed out that, during the test, all

components of one phase winding are stressed at the same voltage, whereas in operation, only a portion ofthe winding is exposed to line-to-line voltage (between bars or coils), while most locations are exposed to

much lower voltage.

6.3.1 Test setup

For global VPI stators, it is preferable to test the stator winding upon completion of the VPI operation andprior to assembly of the machine. The test may be performed after assembly of the machine and completion

of the performance (running) tests; however, it is recommended to remove the end-covers and the rotor toexpose the entire stator end-winding.

For stators wound on-site, it is preferable to carry out the test before the installation of the rotor. If the

diameter of the machine allows, a platform should be installed inside the bore at a safe distance from anyenergized part (including end-arms). From this platform, there should be a direct line of view to both ends

of the stator (CE: connection end, OCE: opposite connection end). A barrier can be installed to ensure that

Increase thickness of center filler as in bullet A.

Top of stator core

Overlap of stress control coating and semiconducting slot coatingLocation of

discharges as in

bullet A.

Location ofdischarges as in

bullet B.

Increase this angle as in bullet B.

CoreSemiconducting slot coating

Stress control coating


27/48



no one gets too close to the winding. Another possibility for large generators is to use a nacelle with a crane

for the observer to move around in the stator.

For stators with a small diameter or horizontal machines of small core length, the observation could be

done from both ends of the machine (CE, OCE) when it is not possible to stand at the center of the core.

If a final coat of paint is to be applied to the winding for mechanical protection, the inspection should bemade before application of this last coat. In the event that external discharge is observed, corrections will

be possible if there is direct access to the semiconducting slot coating and to the stress control coating.

When there are parallel circuits within each phase of the stator winding, all the circuits must be energized

to avoid imposing voltage stresses and consequently producing external discharge activity in the air

clearances (gaps) between the parallel circuits that belong to the same phase. It should be pointed out thatsome phase-neutral crossover locations in the same phase normally exposed to voltage in operation will not

be stressed during the test. The number of such sites is larger for wave windings.

When the neutral point is not accessible (internal Y connection), the test has to be performed on all phases

connected together; and, in this case, no discharge can occur between phase windings under applied test

voltage.

Before the day of the test, obtain a table showing the voltage during normal operation, parallel circuits, andphase of all coils/bars of the winding. An example of such a table is given in Annex E.

Before the test begins, number and mark the stator slots using a permanent marker or a temporary tag suchas an adhesive-backed tape or magnetic strips; this will facilitate locating discharge activities during the

tests. Ensure the temporary tag is removed upon completion of the test. Usually, marking down every tenth

slot is sufficient. It is better to mark down both ends of the slots close to the end of the core.

It is also recommended to identify the first, second, and third line-end coils for each parallel circuit per

phase as these would be the most likely candidates to exhibit external discharges during operation in

service. These positions can also be marked down on paper with a reference to a clock-like positioning foreach of these coils at each end. It may be helpful to place marks (e.g., masking-tape tabs sticking up) in the

core to serve as a reference point using 1 oclock, 2 oclock, etc. as reference markings. In addition, phase

breaks, coil-to-coil spacing in the end-winding where the adjacent coils belong to different phases, could

also be marked.

6.3.1.1 Blackout test on stators in the factory

A pitch-black environment is required to perform this test. This is typically achieved by setting up an

enclosure around the test setup including the stator and the hipot set. The enclosure material used must be

impervious to light. Alternatively, the test may be carried out at night in an area where all the lights are

switched off.

The enclosure should be constructed with 1.8 m (6 ft) minimum clearance imposed from the end of statorwinding (i.e., stator coil noses and circuit rings) at both ends. A fence or physical barrier should be installed

around the ends of the coils so that a safe distance is maintained between the energized coils/bars. Thedistance between the fence and the ends of the coils/bars should be the minimum approach distance

recommended by the controlling authority.

Care must be taken with the enclosure entrance with regard to adequate material overlap, as any light

leakage will compromise the test.


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6.3.2 Test methods

Two test voltage levels may be used: one to test ground clearances, and a second for testing phase-to-phaseclearances as specified in Table 4. Every location showing external-discharge activity must be noted with

the intensity on a relative scale (strong, intermediate, weak). In addition to the physical location of the

discharge sites, the electrical position in the winding must also be determined.

A different procedure is to determine the DIV and DEV for location(s) exhibiting external-discharge

activity (see 6.3.4.3).

On stators assembled on-site, observation of all end-arm locations with a direct line of view should be

made with the corona-imaging instrument in reduced lighting if possible. If not, normal lighting is alsoacceptable but requires more attention to discriminate discharges from reflections and solar UV wave. For

global VPI machines tested in the factory, both visualization with the corona-imaging instrument and the

blackout test can be used.

6.3.3 Test voltage

Since this test on the fully assembled stator winding is intended to confirm that no external dischargesoccur in operation between bars/coils of opposite phase or close to the tightening system in the end-

winding area and at locations with ground clearances, either a single test voltage or two levels of voltage

can be used to evaluate all conditions. The test helps confirm that the clearance is sufficient to eliminate

external discharges up to nominal voltage during normal operating conditions and temperature. Since the

temperature is lower during the test than under operating conditions, the temperature difference can becompensated by increasing the voltage during the test. The increase in voltage is not to compensate for

aging of the material, as each material ages differently and because the voltage distribution along the stress

grading system depends on voltage. Experience suggests performing the test first at the maximum testvoltage shown in Table 4to test bar-to-bar or coil-to-coil clearance. If no external discharges are observed

anywhere in the machine, the test at the lower voltage would not be necessary. This voltage will be

determined by the manufacturer based on the winding diagram of the machine. This value shouldcorrespond to the highest one seen in the machine including the crossover region between top and bottom

planes +15%. A second test can be carried out to evaluate ground clearance and, as shown in Table 4, it is

recommended to use a voltage of 0.66 UN(which corresponds to (UN/3) 1.15 or U0 1.15). This testvoltage would mainly test ground clearances. The 15% increase compensates for both the temperature

difference between factory and operating conditions and the maximum allowable continuous voltage.

Table 4 Recommended test voltage range for fully assembled stator windings

Test voltage of ground clearances Test voltage of bar-to-bar or coil-to-coil clearances

0.66 UN Maximum voltage based on winding diagram +15%

NOTEThese voltage values are based on nominal voltage. They do not intend to take into account transientovervoltage during a fault, disturbance, or overvoltage due to an ungrounded neutral.

If the test is done in the factory for a machine to be installed at an altitude higher than 1000 m, standard

spacing during the test will not ensure the absence of discharges on-site. Reducing the pressure of the air at

higher altitude will give a lower inception voltage than at sea level. For machines operating above 1000 m,a correction of the factory test voltage will have to be agreed upon by both user and manufacturer. An

example of correction factors is proposed in Annex D.

It should be pointed out that, for refurbished stators, the existing clearance with ground and between

connections can be very different from one machine to another. When rewinding a stator with the existing

clearances, it may not be possible to meet the test acceptance criteria at, or close to, the maximum voltage

stated in Table 4. In such a case, the exact voltage level at which to do the test must be determined by theuser and supplier before starting the test.


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It should be recognized that for new machines the required test voltage may have an impact on the design

choice, for example: deeper slots leading to a bigger stator, longer bars leading to a higher stator resistance,

higher losses, and so on.

6.3.4 Test procedure on fully assembled stator windings

Before application of the test voltage, record the environmental conditions including temperature, relativehumidity, and barometric pressure. These parameters may be recorded at the beginning of the test for each

phase winding, particularly when the test duration is relatively long. The start time, finish time, and date

should also be recorded.

The test can be started at the higher test voltage first, followed by lower voltage or the other way around.

Alternatively, a single voltage test can be performed at the highest voltage of Table 4. Test procedures are

explained in 6.3.4.1and 6.3.4.2. For the test at the higher voltage level (single-voltage procedure), only thehighest voltage in Table 4 is applied to each phase winding with the other two grounded while observing all

parts of the end-windings.

Finally, a procedure for the determination of DIV and DEV can be used, which is described in 6.3.4.3.

6.3.4.1 Test with corona-imaging instrument

For testing phase-to-phase clearances, commence the test by energizing one phase of the machine at the full

test voltage shown in Table 4for phase-to-phase clearance evaluation with the other phases and the framesolidly grounded. When full test voltage (for phase-to-phase clearances) is achieved, begin examining the

winding for indications of external discharges. Discharges from windings to grounded structures should be

ignored during this test. The only areas to be evaluated are the gaps between energized phase groups and

grounded phase groups. The gaps between phase groups include coil-to-coil vent spaces on the end-arms,

gaps between top and bottom coil legs, and gaps between coil leads at the phase connections and in theparallel ring areas. Knowledge of which phase is energized and markings or tags previously located on the

windings could expedite identification of the areas to be scanned for external discharges.

Maintain a record of all locations showing external discharges as the test progresses. Details on how to fill

out the recommended data tables are given in Clause 7. Information to be recorded should include bar or

coil leg type (top or bottom bar), end of stator

ieee std 1799-2012 -ieee recommended practice for quality control testing of external discharges on...

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