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

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IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

Electric Machinery Committee

IEEE

3 Park Avenue

New York, NY 10016-5997 USA

IEEE Power and Energy Society

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IEEE Std 1799™-2012

IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

Sponsor

Electric Machinery Committee

of the

IEEE Power and Energy Society

Approved 19 October 2012

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Abstract: The procedure for quality control testing of external discharges on stator coils, bars

and 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 •

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Participants

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

Remi Tremblay, Chair Claude Hudon, Secretary

David Agnew Kevin Alewine Raymond Bartnikas Kevin Backer Stefano Bomben Andy Brown Donald Campbell William Chen Doug Conley Ian Culbert Jeffrey Fenwick Shawn Filliben Nancy Frost Paul Gaberson Michel Gagné Bal Gupta Gary Heuston Richard Huber Marcelo Jacob Aleksandra Jeremic Aleksandr Khazanoy Amir Khosravi Thomas Klamt Inna Kremza Laurent Lamarre Gerhard Lemesch Rimma Malamud William McDermid David McKinnon Charles Millet Glenn Mottershead Beant Nindra Sophie Noel Ramtin Omranipour Howard Penrose Helene Provencher Emad Sharifi John Schmidt Jeffrey Sheaffer Reza Soltani Gregory Stone Chuck Wilson Hugh Zhu

The following members of the individual balloting committee voted on this recommended practice. Balloters may have voted for approval, disapproval, or abstention.

Michael Adams David Agnew Martin Baur Thomas Bishop Stefano Bomben Steven Brockschink Chris Brooks Donald Campbell Weijen Chen Ian Culbert Matthew Davis Ray Davis Gary Donner Gary Engmann Jeffrey Fenwick Jorge Fernandez Daher Sudath Fernando Rostyslaw Fostiak Paul Gaberson Michel Gagné Randall Groves Bal Gupta Werner Hoelzl Claude Hudon Innocent Kamwa Jim Kulchisky Chung-Yiu Lam Benjamin Lanz William Lockley Greg Luri Rimma Malamud William McBride William McCown William McDermid David McKinnon Don McLaren James Michalec G. Harold Miller Charles Millet Jerry Murphy Arthur Neubauer Michael S. Newman William Newman Sophie Noel Lorraine Padden Christopher Petrola Alvaro Portillo Iulian Profir Bartien Sayogo John Schmidt Jeffrey Sheaffer Gil Shultz Reza Soltani Gary Stoedter Gregory Stone James Timperley Remi Tremblay John Vergis Kenneth White Hugh Zhu

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

Richard H. Hulett, Chair John Kulick, Vice Chair Robert M. Grow, Past Chair Konstantinos Karachalios, Secretary

Satish Aggarwal Masayuki Ariyoshi Peter Balma William Bartley Ted Burse Clint Chaplin Wael Diab Jean-Philippe Faure Alexander Gelman Paul Houzé Jim Hughes Young Kyun Kim Joseph L. Koepfinger* John Kulick David J. Law Thomas Lee Hung Ling Oleg Logvinov Ted Olsen Gary Robinson Jon Walter Rosdahl Mike Seavey Yatin Trivedi Phil Winston Yu Yuan

*Member Emeritus

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

Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative

Julie Alessi

IEEE Standards Program Manager, Document Development Malia Zaman

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Introduction

This introduction is not part of IEEE Std 1799-2012, IEEE Recommended Practice for Quality Control Testing of External 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 an insufficient 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 junction between 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 through the 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 have electrical resistivity per unit of surface of 1 × 102_{– 5 × 10}5_{Ohms 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 the semiconducting 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, and acceptance criteria in the factory and on-site.

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IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

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 manufacturing process 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 the main phase terminals

b) confirming proper alignment of the ground plane made by the semiconducting slot coating on the straight 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 foreign objects, 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 machine’s

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IEEE Std 1799-2012

IEEE 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 blackout test, and the use of corona imaging instruments.

1.2 Purpose

The purpose of this recommended practice is to suggest specimen preparation, test parameters, and procedures 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 the event 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 of the referenced document (including any amendments or corrigenda) applies.

IEC 60204-1, Safety of machinery—Electrical equipment of machines—Part 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 Machinery—Risk Assessment—Part 1: Principles.

ISO/TR 14121-2, Safety of Machinery—Risk Assessment—Part 2: Practical Guidance and Examples of Methods 2.

1_{The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc.} 2_{IEEE publications are available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,} USA (http://standards.ieee.org/).

3_{IEEE 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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IEEE Std 1799-2012

3. Definitions

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

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

blackout test: A test performed after eliminating all ambient light, specifically on energized electrical

equipment, 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 surface

discharges 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): The

voltage 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 the

rated 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.

NOTE—The test is used as a proof test of new apparatus, a maintenance test on older equipment, or as one method of evaluating 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 process by 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.

NOTE—Ionization can occur on both internal and external parts of a device. It is a source of radio noise and can damage insulation.

4_{IEEE Standards Dictionary Online subscription is available at:}

http://www.ieee.org/portal/innovate/products/standard/standards_dictionary.html.

5_{Notes 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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IEEE Std 1799-2012

noise: Unwanted disturbances superimposed on a useful signal that tend to obscure the signal’s 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 material

measured 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.

NOTE—Partial discharges occur when the local electric-field intensity exceeds the dielectric strength of the dielectric involved, resulting in local ionization and breakdown. Depending on intensity, partial discharges are often accompanied by emission of light, heat, sound, radio influence voltage (with a wide frequency range) and oxidation if PD 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 black powder, 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., Ménard, P., and Pellerin, J. [B29])6

NOTE—semiconductive slot coating: This alternative terminology, as well as “Slot Corona Protection” and “Conductive 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 is

carried 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 as silicon 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. An overlap between the stress control coating and the semiconducting slot coating is made to provide electrical contact between the two coatings.

NOTE—Other terms for this coating are “end grading system” and “stress grading protection” but are not used in this document.

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IEEE Std 1799-2012

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 in the ultraviolet region (280 nm–405 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 by any 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 in IEEE 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 the capacitance 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. It is 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.

7_{Information on references can be found in Clause 2.}

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IEEE Std 1799-2012

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 π f C V2 (1)

where:

Pr = Reactive power in VA

f = Test frequency in hertz

C = Total test load capacitance in farads

V = Test voltage in volts

It 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 to be 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 several minutes 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. This test, 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 instrument qualification 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 the sensitivity of the corona-imaging instrument is to create a non-uniform field with a needle plane configuration as shown in Figure 1 and 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 under evaluation. 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 to the eye under the same conditions.

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IEEE Std 1799-2012

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 of interest to be used for external-discharge detection.

The dimensions in Figure 1 are 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 similar comparative test between the eye and the corona-imaging instrument.

6. Quality control test of external discharges with corona-imaging

instrument 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 external-discharge spectrum 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°

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IEEE Std 1799-2012

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 high voltages. 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 power plant, 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 safety’s 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 complete darkness 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 observed and 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 user’s 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 machine and 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 user’s 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 is recommended 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 with

1 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 constant light 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 U0, and at operating

temperature. Since the temperature is lower during factory testing than during operation, the temperature difference 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 ages differently 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 the range 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 do the 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 NOTE—These voltage values are based on nominal voltage. They do not intend to take into account transient

overvoltage 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 be grounded.

⎯ 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 strands connected together

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IEEE Std 1799-2012

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 coating should 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 user’s 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 subject to 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 fully assembled 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 user’s 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 altitude higher 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

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IEEE Std 1799-2012

between user and manufacturer. An example of correction factors is proposed in Annex D and 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 such a way that it represents one complete winding pitch. For two-pole and four-pole machines, half-coils can be used 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 6.1.2 for 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-tightening components 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 spacing between 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 voltage and 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 are subjected 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 first one 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 between factory 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 each location, 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 3 —Recommended 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%

NOTE—These 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

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IEEE Std 1799-2012

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 same bore 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 slot coating 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 simulate the 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 3 for test voltages of ground clearances.

As PD could occur if steel elements are too close to the winding, the stator mock-up should also consider simulation 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 6.2.4 at voltages up to and including the test voltage of bar-to-bar clearances defined in Table 3 for coil-to-coil or bar-to-bar clearance verifications and up to the test voltage of ground clearances defined in Table 3 for 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.

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IEEE Std 1799-2012

a) Visible discharges between top and bottom coil legs or bars in the same slot at the junction of the semiconducting 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 the semiconducting 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 should be 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 to be 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 core and 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 bottom coils/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 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 in which 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

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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 bars and 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 the end 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 stators assembled 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 control junctions 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 of the 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 and prior 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 to expose 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 any energized 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 coating Location of discharges as in bullet A. Location of discharges as in bullet B.

Increase this angle as in bullet B.

Core _{Semiconducting slot coating}

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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 be made 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 that some 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, and phase 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 such as 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 for each 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 o’clock, 2 o’clock, 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 stator winding (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. The distance 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.

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

IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

Sponsored by the

Electric Machinery Committee

IEEE Power and Energy Society

IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

Notice to users

Laws and regulations

Copyrights

Updating of IEEE documents

Errata

Patents

Participants

Introduction

Contents

IEEE Recommended Practice for

Quality Control Testing of External

Discharges on Stator Coils, Bars, and

Windings

1. Overview

2. Normative references

3. Definitions

4. Test preparation and safety

5. Test equipment and connections

6. Quality control test of external discharges with corona-imaging

instrument or blackout test