581 -- guide - corona electromagnetic probe tests (tva)

8/11/2019 581 -- Guide - Corona Electromagnetic Probe Tests (TVA)

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Guide

- Corona Electromagnetic Probe Tests(TVA)

Working GroupA1.28


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GUIDE CORONA ELECTROMAGNETIC PROBETESTS (TVA)

Working GroupA1.28

Dan Zlatanovici Convenor (RO)Robert Fenton (US), Kane Hendreson (NZ), Voitto Koko (FI), Nick Kruger (ZA),

Bill McDermid (CA), Bernard O'Sullivan (IE), Mladen Sasic (CA), Smit Nico (ZA),Howard Sedding (CA), Stone Greg (CA), Vincenzo Tartaglione (IT), Alberto Villarrubia

Castellanos (ES), Jrgen R.Weidner (DE), Peter Wiehe (AU), Hugh Zhu (CA),

Copyright 2014

Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use forpersonal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of thepublication for use other than personal and transfer to a third party; hence circulation on any intranet or othercompany network is forbidden.

Disclaimer notice

CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept anyresponsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditionsare excluded to the maximum extent permitted by law.

ISBN: xxx-x-xxxxx-xxx-x


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ISBN: 978-2-85873-276-0


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TABLE OF CONTENTS

1. INTRODUCTION 32. TEST HISTORY. 3

3. TEST APPLICATION FIELD 44. TEST METHODOLOGY 4

4.1. Principle of the method.. 44.2. The test voltage.... 54.3. Scanning the slots 7

5. SAFETY OF THE TEST 96. THE ANALYSIS OF THE RESULTS AND THE WINDING CONDITION

ASSESSMENT CRITERIA. 106.1. General method for analysis of the results...... 106.2. IRIS Power method for analysis of the results 126.3. Assessment criteria. 14

7. METHOD EFFECTIVENESS AND FUTURE DEVELOPMENT15BIBLIOGRAPHY.16ANNEX 1: CORONA PROBE (TVA) TYPE 97 PPM PERFORMANCE..17ANNEX 2: Excerpt from IEEE Std 1434-2000 on the electromagnetic probe...18


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1. INTRODUCTION

This Guide aims to revitalize a method for detecting and determining the location of thepartial discharges at the slot level in high voltage electric generators and motors. In the

distant past, there were several accidents leading to death by electrocution of theoperators of this equipment and as a result, some companies avoided the use of thismethod (according to existing information on the International Generator TechnicalCommunity Forum). The incidents were caused by lack of respect of the safety rules forhigh voltage work. This has slowed the spread and development of this method for testinghigh voltage electrical machines.

In 2010, CIGRE SC A1 has initiated the development of a Guide for this test. After theapproval of TOR by the Technical Committee CIGRE, the WG A1.28 was constituted.This WG organized a questionnaire and a preliminary description of the test that weredistributed to all members of SCA1. 15 from whom responses were received and on their

basis the present Guide was developed. Also, information obtained from the InternationalGenerator Technical Community forum IGTC [14] has been used.

In form-wound bars and coils rated greater than about 4 kV, partial discharges can occurwithin the groundwall insulation or between the surface of the coil or bar and stator coil.Groundwall insulation is the component that separates the copper conductors from thegrounded stator core. These partial discharges, which are sometimes colloquially (butincorectly) called corona, are created by the high voltage stress that occurs in thegroundwall insulation.

According to the IEEE Standard Dictionary 100-1996 [13] , corona is a form of partialdischarges. But the term corona is reserved for the visible partial discharges that can occuron bare metal conductors operating at high voltage, which ionize the surrounding air. Sincepartial discharges within the groundwall insulation are not visible, they should not betermed corona.

If an air pocket (also called a void or a delamination) exists in the groundwall insulation,the high electric stress will break down the air, causing a spark. This spark will degrade theinsulation and, if not corrected, repeated discharges may eventually erode a hole throughthe groundwall insulation, leading to failure.

The off-line and on-line partial discharge tests indicate that partial discharges areoccurring somewhere in the winding, thus, some stator winding insulation deterioration has

occurred. However, these tests do not give any indication of where in the winding theproblem is occurring. TVA probe test can answer this question [9].


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2. TEST HISTORY

By the end of the 1940s, Westinghouse Electric Company (US) developed the first Coronaprobe for locating the partial discharges on the bus bars of large generator windings in thestator iron core area. In the 1960s, the engineer L.E. Smith from the Tennessee Valley

Authority (TVA) developed and published the diagram of a Corona probe made up of aradio frequency antenna tuned to 5 MHz and a meter. The probe and the meter werecommonly named the TVA test. ETI Ltd. manufactured the probe and the meter for 25years.In 1997 the company ADWEL / IRIS has taken over the manufacturing of the probe underthe name PPM 97 (Fig. 1), keeping the original design, just replacing the enclosure [8]

Fig.1. TVA Probe PPM 97 type

3. TEST APPLICATION FIELD

According to the obtained information, the test can be applied to all types of turbogenerators, (cooled with air or hydrogen), hydro generators and electric motors, withoutlimitation of power and nominal voltage. In normal conditions, if the safety rules in section4 are observed, there should be no limit to use the TVA test of any voltage rating ofrotating machines. The PPM 97 probe stick is tested at a test voltage of 12 kV for oneminute. The tip is to be always on grounded surfaces, never in contact with High Voltage.


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Depending on the length of the stator core, a probe with a suitable rod length must beused. For very large turbo-generators (300 MW and large) , because of the length of thestator core, it is possible to put the TVA probe operator in the stator bore (this is discussedin more detail later).

The TVA test can be made on site or in the factory. At the same time, the method shouldbe associated with other normal tests made on the insulation of the stator winding, such as

the on-line partial discharges, off-line partial discharges, insulation resistance, tan, highvoltage (High potential), etc.Generally, TVA test is carried out as an additional verificationafter the tests on-line/off-line partial discharges or as a complimentary test to on-linepartial discharges monitoring.

The majority of users perform this measurement occasionally, when the rotor is removed.Others carry out this test when there are suspicions concerning the insulation condition,resulting from other tests. Some experts recommend a frequency of at least once in fiveyears to perform such a test for each generator.

Normally the TVA testcan not be usedfortheendwindings. For this areait is necessaryto useothertypes ofprobessuch asUltrasonicProbe, Corona Scope, etc. However,undercertain conditions the TVA probe can be used, but the guidelines published in IEEE1434will not be applicable.

4. TEST METHODOLOGY

4.1. Principle of the method

The Corona Probe test aims at detecting and locating the partial discharges in the statorwinding bars (coils or half coils) in the slot area. The test is also called the TVA test. TheTVA acronym comes from the name of the company where this test method wasdeveloped: Tennessee Valley Authority (USA).

In principle, the TVA test consists of supplying each phase of the stator winding in turn withan alternating test voltage of 50/60 Hz, and detecting the Corona discharges (partialdischarges) by means of an electromagnetic probe called Corona or TVA probe, in eachslot of the respective phase.

The probe has a ferrite core and a high frequency coil connected to an amplifying-tuningblock and a peak pulse meter in mA (Fig.2). The probe is placed on top long electro-insulating rod. Rods or 1 to 3 meters are commonly used. The probe is positioned on twostator teeth at a time, to form a closed loop iron circuit around the bars of the statorwinding . The probe will detect the partial discharge pulses occurring in the stator barinsulation that will induce damping oscillations into the coil. The closer the probe is to thepartial discharge location, the higher the detected signal measured in mA. The probeoperates in a MHz frequency range coupling (0.5-10 MHz, centered on 5MHz) [1,2,3,4].


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Fig.2. Principle diagram of the TVA probe1.Stator slot; 2 TVA probe; 3. Peak Pulse meter

4.2. The test voltage

The test voltage is the rated phase voltage 3/UUntest = (about 0.6 Un). For example,

for a generator of 24 kV a test voltage of 13.8 kV is required. For a generator or motor of

6.3 kV a test voltage of 3.64 kV is needed (Table 1).

Some users limit the level of test voltage to the value recommended by the PPM 97 probemanual [8], i. e., a maximum of 8 kV. This is a consequence of most common voltage inNorth America, 13.8 kV line to line. It should be noted that at voltages below 4.5 kV partialdischarge activity is less likely to occur because below this level air will not break down.Therefore the PPM 97 probe manual recommends that the minimum test voltage to be 5kV, which does not exclude the use it for locating partial discharges at 6.3 kV electricalmachines [8].

Table 1. Test voltage according to the rated voltage of the electrical machines

Un kV 24 18 15.75 13.8 10.5 8.7 6.3 3.3Utest kV 13.86 10.39 9.09 7.96 6.06 5 3.75 1.9

Note.For electrical machines with rated voltages of 6-7 kV can use a test voltage of 4.5 - 5 kV (overvoltage).For electrical machines with rated voltages of 3-4 kV TVA test is not relevant.


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The test voltage is applied only on a single phase, the other two phases being connectedto ground. For small power generators and motors, all three (six) phases can besimultaneously energized, using an appropriate equipment. The test voltage must bemaintained for an appropriate period of time. Slot scanning by means of the TVA probecan be started only after the stabilization (stilling) of the partial discharges. This is the so

called soak-in or conditioning period. It is required to stabilize the initially high partialdischarge activity (immediately after application of High Voltage) and allow the dischargesto reach their normal level.

The determination of the time needed for the stabilization of the partial discharges is madeas follows:

after applying the test voltage, a number of minimum 5 slots are scanned and therespective values are noted;

after 10 minutes the same slots should be scanned again; if values different fromfirst values are obtained, wait 10 minutes and the same slots should be scannedagain and the new values should be compared with the last values;

the slots scanning should be repeated every 10 minutes and the results should becompared with the values from the last scanning until the same values will beobtained;

it is considered that the partial discharges are stabilized if the same values areobtained for the 5 slots at two successive measurements.

The time needed for maintaining the voltage is the time until the last scanning iscompleted. For the other phases the same time can be used. This determined time is onlyfor the tested generator and only for the respective test. If the test will be repeated after anumber of years, it is possible that stabilization time may be different.If stability cannot be achieved, this indicates that there is a problem with the insulation

system. In such a case the high voltage test (normal maintenance over potential test forone minute) must be carried out.

4.3. Scanning the slots

The test is generally carried out with the rotor removed from the stator for turbo-generatorsand large motors. However, for hydro generators the test can be performed with the rotorin position if there is sufficient space between the poles or by removing a pole. If the rotoris in place, It is necessary to rotate the rotor periodicvally with an approved method, littleby little. Otherwise the test will be incomplete - will not scan all the slots.

Each slot should be scanned. The majority of the users scan the full slot length, from oneend to another of the stator core. Some users measure partial discharges of each slot onlyin one point situated at about 2 - 3 steel plate packs (core sections) at the end or in themiddle of the core or in 3 points respectively at the ends of the core and in the middle.

The continuous scanning at hydro-generators and electric motors is easy as the length ofthe stator core has dimensions compatible with the size of the probe rod. For very largeturbo-generators, because of the length of the slot, a probe with a longer rod (about 3 m)


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must be used or it is possible to scan half a slot length at each end of the stator core toreduce the total test time, it is feasible to scan simultaneously two halves with two identicalprobes. Another possibility is to place the TVA probe operator in to the bore of the stator.This method requires a highly experienced operator. On generators with a small number ofslots, the slots belonging to the energized phase can be marked in advance. Scanning can

be done only on those slots, in which the coil side or bar is then known to be energizedassuming that only one phase at a time is under test.

On generators with a large number of slots it is very difficult to mark in advance the slots ofthe energized phase. In this case, scanning can be made at all slots, but only the slots withenergized bars will be considered.

It is very important to have the winding scheme and the layout of the bars in the slots, inorder to identify them. Also, the correspondence between winding scheme and the realposition must be identified.

The duration of the test depends on the number of slots and the slots length. According tousers experience, for turbo-generator the test duration is about 2-8 hours. For hydro-generators the duration is 4-10 hours up to one day (24 hours). These times refer to theeffective time of measurement and do not include the time for preparation of the test.

Fig.3. Hydro - generator measurements, Pn= 55 MW, Un= 10.5 kV


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Fig.4. Turbo generator measurements, Pn= 60 MW, Un= 10,5 kV

Fig.5. AC Electric motor measurements, Pn= 800 kW, Un= 6.3 kV

5. SAFETY OF THE TEST


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Due to the fact that during the TVA test the operators are near the stator winding which isunder high voltage, the danger of electrocution is genuine. This has slowed thedeployment and development of the method. The danger comes from the risk that theoperator handling the probe might fall on the wires coming out from the high voltage

transformer or on the stator coil ends if he loses his balance. On the other hand, mentionshould be made that, in case such a breakdown or arcing to earth in the slot occurs, thestator bar voltage falls to zero and, in fact, there is no danger, even if the probe is right infront or near the fault location.

The measures that should be taken in order to protect the operators are the following: The stator core should be connected to earth (if the stator is taken out from its

place);

The meter should be connected to earth;

Threeoperators should participate in the test: one to handle the probe, another toread the meter and the third to register the reading data (Figure 3);

A fourth operator handles the controls for application of high voltage (increase,monitoring, reduction to zero);

The slot scanning should start from the second steel sheet (core segment orpackage) and stop before the last steel sheet bundle (core segment); the front endsof the coil should not be touched by the probe;

The two operators that get in contact with the probe and the meter should wearelectro-insulating gloves;

They should be on a platform with a barrier for the operator handling the probe tolean against it, in case the generator is located on the high technological supportand the operator does not get to winding.

The test operators must stay on the opposite side of the turbo-generator, horizontal hydro-generators and motors terminals. If operators must stay on the terminals end a solidscaffold must be built to avoid the risk of losing balance and falling. For the vertical hydro-generators, the risk is low because the operators stay inside the stator and the terminalsare outside.

The rod carrying the probe should be light and long enough to allow the operator to beable to scan the entire lenght of the slot, from one end to another. If the stator lengthexceeds the length of the rod and the two halves are scanned simultaneously the testshould be carried out by 6 operators, 3 for each end.

For very large turbo generators, the TVA equipment handlers will climb into the stator corewith the stator winding de-energized and earthed. Once the TVA equipment operators areinside the stator core, the stator winding will be energized and voltage increased to therequired test voltage. Once the test voltage is reached, and stabilization time of partialdischarge passed, , scanning of the complete length of the stator slots commences. Caremust be taken that the operators stay within the core and not to touch or climb on thestator end winding with the winding energized. Operators can get out of stator, at the endof the test, only after the winding is de-energized and grounded.


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6. THE ANALYSIS OF THE RESULTS AND THE WINDING CONDITIONASSESSMENT CRITERIA

6.1. General method for analysis of the results

According to IEEE Std 1434-2000 - Guide to the Measurement of Partial Discharges inRotating Machinery [12], the measured quantity is the Peak Pulse Value (ppv) and is givenin mA. The measurement results can be presented in the form of a table with the ppvvalues according to the phase and the number of slots (Table 2) [11].

Table 2. PPV measured values for each slot of the energized phases(TG 60 MW, 10.5 kV, 72 slots)

Phase A Phase B Phase CPhases No.slot PPV Phases No.slot PPV Phases No.slot PPV

CA 7 15 AB 19 5.5 BC 67 8CA 8 10 AB 20 7 BC 68 9.5CA 9 14 AB 21 6 BC 69 8CA 10 9 AB 22 6 BC 70 7.5CA 11 14 AB 23 10 BC 71 10CA 12 28 AB 24 10 BC 72 10AA 13 70 BB 25 18 CC 1 25AA 14 28 BB 26 38 CC 2 16AA 15 8 BB 27 52 CC 3 17AA 16 12 BB 28 34 CC 4 18AA 17 14 BB 29 20 CC 5 24

AA 18 17 BB 30 26 CC 6 18AB 19 22 BC 31 30 CA 7 14AB 20 45 BC 32 15 CA 8 22AB 21 12 BC 33 20 CA 9 16AB 22 17 BC 34 14 CA 10 25AB 23 16 BC 35 20 CA 11 14AB 24 18 BC 36 25 CA 12 8CA 43 14 AB 55 5.5 BC 31 7.8CA 44 9 AB 56 10 BC 32 5CA 45 5 AB 57 10 BC 33 5CA 46 4.5 AB 58 8 BC 34 8CA 47 9 AB 59 6 BC 35 6.8

CA 48 24 AB 60 10 BC 36 9AA 49 42 BB 61 14 CC 37 24AA 50 20 BB 62 14 CC 38 22AA 51 25 BB 63 12 CC 39 22AA 52 15 BB 64 12 CC 40 30AA 53 18 BB 65 15 CC 41 24AA 54 17 BB 66 17 CC 42 28AB 55 12 BC 67 20 CA 43 28


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AB 56 13 BC 68 14 CA 44 28AB 57 10 BC 69 12 CA 45 24AB 58 10 BC 70 15 CA 46 24AB 59 8.8 BC 71 13 CA 47 18AB 60 22 BC 72 14 CA 48 22

At the same time the results can be presented in the form of a chart in Excel, Chart Wizardin two variants: Clusters column (Fig.6) or Scatter with data point connected by lines(Fig.7) . Of course there are other ways to present the results.

TG 60 MW, 10.5 kV,72 slots, phase C, slots 31-48, 67-72-1-12

0

10

20

30

40

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71

Slot No.

PeakPulseVa

lue[mA]

Fig.6. Graphic representation in Excel, Chart Wizard, Clusters column

TG 60 MW, 10.5 kV, 72 slots, phase C, slots:31-48, 67-72-1-12

0

10

20

30

40

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71slot No.

PeakPulseValue[mA]

Fig.7. Graphic representation in Excel, Chart Wizard, Scatter with data point

connected by lines

Of all measured values for a slot (continuously along the slot, in 3-point or in a single point)the highest value should be considered in the evaluation. For slots with two bars of the


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same energized phase, the majority of users assign to the two bars the same measuredvalue. If the measured value is high, the bar toward the air gap is considered to have astate of alarm. Interpretation of the results is made for each bar of the energized slots.

Another method of analysis is to plot the TVA probe values as a function of electrical

position of the bar or coil in the winding. By this method, conclusions may be derived as towhether the winding is suffering from electrical or thermal ageing.

6.2. IRIS Power method for analysis of the results(excerpt from the user manual TVA probe PPM 97 type [8])

If the machine to be tested has a relatively large number of slots, it is possible to arrangethe stator coil readings in small groups, to obtain a smoother curve for evaluation. If thereare several series strings or parallel multi-turn coils per phase, the readings for each coil inthe same position for those strings should be averagedIn this case, a curve can be plotted with coil position as the abscissa (X-axis) versus peakpulse meter reading as the ordinate (Y-axis).

An example of this graph arrangement is shown below in Figure 8. It shows 14 coils fromconnection to neutral and 5/6 turns per coils, in each phase, in a winding containing 216slots phase , ((12x5+2x6)x3=216).

HG 55 MW, 10.5 kV, 216 slots, phase B

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

position coil in series string

PPV

[mA]

phase front

phase back

Fig.8. Coil Position versus Peak Pulse Value average Graph

The Figure 9 presents a graph of the measured Peak Pulse Values for each slot of theenergized phase.


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HG 55 MW, 10.5 kV, 216 slots, phase B

0

10

20

30

4050

60

70

80

1 12 23 34 45 56 67 78 89 100 111 122 133 144 155 166 177 188 199 210

Slots No.

PPV[mA]

Fig.9. Peak Pulse Value for each slot of the energized phase.

If the stator winding to be tested is designed with 30 to 50 coils in each parallel circuitbetween the line end and neutral, the same test procedure is used to probe only thosecoils energized by testing one phase at a time with the other two phases grounded. Thedifference in plotting this data is that the coils are often grouped so that two or three coilswhich are electrically adjacent to each other are plotted as one point. If for example, agenerator has 45 coils from the line end connection to the neutral end, it is easier to plotthe data into a meaningful curve if these coils are grouped together in "threes" to yield a 15point curve. In this example, the three coils connected to the high voltage end are groupedtogether and those readings are averaged to obtain a single point for plotting a curve ofthat phase. The next three coil positions are averaged in a similar fashion. This procedureis continued to the final three coils that are connected adjacent to the neutral end of the

stator phase winding. If it is possible to arrange the stator coil readings in groups of two,this could also be done to obtain a smoother curve for evaluation.

The purpose of the grouping is to yield a reasonable number of points that can be plottedto obtain a representative curve for that phase winding. This curve represents the overallcondition of the insulation in that phase of the winding. Analysis of these test curves,together with a knowledge of winding failures, on-line partial discharge analysis (PDA)readings, winding repairs, will help to provide a more complete evaluation of the presentcondition of the insulating system in the stator windings.

This method of analysis although convenient may result in averaging out problem areas.

But the purpose of the TVA probe test is to identify localized sites of high partial discharge.

6.3. Assessment criteria

According to the IEEE Std 1434-2000 [13] the following limits corresponding to theinsulation system can be used:

- Asphalt mica: 100 mA


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- Polyester mica: 30 mA- Epoxy mica: 20 mA-

In the case of the polyester-mica and epoxy-mica insulation systems, the higher values ofthese limits are frequently caused by partial discharges on the outside of the bars in the

slot. There have been cases where values higher than the upper limits have beenregistered due to internaldelamination.

In IEEE Std 1434-2000 it is also stated:Although the comparison of results from similar machines is frequently the best way ofestablishing appropriate limiting values, some users have found by experience that thecondition of coils/bars should be questioned when the peak pulse readings in milliampspeak pulse at operating voltage exceed the values above

Some specialists use their own evaluation criteria, resulting from their experience butthese are generally not made public.

Different examples of some actual evaluation criteria suggested: Using absolute values of TVA probe readings without considering the electricalposition of the bar or coil in the winding may lead to over pessimistic results. Thatis, high TVA probe readings in bars close to the neutral position are not necessarilyof concern. The value of a TVA probe test is significantly enhanced bymeasurement of the discharge extinction voltage from an off-line partial dischargestest.

If on-line partial discharges measurements indicate an increasing trend then withthe first opportunity of removing the rotor the TVA test is performed

If a phase has more bars with values above the limit of the IEEE, perform a AChigh voltage test

The level of risk (epoxy mica): < 20 mA operation without risk , 20- 120 mAmedium risk operation, >120 mA high risk operation and a AC high voltage test isrequired.

Values between 20-100 mA indicate a state of aging, values between 100-200 mAindicate a questionable state (of care) and values over 200 mA indicate possiblefaults in evolution. Values between 200-500 mA indicate the end of the insulationlife time (IGTC) [14].

This Guide proposes the expansion of the IEEE 1434 criteria with the following values:

20-200 mA possibly aged insulation, questionable state, operation with mediumrisk, analyzes correlated with other industry tests are needed;

>200 mA possible faults in evolution, operation with high risk, the a.c. high voltagetest (high potential or hipot) is recomended at a voltage that sufficiently stresses thewinding (e.g. 1.5 Un).


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7. METHOD EFFECTIVENESS AND FUTURE DEVELOPMENT

Although it takes a long time, the TVA test is not dangerous for stator winding insulation,because, in fact, the applied voltage is the voltage of service.

Experts, who have used this method, have established a good correlation between theTVA test results and the results of other tests insulation: on-line partial discharges, off-linepartial discharges, tan delta, leakage DC current, AC High Voltage. However, the test istreated as an additional test after the other tests of insulation, if the rotor is removed.

The test is carried out by the manufacturers of large electrical machines, the users of largeelectrical machines and the service companies for large electrical machines.However, the test should be done only by experienced specialists of HV laboratories.

Future development of the method is uncertain. Safety issues will continue to exist. Robotic methods could solve this problem.

Also, direct recording of measured values and of the geometric position of the probeon the slot on laptops would be useful.

The present quantitative criteria of the IEEE are not agreed by all specialists.

Should only be performed with combined measurements of a multiple of other teststogether with the dissection of the bars with problems.

There were published very few articles about this method and the results obtained,compared with the method of on-line partial discharges. Perhaps this guide will be anincentive for appliance manufacturer to upgrade the device, for the experts to use morethis test and publish more articles on this topic.

BIBLIOGRAPHY

[1] Dakin T.W., Works C.N., Johnson J.S., An Electromagnetic Probe for Detecting andLocating Discharges in Large Rotating-Machine Stators, IEEE Transactions PAS , vol88, No.3, 1969, pp 251257

[2] Smith, L.E., A peak- Pulse Ammeter Voltmeter Suitable for Ionization (Corona)Measurement in Electrical Equipment, Minutes of the 37thAnnual InternationalConference of Doble Clients, 1970, Section.3-401

[3] Goodwin, T.A., Corona Probe Measurements Taken on Hydro Machines at Grand

Coulee Dam, Minutes of the 38

th

Annual International Conference of DobleClients,1971[4] Vorobev S. E, Gluhov I.S. , Muhina A.A. Vaiavlenie sterjnei s vnutrenimi pustotami

v izoliatii vasokovoltnah ghidrogheneratorov(Identifying bars with internal voids in thehigh voltage insulation of hydro generators) Electriceskie Stantii (Russia) no. 11,1979, pp.40- 43


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[5] McDermid, W., How Useful Are Diagnostic Tests on Rotating Machine Insulation ?,Proceedings of the 19th Electrical Electronics Insulation Conference, Chicago,Illinois,September 25-28, 1989, pp. 209

[6] Timperley, J. E., Corona probe testing of hydro-generators, Minutes of the 56thAnnual International Conference of Doble Clients, Section 7-4.1, 1989.

[7] Timperley, J. E., Improvements in corona probe testing of modern hydroelectricgenerators, Proceedings of the 19th Electrical/Electronics Insulation Conference,Chicago, Ill., IEEE 89CH2788-8, pp. 300304, 1989.

[8] Corona probe model PPM 97, Operating Instructions, Adwel, 1997[9] Stone G.C., Boulter E.A., Culbert Ian, Dhirani H. Electrical Insulation for Rotating

Machines. Publishing House IEEE Press, USA, 2004, 372 p, ISBN 0-741- 44506-1[10] Gheorghiu M., Gheorghiu C., Zlatanovici D. Checking electrical insulation power

equipment. Publishing House Editura Tehnica, Romania, 1984,126 pp,[11] Zlatanovici Dan, Test code of electric generators, vol II,ICEMENERG Publishing

House Bucharest, 2007 pag.152, ISBN 973-1741-01-7[12] IEEE Std 1434-2000 - Guide to the Measurement of Partial Discharges in Rotating

Machinery[13] IEEE Std 100-1996 - IEEE Standard Dictionary of Electrical and Electronix Terms.Sixth Edition

[14] International Generator Technical Community (IGTC),www.GeneratorTechnicalForum.org


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ANNEX 1: CORONA PROBE (TVA) TYPE 97 PPM PERFORMANCE

Continuous wave sensitivity: 0.9 V RMS continuous at 5 MHz gives a full scale

reading on the 100 mA range, with batteries at 8V.

Bandwidth: 0.5 MHz - 10 MHz

Pulse polarity: Reads correctly only on negative pulses, or the negativeswing of ringing pulses.

Minimum pulse length: 35 ns approximately.

Effect of pulse length: Reads approximately 25% higher on a very long pulse(i.e. milliseconds) than on a 35 ns pulse.

Effect of ringing pulses: Oscillatory (ringing) pulses within the bandwidthof the preamp will read higher than a single shortpulse. As the ringing time ncreases, the reading willapproach that of a long pulse.

Pulse repetition rate: Reading is not affected by pulse repetition rate ifgreater than approximately 20 pps.

Effect of battery voltage: Reading decreases approx. 10% as the battery outputdrops from 9V to 7V.

Batteries: Two (2) 9V

Input ranges: 0-10/30/100/1000 mA selection.

Test switch: Either momentary (down position) or continuous (upposition) can be selected by depressing the ON-OFF-ON switch on the side of the instrument.


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ANNEX 2: Excerpt from IEEE Std 1434-2000 on the electromagnetic probe

IEEE Std 1434-2000

IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Machinery

6. Electrical pulse and RF radiation sensing systems(pag.12)

Table 2Off-line partial discharge measurements. Case 5.(pag. 21)Sensors: Electromagnetic probe bridging each slot in turn.

Detector/output: Peak responding meter scaled in milliamps or quasipicocoulombs.Frequency range: Probe tuned to 5 MHz.

Selectivity/attenuation:As a result of winding attenuation at 5 MHz, a partial discharges site can be located to within oneor two slots.User needs to be aware that some partial discharges sites become less active after the test voltagehas been applied for a period of time.If detection of phase related partial discharges sites is required, an appropriate electric stressneeds to be applied to this insulation.Pros and cons:Not always reliable for pinpointing the partial discharges site in a given slot. Does not discriminatebetween partial discharges occurring on the positive and negative half-cycles of 60 Hz. Can usuallytest with hydrogenerator rotor in place.

6.3.1 Electromagnetic probe(pag. 25)

The electromagnetic, or TVA, probe is a proximity sensor that responds to the RF signals radiatedby partial discharges within or outside the stator coils. The search coil probe consists of an 1 minsulated rod with a multiturn coil wound on a ferrite core at one end. A typical coil consists of 15turns of AWG #14 wire wound on a 10 mm diameter ferrite rod, although some users haveemployed a lesser number of turns. The ferrite rod is about 50 mm in length. The sensor coil isattached to a coaxial cable for connection to the peak pulse meter.


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The probe and its connecting cable capacitance form a tuned circuit, which is usually tuned to 5MHz. Frequencies from 200 kHz to 20 MHz have been evaluated. In some machines, a higher orlower frequency was found to be more sensitive than 5 MHz was in detecting partial dischargesactivity. However, standardization on 5 MHz is recommended to permit comparison of databetween machines.

The closer the antenna is to the source of partial discharges, the higher the signal output. It is usedto identify locations of partial discharges within a high-voltage winding. The magnitude of the signalis displayed in relative terms and may not be useful for specifically quantifying the level of partialdischarges. The probe and peak pulse meter cannot distinguish between slot discharge andinternal coil partial discharges, and they do not discriminate between positive and negative pulses.However, if the probe is terminated in an oscilloscope instead of the peak pulse meter, the polarityof the pulses can be determined.

This sensor is used for off-line tests with the winding energized at the normal phase-to-neutralvoltage, or at lower voltages.

11.2 Electromagnetic probe readings (pag.40)

Measurement of partial discharges at individual slots using the electromagnetic probe, tuned to 5MHz, and the peak pulse meter results in readings in terms of milliamps peak pulse. (Othersimilar peak pulse meters are scaled in quasi-picocoulombs.) These readings are not greatlyaffected by the winding configuration, but they do reflect the particular insulation system and thecondition of surface coatings and end arms.

Although the comparison of results from similar machines is frequently the best way of establishingappropriate limiting values, some users have found by experience that the condition of coils/barsshould be questioned when the peak pulse readings in milliamps peak pulse at operating voltageexceed the following values:

Asphalt mica: 100 mA

Polyester mica: 30 mA Epoxy mica: 20 mA

In the case of polyester-mica and epoxy-mica insulation systems, readings in excess of these limitsfrequently are caused by slot discharge, for which corrective measures are available. However,cases have also been reported involving polyester- and epoxy-bonded systems in which readingsin excess of the above limits have been as a result of internal delamination.

These limits apply to ferrite probes and peak pulse meters that have been constructed asdescribed in Smith [B40 - IEEE Std 1434-2000, pag.37] / [2], and where the ferrite probe is held incontact with the stator core, across the slot in which the coil or bar in question is installed.

581 -- guide - corona electromagnetic probe tests (tva)

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