Roceedingr of the 4th international Conference on Ropertier and Applications of Dielectric Materials July 3-8, 1994, Brisbane Australia

Paper 6211

H van der Merwe Eskom Technology Group (T-R-I) south Africa

ABSTRACT The Electricity Supply Commission of South Afiica (ESKOM) and Electricit6 de France (EDF) BIT currently involved in a technical exchange agnement whereby the effectiveness of the EDF method for ststor winding insulation condition assessment,

This method basically entails off-line electrical quadratic rate partial discharge mcasurrments, using relatively low center-&equcncy nmowbandfilters. Initial experience gained through "cnts on huo turbo generntm of Eskom in South Afiica, has shown the ability of this method to .acmately indicate the insulation wndition of statorwindings. On one of the generators tested, excellent comlation was found between the indications of test results and the tindmgs of a stator bar visual inspection and dissection. Dissipation factor and conventional peak-value partial discharge mensuremen& have in the past failed to indicate the extend of insulation damage (discovered through visual inspections) of this specific winding.

World-wide, financial and environmental factors are necessitating refurbishment and life extension (or life management) of power generating facilities nearing the end of their design life. Plant managers can therefore expect an increasing incidence of in-seMce equipment failure. in years to come. The integrity of stator winding insulation in high voltage generators is a crucial parameter in the estimated total reliability of a power generating unit. Detection of partial discharges (PD) is widely seen as an effective method by which the condition of high voltage insulation systems can be assessed, i.e. [Campbell, 11. Across the world, many different methods for the detection of PD pulses in the stator winding insulation of high voltage generators, are k ing employed.

specifically on hvbo generaton. will be investigated.

1. INTRODUCTION

2. CONDITION MONITORING OF STATOR WINDING INSULATION

The Electricity Supply Commission of South Africa (ESKOM) and ElectricitC de France (EDF) are currently involved in a technical exchange agreement whereby the effectiveness of the EDF-method for stator winding insulation condition assessment, with specific emphasis on turbo generators, will be investigated. This method has been discussed in [Audoli, 21, [Audoli. 31 and [Audoli, 4). It basically entails off-line electrical quadratic rate PD measurements, performed on the

JL Drommi ADW Wolmarans EDF @TG) Randburg France south Africa

insulation of high voltage generator and motor stator windings. Each phase winding is in turn stressed with AC voltage up to a maximum of 1.2 times the relevant phase to ground voltage. The remaining phase windings of the machine are grounded to the magnetic core in each case. Coupling capacitors of 0.1 @ and 250 pF at each end of the energized winding (refer to Fig. 1) allows electrical detection of discharges in the winding insulation. A typical partial discharge pulse exhibits Dirac-type characteristics, i.e. it has a very wide-band frequency spectrum. Whilst traveling to the measuring terminal, the higher frequency components of such a pulse are attenuated more than the lower frequency components.

2.1 The 10 kHz and 130 kHz selective measuring frequencies

Measurement of discharges in a narrow fnsuency band, centered around 10 kHz, enables assessment of the winding insulation condition as a whole. Measurement of discharges in a narrow fresuency band, centend mund 130 kHz, enables assessing the winding line side and winding neutral side insulation condition separately (only discharges originating a few bars away from the measuring terminal on the line or neutral side of a phase winding, still contain measurable energy in this band when picked up by the discharge detector).

2.2 The quadratic rate measured quantity Phenomena like mutual electromagnetic coupling, attenuation, resonance and traveling waves, complicate the interpretation of PD test data on high voltage stator winding insulation systems. Furthermore, due to the intricate geometry of these windings, corona-, slot-, surface- and internal discharges can at certain voltages OCCUT simultaneously. Measurement of the highest/* discharge magnitude is therefore not adequate to determine the hamfuhess of each discharge phenomena. Instead, an indication of the combined electrical energy content of any discharge type or combition of discharge types at any stage, can be provided through measurement of the charge quadratic rate quantity. Discharge levels in the solid insulation system of high voltage generators and motors are very high compared to for instance the paper oil insulation system of most

0 ~ 1 3 0 7 ~ . W 0 1994 IEEE

transformers. The discharge quadratic rate quantity for This generator was used for short term peaking these machines is therefore expressed in decibels. generation as well as synchronous condenser operation. For the 10 kHz measurement: Odb = 10-9C2slF1, whilst The above tests were conducted shortly before for the 130 = 1 0 - 1 6 ~ 2 ~ 1 [Audoli, commencement of a planned stator rewind for strategic 241. T~ d e the -6 of one W h i n e comparable old at with that of another machine of the same voltage rating, the time of test. the 10 kHz meabunment is normalised by dividing the Low voltage DC testing indicated that the winding could m~quadraticratequantitywiththecapacitanccof be used normally in the immediate future, i.e. that the winding. winding failure was not imminent: One minute insulation A diagram of the basic test setup used for the PD test &stance = 2600 Mn, polarisation index (pr) = 3.5 and described above, is given in Fig. 1. normalised one minute resorption/discharge current = 0.8

“ I F . By studying the oscilloscope display during the partial discharge test and using the guidelines of [Cigre, 51, the following conclusions were drawn:

Incephon of external discharge8 todc place at about 3 kV. Above 4.6 kV, internal groundwall discharges became

The winding w a ~ more or less 17

L,,,

very prominent. At the inception voltage of internal discharges (about 4.6 kV), considerably higher discharge levels for the 130 kHz measurement were detected from the line side terminal than from the neuttal side terminal. This was true for all three phase windings of the machine, although levels for

Figure 1 Diagram of test setup used for quadratic rate PD tests on generator stator winding insulation

InFig. 1: The HV fransformer is typically a series or parallel resonant test set, used to energise the winding under test to 1.2 times the relevant phase to ground voltage. The Quad Rate Measurer is the quadratic ratc partial discharge detector equipped with the selective filters and a true RMS voltmeter. The Relay Box contains high frequency blocking impedances connected to earth, and relays (the latter is used to switch the d e s i d signal through to the discharge detector). Closing only relay A for instance, will switch all discharge signals passed by the 0.1 pF capacitor on the line si& of the machine winding through to the discharge detector. The Oscilloscope is wd as an auxiliary tool, to help visualising the discharge phenomena (especially noise pulses) picked up by the discharge dacaor.

3. INITIAL TEST EXPERIENCE Low-voltage capacitance (5 V, DC), low voltage polarisation (500 V, DC) and quadratic rate partial discharge measurements (120 % of AC phase to ground voltage), were performed on two turbo generators of Eskom in South Africa.

3.1 Tests on phase windings of a 12 kV, 50 MW gas-turbine generator

A 12 kV. 50 MW gas-turbine generator was tested. It had a 3 phase, 2 pole, 42 slot, double layer stator winding, with a 1-18 coil pitch, diamond coils (lap winding), pole phase groups connected in series, wye/star c o d o n and class-F insulation with class-B permitted temperature rise.

the &dividuaI phases were not consistent. It can therefore be concluded that electrical aging of the groundwall insulation had taken place on the line side of all three phase windings. Table 1 shows the 130 kHz discharge levels detected at the line and neutral side terminals of each phase winding, at approximately 4.6 kV.

Table 1 Discharge levels (db) at 4.6 kV

At the tcst voltage increments betwcen 4.6 kV and 7 kV (i.e. above the inception voltage of internal groundwall discharges), discharge levels detected by the 130 kHz measurement at the tine and neutral ends of the winding respectively, were almost the same. This could indicate that besides the electrical aging on the line side of the winding, the neutral side groundwall was also of inadequate quali& (probably due to thermal and/or mechanical stmscs). Above. 7 kV, exceptionally high partial discharge levels caused saturation of the discharge detector. Note that the rated working voltage of this machine under steady-state conditions is alrcady 7.6 kV. Using the oscilloscope, slot discharges were identified as the main cause for these high levels (a small number of high energy pulses, according to [Audoli, 31 and [Cigre, 51). Slot discharges indicate loose slot wedges and/or inadequate or damaged side packing of the stator bars. The corona varnish of the stator bars may also be eroded. Visual inrpoctions of the winding in the past, confirm these indications.

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3.2 Tests on individual stator bars of a 12 kV, 50 M W gas-turbine generator

Individual stator bars of a 12 kV, 50 MW gas-turbine generator were tested. The bars had been removed from the magnetic core when the stator was rewound some months ago. This generator had identical specifications to the generator discussed above (including the same manufacturer, commissioning year and location). Aftcr a visual inspeaiOn. bars 38, 35 and 22 (all bottom bars) were chosen to be tested. These bars represented a good cross-section of the degree of visible insulation damage. The earth electrode used for these tests was made with aluminium foil. wrapped closely around the slot section of each bar.

Inception of internal pundwall discharges was observed above 3 kV. A considerable drop in PD level with time at maximum voltage indicated groundwall void neutralisation and therefore electrical aging of the line side groundwall. This test interpretation is consistent with the location of bottom bar 38 in the complete winding (5th bar from line side on the blue phase, i.e. subject to approximately 82% of phase voltage under n o d operating conditions). Inception of high energy slot discharges was observed above 6 kV. The PD detector, however. did not saturate. DC tests: The polarisation index (PI) was low (approximately 1.38) and the normalised 1 minute resorptioddischarge current was high (1.57 "IF) This is a bulk indication of poor winding quality.

Inception of internal groundwall discharges was observed above 3 kV. No definite drop in PD level with time at maximum voltage was observed, indicating a lack of groundwall void neutralisation and therefore an absence of electrical aging ofthe line si& groundwall. This test interpretation is consistent with the location of bottom bar 35 in the. complete winding (13th bar from neutral si& on the white phase, i.e. subject to only about 46% of phase voltage under normal operating conditions). Inception of high energy slot discharges was observed above 6 kV. The PD detector, however, did not saturate.

Inception of internal groundwall discharges was observed above 2.6 kV. No definite drop in PD level with time at maximum voltage was observed, indicating a lack of groundwall void neutralisation and therefore an absence of electrical aging of the line side groundwall. This test interpretation is consistent with the location of bottom bar 22 in the complete winding (14th bar from neutral side on the red phase, i.e. subject to only about 50% of phase voltage under n o d operating conditions). Inception of high energy slot discharges was observed above 6 kV. The PD detector saturated above 8 kV. At maxi" test voltage, audible discharges localised at the two ends of the bar. were dimmed.

Tests on bottom bar 38:

Tests on bottom bar 3 5

Tests on bottom bar 22:

Test voltage

0

3.3 Autopsyldissection of bottom bar 22: Audible discharges at maxi" ttst voltage were observed only on bar 22. Also, the discharge detector only saturated when testing bar 22. It was therefore decided to perform an autopsyldissection on this bar (i.e. to remove sections of its groundwall insulation). Autopsy at one end of the bar slot-section: The relevant portion of groundwall insulation did not stick to the winding conductors and was removed easily. Also, a lack of binding material was observed (the insulation did not break when peeled off from the winding conductors). Almost every layer of insulation tape could be separated from one another. Autopsy in the middle of the bar slot-section: The relevant portion of groundwall insulation stuck to the winding conductors and could not be removed easily (the insulation broke when peeled olT from the winding conductors). The binding material formed a reasonably solid structure with the tape layers. Layers of tape could not be separated easily. These autopsy results of bottom bar 22, correlates with the audible discharges discemed only at the ends of this bar during discharge testing. During curing of the bars in the manufaauring process, the insulation binder (probably polyester) of the groundwall could have leaked away through both ends of the bars. No sign of white powder (due to partial discharge activity) could be found in the dissected groundwall sections or on the exposed copper of the winding conductors. This visual observation is consistent with the location of bottom bar 22 in the complete winding (14th bar from neutral side on the red phase) and continns the interpretation of PD test results. Table 2 shows the 10 kHz PD measurement against voltage for the red phase winding of the first 12 kV generator and bottom bar number 38 of the second 12 kV gemtor .

Disc- levels ldbl Redphasewindingl Bottom bar 38

-24 I -28 11 6.9

Saturation 44 9.15 Saturation 50

Table 2 Discharge levels for the 10 kHz measurement (db)

This table, together with the similar PD patterns observed in both cases, shows that the condition of the tested stator bars is a representative indication of the general condition of the tested phase windings. This in turn indicates wide-spread insulation damage in both cases. Note that although the stator bars and phase windings tested were from diffemt machines, these machines were identical in terms of specifications, manufacturer. -

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coounissioning year and duty cycle (the latter approximately).

3.4 Tests on the phase windings of a 20 kV, 600 MW turbo generator

A 20 kV, 600 M W , combined hydrogen cooledlwater cooled hutto generator, was tested during a general outage. The machine was degassed and the terminal bushings included in the test circuit. The stator winding cooling water system was drained, but the F'TFE pipes was not blown dry. The rotor of the machine was r e m d and the endwvers taken off. At the time of tat, the winding was about 14 ycars old, with approximately 80% of generating time. Because of trapped columns of water in Seaions of the stator winding cooling system, polarisation tests (low voltage DC) did not yield meaningful results. Inception of PD oavrred at a test voltage of about 5 kV. Discharge levels in the winding wen found to be low, up to the maximum test voltage. Only external PD of low amplitude and some corona discharge were detected. This indicates that the stator bar packing and slot wedges are in a good condition. Note that this machine has m n t l y been &ged. No neutralisation of internal groundwall voids was observed, i.e. the total discharge level at maximum voltage did not drop with time. This indicates a good quality (solid and well cured) groundwall. There is a noticeable inconsistency of discharge levels between the red phase and the other two phases. This phenomenon has to be monitored in years to come. The measurement discussed here will be used as a reference in this process. It is " m e n d e d that the next PD test on this generator take place within two years.

4. DISCUSSION AND CONCLUSIONS The novel aspeas of the EDF method for stator winding insulation condition aSScSSment have been described. Test results can be summarised as follows: 0 The 20 kV, 600 MW generator stator winding: Test results indicated good quality -tor bar packing and slot wedges. This assessment is backed up by the fact that this winding has recently been rewedged after visual inspection. Due to an inconsistency of results between the red phase and the other two phases, a follow up PD test within two years is " m e n d e d . 0 The 12 kV, 50 MW generator stator winding: Test d t s indicated very high levels of slot discharge. This assessment is backed up by visual inspections in the pasf that have shown bar looseness due to inadequate side packing. The results of the stator bar testing gave a representative indication of the general condition of the tested phase windings (refer to table 2) - Insulation damage in both cases was wide spread, i.e. not lited to only a few bars. Note that although the stator bars and phase windings tested were from different machines, these machines were identical in terms of specifications, manuhchmr.

commissioning year and duty cycle (the latter approximately). At higher test voltages (above 7 kV) on the complete winding, the detector saturated due to high levels of slot discharge. This did not happen when testing the individual bars. In the latter case, the stator magnetic core was simulated by earthed aluminium foil wrapped closely around the slot sections of the bars. It could therefore be concluded that the bars in the complete winding were looser in the core slots than the individual bars were in the earthed wraps of aluminium foil. In the past, dissipation factor and conventional peak value partial discharge measurements have failed to indicate the extend of insulation damage (discovered through visual inspeftions) on the phase windings tested here. The visual findings of two dissected groundwall insulation sections of a stator bar, correlated very well with the origin of audible discharges (only at the ends of the bar), discerned during partial discharge testing: 1. The groundwall insulation in the center of the slot

Seaion of this bar had an adequate amount of binder and was relatively solid. Sticking to the winding conductors was also good.

2. The groundwall insulation at the edge of the slot section of this bar, however, had almost no binder material and showed severe delamination. There was also almost no sticking to the winding conductors.

Tests conducted on two turbo generators of Eskom in South Africa have shown the ability of this method to pick up various forms of stator winding insulation deterioration.

5. REFERENCES 1. campbell, SR etal. : "Roctic01 on-line pmtiol dischmge

tests* Mine genemtors and mot&, Appmwd for pnsentatfon at the EEJPJZS 1993 Summer Meeting.

"And@ of portid dischotges meosuremts ond generotor technology evolution", proacdingr of the 3rd International confauwre on Properties and Applications of Dielectric Mataials (ICPADM ), Tokyo - Japan, July 1991, pp. 687 - 690.

V m - CanaQ July 1993. 2. AuQli, A and D"i, JL:

3. AUQLi A and "mi, JL: "Genemtor and motor stotor monitoring b m d on portid dischme quohtic rote meaawment", IEEE lntaoational Symposium on

4. AuQli, A and Merigot, C: "Use ofdielecbic tests (IS an ddfor h y h generotor maintenonce", Conferemx "d of the IEEE International Symposium on Electrical

5. Cigre Working Group 2 1 03 - Electra no 1 1, "Recognition

El&d Insulation, Baltimorr, J U ~ C 1992, p ~ . 359 - 362.

MOII, To~O~~O - CanaQ, J ~ n e 1990, pp. 379 - 382. ofdischqes", Dazmber 1%9. pp. 61 - 98. 6. ACKNOWLEDGEMENTS

The authors wish to thank Eskom for funding the work reported on here and EDF for making their equipment available for the tests.

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