On-line partial discharge monitoring of statorwindings :comparison of different sensorsPemen, A.J.M.; Leeuw, de, W.; van der Laan, P.C.T.

Published in:Proceedings of the 10th International Symposium on High Voltage Engineering, volume 4, Montreal, Quebec,Canada, 25-29 August 1997

Published: 01/01/1997

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describe the measurement technique, by means of an exam­ple of a measurement on a 650 MW turbine generator.

2. Measurement technique

Unfortunately, for the monitoring of pd's during regular operation, standard pd-detectors can not be used. The main reason for this is the presence of many sources of EM-disturbances in a power plant. Standard pd-detectors operate in a frequency range < 1 MHz. Within this range large interference is caused by the rotor excitation.

In [1] we show that this problem can be solved by using a tunable narrow-bandwidth filter as a front-end for a standard detector. The filter is tuned to a frequency where partial discharges dominate and no interference is present. Our experience is that the best results are obtained at tuning-frequencies between 10 and 30 MHz. As a tunable narrow-bandwidth filter we use a spectrumanalyser in the "zero-span" mode. The output of the spectrumanalyser is connected to a modem pd-detector (Haefely TE571). This detector provides a digital registration of all pd quantities, such as the discharge magnitude, the time of occurrence as related to the 50 Hz voltage, the instantaneous inception voltage, the discharge energy and the quadratic rate. The results can be shown in many ways, as 20 and 3D patterns or pulse-height histograms. Also the detector has built in tools for recognition and classification of pd patterns [2].

Figure 1 shows a result measured in phase " 1" of the 650 MW generator "Hemweg-8". During the measurement the generator was operating at full load. The spectrum analyser is tuned to 20 MHz. The figure shows the phase resolved distributions HQmax (the maximum discharge magi­tude), HQn (the mean discharge magnitude) and H,. (the number of discharges). After calculating and storing a 'fingerprint' of 29 statistical parameters, the measurement can be classified. This means that the result is correlated with a database, which can contain fingerprints of: (i) ear­lier measurements on the same generator, (ii) measure­ments on similar generators, (iii) known defects and (iv) known ageing states. Figure 2 gives the classification of the measurement in Fig.I ("KWARTAAL 4/FASE 1") with reference to an earlier measurement on the same generator ("KWARTAAL 3/FASE 1 "). The correlation between the two measurements is nearly 100 % , which means that the discharge pattern has not changed.

3. Comparison of different sensors

An important part of a pd measuring system is the sensor. For the measurements in the previous Section we used Rogowski-coils, installed around the high-voltage terminals of the generator. Besides Rogowski-coils, we

developed capacitive sensors that are housed inside a stand­ard support-insulator of the isolate-12hase-hus (ipb) (Fig.3). Both sensors are described in [4] and have a good sensitivi­ty, bandwidth, EMC-properties and can easily be installed.

Recently, we installed capacitive sensors in the new 165 MW turbine generator "Roca-3 ". For each phase, two sensors are installed, one at the generator side and one at the step-up transformer side. Besides the capacitive sen­sors, in each phase two other capacitors were available: "ring" capacitors and overvoltage capacitors. The "ring" capacitors are shown in Fig.3c. A metal ring (35 cm. wide) is installed inside the ipb. The ring is isolated from the metal ipb and connected to a N-connector. The over­voltage capacitors are installed inside the enclosure of the generator-circuit-breaker (Fig.4a). The equivalent circuit for all three sensor types is given in Fig.4b. The capacitan­ce values are given in Table 1.

Table 1 values for the three sensor types

sensor Cl C2 R

capacitive sensor 250 pF 10 pF 50 0 ring capacitor 16 pF 182 pF 50 0 overvoltage capacitor 132 nF 8 µF 50 0

(s) (b) (c)

Fig.3 (a) Isolated phase-bus (ipb), with (b) a capacitive sensor inside a support insulator, and (c) a •nng·-capaci­tor inside the ipb

(a) (b)

Enclosure of the generator-circuit-breaker

Overvoltage j_ capacitor

:~tg Fig.4 (a) Overvoltage capacitor house inside the coaxial enclosure of the generator-circuit-breaker, and (b) equiva­lent circuit for capacitive sensors

Figure 5 gives the frequency spectra as recorded in phase W, with a capacitive sensor, a ring capacitor and a overvoltage capacitor. It is clear that the capacitive sensor has the best sensitivity, while the sensitivity of the overvol­tage capacitor is the lowest. The spectra show several peaks caused by partial discharges. These peaks are a result of resonances in the statorwinding or reflections of the signal in the ipb. Preferably, for discharge measure­ments the spectrum analyser is tuned to one of these peaks and an important question is to which peak.

To find the best tuning frequency, for each sensor a serie of zero-span measurements is done. Measurements are done at 30 different tuning frequencies (from 10 to 70 MHz, with an interval of 2 MHz) and the signal level at these tuning frequencies is recorded during several cycles of the 50 Hz voltage. The best results for phase U are given in Fig.6. For all sensors the best tuning frequency is 10 or 12 MHz. At these frequencies, the sensors have the largest sensitivity and no interference is recorded. As can be seen, the capacitive sensor at the generator side Cg has the largest sensitivity, but also the other sensor types give good results.

-50

E' Cx:i ~-60 l S? -70

-80

0

: ;~

·········· - ··-

· ····· · ·· capacitive sensor -- 'ring' sensor · · · · ovsrvoltage capacitor

15 30 45 frequency (MHz)

60

Fig.5 Frequency spectra as measured in phase W of gene­rator #Roca-3 #, by means of J different sensors

G

~ 0 -200

(.) 20 ~ 0 0

T

-20 toot

0 -10~=[ =======~__._ _ _.,,..., _ ___._ _ __,., __ 0 200 400

time (ns)

Fig.7 Waveshapes of a partial discharge in phase V of generator # Roca-3 #' measured simultaneous with 4 different sensors

Figure 7 gives the waveshapes of a partial discharge in phase V, measured simultaneously with all 4 sensors in this phase. From the transit times .1tl and .1t2 it is seen that this signal comes from the generator. In case of a dischar­ge in the step-up transformer, the signal would first show up at sensor Ct. So installing 2 or more sensors in one phase enables the localisation of a discharge. The transit time .1tl is 42 ns (or 12.7 m) and corresponds with the distance between the capacitive sensor at the generator side Cg and the overvoltage capacitor OV-C. The transit time .1t2 is 44 ns (or 13.3 m) and corresponds with the distance between the overvoltage capacitor OV-C and the capacitive sensor at the step-up transformer side Ct. The distance between the ring capacitor and the capacitive sensor at the generator side Cg is 30 cm, and the corresponding transit time of 1 ns cannot be seen in Fig. 7.

As with the spectrumanalyser measurements, the capa­citive sensor at the generator side Cg gives the largest signal. This signal of about 250 m V can easily be detected with a digital oscilloscope. However, the signals from the ring capacitor and the overvoltage capacitor have an ampli­tude of about 20 m V and already show bit-noise. These amplitudes are close to the limit of what can be detected with an oscilloscope. This shows a great advantage of the spectrumanalyser: a much larger sensitivity so that small signals can be detected reliably.

4. Discussion

The results in the previous section show that, although the capacitive sensors have the largest sensitivity, all three sensor types can be used for on-line partial discharge measurements. The measurements can be done with a spec­trumanalyser tuned to a frequency where partial discharges

dominate and less interference is present. This interference has two ways of distorting a measurement: (i) by coupling into the measuring system, and (ii) by coupling into the statorwinding. Coupling into the measuring system must be avoided by state of the art EMC measures [7]. For the measurements in the previous section, all sensor types have good EMC-properties and are shielded by the ipb or by the enclosure of the generator-circuit-breaker. All connectors and coax-cables have a low transferimpedance and the recording equipment is installed in an EMC-cabinet. Cou­pling of interference into the statorwinding causes 'legiti­mate' signals at the sensors, but can be rejected by tuning to the correct frequency.

This tuning technique is not effective when interfe­rence couples into the measuring system, as we noticed during measurements on turbine generator "Claus-B". Here the overvoltage capacitors are installed in cabinets outside the ipb, and have long leads to the ipb and the grounding

. system of the plant. A magnetic flux <I> can couple into the large loop (about 6 m2) formed. This magnetic flux <I> can be a restilt from any source and results in an interference voltage v. = -d<I>/dt.

Figure 8 gives the results for measurements in phase U of generator "Claus-B", for tuning frequencies of 10, 20 and 30 MHz. At all frequencies (also at 12 other tuning frequencies between 5 and 40 MHz) interference is recor­ded. However, off-line measurements show large discharge levels upto 500 nC. The discharges are caused by cracks in the endwindings. The cracks are a result of vibrations of the end-windings due to loose blockings. After consulting the manufacturer it was decided to rewind the generator.

Surprisingly, the on-line measurements gave no indi­cations for the presence of large partial discharges, since interference dominated the measurements. This proves that it is very important to suppress interference as much as possible, preferably by good EMC-measures. The use of a tunable narrow-bandwidth filter (or signal processing) in itself are not enough to reject interference and can only be used for further improvement.

5. Conclusions

Several sensor types (capacitive sensors, overvoltage capacitors and "ring" capacitors) were used for partial di­scharge measurements on a 165 MW turbine generator. The discharges were recorded with a standard pd-detector, preceeded by a tunable narrow-bandwidth filter. The filter is tuned to a frequency where discharges dominate and less interference is present. All three sensor types give good results at tuning frequencies of 10 or 12 MHz; the best sensitivity is obtained with capacitive sensors.

Good EMC-measures are of major importance for on­line pd-measurements. On-line measurements on poorly

installed (from an EMC point of view) overvoltage capaci­tors of a 580 MW generator only showed interference, while off-line measurements showed large discharges of >500 nC.

5 10 15 time (ms)

Fig.8 On-line measurement with a over-voltage capacitor, installed outside the isolated-phase-bus of the 580 MW turbine generator "Claus-B"

Acknowledgements: this research was undertaken at request of the Dutch electric supply companies. The authors gratefully acknowledge the cooperation of EPON, EPZ, EZH and UNA. The contribution of the working group "Generator railsystemen'' is very much appreciated.

References (1) A.J.M. Pemen, P.C.T. van der Laan: "Pitfalls of pd measu­

rements on statorwindings of turbine generators", 7th DMMA, Bath, UK, September 1996, pp.394-397

(2) E. Guiski: "Digital analysis of partial discharges", IEEE Trans.on Dielectrics and Electrical Insulation, Vol.2, No.5, October 1995, pp.822-837

(3] H.G. Sedding, S.R. Campbell. G.C. Stone, G.S. Klempner: "A new on-line partial discharge test for turbine generators", IEEE Trans.on Energy Conversion, Vol.6, No.I, December 1991, pp. 700-706

[4] A.J.M. Pemen, P.C.T. van der Laan, P.T.M. Vaessen: "Sensors for pd monitoring of turbo generators", 28th UPEC, Staffordshire, UK, September 1993, pp.594-597

[5] J.E. Timperley: "Incipient fault identification through neutral rf monitoring of large rotating machines", IEEE Trans.PAS, Vol.PAS-102, No.3, March 1983, pp.693-698

(6) P. Grunewald, J. Weidner: "Possibilities and experience with off- and on-line diagnosis of turbine generator stator winding insulation", 1994 CIGRE Session, paper 11-206, Paris, 1994

[7] P.C.T. van der Laan, A.P.J. van Deursen: "Protection of electronics in high power installations; theory, guidelines and demonstrations", CIGRE Symposium on EMC, Lausanne 1993, paper 600-08

Adresses of authors KEMA Transmission & Distribution P.O. Box 9035, 6800 ET Arnhem, The Netherlands phone +31 26 356 23 22. fax +31 26 351 3683

Eindhoven Univ. of Technology, High-Voltage & EMC group P.O. Box 513, 5600 MB Eindhoven, The Netherlands phone +31 40 247 4433, fax +31 40 245 0735