Zeszyty Problemowe – Maszyny Elektryczne Nr 80/2008 91

Josef Pihera, Oldřich Tureček, Petr Martínek University of West Bohemia, Pilsen

ANALYSIS OF PARTIAL DISCHARGE DETECTED WITH SENSORS

OF EMITTED ELECTROMAGNETIC FIELD

ANALIZA WYŁADOWAŃ NIEZUPEŁNYCH WYKRYWANYCH CZUJNIKAMI POLA ELEKTROMAGNETYCZNEGO

Abstract: Partial discharge measurement and subsequent analyses is one of important and capable test of electro-technological diagnostics. This test evaluates the properties of electrical insulation system of electrical machines and devices. There are a lot of partial discharge test methods based on physical or chemical detection of partial discharge. The detection of electromagnetic field, produced during partial discharge, with capacitive coupler and subsequent data analysis and representation is one of mentioned diagnostic method. The special software program was created for measured data evaluation. This program converts data stored by digital os-cilloscope to more comfortable text formats. The converted data are analyzed by common software Matlab, Statistica, etc. 1. Introduction

Partial discharges are one of the main degrada-tion factors of insulation systems. This fact takes place in aging of electrical machines es-pecially. For the high reliability of the electrical machines it is necessary to detect the partial discharge magnitude within the insulation sys-tem. Partial discharges (PD) can be divided in inter-nal partial discharge and external partial dis-charge. Internal discharges occurs in the bulk insulation. Further diversification of the exter-nal discharges in rotary machines is in separate study of the slot discharges (between stator core and insulation) and gliding discharges. Gliding discharges occur at the ends of winding slots. Another type of external discharge is corona. The main parameters used in the partial dis-charge measurements are apparent charge, pulse count, inception voltage, extinction voltage, phase resolved characteristics, average dis-charge current, discharge power, quadratic rate and total charge amount. The other partial dis-charge measured parameters are pulse repetition frequency, pulse repetition rate, discharge power and largest repeatedly occurring PD magnitude. For the partial discharge measurement, it is possible to use standardized test methods based on coupling impedances [3] or test methods based on detection of emitted electromagnetic field. The second mentioned method [4] employing capacitive sensor was used in presented research.

The basic principle of the capacitive detection of partial discharges is given by the equivalent circuit in Fig.1. The locality of partial discharge is emitting the electromagnetic field. Its electric and magnetic part is proportional to the ampli-tude of the current impulse creating the partial discharge. The capacitive coupler indicates the electrical field. The equivalent circuit contains signal source U and the RC part of an amplifier. The whole ca-pacitive coupler consists of component C1.

Fig. 1. Capacitive coupler equivalent circuit

[10]

Rm

C2

C0 Cc

PD Um U C1 - coupler

U – PD signal CC – coupler capacity C0 – kapacity between coupler and test object C2 – amplifier input capacity C1 – total capacity Rm – amplifier input resisitivity Um – amplifier input voltage

Zeszyty Problemowe – Maszyny Elektryczne Nr 80/2008 92

The measuring device that was used is based on broadband amplification of PD impulses and their consequential integration for apparent charge evaluation [9]. The acquired signal has amplitude correspond-ing to the apparent charge, unifying the shape to rise time of 2 µs and time of duration of 50 µs. The amplifier output signal is then brought to the oscilloscope. So that the obtained data can be further processed or stored. The fibre optic cable is possible to use to bring the signal from the amplifier output to the oscilloscope.

2. Analysis of partial discharges

During tests these PD parameters were meas-ured apparent charge q and pulse count of par-tial discharge. The phase resolved characteris-tics of observed data were analyzed as well. The model setup for partial discharge charac-teristics acquisition were flat specimens (100×100×0,46 mm) of slot insulation (NEN). These specimens were exposed to the pulse voltage with parameters:

� Voltage ±500 V , � Frequency 8 kHz, � Rise time 300 ns,

The measurement of PD was realized under 50 Hz ac voltage of 1,8 kV (4 kV/mm). The elec-trode system presented in Fig. 2 was used for the measurements of PD. The surface dis-charges were eliminated using the conductive rubber electrode (rubber G2 [11]). The elec-trodes pressure to the specimen is given by the construction and it was the same for each meas-urement.

Fig. 2. Electrode and measurement system for

partial discharge testing of flat specimens

The time of partial discharge pattern measure-ment was 16 s (sampling frequency 1 Ms/s, 16 MB of internal memory). The stored data were subsequently analyzed by special designed software in the PC [7]. Software program was created for the analysis of partial discharges within insulation system of electrical machines which are detected by ca-pacitive coupler. The detected signal is stored by digital oscilloscope Agilent Infinium. The data are stored in *.csv format. Main task of de-scribed program is conversion of *.csv format to other more comfortable file formats. These text format files are then processed by conven-tional statistical software Matlab, Statistica, etc. The *.csv data are stored from two oscilloscope channels. Channel one brings signal of apparent charge the other channel brings testing voltage signal. Analyzed data contain information of apparent charge magnitude and apparent charge frequency in dependence on phase position of testing voltage. Histogram of pulse count, average and maximal magnitude of apparent charge in dependence on phase position of testing voltage are analyzed as well. The results of software program analysis are saved in the text file comfortable for subsequent statistical operations.

3. Results and discussion

Figures 3-5 presents the magnitude of apparent charge q and pulse count as the phase resolved characteristics of material NEN aged at ±500 V. Based on the phase position (cca. 30°-80° and 210°-260°), partial discharge pattern corre-sponds to internal discharges [4]. The symmetry of partial discharge pattern (fig. 3, 4) in the positive and negative half of testing voltage cycle is evident. Data for these graphs are obtained from the software described above. The stored signal shows some sampling noise. This signal noise is given due to the oscilloscope bandwidth (2,5 GHz). The mentioned noise is evident in figure 3. Fig-ure 5 presents the 3D partial discharge magni-tudes plotted above the signal noise level.

Sensor PD

Rubber Electrode

Rubber Electrode

Specimen

Metal Electrode

Metal Electrode F

OSC + PC

Amplifier

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Fig. 3. Apparent charge phase resolved char-

acteristics

Fig. 4. Partial discharge pulse count

Virgin state

24h_500V

8h_500V

32h_500V

Fig. 5. 3D interpretation of partial discharge characteristics

0

10

20

30

40

50

60

70

80

0 30 60 90 120 150 180 210 240 270 300 330 360

Phase (°)

Pulse count

-1

0

1

2

3

4

5

virgin state

8 h

24 h

32 h

40 h

0

10

20

30

40

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60

0 30 60 90 120 150 180 210 240 270 300 330 360

Phase (°)

Apparent charge q (pC)

-1

1

3

5

virgin state

8 h

24 h

32 h

40 h

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4. Conclusion

Special software was developed for the partial discharge study. This program analyzes partial discharge measured data obtained by detection with capacitive coupler. The partial discharge signal is stored at digital oscilloscope in file format *.csv. The present method of software analysis is at the very beginning state of solving analysis problem. The software program is realized with universal and open code. This allows imple-menting other analysis of stored signals method in the future. Presented program is able to recognize the phase of partial discharge occurrence at sinus curve of test voltage with precision of one phase degree. The type of partial discharge (internal, external, gliding discharge, corona) is recognized at given test setup thanks to that fact. The experiment proved also the partial dis-charge capacitive sensor as sufficient tool for detection of even small degradation changes in the insulation.

5. Acknowledgment

This research was funded by the Ministry of Education, Youth and Sports of the Czech Re-public, MSM 4977751310 – Diagnostics of In-teractive Processes in Electrical Engineering. The authors are grateful for the support of this program.

6. References

[1]. MENTLÍK, V.; PIHERA, J.; TRNKA, P.; TÁBOŘÍK, O. The influence of pulse stress on main-wall insulation of electrical rotating machines. In Maszyny Elektryczne. Katowice : Branżowy Oś-rodek Badawczo Rozwojowy Maszyn Elektrycznych Komel,, 2007. s. 43-46. ISBN 0239-3646. [2]. MENTLÍK, V.; TRNKA, P.; PIHERA, J. Insulation Materials Under Electrical Pulse Stress. In Annals of DAAAM for 2007 & Proceedings of 18th International DAAAM Symposium. Vienna : DAAAM International, 2007. p. 447-448. ISBN 3-901509-58-5. ISSN 1726-9679. [3]. IEC Standard 270. Partial Discharge Measure-ment [4]. MENTLÍK, V.; PIHERA, J.; TRNKA, P.; MARTÍNEK, P. Partial discharge potential free test methods. In 2006 annual report Conference on elec-trical insulation and dielectric phenomena. Kansas City : IEEE DEIS, 2006. s. 586-589. ISBN 1-4244-0547-5. [5]. MARTÍNEK, P.; LAURENC, J.; PIHERA, J. Typical Patterns of Partial Discharges Acquired by

Measurements on Real Models of Cavity in Solid Dielectric and Instrument Transformers. In Third International Conference on Advances in Process-ing, Testing and Application of Dielectric Materials. Wroclaw, Poland : Oficyna Wydawnicza Politech-niki Wroclawskiej, 2007. p. 245-249. ISSN 0324-9441. [6]. IEEE 1434-2000: IEEE Trial-Use Guide to the Measurement of Partial Discharges in Rotating Ma-chinery [7]. TUREČEK, O.; PIHERA, J. Software for partial discharge analysis from *.csv source file. In Diag-nostika '07. Plzeň : University of West Bohemia , 2007. pp. 393-395. ISBN 978-80-7043-557-1. [8]. Hudon, C., Belec, M. “Partial discharge signal interpretation for generator diagnostics” in: IEEE Transactions on Dielectrics and Electrical Insulation, April 2005, Volume: 12 , Issue: 2, pages: 297-319 [9]. Russwum, D. “On-Site Partial Discharge Moni-toring using the differential LEMKE PROBE LDP-5 and its accessories”, HV Testing, Monitoring and Diagnostic Workshop 2000 [10]. Balogh, J. “Linear and Toroids Inductive Sen-sors and its Use in Electrical Engineering”, Diserta-tion Kosice 2001 [11]. www.charleswater.co.uk [12]. König, D., Rao, N. “Partial Discharges in Electrical Power Apparatus”, Berlin, 1993, ISBN 3-8007-1760-3 [13]. Fenger, M., Campbell, S.R. Pedersen, J.” Motor Winding Problems: Caused by Inverter Drivers” IEEE Industry Applications Magazine Volume 9, Issue 4, July 2003, Pages 22-31