Detection of Defects in On-Load Tap-Changers Using Acoustic Emission Method

Andrzej Cichoń Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

a.cichon@po.opole.pl

Tomasz Boczar Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

t.boczar@po.opole.pl

Paweł Frącz Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

p.fracz@po.opole.pl

Dariusz Zmarzły Institute of Electric Power Engineering

Opole University of Technology Opole, Poland

dzmarzly.@gmail.com

Abstract - The on-load tap changer (OLTC) constitutes one of the main elements of an energy power transformer which ensures the possibility of voltage regulation in an energy power net. Its break-down is a common cause for the transformer cut-off. The paper presents results of scientifically investigative works which aim is the OLTC technical condition estimation by use of the acoustic emission (AE) method. The AE method allows for OLTC diagnosis without the necessity of exclusion the power transformer from operation. The measurements were performed under laboratory conditions. The influence of the degree wear and tear of the OLTC head on the AE signal was evaluated. Gathered signals were analyzed within the time and time-frequency domains. Results analysis in the time domain was performed by use of the Hilbert transform. Furthermore the characteristic times for the particular runs were calculated. Evaluation of the time-frequency domain results was performed by use of short-time Fourier transform.

Keywords - power transformers, on-load tap changer, acoustic emission

I. INTRODUCTION

Over half of average- and high-power transformers in the Polish and European electric power systems have been in use for over 30 years so they have exceeded their expected running time [2]. A many-year failure frequency analysis shows that for units older than 30 years the failure index exceeds 2%, which causes additional and excessive financial load for distribution companies caused by elimination of consequences of failures and compensations for the electric energy undelivered to consumers [2]. The main elements of the transmission grids are power transformers, mostly equipped with OLTCs. A comparative listing with possible OLTC failure causes is shown in Fig. 1. Histograms presented in Fig. 1 indicate that 10 to 40% of transformer failures are caused by OLTC [1,2,5]

Figure 1. Main causes of transformer failures [1, 2, 5]

The diagnosis of the OLTC technical condition is presently based, to a large extent, on the application of the oscillographic method, which consists of registration of the current time runs in the cycle of switching forced by constant voltage. Evaluation of oscillographic tests is possible only after a prior isolation of the transformer under study from voltage what undoubtedly is a disadvantage of this method. The growing demand for electric energy, maintenance works and limited switch-over possibilities resulting from the network structure cause that in practice problems associated with periodical shut-downs of transformers necessary for diagnostic tests are encountered more and more often. Therefore, it becomes important to look for new diagnostic methods for the assessment of the on-line OLTC operation condition, e.g. without shut-down of the transformer under study [3, 4].

Results of efforts undertaken for the influence evaluation of the OLTC degree of wear and tear on the recorded acoustic signals are presented in the paper. The signals are described in the time and time-frequency domains where the characteristic

The work financed with the means from the National Research and Development Center within LIDER program

184978-1-4673-0487-0/12/$31.00 ©2012 IEEE

times, modules of the Hilbert transform and power density spectrograms are determined.

II. THE MEASURMENT SETUP As already mentioned all measurements are performed

under laboratory conditions. The main element of the setup is a tank, dimensions: 50 x 60 x 80 cm - especially designed for the measurement purposes, which enables for location of a tab changer in the middle of the tank. The tab changer is fixed at the bottom of the tank. In order to bring closer the laboratory conditions to conditions present during operation of tab changers in power transformers the tank has been filled with insulation oil. Fig. 1 depicts the measurement tank with tab changer fixed at its bottom.

Figure 2. The measurement tank with tab changer fixed at the bottom

The acoustic signal has been measured by use of a wideband measurement transducer type WD AH 17, made by PAC, which was mounted to the outer surface of the tank with a magnetic chuck. The transducer applied in measurements is of high sensibility (55 dB ± 1,5 dB with respect to V/ms-1) and of wide frequency band from 100 kHz to 1 MHz in range of ± 10 dB. The acoustic signal measured by the transducer has been amplified in a preamplifier configuration type 2/4/6, made by PAC. Further the acoustic signal is forwarded to an amplifier configuration where it is amplified by value of 20 dB. Moreover, the setup includes a band pass filter with cut-off frequencies 10 and 500 kHz. The measurement setup is presented in Fig. 3. a)

b)

Figure 3. Measurement setup: a) transducer WD AH 17 with preamplifier

and amplifier b) band pass filter

III. METHODOLOGY OF MEASUREMENTS The changers degree of wear and tear evaluation has been performed by modeling of their thickness. This have been achieved in a laboratory setup where the thickness of the coupling has been adjusted by use of specialized plates mapping the shape. Measurements have been performed for four coupling thicknesses. Results of experiments are depicted in form of: 1) time runs gathered for the whole measured signal and 2) Hilbert transform modules evaluated for signals limited to first 100 ms. Moreover, based on the time runs the characteristic times have been estimated, of which the estimation method is presented in Fig. 4

Figure 4. The estimation method for the characteristic times of the acoustic signals generated by OLTC

Additional element of the analysis of the coupling degree of wear and tear was the comparison of acoustic signals generated by power changers working with new contacts and with contacts worn out during exploitation. The moving contacts as well as the fixed contacts deform during the switch-over process. This deformation denotes mechanical distortions which lead to deterioration of electrical contact properties and also to generation during the switch-over of a high intensity electric arc. Measurements have been performed in a laboratory setup for a power changer with new contacts and with contacts after exploitation. General view of the contacts applied during measurements is presented in Fig. 5

t [s] t [s]

185

a)

b)

Figure 5. View of new contacts (a) and contacts after exploitation (b) applied

in measurements

Evaluation of gathered acoustic signals has been performed by use of the short time Fourier transform (STFT) by presenting two-dimensional power density spectrograms.

IV. ANALYSIS OF MEASUREMENT RESULTS The time runs and the Hilbert transform modules of acoustic signals generated during measurement adopting the particular contact thicknesses are presented in Fig. 6.

a)

0 0.05 0.1 0.15 0.2 0.25 0.3-2

-1

0

1

2

3

Czas [ s ]

Nap

ięci

e [ V

]U

[V]

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

Czas [ ms ]

Nap

ięci

e [ V

]

T

b)

0 0.05 0.1 0.15 0.2 0.25 0.3-4

-3

-2

-1

0

1

2

3

4

Czas [ s ]

Nap

ięci

e [ V

]

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

3

3.5

4

Czas [ ms ]

Nap

ięci

e [ V

]

c)

0 0.05 0.1 0.15 0.2 0.25 0.3-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Czas [ s ]

Nap

ięci

e [ V

]

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

Czas [ ms ]

Nap

ięci

e [ V

]

T d)

0 0.05 0.1 0.15 0.2 0.25 0.3-2

-1

0

1

2

3

Czas [ s ]

Nap

ięci

e [ V

]

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2

2.5

Czas [ ms ]

Nap

ięci

e [ V

]

Figure 6. Time runs and Hilbert transform modules of the acoustic signals

generated by power changer with contacts which thickness has been increased by: a) 0 mm, b) 2 mm, c) 4 mm, d) 6 mm.

Analyzing the time runs and the Hilbert transform modules regarding to different contact thicknesses one can state that the shapes of time structures do not change. Within extension of the contact the first and the second structure come closer to each other significantly. Numerical argumentation to his phenomenon is presented in Tab. 1 and in Fig. 7.

186

Tab. 1 Comparison of the acoustic signals characteristic times gathered during operation of power changer with different thickness contacts.

Extension of thickness by

[mm]

T1 [ms]

T2 [ms]

Tc [ms]

0 22,0 30,6 52,6 2 21,6 33,4 55,0 4 18,5 36,9 55,4 6 15,2 41,6 56,8

0,0

10,0

20,0

30,0

40,0

50,0

60,0

T1 T2 Tc

22,0

30,6

52,6

21,6

33,4

55,0

18,5

36,9

55,4

15,2

41,6

56,8

Czas

[ms]

Czasy charakterystyczne

0 mm

2 mm

4 mm

6 mm

Figure 7. The acoustic signal characteristic times by operation of power changer with contacts of particular thicknesses

Followed by extension of the thickness the time T1 shortens while the times T2 and Tc prolong. Time T2 indicates the greatest prolongation times which are in the range from 30,6 to 41,6 ms. Changes in the time T1 vary in range from 22,0 ms, for a contact with the least thickness, to 15,2 ms, for a contact with thickness extended by 6 mm. The total duration time of the acoustic signal also prolongs but these changes indicate the lowest range (52,6 – 56,8) ms.

Results of the time-frequency analysis of the acoustic signals generated by changer operation with new constacts and with contacts weared during exploitation are presented in Fig. 8 a)

b)

Figure 8. Power density spectrogram of acoustic signals generated by OLTC

with new contacts a) and with contacts after exploitation b)

In order to depict differences appearing in the time-frequency structures time runs shortened to 120 ms have been applied for the analysis. On both spectrograms it is to observe that the components of the dominating frequencies have similar ranges. These frequencies can be divided into two bands: 1) low with frequencies not exceeding 60 Hz and 2) high including components above 60 kHz. Most similar, with respect to frequencies participation and duration time, are structures appearing at the end of the analyzed time-run. Differences between considered signals relate in general to the first and the second time-frequency structure. Analyzing the time runs and the time-frequency components it is to observe that the first structure in Fig. 8a) contains a clear maximum. Components from the low frequency band indicate the highest amplitude. Components from the high frequency band indicate short duration time and small amplitude. In the first structure, which is presented in Fig. 8b), there are two local maxima to observe. The time-frequency picture regarding to this structure depicts longer duration time of components from high frequency band. Moreover, components from the low frequency band indicate smaller amplitude. Analyzing the second time-frequency structure one can observe that in both considered cases it contains two characteristically maxima. In the case of signals regarding to new contacts these components have similar frequencies participation as in the case of structure nr 1. In the case of signal regarding to contacts after exploitation one can observe clear delamination of particular structures as well as greater participation of components from the high frequency band and smaller amplitude fluctuations in the low frequency band.

V. SUMMARY During the scientifically investigative works performed it

was demonstrated that there exist a possibility of the OLTC diagnosis from the degree of wear and tear point of view under laboratory conditions. There is a necessity for continuation of the works which aim is to confirm achieved results on running

187

objects. These results can pose as basis for on-line diagnosis system that monitors the state of the OLTC.

ACKNOWLEDGMENT The work financed with the means from the National Research

and Development Center within LIDER program.

REFERENCES [1] Cichoń A.: “The Application of the Selected Time–Frequency

Descriptors Detection of the Acoustic Emission Signals Generated by Multisource Partial Discharges”, Acta Physica Polonica A, Vol. 116 (2009), No. 3, pp. 290-293.

[2] Praca zbiorowa pod red. J. Subocza „Transformatory w eksploatacji”., ISBN 978-83-924464-0-8. Energo-Complex, (2007).

[3] Rivas E., Burgos J. C., García-Prada J. C. “Condition Assessment of Power OLTC by Vibration Analysis Using Wavelet Transform”, IEEE Transactions on Power Delivery, Vol. 24, No. 2, April 2009, pp. 687 - 694

[4] SimasFilho E.F., L.A.L de Almeida “Self-Organized Classification of on – load tap Changers acoustic signature” IEEE International instrumentation and measurement technology conference, Victoria, 12-15 May 2008

[5] Skubis J., Cichoń A., Borucki S.: „Praktyczny przykład skutecznej detekcji i lokalizacji wyładowań niezupełnych metodą emisji akustycznej w transformatorze elektroenergetycznym”, Pomiary Automatyka Kontrola vol. 55, nr 1/2009, str. 30 – 33.

188