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Research Note: Dielectric Diagnosis of Water Content in llransformer Insulation Systems A. Kiichler, T. Bedel

Abstract

This paper describes dielectric measurements at transformer insulation models made of Transformerboard bar- riers and oil channels. It was intended to quantify the water content in barriers and to distinguish the influences of water and geometric structure. Recovery-voltage measurements (RVM) did not show a clear correlation with water content, probably because of “parasitic” influences like oil parameters, geometry and parallel resistanc- es. Better correlations were,found for insulation resistances i. e. for polarization currents. Network simulations show that common circuits - normally used for homogeneous materials - are not able to explain the measure- ments. Therejbre circuits had to be adapted to the geometric structures of the insulation. Thereby it could be shown that recovery voltages are dominated by inteqacial polarisation between oil channels and barriers. Sig- nals are strongly dependent on oil resistivities, possibly “masking” the influence of water in the barriers.

1 Introduction

Reliability of power transformers is most important for a save and economic operation of distribution net- works. Unfortunately many transformers are already be- yond their nominal life and are operated in an unknown state of ageing.

Ageing mainly affects the quality of insulation con- sisting of oil and impregnated barriers. In this context the accumulation of water is one of the most severe prob- lems: Water reduces the electric strength of oil and cel- lulose. In solid insulation water also increases the veloc- ity of depolimerisation.

Periodic analysis of oil samples gives information about the state of the oil alone. Solid insulation compo- nents are very hygroscopic, they can be wet, even if the oil is dry.

Up to now there is no commonly accepted method for a non-destructive evaluation of solid insulation components. Therefore it is proposed to use the meth- ods of dielectric diagnosis, i. e. to relate special test volt- ages and currents to the properties of the insulation [ 1-51.

2 Dielectric Diagnosis

Dielectric diagnosis makes use of charging and dis- charging of the test object resulting in polarization and depolarization of the insulating materials. Three differ- ent methods are discussed:

I. After charging and (incomplete) discharging of the insulation an increasing recovery voltage can be measured (recovery-voltage method RVM) be- cause of the remaining polarization. The recovery

voltage reaches a maximum and decreases because of self-discharging by the insulation resistance and other parallel resistances [ 11.

11. Charging and discharging currents (i. e. polariza- tion and depolarization currents PDC) contain in- formation about the properties of the materials and about the geometric structure of the insulation system [2]. Polarization-current measurements are similar to insulation-resistance measurements.

111. Instead of time-domain measurements it is also pos- sible to perform a frequency-domain analysis (FDA) by measuring of capacitances Cand loss fac- tors tan 6 as a function of frequency in a low fre- quency range [9].

It is quite clear that polarization and depolarization currents, recovery voltages, capacitances and loss fac- tors are influenced both by material properties and inter- facial polarization depending on the geometry of oil channels and Transformerboard barriers.

Actual RVM interpretation does not distinguish between water content w in the barriers and geometric influences [6] . Therefore both parameters were varied in specially developed insulation models and analyzed by RVM and insulation resistance measurements. Results are in agreement with dielectric measurements on power transformers [9].

3 Insulation Models

Experiments were performed on insulation models made of Transformerboard-discs separated by an oil channel, Fig. 1. The insulation geometry was varied by three different spacings: d = 0 mm, 4.8 mm and

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Fig. 1. Schematic drawing of the insulation model

9.6 mm. Seven different pressboard moisture levels wereinvestigated: w=O%, 1.7%,2.7%,3.7%,4.4%, 6.2 % and 11.6 %.

The pressboard discs were dried by hot air (5 h at 90 “C) and vacuum treated ( 12 h at 100 “C). The desired moisture level w was adjusted by observation of increas- ing weight, caused by absorption of humidity from am- bient air. Electrodes and barriers were assembled under dry oil (water content approx. 5 ppm). Impregnation time was nearly three days.

4 Measurements

Every arrangement was analyzed by a series of die- lectric measurements such as capacitance, loss factor, RVM and insulation resistances. - RVM were performed with an automatically con-

trolled test setup, consisting of a 500-V DC source, a switching unit, a high-impedance electrometer and a signal-processing PC. The test object is charged and polarized by a charging voltage U, for a charging time t,. A short circuit during discharging time td re- sults in a partial depolarisation. After that the remain- ing polarization results in a recovery voltage u( t ) , Fig. 2 (left). So called “polarization spectra” were taken by a se- ries of RVM measurements with a constant ratio of charging and discharging times. According to the proposals in literature rChd = 2/1 was chosen [l]. Maximum recovery voltages U,,, were plotted as function of charging times (t, = 1 s, 2 s, 5 s, 10 s, 20 s, 50s lOOs, 200s, 500s), Fig. 2 (right).

- Insulation resistances Rins were measured by a tera- ohmmeter at 500-V DC and recorded over 30 min. Values were taken at 20 min for comparison purpos- es. The transient behaviour of the measured values reflects the transient changes of polarization cur- rents which also are proposed for transformer diag- nosis [2].

r, +

Fig. 3. Polarization spectra of homogeneous test samples (dOil = 0 mm) with different water content w ( U,,,ax: maximum recovery voltage; t,: charging time)

100 V 80 70

50

30 20 10

t 6o urn,, 40

1 w=O%l I

100 s 1000 r, 4

Fig. 4. Polarization spectra of multi-layer test samples (&I = 4.8 mm) with different water content w

5 Results

5.1 Recovery-Voltage Measurements

Polarization spectra for different geometric arrange- ments (&, = 0 mm and doi, = 4.8 mm) are very different, Fig. 3 and Fig. 4. It may be assumed that these differ- ences are caused by interfacial polarization between oil- channels and barriers.

Additionally polarization spectra show a strong in- fluence of water content in the barriers, but a clear ten- dency cannot be seen. Therefore these measurements cannot verify that the position of the maximum in the po- larization spectrum is an indication for a specific water content in the barriers.

Obviously recovery voltages and polarization spec- tra are affected by some other factors, possibly by para- sitic resistances parallel to the test objects. It should fur- ther be noted that the water content w was well defined in the barriers, but not in the oil. Therefore interfacial po- larization could be influenced in an unknown way.

These problems are even more severe in complex transformers where surface resistances of bushings have to be regarded and where equilibrium states of water contents cannot be assumed.

5.2 Insulation-Resistance Measurements

Fig. 2. Single RVM measurement (left) and complete “polarization spectrum” (right)

It was found that there is a significant correlation between insulation resistance Rins and water content w

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t Rim

25 Ti2 20

15

10

5

0 0 1 2 3 4 5 6 7 8 9 10 % 1 2

W-

Fig. 5. Insulation resistance R,,, of insulation models as a function of water content w and barrier distance d,,,,

in the barriers, Fig. 5. These resistances are not depen- dent on the geometric structure, i. e. on the width of the oil channel d,,+

These results can be interpreted as follows: After 20 min polarization currents have come more or less close to steady-state conditions. Measurements mainly reflect the series resistance of barriers and oil channels. The conductivities of the different materials differ by some orders of magnitude. Therefore the total resistance is dominated by the high resistance of the barriers.

6 Simulations

The above described results indicate that interfacial polarization has a dominant influence on recovery volt- ages u( t ) , insulation resistances Ri,, and polarization currents in oil-barrier-systems. Therefore equivalent circuits must not treat the insulation system as a homo- geneous material, Fig. 6 (left). Each layer (oil and bar- riers) has to bedescribed by its own circuit, Fig. 6(right).

Already a very simple circuit, only consisting of ca- pacitances and resistances for each type of layer, gives

Fig. 6. Conventional “black-box’’ circuit (left) and circuit fitted to barrier geometry (right)

“ I I calculated 1

fl l 1 10 100 s lo00

t, + Fig. 7. Measured and calculated polarization spectra for an oil-barrier insulation model = 9.6 mm)

good simulation results in comparison with measured recovery voltages, Fig. 7. Differences may be due to un- defined resistances in parallel to the test arrangement. This confirms that recovery voltages are very much in- fluenced by the geometric structure and by the proper- ties both of barriers and oil channels.

7 Conclusions

- Insulation-resistance measurements and polarization- current measurements seem to have a good potential to distinguish the influences of geometric structure and water content in the baniers. Measurements can be performed in a comparatively short time.

- In RVM measurements it is difficult to distinguish the influences of both parameters. Additionally other factors can not be neglected, e. g. the properties of the oil and undefined parallel resistances.

- In any case, interpretation of dielectric measure- ments requires equivalent circuits which are fitted to the geometric structure of the barrier system. “Black- box” circuits, often used for homogeneous dielectric materials, are not sufficient.

- Effective simulation and interpretation of dielectric measurements requires deeper investigations of the properties of all materials.

- Recently measurements on transformers and simula- tions were reported, comparing RVM, PDC and FDA analysis [9]: It was found that all methods were sen- sitive to humidity. FDA and PDC analysis gave iden- tical results and were less sensitive to systematic er- rors than RVM measurements [9]. These conclusions are in good agreement with the results of laboratory experiments described above.

8 List of Symbols and Abbreviations

capacitance barrier distance, width of oil channel insulation resistance time, charging time, discharging time loss factor charging voltage maximum recovery voltage recovery voltage water content in the barriers

DC direct current FDA frequency domain analysis PDC polarization/depolarization currents RVM recovery-voltage method

References [ I ] Csepes, C.: RVM measurements on oil-paper insulation

models and real transformers. Paris/France: CIGRE TF 15.01.09, 1999

[2] Der Houhunessiun, K; Zuengl, W : Vor-Ort-Diagnose fur Leistungstransformatoren. Bull. Schweiz. Elektrotech. Ver. (SEV) 23 (1996) pp. 19-28

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tric Measurements o n Power Transformers. Paris/France: CIGRE Session I998

141 Suhu, 7: K. ; Danmizo, M.; Yao, Z. 7:: FieldExperience with Return Voltage Measurements for Assessing Insulation Condition in Transformers and its Comparison with Accel- erated Aged Insulation Samples. Int. Sympos. on High Volt- age Engng. (ISH), London/GB 1999, Rep. 5.140.Sl6

51 Porzel, R.; Sturm, M.: Dielektrische Diagnostik von Hoch- spannungs-lsolierungen. etz Elektrotech. Z. 1 16 (1995) no. I0,pp. 18-29

[6] Kuchler, A . J . ; Baehr; R.; Zuengl, W S.; Breitenbauch, B.; Sundermann, U . ; Kritische Anmerkungen zur Feuchtig- keitsbestimmung von Transformatoren mit der ‘Recovery- Voltage-Methode’. E1ektriz.-wirtsch. 95 (1996) no. 19,

171 M o w r ; H . R: Transformerboard. Rapperswil/Switzerland: Weidmann AG, 1979

[8] Kruuse. C.; Gasser; H. R; Huser, J . ; Sidler A.: Effects of Moisture in Transformerboard Insulation and the Mecha- nism of Oil Impregnation of Voids. Int. Conf. “transform 98”. MunichKermany 1998

191 Gufkrt , U.: Adeen. L.; Tapper; M . ; Ghasemi, I?; Jiinsson, B.: Dielectric Spectroscopy in Time and Frequency Do- main Applied to Diagnostics of Power Transformers. 6th Int. Conf. on Properties and Appl. of Dielectric Materials (ICPADM), Xi’an/China 2000

pp. I 238- 1 245

Acknowledgements The authors are grateful for valuable technical support and contributions by Mr. Franz Klauer and Mr. Artur h k s a . Transformerboard samples were supplied by Weidmann Transformerboard Systems AG in Rapperswil, Switzerland. This work was financially supported by the German Federal Department of Education and Research (BMBF).

The Authors

Prof. Dr.-Ing. Andreas Kuchler ( 1 953), VDE, is Professor for power engineer- ing and high-voltage technology and head of the high-voltage laboratories at the University of Applied Sciences FH Wurzburg-SchweinfurVGermany. He received his Dip1.-lng. degree in 1980 and his Dr.-Ing. degree in 1986, both from the University of Karlsruhe/Ger- many. Until 199 1 he was R&D manag- er of F&G Hochspannungsgerate

GmbH (now HSP) in Cologne/Germany. Now his research interest concentrates on design and diagnosis of insulating systems, e.g. for transformers and for HVDC applications. (FH Wurzburg-Schweinfurt, Ignaz-Schon-Str. 1 I , 9742 1 Schwein- furVGermany, Phone: +49 97 2 I 940-8401-8 02, Fax: +49 97 2 1 940-8 00, E-mail: akuechler@fh-sw.de)

~~

I

Dipl.-Ing. (FH) Thomas Bedel (1972) received his diploma degree in electri- cal engineering from the University of Applied Sciences FH Wurzburg- Schweinfurt/Germany in 2000. Now he is working as a project engineer with Heraeus Quarzglas GmbH & Co. KG (Heraeus Quarzglas GmbH & Co. KG, Quarzstraae, D-63450 Hanau, Phone:+4961 81367-6616,Fax:+49 6 I 8 I 3 67-240, E-mail: thomas.bedel @ heraeus-quarzglas.com)

Manuscript received on September 12, 2000

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