Characterization of Performance of Paper-Oil Insulation System of Power Transformers in Presence of Copper Corrosion

Transcription

1 Characterization of Performance of Paper-Oil Insulation System of Power Transformers in Presence of Copper Corrosion * Faheem Ahmed Khan 1, J. Sundara Rajan 2 Senior Member IEEE and ** Mohd. Z. A. Ansari 1, Member IEEE 1 Dept of Electrical and Electronics Engineering, Ghousia College of Engineering Ramanagaram, Karnataka, India * faheem_khanb@rediffmail.com, ** zahedansari@gmail.com 2 R & D Management Division Central Power Research Institute Bangalore, Karnataka, India sundar@cpri.in Abstract Corrosion of copper conductors due to sulpher in transformer oil is a serious problem affecting power and converter transformers. Though the reactive and corrosive sulpher components are of very low order in fresh transformer oils, they pose a serious problem as the transformers age. These reactive and corrosive sulpher components react with copper forming copper sulphide, which migrates from conductor surface to outer layers of paper. Since copper sulphide is conductive, it affects the voltage distribution of insulation. Experimental study has been carried out to investigate the role of copper sulphides in leakage current and breakdown of paper insulation. Keywords- Surface discharge, transformer oil, copper corrosion, voltage distribution. I. INTRODUCTION Transformers are vital parts in power systems. Analysis of failed transformers has shown that presence of high sulpher content in transformer oil has played a major role in the failure of power and converter transformers all over the world [1]. These failures are not confined to a particular weather condition. Fresh oil will have very low quantity of mercaptan and total sulpher components but the values change very rapidly during service conditions. This is influenced by various parameters that come into play during normal life of transformers [2 6]. High temperature is recognized as the main driving force but it is not always a necessary condition for failure to occur. The present experimental work carried out deals with understanding of (i) Reaction of corrosive oil with paper (ii) Formation of copper sulphide (Cu 2 S), (iii) Migration of Cu 2 S to the inner most layer of paper insulation and its influence on dielectric strength and (iv) Correlation of these factors to breakdown of transformer. II. EXPERIMENTAL METHOD Two brass hemispherical electrodes of 25 mm diameter, fixed on either sides of an acrylic cell are used for measurements. Half a litre of fresh and virgin oil is 81 filled in test cell such that the electrodes are fully immersed. One end of electrode is connected to high voltage (HV) and the other electrode is grounded. The electrodes are movable such that the gap distance between them can be varied. A variable ac voltage is applied to the electrodes through a motorized auto transformer and stepup HV transformer. A high precision ammeter was used to measure the leakage current. A. Preparation of insulation paper for test and method A set of paper samples of size 8cm x 12 cm and thickness 50 μm were prepared and thickness was ascertained to be 50 +/- 5 μm. These samples were dried in a hot air circulating oven at 80 C for 6 hours to remove moisture. The method adopted for breakdown measurements are: i. Breakdown voltage was determined after recording - characteristics for fresh and virgin oil. ii. Different amount of Cu 2 S was added to fresh and virgin oil and breakdown voltage was determined after establishing voltage-leakage current characteristics. iii. Different quantity of Cu 2 S was applied to insulating paper and its effects on breakdown voltage and leakage current were determined. B. Simulation of Cu 2 S formation in oil in the laboratory 2 grams of Cu 2 S is added to one litre of fresh transformer oil and stirred gently so that Cu 2 S is mixed uniformly. This was done to simulate the problem of Cu 2 S in the laboratory. Similarly, concentration of Cu 2 S is increased to 4 g /litre, 6 g /litre, 8 g/litre, 10 g /litre, 12 g/litre and 14 g/litre and the above tests are repeated. C. Insulation Paper with different quantity of copper sulphide In this method Cu 2 S is applied to different layers of paper insulation simulating Cu 2 S formation and migration into first, second, third, fourth and inner most paper layers.

2 The measurements carried out in this experiment are of breakdown voltage and leakage current for different layers of paper insulation placed on either high voltage (HV) or low voltage (LV) electrodes. III. RESULTS AND DISCUSSIONS A. Breakdown in Case of Fresh and Virgin Oil Initial studies were carried out on fresh and virgin oil to determine their dielectric characteristics and also to benchmark their properties. The result of leakage current as a function of applied voltage is shown in Table I. Fig. 1 represents the leakage current as a function of applied voltage for fresh and virgin transformer oil. The voltage was increased from 1kV to 11kV in various steps and corresponding leakage current has been tabulated for different electrode gaps. TABLE I VIRGIN OIL Electrodes, d=0.2 to 0.4 cm Electrodes, d=0.8 to 2.0 cm BD* *Breakdown The leakage current varies from about 0.1mA to 1.3mA irrespective of electrode gap distance. However, for a distance less than 0.4cm a voltage more than 10kV results in breakdown of the gap. Thus it is observed that the leakage current varies linearly with applied voltage, but at lower gap distances breakdown occurs at voltage less than 10kV. When the stress is about 25kV/mm or higher, breakdown is observed for distance of 0.2 to 0.4cm.The above experiment gives an idea of the performance of pure transformer oil used. However, in the presence of paper, these characteristics alter. Hence a single layer of paper was introduced in between the electrodes and the study was repeated. These results are shown in Table II. Even in this case the leakage current varies linearly from 0.1mA to 1.2mA and no breakdown was observed even with increase in electrode gap distance up to 2cm. Thus the presence of electrical grade paper does not influence the leakage current irrespective of its presence on BDV in kv Fig. 1: Variation of leakage current as a function of breakdown voltage for different electrode gap distances TABLE II VIRGIN OIL WITH PAPER PLACED ON EITHER HV OR LV Electrodes, d=0.2 to 0.4 cm Distance: cm Distance: cm Leakage in ma Electrodes, d=0.8 to 2.0 cm LV or HV electrode. Interestingly, there is no significant change in leakage current due to inclusion of electrical grade paper. This is confirmed by a comparison of results in Tables I and II. The studies on fresh and virgin oil and oil containing paper shows that the oil characteristics remain unaltered and the presence of paper has no bearing on the V-I characteristics for a gap distance within a certain value. With addition of more number of papers (1 to 4), no change in leakage current was observed. The representative result of leakage current for two layers of paper are tabulated in Table III for electrode gap up to 2cm. This is true irrespective of whether the paper is towards high voltage or low voltage with voltage varying from 1kV to 10kV. 82

3 TABLE III VIRGIN OIL WITH TWO LAYERS OF PAPER PLACED ON EITHER HV OR LV Electrodes, d=0.2 to 0.4 cm Electrodes, d=0.5 ( kv) ( kv) B. Effect of Cu 2 S on Breakdown Cu 2 S was added to fresh and virgin oil in different quantities and investigations were carried out. Table IV shows the V-I characteristics for a gap distance of 0.5 cm and Table V for a gap distance of 1.0 cm when 2g /litre of Cu 2 S is added to transformer oil. Breakdown was recorded at 9.2kV for gap distance of 0.5cm, whereas in all other cases no breakdown was observed. This clearly shows that the electrical stress is above 25kV/mm in this configuration. Here the most significant observation of V-I characteristics is the random variation of leakage current that occurs at a concentration of 14gms of Cu 2 S/lit, when the paper was placed on LV side. These characteristics clearly show that there is partial discharge in liquid medium due to random motion of the minute particles of copper sulphide. The instantaneous values of electric field may also be affected by the movement of these particles. Hence leakage current keeps fluctuating. These fluctuations are different at different places of inter-electrode area. As the electrode gap distance increases, the fluctuation in leakage current also increases. The fluctuations are visible at very low voltage. Duration of these fluctuations increases with increase in the gap distances. The breakdown of the gap is closely followed by breakdown of electrical grade paper. In fact there are multiple breakdowns of papers indicating that the particles of Cu 2 S migrate towards the LV electrode and thereby cause multiple breakdowns. Significantly the increase in leakage current is minimal even at very high concentration on Cu 2 S. The problems that can be encountered due to presence of Cu 2 S are intermittent discharges and large fluctuations in leakage current. Thus leakage current varies non-linearly with applied voltage in the presence of Cu 2 S. The randomness in leakage current is also dependent on oil present in the inter-electrode area. Smaller the electrode gap, more are the fluctuations in the leakage currents. With increase in gap distance to 1.0 cm, the variation in leakage 83 current is observed up to an applied voltage of 4kV. Therefore there is correlation between the concentration of Cu 2 S, electrode gap spacing and applied voltage. In Table VI, the variation of leakage current with applied voltage is shown for a Cu 2 S concentration of 14grams/lit. TABLE IV VOLTAGE VS LEAKAGE CURRENT CHARACTERISTICS OF TRANSFORMER OIL WITH 2g/LITRE OF Cu2S AND PAPER PLACED TOWARDS EITHER HV ORLV FOR GAP DISTANCE OF 0.5 Cm Electrodes, d=0.5 cm Electrodes, d=0.5 cm BD TABLE V VOLTAGE VS LEAKAGE CURRENT CHARACTERISTICS OF TRANSFORMER OIL WITH 2 g//litre OF Cu 2S ADDED AND PAPER PLACED ON EITHER HV OR LV FOR GAP DISTANCE OF 1.0 cm Electrodes, d=1.0 cm Electrodes, d=1.0 cm

4 TABLE VI VARIATION OF LEAKAGE CURRENT AS A FUNCTION OF VOLTAGE APPLIED BETWEEN THE ELECTRODES WHEN 14 g/ litre OF Cu 2S IN OIL WITH PAPER PLACED ON LV Electrodes, d=0.4 cm kv ma Electrodes, d=0.4 cm kv ma BD It is observed that from 3kV to 5kV there is a large increase in leakage current with paper placed on LV as compared to the case with paper placed on HV. With paper layers on both HV and LV, the results obtained have confirmed the hypothesis stated above. There is no increase in leakage current and even the fluctuations in the leakage current are not significant and no breakdown was observed up to 10kV. These results have conclusively proved that the Cu 2 S doesn t contribute towards leakage current but creates alternate paths for breakdown. They also affect the stress distribution in oil resulting in micro-discharges and breakdowns. The cumulative effect of these processes leads to breakdown of oil gap which in turn triggers the breakdown of paper. This is confirmed by multiple breakdown spots observed in electrical grade paper. C. Effect of CuS on Paper Insulation BDV in kv Number of layers Fig. 2: Variation of breakdown CONCLUSIONS voltage as a function of paper layers (1) From the studies on pure oil, the electric strength of oil with paper was observed to be 25kV/mm and in the presence of Cu 2 S, it is reduced to 10 to 15kV/mm. (2) Oil contaminated with Cu 2 S does not show much sensitivity to leakage current. Hence presence of Cu 2 S in oil becomes difficult to detect under normal working conditions of transformers. (3) Presence of Cu 2 S increases the leakage current significantly for a concentration of 10g /litre. (4) Cu 2 S reduces the intrinsic breakdown voltage of paper by nearly 80% in some cases. (5) Cu 2 S concentrations up to 2g/litre in oil do not show any abnormal variation in leakage current or BDV. Similarly concentrations above 14g/litre did not increase the leakage current. (6) The number of Cu 2 S affected paper layers influence the BDV much more than the location of Cu 2 S on paper either towards HV or LV electrodes. ACKNOWLEDGMENT The authors would like to thank the authorities of Ghousia College of Engineering, Ramanagaram and Central Power Research Institute, Bangalore for all the cooperation and encouragement in carrying out this work. A small circular area (6 cm 2 ) was covered with Cu 2 S of different concentration on electrical grade paper and breakdown strength was studied. Breakdown studies were also carried out with Cu 2 S present on different layers. It was observed that the leakage current increases linearly as the Cu 2 S migrates into more layers of papers. As the concentration of Cu 2 S increases, the breakdown strength decreases. The leakage current is higher when more number of layers are placed towards LV and BD takes place normally at the circumference of round conductors. Fig. 2 shows that the breakdown strength of paper reduces drastically as Cu 2 S is migrated into more number of layers. 84

5 REFERENCES [1] J. Sundara Rajan, C. Jayarama Naidu, K Dwarakanath and A.K Tripathy Studies on the effects of sulpher in transformer oil on transformer components, XV International symposium on high voltage Engineering, Slovenia, August [2] Copper sulphide in transformer insulation Report from task force A2-31, ELECTRA, February [3] Lewand.L. Investigating copper sulphide contamination in a failed GSU transformer Proc. of the 72 nd annual Doble Conference of Doble Clients, Boston, M.A. Insulating materials session, [4] Claes Bengtsson, Jan Hajek, Mats Dahlund, Lars Patterson, Karin Gustafsson, Robot Leandersson, Arne Hjortsberg Oil corrosion and conducting Cu 2 S deposition in power transformer windings CIGRE 2006 session, paper A [5] Jan Hajek, Mats Dahlund, Lars Patterson, Quality of oil makes the difference, ABB discovers the solution to transformer breakdowns, ABB review. Nr , pp [6] B.P.Singh, T.S.R. Murthy, G. Jayaraman, A. K. Adikeshalu et al. Effect of Mercaptan sulpher on insulation performance of HVDC converter transformers CIGRE 2006 paper A