Effect of different shapes of electrodes on bridging in contaminated transformer oil

Abstract

Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles were always formed bridges between electrodes under DC electric field. AC field does not induce any bridging. Combination of AC and DC enhances the bridging dynamics. The bridges were thicker or more particles attracted with more uniform electric field (spherical electrode) than diverse electric field (needle-plane). During the operation oil is in contact with metal, iron core and pressboard insulation. Contaminants such as metal filings or cellulosic residue can be formed in the oil, especially for transformers with aged paper insulation. Non-uniform fields are present within the transformer. These contaminants tend to move towards high field regions due to dielectrophoresis (DEP) forces (2). Once in the high field regions, these particles may acquire a charge when in contact with different parts of the transformer and contribute to a leakage current (3). They also could form a bridge over a period of time. The bridge may potentially act as a conducting path between two different potentials within the transformer structure, leading to partial discharges or insulation failure. We have reported previously the bridging with bare spherical electrode under different combinations AC and DC electric fields as well as mathematical model of the bridging (2-6). This paper focuses on the effects of strong non-uniformity of an electric field in the vicinity of sharp edges and how it affects the bridging. The experiments have been carried out to investigate the particles' accumulation between two different shapes of electrodes under different conditions: AC, DC and AC biased with DC voltages. The paper presents the details of the experimental setup and explains the observed results. Three different levels of AC and DC voltages were investigated for these experiments for needle-plane and spherical sets of electrodes. Different levels of contaminations were also examined. Optical images of bridge formation process were taken and conduction current was also recorded simultaneously during all the tests. Pressboard fibers were produced by rubbing an unused pressboard typically used in high voltage transformer by different sizes of metal hand files. Different sizes of sieves were used to separate the fibers by width rather than length i.e. 250-500 μm, 150-250 μm, 63-150 μm and less than 63 μm. All the four sizes of particles were tested under DC electric field. Only the results from 150-250 μm and 250-500 μm tests are discussed in this paper. Only 63-150 μm size particles were investigated for AC and DC biased AC experiments. The contamination levels for each size of particles were 0.001%, 0.002%, 0.003%, 0.004%, 0.006%, 0.008%, 0.016% and 0.024% by weight. For DC experiments only 0.001% to 0.004% contamination levels were investigated whereas for AC all the above contamination levels were used. In the case of DC biased AC, only 0.024% which was the highest contamination level was tested along with a pure 3 kV DC voltage.