The article concerns the problem of electrical discharges developing in transformer oil under lightning impulse (0,83/50 μs) of both polarities. The results of experimental studies performed for two setups: HV electrode-grounded plane, having different electric field distribution, were discussed. For both setups, two types of HV electrode with identical outer dimensions: bare and paper insulated, were used. The results of research lead to the conclusion that the oil, not the surface of electrode or insulation wrapping, is the place of weak points of insulation system. Simultaneously, dynamics of electrical discharge development in the case of insulated electrodes is different than in the case of bare ones.
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