Effect of Repeated Impulses on Transformer Insulation

Power transformer plays a vital role in electrical power system. Many of the transformer failures reported are due to insulation failure in windings. The sources of failures are mainly due to impulse overvoltages of different waveshapes and also due to repeated application of impulses. In this paper an attempt has been made to analyze the degradation of transformer insulation for repeated applications of different impulse voltages. As oil impregnated paper (OIP) constitute major insulation of transformer winding, detailed experimental analyses are carried out for OIP insulations using standard test cell for impulse waves of varying front times from 0.064 μs to 1.4 μs and tail times from 6.0 μs to 67 μs. Exponential mathematical models for the voltage-number (V-N) characteristics are derived and used for predicting the number of impulses that OIP can withstand for any wavefront and wavetail. The accuracy of the models are checked with the experimental values.