Effect of fast transients on distribution transformer insulation: switching transients modelling and material research

Worldwide many transformer insulation failures have been reported caused by switching operations, while those transformers had previously passed all the standard tests and complied with all quality requirements. The problem is mostly associated with high-frequency overvoltages generated during opening or closing of a switching device. Therefore, a numerical model of a transformer and a circuit breaker are first of all combined and weighted against laboratory measurements. To improve the accuracy of the overall model, the transformer model is extended with a frequency dependent loss model. Secondly, it is important to know the sensitivity of the electrical insulation against these high frequency disturbances superimposed on the 50 Hz main voltage. To investigate this, material tests using an extended Tesla transformer are performed on 2 types of insulation materials: cast resin and paper-oil insulation.