Non-Standard Lightning Impulse Voltage Waveforms Applied to Shell-Form Transformers

Lightning impulse tests on power transformers are often performed with non-standard voltage waveform parameters, which could result in overvoltage within windings that are different from calculated. This paper illustrates the impact on the theoretical overvoltage on shell-form transformer windings, based on varying front and tail times, as well as different relative overshoot magnitudes. The results suggest that the transformer insulation might be understressed and/or overstressed, depending on the applied waveform. The paper concludes that present front time tolerance in international standards might need further consideration, and that the combination of long front times with high relative overshoots may result in severe overstress in high voltage windings.