Evaluation of fast-front overvoltages arising at a 20/0.4 kV distribution transformer

Fast-front overvoltages arising at or transferred to the medium-voltage and low-voltage terminals of a 20/0.4 kV distribution transformer are evaluated with the aid of ATP-EMTP simulations. The effects of lightning crest current and of the transformer and load grounding resistances on the computed overvoltages are investigated. The protection against lightning surges offered by surge protective devices installed close to the low-voltage terminals of the distribution transformer is evaluated. The peak of the overvoltage arising at the medium-voltage terminals of the distribution transformer may exceed the corresponding insulation level even when surge arresters are operating close to the latter. The peak of the overvoltage arising at or transferred to the low-voltage terminals of the distribution transformer may exceed the corresponding insulation level when surge protective devices are not installed. When surge protective devices are installed, the overvoltages are significantly reduced and practically unaffected by transformer and/or load grounding resistances.