Voltage stresses in a distribution transformer under nonideal switching conditions

Failure of a 1600 kVA molded type distribution transformer due to switching surges is investigated through computer simulations. The transformer was supplying a motor bank in an industrial plant from a 20 kV busbar. A three-phase PSPICE model is built to simulate the system transients and internal voltage stresses on transformer windings during switching operations. Simulation of various switching conditions involving current chopping and multiple reignition phenomena have shown that severe internal overvoltages may arise between adjacent coils of the high voltage windings, while the terminal voltage of the transformer remains below the guaranteed basic insulation level. In such cases surge arresters can not provide a sufficient protection since high frequency oscillations may cause partial winding resonance. Simulations have shown that an RC filter installed in the HV terminals of the transformer reduces the internal voltage stresses below the guaranteed limits. The filter also reduces the possibility of arc reignition between the vacuum circuit breaker contacts.