Flashovers at a 33-kV filter reactor during energization

For an extrusion plant 33-kV filter circuits are installed to absorb harmonics of the 5th and 11th order in the electric power supply. During the system start-up of the filters flashovers occurred between the two coil parts of the reactor belonging to the filter for the 5th order harmonic in two of the three phases.This paper shows how overvoltages can be produced during energization of the filter, which may cause flashovers at the filter reactor by means of digital simulations of switching transients. The system components including the filter circuit are represented in detail using ATP-EMTP. Both filters are fed by a relatively long 33-kV cable. The saturation characteristic of the filter reactors plays an important role in initiating repetitive high transient voltages. The neutral point of the filters is isolated, since the neutral of the feeding 33-kV system is not grounded.The overvoltages across the reactor obtained by a statistical analysis of switching surges are far higher than the amplitude of the withstand test voltage performed at the factory. Severe overvoltage stress of the filter reactor may also happen, when the filter will be re-energized shortly after it has been switched off, so that the filter capacitors remain charged. In this case, the highest overvoltage of 83.9 kV is expected across the filter reactor.