Controlled switching of transformers - effects of closing time scatter and residual flux uncertainty

Controlled switching taking into account the residual flux can - in theory - completely eliminate transient transformer inrush currents. Real factors such as closing time scatter of the circuit breaker and uncertainty in the residual flux measurement significantly reduce its performance in the field. To quantify the acceptable tolerances of these factors if no inrush currents have to occur, a systematic inrush current study was carried out in the laboratory. Instantaneous switches and ideal residual flux measurement in combination with a controller unit are used to emulate the scatter respectively the uncertainty by presetting deterministic deviations. The analyses were carried out for ldquorapid closing strategyrdquo and ldquodelayed closing strategyrdquo and showed that the acceptable tolerances for both strategies can be evaluated independent of the controlled switching strategy with the inrush current studies of the first phase that is energized.