Influence of Breaker's Grading Capacitor on Controlled Switching for Unloaded Transformer

Abstract

Controlled switching is an effective measure to suppress inrush current of unloaded transformer, and measurement of residual flux is the key to realize controlled unloaded transformer switching. In this paper, influence of circuit breaker's grading capacitor on controlled switching for unloaded transformer is studied. Firstly, theoretical analysis shows that the residual flux in transformer core is a periodic dynamic value rather than a steady value as general think, after the transformer is switched with a circuit breaker equipped with grading capacitor. Secondly, simulation analysis and experiment research on the influence of circuit breaker's grading capacitor on the residual flux in transformer core are conducted. Results demonstrate that a larger grading capacitor decreases residual flux level. Finally, a controlled switching strategy is proposed, taking average value of dynamic residual flux during full period as the residual flux level to determine optimal energization instant. Simulation and experimental results validate its effectiveness. The research results are constructive to controlled unloaded transformer switching.