Minimising transformer inrush current under variable load conditions by controlled switching with automatic search

Energisation of power transformers through circuit breaker switching gives rise to inrush currents due to magnetic saturation of the transformer core, which can lead to undesirable voltage transients and false tripping of protection functions. Inrush currents can be effectively eliminated by controlled switching if the closing instant for each phase is selected such that the prospective magnetic flux generated by the applied voltage matches the remanence in the transformer core. The remanence is determined by the circumstances at the last de-energisation event which includes the magnitude and power factor of the interrupted load current and the presence of de-magnetising effects. In a dynamic grid, the remanence of transformers can be expected to vary significantly over time, which prompts for a self-adapting controlled switching strategy. In this paper we present an automatic search method to find and maintain the closing phase angles that minimise the inrush current under variable load conditions. Laboratory results are presented where a 0.9 MVA 20/0.69 kV dry-type transformer is energised and de-energised by a vacuum circuit breaker with independent pole control where the low-voltage load irregularly changes between resistive load, inductive load, and no-load.