After a power transformer is de-energized, the residual flux trapped in its core is unknown; consequently, re-energization often produces severe inrush currents whose peaks may reach 6–8 times the rated current. To protect the power system and equipment, controlled closing is commonly employed. This approach, however, demands precise control of the closing angles. This paper therefore investigates synchronous-closing inrush-current suppression strategies and the operating-time characteristics of three-pole mechanically linked circuit breakers. First, based on the residual-flux evolution, a synchronous closing inrush-current suppression strategy is proposed and requirements on circuit-breaker closing-time accuracy are quantified. Next, the closing pre-strike process are analyzed for breakers. Then a closing-time compensation algorithm tailored for synchronous closing inrush-current suppression is proposed. Finally, field tests under varying operating conditions validate both the algorithm’s ability to compensate for the breaker’s operating-time scatter and the effectiveness of the compensated inrush-current suppression strategy.

Inrush-current suppression,no-load closing,pre-strike process,SF₆ circuit breaker,transformer