Controlled switching technology can effectively suppress the inrush current of unload power transformer. Conventional switching strategy only works with independent-pole-operated circuit breakers. There are few researches about controlled switching strategy for three-pole-operated circuit breakers. For this reason

we analyzed the basic principles of controlled switching strategy for unload transformers

studied the change rules of dynamic core flux during the energization of unloaded power transformers with ganged three-operated circuit breakers

and proposed a controlled switching strategy for ganged three-operated circuit breakers. Based on the PSCAD/EMTDC software

the proposed controlled switching strategy was simulated

and the simulation results prove the effectiveness of this technique. The experimental platform was built

and the tests of random and controlled switching were conducted for unloaded transformers. The simulation and experimental results show that the inrush current caused by random switching is 6.3 times the rated current. For the ganged three-operated circuit breaker

the proposed controlled switching strategy is used to energize

the unload transformer magnetizing current can be suppressed to 2 times the rated current

and the suppression effect is obvious.