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High inrush current severely threats the safe operation of the power system and transformers, which may lead to mal-operation of overcurrent protection device, internal short-circuit fault in transformers. Phase-controlled switching technology is an alternative method to reduce inrush current effectively by controlling the front-mounted circuit breaker (CB) to break or make at the predetermined phase angles of current or voltage. In this paper, a new simplified phase-controlled switching strategy is proposed for three-phase unloaded transformer with Y-connected windings in the primary side, which can limit the inrush current under 46.7% of the captured maximum inrush current. Meanwhile, this strategy is independent on the residual fluxes in the iron cores. The optimal making angle is obtained by theoretical analysis based on an equivalent circuit model of unloaded transformer. The comparison of the proposed method and the random switching approach is also presented with considering residual flux. A simulation model is established to calculate the amplitudes of inrush currents against various residual fluxes when using the proposed phase-controlled switching strategy and random switching method. The simulation results show that the proposed strategy can limit the inrush current under a low value. A test platform with high and low voltage instruments is built and some measurements are carried out. The experimental results are in good agreement with the theoretical analysis and simulation results, which verifies the effectiveness and practicability of the proposed strategy. [ABSTRACT FROM AUTHOR]

Proper use of phase-controlled switching technology by controlling the making or breaking of the circuit breaker (CB) at predetermined phase angles of current or voltage can improve the service life of the CB, avoid relay protection device from mal-operation, and reduce the impact on the power grid when power equipment or load is energized or taken out. This paper proposes a novel phase-controlled switching method to address the influence of residual flux for three-phase wye winding unloaded transformers with three limbs, such as large capacitor variable frequency transformer or mine explosion-proof transformer. The optimal operation phase angle for CBs is firstly obtained via theoretical analysis. A model based on the alternative transients program/the electromagnetic transients program (ATP/EMTP) is then built to simulate the transient procedure of a power transformer during breaking and making. The simulated results confirm that the proposed switching strategy can reduce the excitation inrush current effectively when the unloaded transformers are energized. The experimental results reported in this paper are used to validate the accuracy and effectiveness of the phase-controlled switching strategy. [ABSTRACT FROM PUBLISHER]