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Flux Error Function Based Controlled Switching Method for Minimizing Inrush Current in 3-Phase Transformer

Energization of the transformer leads to large magnetizing current called inrush current. It results in reduction in transformer life, reduction in circuit breaker life, and power quality issues in the system. For low and medium voltage level systems in generation and distribution, usually, gang operated circuit breakers are employed, which do not allow independent pole operation. This makes controlled switching more challenging in such systems. This paper proposes a controlled switching technique to minimize the inrush currents for a 3-phase transformer using gang operated circuit breaker. Optimum points for controlled closing and opening are obtained from a flux error function calculated from residual and prospective fluxes of the transformer. Practical challenges due to error in voltage measurements, variation in mechanical closing time, transients in flux, etc. and their solutions are also discussed in this paper. The proposed technique is validated by Controller Hardware-In-Loop (CHIL) test and a scaled-down laboratory test on 5 kVA, 210/400 V, Delta-Star transformer. In both cases, the inrush current is limited to 1pu, demonstrating the effectiveness of the proposed scheme. Furthermore, a field test is carried out on a 500 kVA, 6.6 kV/433 V, Delta-Star distribution transformer to confirm the viability of the developed switching technique.