Reduction of Three-Phase Transformer Inrush Currents Using Controlled Switching

This article proposes a new approach for reducing inrush currents in three-phase three-legged stacked-core power transformers. Although magnitudes of inrush currents may not be as high as short-circuit currents, they are extremely detrimental to normal operation of power transformers. In this article, a new controlled switching scheme that takes into consideration both core saturation and residual flux is proposed to reduce inrush currents. Also, a transient model based on the duality transformation between electric and magnetic circuits is proposed, where a nonlinear inductance is used to simulate core saturation. The proposed scheme is demonstrated on a real transformer by both laboratory experiments and performing simulations. The Alternative Transients Program is used to simulate a wide spectrum of possible operating conditions. The experimental and simulation results show that inrush currents are significantly reduced after utilizing the proposed scheme. Therefore, the proposed controlled switching scheme will result in relaxing inrush-current-related constraints in designing and manufacturing three-phase power transformers, which, in turn, will reduce their cost.