Mitigation of magnetic inrush current during controlled energization of coupled un-loaded power transformers in presence of residual flux without load side voltage measurements

Abstract

This paper presents a novel approach for minimizing magnetic inrush current to a level as low as no load current during controlled energization of coupled un-loaded power transformers in presence of residual fluxes without employing load side voltage measurements. The suggested method has been evaluated for power transformers having various winding connections, design configurations (magnetically dependent and independent type), its side of energization (Δ or Y) and different level & polarities of residual fluxes. The effect of breaker performance in terms of variation in dielectric properties and operating time of circuit breakers has also been explored. The method is applied successfully on various 765/400/33 kV power (auto) transformers with delta connected tertiary winding on an existing Indian power transmission network and results, for a couple of cases, are presented. Based on simulation results, it has been observed that the inrush current for a number of transformers has been managed below 5% of its full load current rating. This validates applicability of the proposed technique in real system applications.