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

• The proposed scheme is for magnetic inrush mitigation based on signature identification.
• The proposed scheme does not require transformer side PT measurements.
• It can be used for to all major connection and design configurations of transformers including coupled transformers.
• It can effectively reduce inrush to a very low level of 10% of full load current.
• The scheme is successfully used to mitigate inrush for a large number of coupled transformers at 765 kV level.

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.