Optimal operation of paralleled power transformers

Abstract

Parallel operation of power transformers is a common practice. Interest is placed on minimizing the reactive current circulation between transformers due to mismatching of electrical properties. Several control schemes have been applied over the years to ensure efficient operation of paralleled transformers with OLTC. These methods rely either on field measurements of circulating currents or on classic Master/Follower approach and usually assume that the paralleled transformers have matching impedances and OLTC positions. On the present work a generalized approach is implemented calculating the circulating current and minimizing losses directly at the System Operation Office and using the distribution system SCADA to control tap position of the paralleled transformers. This permits the determination of optimal tap matching between transformers and calculation of losses under emergency situations where coupling of transformers with slight impedance mismatch might be considered. In the proposed method a map of circulating current is calculated based on the transformer impedance, X/R ratio and OLTC positions, then the optimal tap positions (minimizing losses) under parallel operation are determined.