MV/LV parallel transformers: an experimental study on circulating currents

When a high value load is to be supplied and greater system reliability is required, the medium voltage power supply is generally used with a low voltage conversion by means of two or more MV/LV transformers. This configuration is also more suitable for maintenance or resilience aspects. In Europe, in case of use of these distribution transformers, the windings configuration is Delta (primary winding) and Star (secondary winding). To obtain a common ground reference for the three phases and neutral system, the star point is grounded. In case of parallel transformers, this ground point is common. Furthermore, when using transformers in parallel, a proportional reduction of the short-circuit impedance is obtained with the consequent increase in the short-circuit current during faults and the need to adopt cables and protections that consider this aspect. From the other hands, the use of parallel transformers may introduce an increasing of the no load losses. In transformers parallel configuration system, if the transformation ratios are not identical, a circulation current is generated. This current is proportional to the voltage difference divided by the sum of the impedances along the path comprising windings and ground connections of the star centers. In a distribution substation with two parallel transformers of identical characteristics, the circulating currents in each transformer are of equal amplitude and in opposite directions. Few studies have investigated what happens at the connection points between the star centers of the transformers and the earth bar of the secondary substations. This paper gives a contribution on the evaluation of the circulating currents based both on theoretical considerations and experimental measurements.