Solid state transformer parallel operation with a tap changing line frequency transformer

Increase in load demand require upgrade of distribution transformers and significant investments. An alternative to a complete overhaul of the distribution transformer is to operate a solid state transformer in parallel with the line frequency transformer. This allows functionality to either share the load or accommodate overload conditions. In addition, other options for superior distribution system voltage control may be present. This paper focuses on the parallel operation and control strategy for a 33 kV/415 V, 200 kVA distribution transformer with a 200 kVA parallel solid state transformer. Two control strategies, namely parallel load sharing operation and overload accommodation are investigated. Simulation results are presented for the parallel operation of a SST with a LFT and the effectiveness of the two strategies are discussed.