An intelligent transformer enabled medium voltage AC network for loss minimization and node voltage improvement

Abstract The research work presented here discusses about design and integration of intelligent transformer (IT) to the 15 kV/0.4 kV overloaded distribution feeder in Wolaita Sodo Town. The feeder contains 113 distribution transformers with different capacities ranging from 50 kVA to 1250 kVA. The proposed intelligent transformer is a three-stage modular type using phase shift multicarrier modulations for MMC front end converter. Its middle stage is dual active bridge having input series output parallel (DAB-ISOP) configuration using a phase shift modulation. The smart transformer’s back end uses an MMC inverter using interleaved phase shift modulation. The distribution feeder is modeled in MATLAB/Simulink and the designed intelligent transformer is allocated to it using voltage stability index (VSI). The peak load data, feeder data and transformer data of the distribution system under study are obtained from the utility office. The existing distribution feeder’s performance was evaluated by using backward-forward sweep load flow simulation for peak loading condition. The line flows and voltage profile at all nodes were observed and candidate nodes for enhancement were identified. Placement of modular intelligent transformer by using voltage stability index (VSI) was conducted to improve the voltage profiles, minimize reactive power flows and losses. Based on the simulation results for base case scenario, total power loss of 936.5 kW is obtained. The minimum node voltage of 0.81PU and maximum reactive power flow of 10.9MVAr were observed for the base case simulation. Placement of intelligent transformer at weak node 67 and node 111 improves the node voltages to have a minimum value 0.965 PU and maximum value of 1.05PU. The total power loss is reduced to 511.5 kW (54.5% improvement) after integration of intelligent transformer