Partial Start-up Scheme for ST to Reconnect to MV Grid from LV Side Islanded Operation

This paper proposes a partial start-up procedure for a smart transformer (ST) to changeover from low voltage (LV) side islanded operation to grid connected mode. In the islanded operation of ST LV side, the ST LV converter is operational but the other two converters of the ST namely the ST medium voltage (MV) converter and ST isolated dc-dc converter are shut down. For the reconnection to the MV grid, these converters have to be switched on to enable power flow. Thus while one portion of the ST is already operational, the non-operational parts have to be started without affecting or interrupting the operation of the LV grid. This calls for a proper stage by stage start-up procedure to ensure that the converters become operational maintaining the staring inrush currents within rated limits of the converters. A reverse start up scheme is proposed where the ST MVdc bus is charged by drawing power from the battery energy storage system connected to the LVdc bus. Subsequently the ST MV converter is started to connect the LV side to the MV grid. Simulation results are incorporated which verify the proposed start-up scheme.