Partial Startup Scheme for Smart Transformer in Meshed Hybrid Islanded Grid Operation

Smart transformer (ST)-based low-voltage (LV) meshed hybrid grids provide various power flow paths and ensure continuous power supply to loads. The meshed hybrid microgrid can continue supplying to LV loads even during islanding operation. In such a case, the ST LV converter is operational while its medium voltage (MV) converter as well as the isolated dc–dc converter are shut down. This article proposes a step-by-step startup method for these two converters of the ST to establish reconnection of the meshed hybrid LV microgrid to the MVac grid while keeping the ST LV converter functional. The proposed method involves charging the MVdc bus to 85% of the nominal value from the LVdc bus, which is already energized. Next, the ST MV converter is energized to build the MVdc voltage to the final value. Once it is established, the power flow is started, and the transition from islanded to grid-connected mode of the meshed hybrid microgrid is established. To realize the entire starting process, a centralized controller is proposed. This controller gives the
on
/
off
commands to the various converters and circuit breakers in the system as per the proposed partial startup algorithm. Stability studies of the ST isolated dc–dc converter are carried out to ensure that the system maintains stability during the starting process. Simulation studies and experimental validation have been performed to verify the proposed starting scheme.