Grid forming control for solid-state transformer operating in islanding and grid connected conditions

Solid-state transformer (SST) has recently been considered as one of the key components for future distribution networks with enhanced controllability and operating flexibility. This paper proposes a universal control scheme for SST's low voltage DC/AC converter such that the same grid forming control structure can be used during both islanding operation and grid connected state to the adjacent feeder through the conventional normally open point (CNOP). This ensures smooth transition between different operation states without the need for control mode switching. To reduce the coupling between active and reaction power, an additional compensation control using active and reactive power feedforward compensation to dq-axis current components of the DC/AC converter is proposed. Simulation results are provided to confirm the overall SST performance and effectiveness of the proposed control schemes during power reference changes when the SST is operated in grid connected mode through CNOP.