A Microprocessor Resource-Saving Dual Active Bridge Control for Startup and Restart of Three-Stage Modular Solid-State Transformer

The three-stage modular solid-state transformer (SST) is a typical SST with a non-trivial start-up process. Specifically, special controls must be used to reduce the inrush current in the DC/DC stage during the start-up process of these SSTs. For the DC/DC stage based on the widely studied single-phase-shift dual active bridge (DAB), no inner-phase-shift control is required in its normal operations. In this case, using the existing inner-phase-shift control to reduce the inrush current increases the PWM channel requirements of microprocessors and the complexity of the control system. To address this problem, this paper proposes a long-dead-time control with easy implementation to reduce the inrush current without introducing additional PWM channel requirements. Fixed-dead-time and varied-dead-time controls are also proposed to reduce the inrush current when restarting three-stage modular SSTs with small cell capacitors without increasing the PWM channel requirements. The varied-dead-time control is faster than the fixed-dead-time one. The magnitude of the DAB current under these controls is calculated, and the overall time for fulfilling the proposed controls is estimated. The simulation and experiment on a 510 V/380 V 500 kW cascaded-full-bridge SST reveal that the proposed controls can effectively reduce the inrush currents during the start-up and restart processes of SSTs.