Predictive Direct DC-Link Control for Active Power Decoupling of A Single-Phase Reduced DC-Link MV Solid-State Transformer

This paper presents a fast predictive control method for active power decoupling of a reduced dc-link medium-voltage (MV) solid-state transformer (SST). Typical configurations of the SSTs are multiple series-stacked modules to interface the MV, which requires the modules to be single phase in nature. One critical issue in the single-phase SSTs is 120 Hz power fluctuation. Traditionally, large passives are used to buffer the 120 Hz power for 5% ripple on the dc-link, which significantly increases footprint and cost. This paper proposes active power decoupling strategy for the MV SST, i.e., adding an additional capacitor port to buffer the 120 Hz power fluctuation with 30% ripple on the capacitor port for smaller capacitance requirements. Moreover, significantly, different from conventional single-phase converters with small dc-link ripple, the dc-link itself is sized for 30%-60% pk-pk switching-frequency ripple to further minimize the dc-link passives in this paper. For such a converter with small passives, the control should be fast enough to ensure small over/undershoot on the dc-link during transients for safe and reliable operation. To address the challenge, direct dc-link regulation is proposed rather than traditional indirect dc-link regulation in terms of control architecture. Based on this control architecture, a model based predictive control method, which updates every switching cycle for a deadbeat response on the dc-link, is proposed. Simulation and experimental results at 2 kV verify the effectiveness of the proposed direct dc-link predictive control method for active power decoupling and reduced dc-link in the MV SST application.