Fault-Tolerant Control of Electric-Spring Enabled Solid-State Transformer Under Dual Active Bridge Failure

The electric-spring (ES) technology is recently integrated into a solid-state transformer (SST) to support the power grid at the distribution voltage level and provide an 800-V dc grid for large-scale electric vehicle (EV) charging infrastructure. The ES-enabled SST (ES-SST) studied here consists of a diode-clamped converter (DCC) and several dual active bridges (DABs). The failure of one DAB could pose a big challenge to the balance of the dc-link capacitors and threatens the operation of the whole system. Existing voltage-balancing methods are not suitable for the faulty ES-SST due to the highly uneven distribution of the capacitor output power. In this article, a fault-tolerant control method is proposed to keep the capacitor voltages balanced under the DAB failure and maintain the operation of the system. The proposed control features a modulation algorithm to maximize the balancing capability of the DCC, the insertion of a zero-sequence voltage offset in the ac voltages, and the deliberate generation of reactive power. A numerical tool is also developed to predict the operability of the faulty system and design the controller. Simulation and experiments are conducted to verify the proposed control.