Fault control and line protection strategy for LVDC microgrids based on modified high-frequency-link DC solid state transformer

Fault current with high amplitude and rate of change, together with the vulnerability of electronic devices, poses a great challenge to the line protection in low voltage DC (LVDC) microgrids. A fast fault control and protection method are proposed for LVDC microgrids, based on the modified topology of the high-frequency-link DC solid state transformer (DCSST). By installing two bidirectional antiparallel thyristors in the DCSST and isolation devices at the interface converter side, the freewheeling diodes are protected and current zero-crossings occur due to the submodule capacitor discharge during the fault. Then the line protection could be realized by improved current differential protection, and the mechanical circuit breaker can be tripped around current zero-crossings. It alleviates the high requirement for fast tripping devices (such as solid-state circuit breakers) and fault identification within several milliseconds. The scheme combines the converter fault control with line protection to ensure slow but more economic and reliable fault identification for LVDC grid protection. The proposed strategy is verified by PSCAD/EMTDC simulations and the results show that the fault-tripping-recovery process only takes few tens of milliseconds.