Transient stability enhancement of DC-connected DFIG and its converter system using fault protective device

Transient stability of doubly-fed induction generators (DFIGs) is a major concern in both AC and DC grids, and DFIGs must stay connected for a time during grid faults according to the power grid requirements. For this purpose, this work proposes an overcurrent and overvoltage protective device (OCV-PD) to ensure that DC-based DFIG system can stay connected and operate well during the faults. Compared with a series dynamic braking resistor (SDBR), two aspects are improved. First, a two-level control strategy and DC inductor circuit are used to ensure that the OCV-PD can limit the current impulse to protect DFIG system during an overcurrent fault. Second, the OCV-PD can protect system from overvoltage fault which a SDBR cannot do. Simulation results verify its validity and feasibility, finding that for overcurrent protection the OCV-PD outperforms a SDBR with an average decreased index of 3.29%, and for overvoltage protection it achieves an average index of 1.02%.