DC Fault Current Blocking With the Coordination of Half-Bridge MMC and the Hybrid DC Breaker

In this article, a modified hybrid dc breaker and a half-bridge modular multilevel converter (MMC) are employed to interrupt the dc fault current in a high-voltage direct current transmission system. In the hybrid dc breaker, parallel to the main branch, a series connection of an energy-absorbing capacitor and a Thyristor stack is employed. When a dc fault current is recognized, the Thyristor stack is triggered and the load commutation switch in the main branch is turned off. In addition, in the fault clearing state, the MMC is programmed to generate zero voltages at all arms. Using this mechanism, a second-order RLC circuit with zero input is formed, and the fault current is cleared in a very short time. A freewheeling path is provided to bypass the dc line when the equivalent inductance seen from the dc line is large. In contrast to conventional approaches, the peak of dc fault current is limited to a specific limit and the necessity to ultrafast mechanical switches is avoided. In addition, the amplitude of arms currents and corresponding insulated-gate bipolar transistors are kept in the safe operating area. A detailed circuit analysis of the new method is given and design formulas are extracted. Simulation results are provided in PSCAD/EMTDC environment, and experimental results are presented for a scaled-down prototype.