Fault Protection Strategy and Switching Overvoltage in a ±500 kV MMC-HVDC Grid

The Zhangbei ±500 kV modular multi-level converters (MMC)-based high voltage direct current (HVDC) project to be built in China will use overhead transmission lines with dc circuit breakers (DCCBs). However, the operating characteristics of DCCBs are still not clear. In this paper, a detailed hybrid DCCB model and the fault protection strategy are built to study the influence of switching overvoltage. The results indicate that fault protection strategy can protect the equipment and causes the overvoltages changing based on the fault location. For the fault without protection, the overvoltage of three-phase grounding fault at the valve side of converter is the largest, which is 1295.2 kV (2.6 pu). For the fault with protection, the overvoltage of line-to-line fault is the largest, which is 1206.8 kV (2.41 pu). The theoretical analysis indicates that the outlet voltage of the converter increases when the DCCBs open because it is the superposition of the voltage of the line reactance and the residual voltage of the arrester, which is consistent with simulation analysis. The results can provide references for the application of DCCBs and construction of MMC-HVDC grids.