Protection Method for Continuous Operation of Wind Power Plants in a Mechanical Circuit Breaker-Based Multi-Terminal HVDC System

Multi-terminal high-voltage dc (HVDC) transmission system is a promising approach to connect offshore wind power plants (WPPs) to onshore ac grids. However, there is no standardized protection method against DC faults. As one of its protection methods, mechanical dc circuit breakers (DCCBs) have the potential to improve supply reliability against dc faults while avoiding a cost increase. Nevertheless, due to their relatively slower operation, the blocking of half-bridge-based modular multilevel converter (HBMMC) is often required. In offshore ac collecting system, where the HBMMC maintains the grid voltage, such converter blocking can destabilize the grid voltage and lead to shutdowns of offshore WPPs. Large scale shutdowns of offshore WPPs may have a negative impact on onshore ac grids. Therefore, this article proposes a protection method that enables the continuous operation of offshore WPPs while using mechanical DCCBs. The proposed method focuses on the backbone HVDC configuration connecting multiple onshore and offshore terminals, and applies different fault clearing methods across the terminals. At onshore terminals which form a loop configuration, mechanical DCCBs selectively isolate the faulted section. At offshore terminals which form a radial configuration, reconfiguration is employed to reroute power transmission from the faulted line to the healthy line. These operations are coordinated based on the fault ride-through (FRT) capability of offshore WPPs and realizes their continuous operation. The proposed method is verified by an experiment using the scaled-down three-terminal HVDC system.