Analysis and protection of HVDC systems subject to AC and DC faults

Voltage source converter-based (VSC) high-voltage direct-current (HVDC) system is emerging as an important technology for transmitting bulk power over long distances and integrating large-scale renewable energy. However, there are still a number of technical challenges. One of the critical aspects is the fault protection. This thesis focuses on the fault behaviours of the VSC HVDC systems and the connected ac systems when the HVDC system subject to ac or dc faults. In relation to the dc side faults, the operating characteristics of dc circuit breakers (DCCBs) and the application of DCCBs in dc grids were studied. The factors, such as the current limiting reactor and the surge arrester, which affect the performance of a DCCB to interrupt dc fault currents were investigated through simulations. In order to test the effectiveness of utilising DCCBs to isolate dc faults, experimental validation was carried out by applying DCCB prototypes in a three-terminal VSC HVDC test-rig. Different dc fault protection approaches were applied in a meshed dc grid which connects to an ac transmission system with two connection points. This aims to investigate the impact of different dc protection approaches on the stability of the integrated ac/dc system. The dynamic interactions between the ac and dc systems were studied further. In terms of converter ac side fault, single-phase faults which occur in the area between the valve and the interface transformer were studied. The fault characteristics of the valve-side single-phase fault in three converter station configurations were analysed in detail. Fault protection strategies based on the theoretical analysis were proposed and validated through simulations. The study of this research work is expected to contribute to the design of ac and dc fault protection of HVDC systems