Research on Transient Overvoltage Characteristics of VSC-HVDC Transmission System Connecting to Large-scale Offshore Wind Farm

China is rich in offshore wind power resources. With the development of offshore wind power resources near the coast and the continuous improvement of technology, the development and utilization of offshore wind power resources away from the coast has become an inevitable trend. Voltage sourced converters high voltage direct current (VSC-HVDC) transmission is the best technical method to complete the collection and transmission of offshore wind power resources away from the coast. The transient overvoltage level of the VSC-HVDC system is an important factor in determining the insulation level of the equipment, air clearance and project costs. This paper adopts PSCAD/EMTDC simulation analysis method, and conducts research based on a real offshore wind power transmission project via VSC-HVDC under construction. This paper analyzes the characteristics of typical faults, as well as the transient overvoltage levels at key locations of both offshore converter station and onshore converter station under different operating conditions. The influences of different operating conditions on the overvoltage level are analyzed. Studies have shown that due to the capacitance effect of long-distance DC submarine cables, the fault characteristics and transient overvoltage duration of the DC system are far from the previous VSC-HVDC projects. Protection strategy, transmission power, switching strategy of DC energy dissipation device will all affect transient overvoltage. The feature of cable overvoltage is studied. The maximum transient overvoltage levels between terminals, phases and poles are obtained. The research conclusions can provide a theoretical basis for the development of equipment and the layout of converter stations. Furthermore, it also provides a reference for the follow-up offshore wind power transmission projects.