Study on the recovery strategy of Multi-Terminal HVDC after the fault

The Multi-Terminal HVDC (MTDC) technology is one of the most important meathods to realize cross-regional and long-distance power transmission. The MTDC system generally takes use of power electronics technology based on the Voltage Source Converter and Pulse Width Management technology. Compared with the traditional DC transmission system, the MTDC system suppresses the generation of harmonics in the system. In fact, active power and reactive power are independently decoupled to realize four-quadrant operation in the MTDC system. At the same time, the rapid development of MTDC systems brings a big challenge to the fault isolation technology and the recovery strategy of the MTDC system. At present, the technology of DC circuit breakers in China is still not mature, and the converter commonly used in projects can not realize the isolation of DC faults in a very short time. Besides, after faults are cleared, the DC system needs to be precharged to resume itself. Thus, technology we make use of nowadays will be relatively weak in terms of the fault isolation technology and the recovery/restart strategy. At present, most research of domestic and foreign scholars mainly focus on fault isolation technology of the MTDC system. Fortunately, lots of outstanding results have been achieved in related fields. However, there are few studies on the recovery strategy of MTDC systems. Therefore, based on the analysis of characteristics of the DC fault appeared in MTDC systems, a four-port ring network DC transmission system model is established to study the main questions in the recovery strategy of MTDC system after the DC fault.