DC fault analysis and detection for offshore wind farms integration via MTDC

Two-level VSC(Voltage Source Converter) and half-bridge MMC are lack of the ability to control fault DC current during DC fault. Therefore, in MTDC(Multi-Terminal DC) system, it is crucial to immedia-tely identify and isolate the fault during the first period of DC fault, namely, capacitor discharging stage, other-wise the overall system might be out of operation due to the fast blocking of converter valves. The initial DC fault current analysis during capacitor discharging stage is the prior issue for fault detection in MTDC system. A transient equivalent model is proposed for fault analysis of MTDC system, in which only high-frequency components of the fault network are reserved, so the analysis in the failure period can be simplified. The analytical expression of fault current of faulty and healthy lines can be deduced based on the proposed model. Based on the current difference between fault lines and healthy lines, a fault detection method based on the average value of transient current is further proposed, which has the advantage of rela-tively low computation burden, high fault resistance tolerant ability, and relatively low sampling frequency. Numerical studies carried out in EMTP verify the accuracy of the proposed transient equivalent model and the effectiveness of the proposed fault detection method. © 2020, Electric Power Automation Equipment Press. All right reserved.