DC Fault Analysis and Transient Average Current Based Fault Detection for Radial MTDC System

It is of great importance for multi-terminal DC (MTDC) system to immediately isolate the DC fault line within several milliseconds after DC fault due to VSC or half-bridge based MMC converters with limited ability to control DC fault current during DC fault. Failing of the DC fault isolation during 'Capacitor Discharging Stage' may lead to the overall MTDC system out of operation with the fast blocking of IGBTs. Therefore, initial DC fault current analysis is essential for system protection design, especially for fault detection. Accordingly, a transient simplified model is first proposed with only high-frequency components of the fault DC network reserved. As a result, the original complex DC network is largely reduced and the fault DC current with several milliseconds can be clearly expressed based on the proposed model. According to the distinct differences between the current of faulty cable and that of healthy cable, a non-unit DC fault detection method utilizing the transient average value of line current with low computational complexity and high-sensitivity is further proposed. Numerical simulations based on PSCAD/EMTDC have been carried out, which well-demonstrate the accuracy of the proposed transient simplified model for initial DC fault analysis and the effectiveness of the proposed transient average current based fault detection scheme. Compared with the traditional change of rate of current based protection method, the proposed one stands out for relatively low sampling frequency, high fault resistance tolerant ability, and robustness against outside noise and data missing.