A Fast Protection Relay for MMC-Based DC Grids Robust to Cyber-Physical Anomalies

Half-bridge modular multilevel converters (HBMMCs) are widely applied in modern DC grids, where converter substations have a cyber-physical structure similar to that of a typical intelligent substation. One major issue for HBMMCs is the lack of sufficient fault interruption capability. Hence, a reliable protection relay is necessary. However, conventional relays need a very robust cyber-physical system, and unintended events are not rare in intelligent substations. In this paper, several types of cyber-physical anomalies and their impacts on protection relays are first analyzed. Then, a novel protection relay is proposed based on the difference between the theoretical and the calculated modal measurements. To eliminate inter-substation asynchronization, the Bhattacharyya distance is introduced to quantify the above graphic feature in a sliding time window with high robustness to noise contamination; also, historical data is employed to identify and erase upcoming abnormal samples in real time. Case studies validate the robustness against external faults, power order change, and cyber-physical anomalies. Additionally, the proposed protection relay is highly sensitive to internal faults. Discussions are also conducted to investigate the quantitative capability against abnormal samples, to determine the scope of applications for different DC lines and to compare the proposed protection relay with several state-of-the-art methods.