An Enhanced Current Differential Protection for AC Transmission Lines Connecting MMC-HVDC Stations

The peculiar fault current signatures of modular multilevel converter-based high voltage dc transmission (MMC-HVdc) system degenerate the operating performance of conventional current differential protection (CDP). CDP may incur low sensitivity or maloperations, especially when MMC-HVdc stations operate in rectifier mode. The performance problem of CDP in the presence of MMC-HVdc stations is analyzed. To address this issue, an enhanced CDP (ECDP) based on amplitude and phase correction factors is presented. The basic principle of ECDP is illustrated and the performance comparison between CDP and ECDP is provided. Furthermore, to avoid unwanted operations caused by abnormal data under external faults, abnormal data index is designed to identify abnormal data. Saturation index is proposed to improve the security of ECDP in the case of CT saturation. Compared with CDP, ECDP has a faster respond speed for internal faults. ECDP can perform well under different operation modes of MMC-HVdc station and various fault conditions. Therefore, ECDP is a promising candidate to protect transmission lines connecting MMC-based converter stations. The performance of ECDP is evaluated using PSCAD/EMTDC and RTDS.