Time-domain impedance pilot protection of MMC HVDC transmission line based on modulus characteristics

Flexible HVDC transmission systems provide flexible and efficient solutions for new energy grid integration and asynchronous interconnection of AC systems, and represent a major trend in the future development of power grids. However, the current pilot protection for DC lines suffers from a relatively long time delay (approximately 1.1 s), resulting in poor speed performance. To address this issue, this paper starts from the converter station structure of MMCHVDC transmission systems. First, based on the series-parallel characteristics of arm resistors, inductors, and sub-module capacitors, the DC side of the converter station is equivalent to an RLC series circuit. Second, through frequency response analysis, it is found that the impedance of this equivalent circuit exhibits inductive characteristics in the fault transient frequency band (>1 kHz); further analysis shows that during internal line faults, the converter stations at both ends are equivalent to negative impedances, while during external faults, one end is equivalent to a negative impedance and the other to a positive impedance. Based on this, a pilot protection method for MMCHVDC lines based on modal characteristics and time-domain impedance is proposed, which for the first time introduces the correlation analysis of fault components of modal voltage and current into MMCHVDC line protection. Finally, the adaptability of the proposed method was verified using PSCAD/EMTDC. Experimental verification shows that the proposed method exhibits excellent robustness in a strong noise environment of 20 dB, can reliably detect high-resistance ground faults up to 500Ω, and has a protection operation time of only 3 ms, demonstrating good speed performance.