Adaptability analysis of distance protection in flexible low frequency ac outgoing transmission line of wind power system

The extension of offshore wind power to medium and far seas has promoted the development of flexible low-frequency AC transmission (FLFAC) technology. However, the fault responses of the power electronic interface-type power sources (wind turbines and modular multilevel matrix converters) connected at both ends of the low frequency (LF) line are actively regulated by control strategies, which are essentially different from traditional power frequency (PF) AC transmission systems, leading to the adaptability bottleneck of traditional relay protection. Existing studies have insufficient analysis on the adaptability of distance protection in FLFAC systems, especially lacking a systematic comparison of different types of distance protection. Therefore, this paper focuses on the adaptability issue of distance protection in FLFAC systems. It first elaborates on the basic principles of power frequency distance protection, power frequency fault component distance protection, phase comparison distance protection, and time-domain distance protection. Combined with the fault characteristics of low-frequency systems, it theoretically analyzes the influence mechanism of power electronic source fault responses on each protection. Then, it verifies the adaptability of protection on the wind farm side and the M3C low-frequency side through simulations. The research shows that time-domain distance protection has the optimal reliability under complex working conditions, which can provide a quantitative basis for the selection of FLFAC system protection schemes, has practical value for improving the reliability of system protection and engineering implementation, and also lays a theoretical foundation for subsequent protection improvement.