Survey of Fault Analysis and Relay Protection of Flexible Low-Frequency Transmission System

The flexible low-frequency AC transmission technology based on the voltage source AC-to-AC converter makes large-scale networking easy, which can enhance the line transmission capability with its flexible regulation and control functions like power flow regulation, so it is a new transmission technology worthy of in-depth research. The fault characteristics of the flexible low-frequency transmission system (LFTS) are affected by topological structure and control strategies, possibly resulting in a problem of adaptability of conventional protection methods, i.e. relay protection faces a serious challenge. This paper presents an analysis of the basic structure and fault characteristics of the flexible LFTS based on the modular multilevel matrix converter (M3C). On this basis, the adaptability of the relay protection technology to the flexible LFTS is summarized. To solve the problem of adaptability inadequacy of some protection technologies, the research prospect of key protection technologies adapting to the LFTS is made, such as fast amplitude calculation method, sampling value differential protection and time-domain distance protection.