1.Key Laboratory of Control of Power Transmission and Conversion, Ministry of Education (Shanghai Jiao Tong University), Shanghai 200240, China;2.State Grid Hubei Electric Power Co., Ltd., Wuhan 430077, China

The multi-terminal flexible DC transmission system has high control flexibility, and the control and protection are closely linked when the system fault occurs. With consideration of the response characteristics of the control process variables with different control strategies of flexible DC transmission, the magnitude relationship of the control signal transient energy for control signal is used to identify internal and external faults, and the transient energy ratio of the positive and negative poles is used to select the fault pole. A protection scheme for detecting DC transmission line faults based on characteristic signals of outer loop power control is proposed. The scheme makes full use of the rapid response capability of converters. It is simple to implement, has a fast failure detection time, and facilitates the integrated design of control and protection. It can realize the adaptive protection of different control strategies and control parameters. Finally, the effectiveness of the proposed protection scheme in fault identification and failure pole selection is verified through a large number of simulations, and the reliability of the protection is verified with the influence of various factors.