Research on line pilot protection of dual-end weak-fed AC system based on membership degree discrimination of transient impedance model

At the dual-end weak-fed AC system with a significant share of renewable energy integration, flexible DC technology is mostly used in the form of delivery. In such a system with weak feedback at both ends, the influence of the control strategy makes the fault characteristics change essentially. Therefore, it is very important to study the fast protection that is not affected by the control strategy. Among them, the single-ended protection cannot protect the full length of the line and requires high sampling rate. The pilot protection can identify the fault more reliably. However, the existing pilot protection methods have problems such as dependence on detailed system parameters, strict communication synchronization, and complex algorithms. Therefore, A pilot protection based on logical quantity transmission, which does not depend on a large number of parameters, is proposed. Firstly, it is theoretically analyzed that the transient characteristics between fault transient voltage (FTV) and fault transient current (FTC) satisfy the negative resistance-inductance series model when a positive fault occurs in a specific frequency band. When a reverse fault occurs, it satisfies the positive resistance model relationship. Secondly, the frequency characteristics of each impedance are analyzed and the generalized S transform is used to extract the specific frequency band of the signal. Then, the model error is introduced to measure the model membership degree (MMD). The fault direction is identified by the error difference between the two fault models. Finally, based on the above theory, a directional pilot protection that relies only on logic transmission is proposed. PSCAD / EMTDC simulation results show that the proposed method can correctly identify internal and external faults at a sampling rate of 10 kHz. It can withstand 300 Ω fault resistance and 20 dB noise.