Adaptive positive-sequence fault-component directional relay for the two-terminal weak feed AC system based on positive-sequence impedance reconstruction control

The flexible DC transmission system for renewable energy has become a key solution for the large-scale integration of renewable energy into the power grid. This system typically consists of a two-terminal weak-feed AC system, which relies entirely on power electronic equipment. In the event of a transmission line fault, the fault ride-through (FRT) control strategy implemented by the converters at both ends causes changes in the phase angle of their equivalent impedance, thereby reducing the sensitivity of the directional relay. First, this paper theoretically derives the phase characteristics of the positive-sequence equivalent impedance (PSEI) at both ends of the two-terminal weak feed AC system under FRT control, and performs an adaptability analysis of the positive-sequence fault-component-based directional relay (PFDR) in the two-terminal weak feed AC system. Second, at the control strategy level, a positive-sequence impedance reconstruction (PSIR) control strategy based on the FRT framework is proposed. This strategy not only enhances the performance of PFDR but also meets the FRT requirements of the two-terminal weak feed AC system. Third, at the protection principle level, an adaptive-function-based PFDR is proposed, which enhances the fault characteristics at the protection principle boundary, thereby improving the adaptability and sensitivity of PFDR under complex fault conditions. Finally, an improved PFDR is proposed for fault direction detection in two-terminal weak feed AC system. The proposed PFDR combines the PSIR control strategy with an adaptive-function-based tuning principle. This approach enhances the performance of fault detection while maintaining the system’s stability. The simulation results demonstrate that the proposed protection scheme can reliably identify the fault direction in the two-terminal weak feed AC system, even under fault conditions with a 300 Ω fault impedance and 30 dB noise interference.© 2017 Elsevier Inc. All rights reserved.