An adaptive restart strategy based on double-ended split-frequency active injection for three-terminal hybrid HVDC systems

The restart strategy for HVDC transmission systems is essential to improve the power supply reliability. The active fault property identification can be realized by injecting characteristic signals. However, the existing signal injection strategy is not fully suitable for the hybrid HVDC transmission system. Additional control strategies of converters, parameter selection of injection signals, and fault property identification criteria need to be further studied. An adaptive restart strategy for multi-terminal hybrid HVDC systems based on double-ended split-frequency injection is proposed in this research. The additional control strategies of LCC and MMC are designed to realize the active injection of sinusoidal current signals with different frequencies and amplitudes. The expressions for the injected current propagating to the end of the line under temporary and permanent faults are deduced to construct the fault property identification criterion. Based on the quantitative analysis, the injection frequencies and amplitudes are selected, and the floating thresholds are calculated, realizing the adaptive restarting of the three-terminal hybrid HVDC transmission system. Simulation results show that the proposed adaptive restarting strategy can accurately identify the fault property at different poles and locations, and the proposed adaptive restarting strategy has the capability of identifying the fault property with transition resistance up to 800 Ω and has the noise interference immunity capability up to 20 dB, which shows good reliability.