Research on Low-Voltage Current-Limiting Strategy for Single-Phase-to-Ground Fault Differential Protection in MMC-HVDC AC Collection Lines

After a fault occurs in Renewable Energy Transmission via MMC-HVDC System AC collection line, in order to prevent the MMC converter station from being disconnected from the grid due to overcurrent tripping, a low-voltage current-limiting strategy is adopted to restrict the fault current. However, the impact of the low-voltage current-limiting strategy on the differential protection performance of the collection line still needs further research. To address this issue, this paper first analyzes the effect of the low-voltage current-limiting strategy on the sensitivity of differential protection. When a metallic short-circuit occurs in the collection line, a smaller voltage reference value must be selected. However, in the case of a non-metallic short-circuit, a smaller voltage reference value causes differential protection to fail to operate. Based on this analysis, this paper further proposes a low-voltage current-limiting control strategy that ensures reliable operation of differential protection under single-phase ground fault conditions. The ratio of differential current to braking current is incorporated into the calculation of the voltage reference value, enabling the differential protection to operate reliably while adaptively lowering the voltage reference value to ensure the safety of the converter station. Finally, an electromagnetic transient simulation model is constructed in PSCAD/EMTDC, which verifies the correctness and feasibility of the proposed strategy.