Transient Voltage Security Assessment of Sending End System during LCC-HVDC Commutation failure based on Quasi-Steady-State Modeling

High Voltage Direct Current (HVDC) transmission stands out as a highly efficient solution for the long-distance delivery of renewable energy. However, the transient process of Commutation Failure (CF) in HVDCs often leads to under-or over-voltage issues in the sending-end AC systems. These voltage fluctuations can trigger fault ride-through events in renewable units, potentially causing widespread generation losses in systems heavily reliant on renewables. To comprehensively analyze this phenomenon, our paper introduces a novel voltage security assessment approach designed to calculate transient under-and over-voltage levels during the CF process without resorting to time-domain simulations. Our approach involves the direct solution of algebraic equations corresponding to two critical quasi-steady states. The first state, denoting the minimum voltage level, is determined by the zero-crossing point of DC voltage at the rectifier side. The second state, representing the maximum voltage level, is defined by setting the DC current to zero. These states are characterized by two sets of nonlinear algebraic equations that can be numerically solved. To validate the effectiveness of our method, we apply it to a benchmark HVDC sending-end test system and compare the results with detailed time-domain simulations. The findings demonstrate that our method, by simply computing two quasi-steady states, achieves accurate analysis of under-and over-voltage, exhibiting a relative error of less than 1%.