Online power system short-circuit ratio estimation using LCC-HVDC reactive power perturbations

Short-Circuit Ratio (SCR) is a well-established indicator for understanding the strength of the power system in relation to LCC-HVDC. Traditionally, SCR is calculated offline based on predetermined generation plans and network configurations. However, the inherent variability of renewables introduces complex and stochastic dynamics in both generation and transmission, making offline-calculated SCR unable to accurately reflect the state of the power system during operation. Therefore, this paper proposes an online SCR estimation method based on LCC-HVDC perturbations to support online perception of system strength and benefit operational decision-making. Through increasing (or decreasing) DC voltage in coordination with decreasing (or increasing) DC current, the LCC-HVDC system acts as an excitation source that introduces reactive power perturbations to the AC system while maintaining active power transmission. The resulting states of the AC system yield a set of nonlinear equations characterizing its behavior, which can be solved to derive the SCR. In addition, due to the coupling of the LCC rectifier and inverter, the perturbations functions for both the sending-end and receiving-end grids, achieving high efficiency in concurrently calculating the SCR at both terminals. For verification, CIGRE HVDC benchmark system is simulated using PSCAD/EMTDC. The results indicate that the online estimated SCR for both sending-end and receiving-end grids closely aligns with the theoretical values, exhibiting an error of less than 5%.