Load-Preserving Short-Circuit Ratio for Voltage Stability Assessment

Short circuit ratio (SCR) is a popular index to assess voltage stability of HVDC-infeed systems. In recent years, many HVDCs have been constructed to directly feed into load centers, but SCR does not explicitly reflect the impact of loads, potentially leading to misinterpretation of voltage stability. This paper demonstrates that the issue can be very severe in some scenarios. Some power systems unexpectedly get unstable when their SCR is increased, and get stabilized when their SCR is reduced. Some systems even operate stably with SCR<1. This phenomenon arises because most SCR-based indices rely on the Thevenin equivalent circuit, which absorbs AC loads into the equivalent source. Such structure cannot appropriately reflect voltage stability involving load participation. In view of the problem, this paper proposes a load-preserving equivalent circuit, taking the place of the Thevenin circuit in SCR calculation. Further, a load-preserving SCR (LPSCR) is proposed. It retains the distinct physical meaning of SCR, and is enhanced in accuracy. In addition, several advanced, load-preserving indices are introduced to replace classical indices such as ESCR and CSCR, enabling a more accurate assessment of system stability from multiple perspectives.