A Fast Local Index for Online Estimation of Closeness to Loadability Limit

Closeness to the nose point of P-V curve, or loadability limit (LL), and voltage instability is one of the main concerns of modern power systems. Hence, online evaluation of LL has attracted the interest of researchers in recent years. This letter introduces a simple local index for online estimation of closeness to LL, using two consecutive scalar local measurements. This index is achieved based on the fact that variation of the load apparent power is zero at LL. The index is suitable for implementation in numerical relays to stimulate local control actions to prevent voltage instability, and in the system control center for system-wide coordinated analysis and control measures. Simulation results on the IEEE 30-bus test system show the suitability of the proposed index.