Voltage stability of a power system: Development of a proximity index for predicting voltage collapse

Due to economical and environmental constraints, the increasing interconnection between power systems and the usage of existing transmission lines have led system operations closer to the thermal and/or stability limit. These system operations can contribute to voltage instability, which may develop into a voltage collapse situation, caused mainly by the insufficient reactive power support. In this research, we propose an easily computable proximity index for predicting voltage collapse of a load bus using only measured values of the bus voltage and power; its value can be used as a measure of closeness of the current operating point to the stability limit point at the load bus. Based on a simplified power system, the index is computed from algebraic expressions derived from the tangents to the load S-V curve and the network S-V curve at the operating point. To construct these two curves, the load is modeled by mathematical expressions derived from a relationship between bus voltage and bus power, and the rest of the power system as viewed from the load bus is proposed as the network model represented by a simplified Thevenin equivalent. The Thevenin equivalent parameters are periodically updated using a recursive least squares algorithm as system operating conditions vary. The algorithm for estimating those parameters was tested, and test results verified the feasibility of such a scheme. Under assumptions of voltage-dependent loads and reactive power limits on generators, we demonstrate the applicability of our proposed method on two power systems, the Ward-Hale 6 bus system and the IEEE 14 bus system. The results obtained verify its applicability, as well as its accuracy and the simplicity. With two threshold values defined in the method, the index can identify the maximum power point and a marginally stable operating point which may extend beyond the maximum power transfer point. Since the index value obtained can determine the strength of the stability of the current operating point, the method can be applied to continuously monitor the voltage stability of the load bus