A Generalized Voltage Stability Indicator based on the Tangential Angles of PV and Load Curves considering Voltage Dependent Load Models

The majority of traditional techniques for voltage stability assessment do not account for load power-voltage characteristics and assume a constant PQ load model, which is proven unreliable especially if the system load contains a large percentage of induction motor loads. This work presents a generalized voltage stability index which has two important features. First, it is applicable to general load models including ZIP, exponential and induction motor loads. Second, it can be used for both measurementbased and model-based voltage stability assessment. The key idea of this index is that at the collapsing point, the system P-V curve and the load P-V curve share the same tangent line. Thus, the tangential angle between the two curves is an appropriate indictor of proximity to voltage collapse when voltage-dependent loads like ZIP and motor are considered. The performance of the proposed index is tested on both the IEEE 14-bus system and the 181-bus WECC system. The results show that the proposed index gives a good and reliable assessment of system stability. In addition, it demonstrated a consistent behavior, always reaching a value of 0.0 at the stability limit which enables the definition of a threshold for taking corrective measures.