Assessment of Static Voltage Stability Limits as Affected by Composite Load Models

Classically, with constant PQ-model for power system loads, the nose-point of a PV-curve is taken as the critical point for power system voltage stability. However, this point reflects the transmission system power/voltage limits neglecting the actual sensitivity of loads to system voltage changes. In this paper the effect of composite dynamic load models, represented by ZIP-models, on both the critical voltage and the critical loading of power system voltage stability is evaluated based on continuation power flow (CPF) method. The impact of various proportions of the ZIP-model for power system loads on static voltage stability is evaluated. Moreover, different operational scenarios under normal and contingency conditions are analyzed to investigate the effect of various load models on static voltage stability. The results show that composite load models are reflected as an enhancement of power system static voltage stability through reduction in the critical voltage value. Therefore, significant reduction in the reactive power support requirements can be estimated with loads represented by ZIP-models instead of constant PQ-model.