Analysis of Composite Load Models on Load Margin of Voltage Stability

While it has been widely accepted that the constant P-Q leads to conservative load margin calculation of voltage stability, lately the studies show that induction motor loads can lead to an even more conservative load margin calculation. This paper analyzes the impacts of the composite load model (the composite of ZIP and induction motor load models) on the load margin of voltage stability. In order to take into consideration the composite load model, a new set of parameterized power flow equations is derived. A procedure of integrating the composite load model is integrated into CPFLOW to model the load increase. Numerical studies on test system are conducted and numerical results indicate that the load margin with the composite load model can be significantly less than that with the constant P-Q load model. These numerical results are contradicted to the belief that the constant P-Q leads to conservative load margin calculation of voltage stability.