Undervoltage load shedding for induction motor dominant loads considering P,Q coupling

A technique is presented that is suitable for undervoltage load shedding (UVLS) of dominant induction motor loads in power systems. To derive the algorithm, the induction motor voltage-dependent characteristics are formulated along a line parallel to the previously proposed generic dynamic load model. The load model is derived from the third-order induction motor model and is expressed in terms of real power, reactive power and slip, while retaining its dynamic characteristics. For the period following the initial induction motor transients (t > 3s), the model has been further simplified, explicitly representing the real and reactive power coupling in load dynamics. In the event of a voltage unstable situation, at a given voltage, the proposed UVLS criterion can be used to calculate the amount of load to be shed at any time to arrest voltage instability. It is shown that, when the induction motor dynamics are approximated using the first-order generic dynamic load models proposed in literature, the predicted amount of load to shed is approximately 2-3 times the amount obtained using the proposed method. This is owing to the real and reactive power coupling in the load dynamics. The proposed method is general and can be applied to any power system, provided the proportion of induction motor loads is approximately known.