Voltage stability and voltage recovery: load dynamics and dynamic VAr sources

Voltage instability may occur from a variety of phenomena. Two of the most frequent are: (a) voltage recovery following typical faults on a power system and (b) voltage collapse during transient swings of the power system. We propose approaches to deal with these phenomena consisting of identifying the phenomena involved and proper response of control devices in the system such as dynamic VAR sources to mitigate the problem. Most of the times the phenomena are tolerable assuming that there are no secondary responses such as tripping of motors, switching of reactors etc. that tend to aggravate the problem. We also propose tools for reliable simulation of the phenomena involved. Specifically, a modeling approach is proposed, coined the time continuation method, that combines the efficiency of power flow techniques and capabilities of transient stability models. The proposed methodology is based on a quadratic model of the electric power system, including generators, voltage regulators and dynamic loads consisting mainly of induction motors. It is shown that the proposed method and model explains recent observed voltage stability and voltage recovery phenomena. Numerical experiments with visualizations are presented for the purpose of quantifying the phenomena and identifying the major parameters affecting these phenomena. It is shown that proper combinations of static and dynamic reactive power sources can alleviate the risk of voltage instability. Finally, the paper proposes a methodology for selecting the location and size of a mix of static and dynamic VAR resources for the purpose of alleviating the risk of voltage instability and slow voltage recovery