Analysis on Mechanisms of Mid and Longer-Term Voltage Instability

Mid and longer-term voltage instability are usually caused by the continuing increase of the load, and is characterized by a period when the voltage drop down quite slowly. In this paper, the small disturbance method on how the static voltage stability region concerning different load characteristics that can be fixed on the system characteristic curve and the phenomenon of the power system operating at the abnormally low voltage during the times of mid and longer-term voltage instability is studied. Whether certain kind of load can operate at the lower part of the system characteristic curve depends on its static character. The mechanism of the long term voltage instability will be discussed also. Now there are two kinds of mechanism of voltage instability, the reactive power imbalance mechanism and the active power imbalance mechanism. The former one considers the reactive power imbalance the cause of the voltage instability while the latter one considers the active power imbalance the cause of the voltage instability. The paper discusses the relations between these two kinds of mechanisms and points out that they should not be treated separately. Actually there exist close links between two of them. Integrating these two analysis methods can help to understand the essence of the phenomenon more clearly.