Voltage Stability, Voltage Collapse and Voltage Security are the main concepts of power systems.
The ability of power system to maintain the voltage such that, an increase in load demand has met by the increase in power, where the control and proactive devices tries to correct the voltage instability may leads to voltage collapse.
It is characterized by initial slow progressive decline in the voltage magnitude of the power system buses and final rapid decline of voltage magnitude.
Voltage Collapse can be defined as the inability of the power system to supply the reactive power or by an excessive absorption of the reactive power by the system itself.
The ability of the power system not only to operate stable but also to remain stable even when the load increases it often means the existence of considerable margin from an operating point to the voltage instability point.