Voltage stability refer to the ability of power system to maintain steady voltages at all buses in
the system after being subjected to a disturbance from a given initial operating point. The system
state enters the voltage instability region when a disturbance or an increase in load demand or
alteration in system state results in an uncontrollable and continuous drop in system voltage.
A system is said to be in voltage stable state if at a given operating condition, for every bus in
the system, the bus voltage magnitude increases as the reactive power injection at the same bus is
increased.
A system is voltage unstable if for at least one bus in the system, the bus voltage magnitude
decreases as the reactive power injection at the same bus is increased. It implies that if, V-Q
sensitivity is positive for every bus the system is voltage stable and if V-Q sensitivity is negative
for at least one bus, the system is voltage unstable. The term voltage collapse is also often used for
voltage instability conditions. It is the process, by which, the sequence of events following voltage
instability leads to abnormally low voltages or even a black out in a large part of the system.