VARYING MODES OF VOLTAGE INSTABILITY

In reported incidents, varying modes of voltage instability have occurred and each has presented in a different manner. To acquaint power engineers with the different types, two incidents are described which each commenced with a type of system voltage instability and culminated with radial voltage instability. In one case, events following the loss of a heavily loaded generator presented as "classic" system voltage instability. In this incident operators were able to shed load, but not fast enough to prevent the loss of other generators so that the situation reverted to radial voltage instability. In the second incident there was no initiating disturbance but extremely hot weather led to high levels of air conditioning load. This created a situation of "slow" system voltage instability where the system remained intact 12 minutes after the last shunt capacitors had been switched in. A more serious collapse was avoided when radial voltage instability led to an exponentially increasing voltage reduction at three 500kV substations. The quickly altering voltages and currents triggered impedance protection on the 500kVlines feeding them and all consumers at the three substations were interrupted. In neither of these two incidents were operators properly aware of the situation. The analysis of the incidents provided in this paper can provide important operating insights, showing the value of timely load shedding and the need to allow a 25% margin from the radial voltage stability loading limit

Keywords:

System Voltage Stability , Radial Voltage Stability , Rotor Thermal Reserves , Rotor Heating , Operating Margin , Operator Training , Load Shedding