Faults on the Feeders to Induction Motors
A common cause of the stalling of induction motors (or the low-voltage releases operating and removing them from the supply) occurs when the supply voltage is
Figure 8.18 Voltage collapse on the network shown in Figure 8.10
either zero or very low for a brief period because of a fault on the supply system, commonly known as 'voltage dip' or 'voltage sag'. When the supply voltage is restored the induction motors accelerate and endeavour to attain their previous operating condition. In accelerating, however, a large current is taken, and this, plus the fact that the system impedance has increased due to the loss of a line, results in a depressed voltage at the motor terminals. If this voltage is too low the machines will stall or cut out of circuit.
Steady-State Instability Due to Voltage Regulators
Consider a generator supplying an infinite busbar through two lines, one of which is suddenly removed. The load angle of the generator is instantaneously unchanged and therefore the power output decreases due to the increased system reactance, thus causing the generator voltage to rise. The automatic voltage regulator of the generator then weakens its field to maintain constant voltage, that is decreases the internal e.m.f., and pole-slipping may result.
The control circuits associated with generator AVRs, although improving steady- state stability, can introduce problems of poorly damped response and even instability. For this reason, dynamic stability studies are performed, that is steady- state stability analysis, including the automatic control features of the machines (see Section 8.7). The stability is assessed by determining the response to small step changes of rotor angle, and hence the machine and control-system equations are often linearized around the operating point, that is constant machine parameters and linear AVR characteristic. The study usually extends over several seconds of real-system time.