In vice versa, we face to reverse power phenomena or motoring conditions. Motoring is damaging to the prime mover, not the generator. Motoring can result from operator error, a failure of the generator breaker to open during shutdown, or because of a mechanical failure. Motoring a generator that is under the control of the automatic voltage regulator does not threaten the power system. Of course, this assumes that the system can withstand the loss of the failed unit’s generating capacity. Because the field circuit is unaffected by the loss of the prime mover, generator Var loading and terminal voltage remains unchanged immediately following the power reversal.
Var loading will then adjust to the level necessary to maintain the regulator voltage set point without kW output. If the generator were initially operating at a lagging power factor (Vars out of the generator) an increase in Var output would be expected to accompany motoring. If the generator was operating at a leading power factor prior to the loss of input power, Vars into the generator would decrease.
The rate of Var adjustment will vary. Loss of a steam turbine will result in a slow decline in output power and a slow change in reactive power as steam pressure at the turbine decays. An engine-driven generator has little stored energy and power output would cease immediately upon loss of the engine. The resulting Var adjustment would be rapid, determined by exciter response and generator time constants.
If the generator were operating on the manual voltage regulator near rated load prior to motoring, damaging conditions will arise. The manual regulator will hold field current at the value required for rated load. This current will substantially exceed requirements without kW output.
The results may include Var output, terminal voltage, and, possibly, system voltages far above safe limits. These adverse consequences underscore the need to operate on automatic voltage regulator control when synchronized to the power system.
When the generator field excitation remains in service, the generator will act as a synchronous motor without damage. Generator damage may occur if excitation is lost coincident with the loss of the prime mover. Under this condition, the generator can lose synchronism and act as an induction motor driving the prime mover at less than synchronous speed.