When a synchronous generator or alternator operating under the load and when suddenly input power to the shaft of the generator lost which the machine is connected to the power system, then the machine acts as motor. This motoring mode of operation of generator or alternator is not desirable especially to the prime mover (turbine) running the generator. Hence machines have to be protected against motoring mode of operation of the generator. Motoring mode of operation of generator can be because of the operator error, or can be due to failure of the generator breaker to open during shutdown or can be because of the mechanical failure.
Motoring Operation of Generator:
When synchronous generator losses shaft power from the prime mover (turbine) and is connected to the power system with Automatic Voltage Regulator (AVR) in service, synchronous generator will run as synchronous motor and continue to operate without any damage. When the alternator losses power from prime mover the terminal voltage of the alternator remains unchanged following the power reversal as the field circuit is unaffected. Reactive power (Var) loading will be adjusted by the AVR system to maintain the terminal voltage. If the generator is operating at lagging power factor output before the motoring, an increase in the Var loading is expected. On the other hand, if the generator is operating at leading power factor prior to the operation as motor decrease in Var loading is expected. Loss of steam turbine power will reduce the electrical power (real power and reactive power) output slowly (decay slowly) due to the high inertia of the steam turbine. In the case of engine-drive generators the inertia will be less and hence the power output decay will be faster. The Var adjustment would be based on the response time of the exciter system time constants.
When the generator excitation also losses power during loss of prime mover shaft power (eg: manual AVR), synchronous generator losses synchronism and will operate as induction motor and run at speeds lesser than the synchronous speed. The speed difference between the synchronous speed (stator magnetic field) and rotor speed will induce currents in the rotor. These induced currents are of lower frequencies (slop frequency). This can cause damage to rotor of the synchronous machine.
Steam Turbine Limitations for Motoring:
Motoring operation is generally dangerous for prime mover (steam turbine) compared to electrical generator. When turbine is running, windage losses generated in the steam turbine cause significant heating in the steam turbine blades. During normal operation, this heat is removed by the steam flow. But during motoring mode of operation, heat generated cannot be removed efficiently. This inadequate heat removal results in the turbine blade distortion and softening. Large diameter turbine blades have higher tip velocities and hence more windage losses implies more heat generated at the Low Pressure (LP) turbine near the exhaust.
Motoring operation does not only results in damage of the turbine blades. During motor operation the loss of steam flow does not remove the heat generated due to windage losses but also reduces the ability to equalize heating. This results in creation of localized hot spots which can cause stress and distortion within the steam turbine. This distortions generated can cause rubbing between the components which creates more heat and results in wear and tear of the steam turbine.
Electrical Protections against Motoring – Generator:
When the turbine power input to the synchronous generator or alternator is lost, the generator continue to operate as motor and the power required to the run the motor will be supplied from the grid. Hence a power relay at the generator terminals can able to detect the reverse power and can be trip the generator. The type of prime mover (steam turbine or gas turbine) determines the sensitiveness of the relay.
The power consumed by the gas turbine generator will be as high as 100% kVA rating of the machine for single shaft and about 10 to 15% rated power for double shaft installation for motoring operation of the generator. Hydro turbine generators requires power about 0.2 to 2 % of the rated kVA and in the case of diesel generators power required will be about 15% to 25%. Steam turbines require about 0.5 to 3% of the rated power for motoring operation. Generators manufactures specifies the motoring power and permissible duration of time for motoring operation. The sensitivity of the reverse power plays an important role dependent on the type of application (turbine employed).
Mechanical Protections against Motoring – Steam Turbine:
Differential pressure switches, Control oil system monitors, and exhaust hood temperature detectors are employed at the turbine side to detect the motoring operation of the generator for tripping the turbine.
Measuring the differential pressure across the HP turbine can give the direct measurement of steam flow. A pressure switch which is set to operate in the case of no load steam flow to the turbine is a good method to detect the motoring operation of the generator.
Monitoring the pressures at turbine trip oil system and governing oil system can give the indication related to motoring operation of the generator
Temperature monitoring at the turbine exhaust hood can give indication related to motoring operation of the machine