Volts per Hertz protection needs generally arise from different situations than those for which turbine underfrequency protection is provided. Turbine-generator shutdown with the automatic voltage regulator left out of service, sudden load rejection with the automatic voltage regulator out of service, and manual excitation adjustment during startup with faulty metering are events that support the need for volts per Hertz protection.
The protection methods discussed here illustrate volts per Hertz protection schemes that provide protection for both the generator and transformer on unit connected generators. Modern transformers withstand relatively low overexcitation since the flux density is already high at rated values. The most commonly employed methods are discussed.
For hydrogenerators, generator high overvoltages may occur without exceeding the overexcitation capability of the generator. During load rejection the overspeed of hydrogenerator could exceed 150% of rated speed.
If an overvoltage condition is the result of a proportional increase in frequency, the V/Hz relaying will miss the event because the ratio of V/Hz may not have changed. It is general practice to provide overvoltage relaying to shutdown the generators to protect for these high-voltage levels if governor and excitation control fail to operate properly.
Volts per Hertz limiters in generator excitation system controls
The V/Hz limiter is a control feature within the automatic voltage regulator that limits generator field current to hold the generator output voltage to a safe V/Hz value. The limiter will limit the output of the machine to a set maximum V/Hz no matter what the speed of the unit.
This limiter functions only in the automatic control mode. To provide protection when the unit is under manual control, the limiter may also have a relay signal output which will activate any additional protective circuits to trip the generator.
In addition to a V/Hz limiter in the excitation control, it is recommended practice to provide separate V/Hz relaying to protect the unit transformers and the generator. This protection should be as independent as possible from the V/Hz control in the excitation system. If possible, it should be supplied from different VTs. Independent V/Hz relaying provides protection for voltage regulator malfunction or if the regulator is out of service.
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