A block load is the sudden addition of a large load to the generator. If a large block load is applied to a generator the engine that drives the generator will also be burdened with additional load.
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| Illustration 1 | g01732098 |
(1) AVR reduces excitation voltage after frequency drops below a preset frequency. (2) Frequency knee point (3) The Steady state operating point (nominal frequency) | |
The engine will require more fuel in order to maintain the RPM that is required for the generator to operate at the nominal output frequency. The engine cannot respond to the generator load in time in order to prevent the output frequency of the generator from dropping. In order to speed up the recovery time of the engine, the AVR will reduce the voltage output of the generator. This reduction in voltage output will occur at a predetermined frequency threshold. This frequency threshold is called the knee frequency. The knee frequency is selectable on the R250 AVR. The setting for the knee frequency will be determined by the type of loading that is anticipated for the generator.
The AVR reduces the voltage output of the generator by reducing the excitation current to the generator. When the voltage output of the generator is reduced, the active load on the generatorand on the engine is reduced. The AVR will continue to drop the generator voltage at a rate that is proportional to the loss of frequency from the generator. This type of control is called Volts Per Hertz (V/Hz). The AVR will continue to reduce the voltage output from the generator until the engine begins to return to the correct RPM.
The amount of reduction in the voltage that occurs with knee frequency operation is determined by the amount of loss in frequency on the output of the generator. As the engine begins to return to the nominal operating RPM, the frequency and the voltage of the generator will return also.
Care must still be taken in order to keep block load applications small enough so that the engine can recover in acceptable time. Block load recovery with V/Hz is also susceptible to oscillation.