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Does the Increase in Power Factor Affect the Generator

Oct. 15, 2019

Too high or low power factor will affect generator operation, mainly at full load.

Power factor cosφ=active power/apparent power


When the active load is full, the power factor is too high, that is, the reactive power is too low, reducing the reactive power margin of the system, which will affect the stability of the generator. Although the economy is improved, in the long run, this is in exchange for increasing the probability of accidents. In the event of an emergency, the generator may not be able to withstand small disturbances or oscillations, may be out of step.


In addition, the low reactive power will cause the generator terminal voltage to drop, which will affect the auxiliary power. The current drawn by the motor rises, causing the voltage to be lower, creating a vicious circle that may cause the entire system to lose its stable operation and collapse.


1. Too high power factor will also increase the chance of the generator phase operation, and make the generator end heat easily.

2. If it is too low, the reactive power is too high, the excitation current rises, the rotor winding temperature rises, and the service life is shortened.

3. Too low causes the genset voltage to rise, the magnetic flux density in the core increases, the loss increases, and the core temperature rises.


When the generator is running under the rated load, the power factor is too low, the excitation current and stator current of the generator increase, which will cause the equipment to heat up, increasing the chance of equipment aging, switch tripping and so on.


In the normal operation monitoring, it should be adjusted according to the voltage. More reactive power should be generated when the voltage is low, and less reactive power should be generated when the voltage is high. By adjusting the ratio of active power and reactive power, control voltage and running current, ensure that the generator runs under safe and economic conditions.


diesel generator set


Generator power factor adjustment


1. The power factor of most generators is 0.8, and the individual power factor can reach 0.85 or 0.9.

Under normal circumstances, when the power factor changes from the rated value to 1.0, the output of the generator can remain unchanged, but in order to maintain the static stability of the system, the required power factor cannot exceed 0.95, that is, the reactive load should not be less than 1/3 of the active load. When the power factor of the generator is lower than the rated value, the rotor temperature will increase due to the increase of the rotor current. At this time, the load should be adjusted to reduce the output of the generator. Otherwise, the rotor temperature may exceed the limit.

Therefore, the duty personnel must pay attention to adjusting the load so that the rotor current does not exceed the allowable value at the inlet temperature of the cooling air. In general, the power factor is around 0.8-0.9! This is based on the power factor parameters specified by the unit and the requirements of the power grid. If the genset is a peaking genset, it may be different during the day and night.


2. If more reactive power the generator set generates, the smaller the power factor will be. When the generator output power remains unchanged, the voltage at the generator end will rise. The more reactive power, the excitation current will increase, the stator and the rotor temperature of the unit will rise, if too high, the insulation of the two may also be affected. Conversely, if the power factor is too high, the reactive power generated by the unit is very little, and the terminal voltage will also decrease, which will reduce the stability of the operation and affect the operation of the generating set.

Therefore, when the unit is in operation, pay attention to the terminal voltage at the specified value and ensure that the unit does not run in phase.


3. In order to ensure the stable operation of the diesel generator, the power factor of the generator should generally not exceed 0.95 of the late phase, or the reactive load should be less than 1/3 of the active load. If the generator automatically adjusts the excitation device to be put into operation, the generator can run for a short time with a power factor of 1.0 when necessary. Long time operation will cause the generator oscillation and out of step. At present, large units are basically not allowed to enter phase operation, and some large units are undergoing phase-in phase tests, the operating personnel should timely adjust according to the situation of the genset.


The tests shows that when the power factor is equal to 0.7, the output of the generator will be reduced by 8%. Therefore, in the operation of generator, if the power factor is lower than the rated value, the on-duty personnel must adjust it in time so as to bring the output to the allowable value as much as possible, and the rotor current shall not exceed the rated value.


The generator voltage variation within ±5% of the rated value is allowed for long-term operation, and the voltage is reduced by 5%, and the current can be increased by 5%. This is to consider the voltage reduction will reduce the iron consumption. If the voltage is too low or too high, it will have an effect on the current operation. First, if it is too high, the temperature rise of the rotor windings may exceed the allowable value. The voltage is generated by the magnetic field reaction. The strength of the magnetic field is related to the magnitude of the excitation current. If the active output is kept constant and the voltage is increased, the excitation current is increased, so the temperature rises.

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