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Reverse Power Operation and Protection of Diesel Generator

Sep. 20, 2019

In principle, diesel generators can operate at reverse power. But in fact, when the reverse power runs, the generator will become a motor, which not only cannot send out electricity, but also consumes itself, and uses the electricity of the grid. Long time reverse power operation may cause winding burnout. Therefore, in general, it is not allowed to operate the generator inversely for a long time.


The reason for reverse power of diesel generators

Generators can not be synchronized completely at the moment of closing, because the voltage can not be completely uniform or the phase angle frequency can not be fully synchronized, and there may be fluctuations in the power grid, it absorbs a little electricity from the grid, that is, there is a temporary reverse power phenomenon, but soon when the load is brought up within the delay time of the reverse power protection settings, reverse power will disappear. But if the load is too slow, it is likely to expand the reverse power phenomenon and cause diesel generator tripping.


Another possibility is that the sudden drop or disappearance of oil pressure or other faults may cause the main valve to close incorrectly, and at this time, the precise protection and refusal to operate will lead to the generator terminal switch not tripping, which will certainly lead to reverse power action.

 Yuchai diesel generator

At present, two sets of independent reverse power protection devices are generally installed on large generators, that means program tripping reverse power and simple electrical reverse power protection, to limit the reverse power of generators, which generally does not exceed about 10% of rated active power.


Generator reverse power protection, also known as power direction protection, when the generator appears reverse power (the external power points to the generator, that is, the generator becomes the motor working condition), reverse power protection action circuit breaker will trip. Three-phase voltage and two-phase current signals need to be collected.


Generally speaking, the power direction of the generator should be from the generator to the bus, but when the generator loses excitation or for some other reasons, the generator may be converted to the motor operation, that is, to absorb active power from the system. This is the inverse power. When the inverse power reaches a certain value, the protective action of the generator is either signaling or tripping.


Reverse power protection action characteristics

1.The stator current exceeds the normal value and the ammeter pointer will be severely blocked.

2.The pointer of the stator voltmeter will swing rapidly.

3.Active power meter pointer swings on the whole dial.

4.Rotor ammeter pointer fast swing near normal value.

5.The generator emits a whistle, and the change of the whistle corresponds to the swing frequency of the instrument pointer.

6.The instruments of other parallel generators also oscillate accordingly.


It is not certain how much power the generator absorbs before tripping. It is decided by the setting value of the protection device, which is usually a quadratic value.


Adjustment of reverse power phenomenon caused by frequency

If the frequency of two diesel generator sets is different and larger difference, the instrument (ammeter, power meter) will show the value, the current of higher speed diesel generator set will displays positive value, power meter also displays positive power value. Otherwise, current and voltage value are negative.

At this time, the generator will adjust the speed (frequency) of one of the diesel genset, according to the indicator of the power meter, adjust the indicator of the power meter to zero. Make the power indication of the two units both zero, so that the rotational speed (frequency) of the two units is basically the same. However, when the ammeter still has the indication, this is the reverse power caused by the voltage difference.

Adjustment of reverse power phenomenon caused by voltage difference

When the power meter indication of both units is zero, and the ammeter still has the current indication (i.e., the reverse-positive indication), the voltage adjustment knob of one of the generating sets can be adjusted. According the indicator of current meter and power factor to adjust. Adjust current meter indication to zero, after current meter has not any indication, at this time, according the indication of power factor meter to adjust power factor to lagging more than 0.5. Generally, it can be adjusted to about 0.8, which is the best state.


Above is our sharing information about reverse power operation and protection in diesel generator, hope it is helpful to every one. Starlight Power is a manufacturer of diesel generator set, such as standby generator, emergency generator etc., covering Cummins, Perkins, Volvo, Deutz, Yuchai, Shangchai, Weichai etc. Contact us sales@dieselgeneratortech.com.

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