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What Causes Voltage Drop in Diesel Generator Set

Jun. 03, 2020

When using diesel generator set, maybe there is voltage drop problem, today Starlight Power will share the reasons.

It is normal that the voltage drops 5% when the generator set is fully loaded. But if voltage drops too much, we should find out the fault in time to ensure generator set running normally. 

What should we check?

1.Check whether speed and frequency of diesel engine drop seriously.

2.Check whether there is poor contact between rotor carbon brush and wiring.

3.Check whether the excitation regulator works normally(Whether the excitation current increases automatically with the increase of load).

4.Check whether the power factor of the generator road is too low.


Even though there is voltage drop, but we should take different measures according to different situations, here is an example:

If the output voltage of 50KW diesel generator is 400V, the output voltage is normal when there is no load, but when running on load, the output voltage drops suddenly until it cannot work normally.

According to above problem, we can confirm the reasons:

1.The load is large. The excitation coil in the generator is burnt. You can calculate the capacity of your generator, then compare with the load capacity. If the power factor is 0.8, the rated apparent power of your generator is 50/0.8 = 62.5kva. If the generator set is standby unit, even if the power does not exceed the generator, when you start the load, the voltage at the moment when the load starts (especially the inductive load) will be too low, causing failure to work properly.

2.There is a problem with the excitation of the generator. Check whether the excitation is normal. If the voltage is normal without load, it means that there is no problem in your rotor circuit. It is mainly to check the device providing excitation current.

 1100KW Perkins DG Set

If the generator voltage is getting lower and lower, what is the reason and how do deal with it?

1.Speed of diesel engine is too low.

 Adjust speed to a rated value.

2.Some rectifier diodes of the generator are broken down.

Check and replace the broken diode. 

3.The contact surface of the brush is too small, the pressure is insufficient, and the contact is poor.

If the surface of the commutator is not smooth, polish the surface of the commutator with emery cloth at low speed, or adjust the spring pressure.

4.Short circuit or ground fault in stator winding or excitation winding.

Check fault and remove fault.

5.The return brush of exciter is not at the neutral line position, or the spring pressure is too small.

Adjust the brush to the correct position, replace the brush and adjust the spring pressure.

6.Excessive resistance of excitation circuit.

Reduce the resistance of the field rheostat to increase the excitation current. For the semiconductor excitation generator, check whether the additional winding joint is broken or wrongly connected.

There are ways and steps to regulate the voltage of diesel generators. The adjustment methods is not difficult, but you need to know the voltage range of diesel generator and the acceptable fluctuation range first. As long as the setting is good, make the generator and voltage regulator cooperate well, the unit will run normally.

Diesel generator voltage should meet the following requirements:

1.Voltage setting range

Voltage setting range refers to the range of the max. and min. voltage that can be achieved by adjusting the manual or automatic voltage regulator on the control panel when the generator operates at rated speed and no load. In general, the no-load voltage setting range is 95%-105% of the rated value.

2. Steady state voltage regulation rate

Steady state voltage regulation rate refers to the change rate of steady state voltage of synchronous generator under all load conditions from no load to rated load. Under normal circumstances, the steady-state power adjustment of synchronous generator is different under different load conditions. When the inductive load is added to the generator, the steady-state voltage value after the load change is lower than the no-load setting voltage; 

When the capacitive load is added to the generator, the steady-state voltage value after the load change is higher than the no-load stable voltage. In general, the higher the regulation accuracy of the voltage regulator, the smaller the deviation of the no-load setting voltage, and the smaller the steady-state voltage regulation rate.

3. Transient voltage regulation rate

The transient voltage regulation rate refers to the change rate of the transient voltage in the process of suddenly increasing or decreasing the load when the generator is no-load and the beamer voltage reaches the rated value.

4. Voltage stabilization time

Voltage stability time refers to the time required from sudden load to voltage stability within the specified range when the generator brand is wearing and the voltage reaches the rated value, expressed in S. According to the national regulations, the voltage stability time of class I, II and III power plants is 0.5-1s, and that of class IV power plants is 3S.

5. Voltage fluctuation rate

Voltage fluctuation rate refers to the voltage fluctuation degree of synchronous generator when the load is constant.

6. Steady state frequency regulation rate 

The steady-state frequency regulation rate refers to the ratio between the frequency stability difference of synchronous generator and rated frequency before and after load change.

7.Transient frequency regulation rate

Transient frequency regulation rate refers to the ratio of the difference between the transient frequency and the frequency before the load change and the rated frequency when the generator unit suddenly adds or subtracts the load.

Maybe you like the article: 

Causes of Overvoltage in Power System

The Cause of Undervoltage Fault of Diesel Generator Set

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