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Low Frequency of Diesel Generator Set

Aug. 29, 2018

Frequency is one of the criteria for the stability of power supply. If the frequency of generator set varies greatly, it will cause damage to electrical equipment and lead to machine failure or safety accident. Therefore, generators set should be maintained at rated frequencies as far as possible. The rated frequency of most country is 50 Hz. However, due to the increase and decrease of load in power system, sometimes the rated frequency can not be maintained under the condition of peak load. When the frequency range of the system is 50 ±0.5Hz (the power system capacity is ±0.2Hz at 200MW and above), the generator can operate according to the rated capacity.

Low Frequency of Diesel Generator Set.jpg


If the frequency of the system is too high, the speed of the generator will increase, which will increase the centrifugal force of the generator rotor. When it is serious, it can cause damage. There are few cases where the system frequency is too high during normal operation. It is easy to run when the frequency of the system is reduced and will be maintained for a period of time. If the frequency is too low, the generator's output will be limited. When the rotating speed of the generator is reduced, the ventilation at the end of the generator will be reduced and the cooling conditions will become bad, which will increase the temperature of the winding and iron cores and reduce the generating force of the genset.



For independently operated generators, the frequency can be changed by adjusting the rotational speed. If connected to a large power grid, the generator set will also be kept at rated speed because the frequency of the large grid is 50 Hz, which is determined by the frequency of the large grid.



What are the adverse effects of the low frequency of diesel generator set?


The frequency of diesel generator has an appropriate range, we need to carry out the necessary frequency adjustment. The influence of low frequency on electrical equipment is extremely unfavorable. However, many customers do not attach importance to this problem, the frequency of random transfer of generating set. So, what effect will the low frequency bring to the diesel generator set?


Negative effects:


When running at low frequency, the ventilation of generator will be reduced. In order to maintain the normal voltage, the excitation current should be increased to increase the temperature rise of the top and rotor of the diesel generator set. In order not to exceed the temperature limit, diesel generating units had to be reduced.


Low frequency operation will increase the pressure of turbine blade, cause blade resonance, shorten blade life and even make blade break.


When the frequency is reduced, the reactive power load in the system will increase, which will lead to the decrease of the voltage level of the system.


During the low frequency operation the core loss and excitation current of the transformer will increase due to the increase of magnetic flux density. In order not to exceed the temperature rise limit the transformer load has to be reduced.


After the diesel generator is started, the generator generates electricity, but the frequency of generation is only up to 45 Hz.


Fault analysis:


This kind of fault appears to be a fault inside the generator or the control part of the generator, but in fact it is caused by the diesel engine fuel supply system or the mechanical failure of the diesel engine that the diesel engine speed does not reach the rated speed, resulting in the generation frequency of less than 50 Hz.


Cause of failure:


1. Diesel engines have individual cylinders that work poorly or not.

2. Low pressure partial blockage.

3. The quantity of oil supplied by the high pressure oil pump does not meet the requirements of the machine or the internal plunger of the high pressure oil pump is jammed.

4. There is air in the oil path.

5. The overhaul of the connecting rod bolts of the diesel engine is too strong, causing the connecting rod bush to be too tight with the journal.

6. Poor work of oil pump.

7. Individual injectors do not inject oil.



Troubleshooting method:


1. Start the diesel engine up to about 700r/min, touch the exhaust pipe of each cylinder by hand. If the exhaust temperature of 6 cylinders is found to be basically the same, it shows that each cylinder works normally and can exclude the possibility of poor working and non-working of individual cylinders.


2. After shutdown, check the oil pump oil filter net, no impurities in the power filter, it should be said to work normally


3. The oil pressure test is carried out on the oil pump. The oil pump outlet pipe is removed first, and then the left hand is used to block the oil outlet of the pump, and the right hand repeatedly presses the pump handle to carry out the oil pressure test on the oil pump. The oil pressure of the generating oil pump is basically normal by checking, but the oil inspection tube of the high pressure oil pump drips diesel oil out


4. Remove side cover plate of high pressure oil pump after installation of oil pump outlet pipe. Then start the diesel engine and see if the diesel oil leakage between the plunger and pump body. If there is no electricity leakage during the inspection, the diesel oil can only be considered to be channeling oil into the pump body of the high pressure oil pump through the gap between the plunger and the plunger sleeve in the work. This causes the low pressure fuel supply insufficient, causes the diesel engine power to drop, the speed does not reach the rated speed.


5. Start the diesel engine after replacing the oil pump and improve the speed of the diesel engine. After the longitudinal handle reaches the requirement of the maximum rotational speed, the speed of the tachometer is 1510r / min, the exciter is excited, the generator is generated and the frequency meter is shown to be 50.5 Hz. Reduce the speed to the generator frequency of 50Hz, the equipment power supply is normal, the fault will be eliminated.




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