High Frequency and voltage hunting in 80 KW DA

[Ulterior24]

We have a 80 KW Diesel alternator which was running satisfactory with all load combination. However, suddenly it has an over spees trip when switching on one AC machinery. After that we ran it on lesser load and suddenly observed hunting in frequency and voltage and the DA tripping when running. Can anyone help on this

 

[waross]

A description and electrical specs of the "one AC machinery" may help.
Your fuel control valve may be sticking.
With many electronic governors send an electrical signal to the fuel control valve. The greater the load, the greater the voltage or current of the control signal. When the load decreases, the control signal is reduced and a spring reduces or closes the fuel control valve.
If the valve is sticking due to dirt or dry fuel, the generator will go over-speed when the load is reduced.
As the signal is reduced more, the closing force of the return spring becomes greater and the valve may abruptly drop to a lower setting, bringing the generator back to intended speed, only to stick in that position, go over-speed, and drop again.
Try adding a fuel additive that contains a lubricant.
Not all additives contain lubricant, some additives only suspend water, and/or raise the cetane rating of the fuel.
If your fuel is dirty or there are a lot of hours on the set, the problem may be sludge and/or dirt build-up in the valve.
I will leave it to others to describe methods for cleaning a dirty fuel control valve.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Ulterior24]

The AC load is an Air conditioner. Basically an AC induction motor driving the Air conditioning compress.
Presently the Alternator is tripping at any load view hunting in frequency and voltage.will be checking the fuel line and governor settings but can the AVR also be at fault.

 

[waross]

I have seen similar hunting from a failing AVR;
EXCEPT OVERSPEED.
A failing AVR may vary the voltage and as a result vary voltage dependent loads. The result may be small speed variations, but seldom overspeed, and not overspeed to the point of tripping.

Let me explain the action of many fuel control valves:
Imagine a solenoid with an electromagnet acting in one direction and a return spring acting in the opposite position.
When the control circuit senses an increased load, the control signal increases the current and the solenoid core compresses the spring further.
When the load is reduced, the current is reduced and the spring pushes the solenoid core in the opposite direction.
BUT,
It dirt or dry fuel causes stiction, the spring may not be able to return the solenoid core as it should.
So when the load is reduced but the fuel is not reduced the engine overspeeds.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

By the way.It is normal for the speed/frequency of a set of that size to vary from 100% at full load to 1.03% at no load.
That is 51.5 Hz or 61.8 Hz at no load.
There will be a frequency spike whenever a load is removed as the controls react, but not enough to trigger an overspeed trip.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

As Bill says, it's your engine governor malfunction/bad fuel since AVR's do not change frequency, only the voltage.

Muthu

http://www.edison.co.in