Generator Frequency hunting

[jalover]

Hi to everyone, i need some technical assistance regrading my project, i have small genset rated 230kw, during operation underload the frequency is hunting.
Can anyone advise me what are the possible causes of this, still the genset is working but the frequncy is not steady.
thanks in advance.

 

[rovineye]

Are you operating isolated or paralled?

What kind of governor does it have? It can be the integral adjustment of the PID. Integral controls the output to be proportional to the amount of time that the speed error is present. It prevents slow hunting at steady state.

I have seen things jammed in the governor linkage area that prevented free movement as well.

Is this new? If not, pilot valve bushings can be worn. Too thin of a governor oil can cause similar problems.

IS it a rythmic hunting or random? Is the load steady?

 

[waross]

The first place to check is the droop of the governor. (proportional band) There should be 3% drop in generator speed from no load to full load. 61.8Hz unloaded, 60 Hz loaded.
It may also be one of the issues that rovineye suggested.
It may be a dirty fuel filter.
It may be a stability problem with the Automatic Voltage Regulator, but AVR problems are more probable at light loading and often diminish as the set is loaded.
How much is the frequency drifting? Is the voltage and current steady?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rovineye]

Bill,
I also thought about droop which is why I asked how steady the load is and if it is paralleled with anything. I have not seen droop (or lack there of) cause hunting, usually more of an overload or wild load swings (but this won't happen in isochronous). We just need a bit more info.

 

[waross]

Droop is equal to proportional band plus offset. At 60 Hz, the droop is set at 3% or 1.8 Hz. Offset is used to put the setpoint at 61.8 Hz so that the frequency is at 60 Hz at full load. The main speed control or adjustment is used to add the offset. Older sets with non-electronic governors often had only droop.
Droop plus reset is equal to PI (proportional plus reset or integral) This is isochronous mode.
In either droop (P) or isochronous (PI) mode too little droop will cause instability.
When the load increases, both droop and isochronous lose speed. With isochronous, the reset or integral function corrects the speed back to the set point.
Too much integral (reset) may also cause stability problems.
Add to the list of possible causes thick oil and cold oil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rovineye]

Bill,
Agree. What I should have said is I have not seen improper droop cause hunting if not in parallel. At least not as I think of hunting (rhythmic oscillation).

I only go back as far as 2301s, so I have not worked with non-electronic governors that only have droop. I only work around machines being commissioned or recently commissioned, not repair. And certainly nothing as small as jalover is asking about.

 

[edison123]

Bill

Why would AVR cause frequency hunting, which is speed related ?

 

[waross]

If the voltage is fluctuating, the current will usually be fluctuating also. Watts is voltage times current so the mechanical load on the prime mover will also fluctuate. The fluctuating load will cause a slight frequency variation.
When I set up an AVR one of the first things I do is set the voltage and stability, so I don't get to see a lot of unstable AVRs. The worst case I have seen was an 18 kW set. I was out of town and another expert was called. He diagnosed wildly fluctuating voltage as moisture and said he needed to take the alternator to his shop to dry it out. To that end he completely dismantled the set. When I arrived he had the set in pieces in the back of his truck. I meggered the stator, exciter and rotor. All showed good. He claimed that furthermore the set was burned out as evidenced by the inside being totally black. I guess he had never seen black paint before. He refused to reassemble the set. He unloaded the components and left. I reassembled the set and started it. There were wild voltage swings. Had I applied a load I am certain that there would have also been frequency swings. I gave the stability adjustment on the AVR 3/4 of a turn and the set as been working well for years now.
Increased loading will often dampen out AVR instability but in this case the owner said that the voltage was swinging badly even under load. Had he known, I am sure that he also had frequency swings.
Ah the adventure of working in the third world!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Ah Bill, I have always loved your field stories. I could sorta see a hunting voltage could lead to hunting kw in turn leading to a hunting governor and related speed and hz swings.

 

[BigIanH]

First thing I would need to know about th egenset is it's type of governor/speed controller.
Instability on a diesel set with an electronic governor is usually too much gain on the governor control (PID), try reducing the gain pot slightly and monitor the reaction.
Beware that this can also make matters worse, so baby steps with any adjustments! (mark th eoriginal positions of any pots you change with a marker so that you can return to original settings if it all goes wrong.
If reducing the gain settles the machine down, make sure that you shut it down and do a re-start as reducing the gain can often cause overspeed on starting.
It's always a compromise and it takes time and experience to get it spot on.

Also as someone else mentioned, check the throttle linkages are free moving with no tight spots, check all electrical connections to actuators.
If all else fails, reset the magnetic pick up gap and check the voltage output.

Happy hunting (sic)

 

[jalover]

thank you guys for all your valuable info. im sorry for the late response , been very busy the past days & camr from out of town travel.
this is the latest. when we start genset under load the freq rises very fast after 2-3 hrs from 50 to 51-52 hz. The voltage , current & load steady. i try to reduce the hz manually down to 50 , then after 2-10 mins, the situation exactly the same. the genset has a tendency of raising the hz so we need to reduce it manually. but most of the time in 24 hrs everything is ok with small deviation. any thoughts re on this problem. is it something to do with a governor or avr issue? appreciate all your help. thanks a lot

 

[waross]

Check your frequency meter. I apologize for not suggesting this sooner. I have seen a lot of flaky frequency meters on gen sets. Another possibility with a mechanical governor is something coming loose inside the governor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jalover]

thanks warros for your help, i think freq meter is working well, because when the gen set over 52 hz the protective device work giving an alarm and more than that the genset acb tripped off. you are right fault may lies on the governor of the genset. we will check further.thanks a lot.

 

[BigIanH]

This is very unusual, you say it runs ok for 2-3 hours and then the Hz/Rpm start to rise to above 52Hz?
Can you see the throttle increasing the fueling to the engine as the speed increases?
If the throttle actuator is increasing the fuel, then you need to look at the governor.
If the throttle is trying to reduce the fuel and the speed continues to rise, I would take a look at either fuel quality (probably not the problem with a deisel).
Is there a possibility of any flammable gases in the engine area that the engine may be using for fuel? this is a longshot, stab in the dark by the way!

I assume that this is "island" load, isolated from the grid?
I have seen problems with synchronising machines that are running island load but are sensing their own voltage as mains and attempt to synchronise to themselves, that can cause under/over speed or hunting....

If you tell me more about the governor/actuator/ control system I may be able to help more.

Good luck.

 

[waross]

I had a problem with a set with an electronic governor and an electrically controlled fuel control valve. The electric operator was power to open and no power to close. The position was determined by the supplied current. The valve was sticky. When the load increased, the operating force would overcome the sticktion. When the load decreased the governor would reduce the current to the operator but the valve would not close. The result was over-speed/over-frequency. If the over frequency occurs when the load is reduced slightly, check your fuel valve and or linkage for sticktion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigIanH]

Good call.
I have seen similar occurrences on machines with long hours or harsh environments/poor maintenance.

 

[jalover]

Thank you all guys for valuable help. Im using mechanical governor and this is stand alone genset with only 240kw. cummins generator with mechanical governor linked into actuator motor ( rev-fwd to increase & decreace speed) the governor is made by bosch.

 

[PAFred]

I think you have a governor stability issue. Depending on whos governor your using and or type you should look at the actuator compensation , and or gain settings. If your unit is an older Detroit with an SG type governor there is an adjustment inside that requires the top be taken off to gain access to. In this case you would need to loosen an allen lock bolt and move a sliding fulcrum to one side or another by very small amounts to bring the hunt out but this bein said an SG governor is a 2% droop governor which you can't get rid of intirely, if it's an PSG you can get rid of all hunting (with pateints).

 

[waross]

jalover;
Watch for a slight bump in the load. A sticky governor or linkage may open the fuel to compensate for the 'bump" and then not return all the way. Look for a worn or dry bushing on an actuator shaft.
The actuator motor should not be a problem. It just sits there until you send it a command to raise or lower the speed.

Bill
--------------------
"Why not the best?"
Jimmy Carter