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Problem with strange trip 4

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Denan

Electrical
Jul 19, 2006
66
Hi all,

I have a problem with a CHP plant, strange gen.CB trips.

The situation is like this:

First I have to say that the gris in the area is very bad, unstable. there are two similar plants, one (our, with the problem) has one genset, and the other (neighbour) with two gensets. If we are on the grid (both plants are running on the grid only, no island mode etc.) and the neighbour starts his two engines our engine often trips. But if the neighbour is running first and we start after, everything is OK. And, also the trip does not happen every time, but most of the times. I attach some trendings (I am not there, but got it from my colleague), as well as AVR settings.
Hopefully someone has an idea of what could cause these problems.
As I mentioned, the grid is very unstable, and the voltage is usually little bit higher than nominal (Un=10,5kV)

Regards,
Denan
 
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Hi Denan.
What are protective functions tripped?
ROCOF, Voltage or var export?
Regards.
Slava
 
I suspect that your neighbor is making a sloppy synchronization. He may be coming in a little ahead, or a little behind, or he may have his initial voltage setting high or low.
The poor grid leaves you at his mercy.
If he is coming in late, you may get reverse power trips.
If he is coming in early, you may get over current trips.
If his voltage setting is off, you may get excess VAR trips or excess current trips.
Try to determine what your protection is tripping on and increase the settings.
In a perfect world, you could work with the other plant to resolve their synchronizing issues, but if that is not feasible, the workaround is to treat the symptoms. Apply protection settings that will tolerate the transients caused by the sloppy practices at the other plant.
You may be able to safely increase the over current setting, it looks a little low.
Try to identify the parameter that is causing the trip and evaluate whether that setting may safely be changed.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
I am not sure what was the trip, I asked the guys over there today, but did not get an answer yet.
Will try tomorrow again.

Have a nice evening until then!

D.
 
Hi again,

today I got some more info regarding this. Trip comes with "var import". The limit is 30%, 2sec., and in case of trip the value is 35,7%...

 
So.
What is a next steps.
it's var import== loss of excitation/maybe possible say pole slip/loss of synchronizm. Bad situation for the generator.
only two option:
1. some connection with second plant.
2. change setting: VAR's or delay? how much change?
What say generator mnf.? Who responsible for the setting?
Firstly, I'm stronglu recommens ask generator mnf., maybe generator have additional winding for VAR absorb.
Second check option for AVR connection with second plant.
Best Regards.
Slava.
BTW, Bill said about little low setting for overcurrent function. For my pinion, you need revise sevral settings.
 
It is possible that your neighbor is coming online with too high a voltage setting. You may be running with too little excitation. If you are running at a low power factor on you will be more sensitive to a sudden export of VARs from your neighbor.
Step 1:> Review your excitation level. You may be running at too low a power factor for local conditions. This will make you susceptible to a VAR import trip.
Step 2:>Rewiew your protection settings. A small increase in the time setting for VAR import may be all you need.

You mentioned that the line voltage is high. If your AVR is set for what the line voltage should be rather than for what the line voltage actually is, you may be under excited.


Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Hi.
As Bill said.
Step 2:>Rewiew your protection settings. A small increase in the time setting for VAR import may be all you need.
Bill, what is a small increase, one, two second?
Please see, Denan, have also little low setting for undervoltage protection. If it's low power factor, next time with longer var import protection delay will possible trip of undervoltage protection.
for me 30% for 2 second is not bad setting.
I think need review of AVR only.
Regards.
Slava
 
Thanks for the comments guys!

I asked our guy to talk with the client as Bill suggested, but probably not easy to do anything about it.
Another thing I asked him to do is to ask the client to run with PF lower than 1 (which they have now). But in that case they will "loose" some kW - and money, and I am not sure they are willing to do that.
Also, I told him not to increase var import setting more than 32% (the limit of the gen.). Rather some small increase of timer, up to 4sec.

All this happened friday, so I have to wait and see what happened and if any of this helped.

Bill, grid voltage is 10,5kV, and as we can see from the trends, it is closer to 10,6kV, sometimes more than that.
But your tips regarding sloppy synch from the neighbour makes sense to me now, because our plant is running with PF=1. By the way, PF=1 is normal PF for the area (Holland). All the plants we have there are running with PF=1. But I believe since the grid on this one is poor, they have problems.

I will keep you informed on the situation...

 
Denan,

PF-1 is a bit of concern when it happens to be operating power factor of a generator.
Generator field is weak at that power factor and is likely to trip (out-of-step) for any disturbances in the grid. Working the generator at 0.8Lag is recommended (I hope your generator system is rated for that).
The alternative could be a rapid response AVR (by way of current compounding or other principles).
 
Thanks for the comment Raghun,

Yes, generator is rated for PF=0,8, but is supposed to run as close as possible to 1. The owners do not have to produce any vars, and they get paid for kW, not var, so that is the only reason why PF is 1. As I said, normally this is not the problem, but in this case the grid is very bad, causing the problems (or maybe not causing, but contributing to it).

 
Hi.
Star to Raghun!!!!
Denan, please explain to your customer:
kW is function of prime mover, not of PF.
PF=0.8 is recommendad by gen. mnf, is issue of generator stability.
You must some vars.
I think, we had some topic on the issue before few days or weeks.Regards.
slava
 
raghun,

A generator should have no problem making base load at unity power factor. If there are stability issues such as loss of synchronism during a grid distrubance then the AVR tuning must be very bad. Pushing the field current up to reach a 0.8 lag power factor will cause the local system voltage to rise if the grid is weak, and it is a little high already.

Denan,

What are the relative sizes of the machine you are responsible for and the machines at the neighbouring plant? I am going to guess that those at the neighbouring plant may be larger than yours.


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If we learn from our mistakes I'm getting a great education!
 
I was mistaken before, I thought the generator runs with PF=1, but it is actually 0,98, which should be fine I suppose.

The neighbours generators are more or less the same size as our, our is 3000kVA.

The guy told me today he changed var import setting to 31% with 7 sec. and these few days there were not problems so far.
 
Well.
I can say only one:
instead fix problem and solve it, again change setting : instead 2sec to 7sec. Why???? No answers.
Next time will change setting to 20sec and peak up to 40%.
And will finish with damaged generator.
It's classical variant of out of step situation, two same genrators work one with second ( and very unstable/weak grid). I'm sure possible solve this problem with some parallel operation of AVR of both plants, but sorry isn't solution change setting.
Best Regards.
Slava.
BTW, Denan could you please check what is a setting of var imports or out of step at the second plant.
 
Denan.
Additional Q.
What is a delay setting of 40 ( loss of excitation) function?
I don't see it in your attached setting.
Regards.
Slava
 
Slava, thanks for your comments!

Unfortunatelly it is not possible to connect AVRs with the other plant, or at least it is not very easy (I have not seen the other plant, I don't know how far it is, but I know it is not close). The other thing is that these two plants are from two different suppliers, and thus difficult to make changes. But I asked our guy to talk to the owner and the neighbour to try to work something out. Hopefully when the neighbour hears about these problems, he will try to help.
Don't misunderstand me, I agree with you, changing the settings is not the best solution.
 
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