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Unitrol 6800 static exciter voltage fluctuation

Unitrol 6800 static exciter voltage fluctuation

Unitrol 6800 static exciter voltage fluctuation

Hi guys,
as above i have a ABB Unitrol 6800 static exciter on a 40 MW generator with big voltage fluctuations.
The fluctuation is either in Current and this is a first indication it is not a display problem but it is real.

Furthermore another indication of real problem is related to MW generated from generator are fluctuating accordingly with voltage of Unitrol.

The exciter work with 49V and the fluctuations are until 15V

Have you any idea about?


RE: Unitrol 6800 static exciter voltage fluctuation

What's the generator connected to? If it's running against a large utility system the possible causes are very different than if it's the sole source to an isolated plant load.

RE: Unitrol 6800 static exciter voltage fluctuation

Hi davidbeach,
sincerly i don't think this could be the cause because we have other 2 generator (so... 2 40MW Gas turbine and 1 35MW steam turbine) but only in one of this we have this problem and the AVR reveal voltage fluctuation only in one Gas Turbine.

RE: Unitrol 6800 static exciter voltage fluctuation

One expert has tried to help you but you won't answer a simple and important question.
Your best recourse is to call ABB and have a service technician come out and fix your problem.

"Why not the best?"
Jimmy Carter

RE: Unitrol 6800 static exciter voltage fluctuation

I'm sorry waross,
i did not want to be rude... sorry.
We are power plant and we produce energy for the national Electric Company.

The generators are connected directly to transformer then elevated from 11KV to 132KV and put on national electrical line.

The transformer have 2 different output one for 132KV and one for power station load at 6 KV.

The 6KV output have another transfornmer connected to every 380V loads.

Thinking this could be enough i have at your disposal for every clarifications.


RE: Unitrol 6800 static exciter voltage fluctuation

Is this a steam turbine or a gas turbine?
Variations in MW on a parallel set are generally a mechanical problem. That is the governor of fuel supply. When the input energy to the prime mover varies, the AVR will be changing its output trying to match the MW output.
Is the AVR in power factor mode?

"Why not the best?"
Jimmy Carter

RE: Unitrol 6800 static exciter voltage fluctuation

This is a steam turbine and it is not in Power factor mode.
But the output MW should be only related to a AVR voltage fluctuation problem instead of mechanical problem?
Should be for instance some card loose or some reference value fluctuating?
There are some test we can perform with machine running? ex oscilloscope waveform or Others?

Thank you

RE: Unitrol 6800 static exciter voltage fluctuation

If the generator is connected to a fairly stiff system and you are having both real power and field voltage issues I think that your problem starts in the governor rather than in the AVR; that the AVR is chasing a problem elsewhere.

When connected to a large system with multiple generators any change in real power that isn't shared among all units is likely a governor issue. The AVR can't realistically change the voltage, but it can change the reactive power output. The real power changes are going to change the voltage drop across the GSU and the AVR is likely trying to correct for that.

RE: Unitrol 6800 static exciter voltage fluctuation

The unit may be starving for steam.
Check your steam pressure and steam flow.
I am going to guess that the steam is produced by the exhaust heat of the gas turbines. If the load drops, then the total exhaust heat drops and you don't have enough steam for full output of the steam turbine. When the power output of the set drops, the voltage drop across the transformer drops. The 132 kV is set by the grid, so with less voltage drop across the transformer the voltage on the generator side of the transformer rises. The AVR sees the voltage rising and cuts back its output to try to bring the voltage down.
You may be trying to get more MW out of the set than you have steam to support.
Try setting the output of the steam turbine 10% lower.

"Why not the best?"
Jimmy Carter

RE: Unitrol 6800 static exciter voltage fluctuation

I made Mistake because the system with the problem is a GAS turbine and Not Steam Turbine.
Anyway i show you a sample of the problem.
on the image there are very few little problem (sorry i'm new in this plant so i don't know yet how to load previous data in long time) but consider when the problem is present the voltage fluctuations are more and more.

RE: Unitrol 6800 static exciter voltage fluctuation

Looks like voltage on a transmission system. I can't read the vertical scale to know for sure, but that looks pretty ho-hum to me.

RE: Unitrol 6800 static exciter voltage fluctuation

What is the time base? Minutes?, Hours?, Days?

"Why not the best?"
Jimmy Carter

RE: Unitrol 6800 static exciter voltage fluctuation

Probably this is clearer.
Voltage scale min 34 - max 47 Volts
Current Scale min 8 - max 11 Ampere

Timespan is from 3 until 17.30

RE: Unitrol 6800 static exciter voltage fluctuation

If that's field currents and voltages we still don't know much. What does the generator output look like while this is happening? Terminal voltage, real power, and reactive power would provide a more meaningful story. Is there a PSS on the unit? They can cause lots of activity in the field that doesn't appear as readily in the stator.

I'm still inclined to think that you're looking at the AVR's response to external conditions rather than seeing the AVR cause trouble. Part of that is based on assuming that "National Electric Company" implies a large stiff system. If this unit is a significant portion of the total capacity then other conclusions could be drawn.

RE: Unitrol 6800 static exciter voltage fluctuation


Davidbeach, i don't know really if PSS is present in the unit and below a picture about problem and in attachments you found layout of the plant in order to give you much indications as possible.
I don't know exactly where the problem cause reside and i'm open at every considerations. The active power of the system still constant because forced by active power of other (stibilized .. but i'm not sure).
looking at generator voltage it is possible to see it follow exciter voltage drops.

Sincerly my opinion is to have a problem on:
  • AVR cards - probably some loosenes inside cards or power line instability
  • PT malfunction - connections loosenes and/or malfunction (less probable) of it

RE: Unitrol 6800 static exciter voltage fluctuation

What did it look like 6 weeks ago? What did it look like 6 months ago?

What is the generator control mode?

What are other units in the plant doing at the same time?

What evidence, not traces on a screen, is there that this is a problem and not just a normal response to beyond the GSU system dynamics?

You may have a real problem, but I've seen those types of traces with that much, or more, noise on the exciter channels where everything is just fine. Throw a PSS into the mix (a unit that size would almost certainly have a PSS here) and the signal gets even noisier looking.

RE: Unitrol 6800 static exciter voltage fluctuation

Looking at the voltage scale on the Y axis, the voltage disturbance at the machine terminals is not large. There's an approximate correlation between rising reactive power export, with a corresponding drop in terminal voltage, in response to which the AVR increases the field current. If there's a PSS in there then the whole thing becomes considerably more complex. In the UK I'm not certain that we'd see a PSS applied on a 30MW set.

I'm not familiar with the Unitrol AVR but I wouldn't worry too much about the exciter voltage: the field current is almost certainly the controlled variable in the AVR's inner control loop, not the voltage. The voltage will swing significantly because of the highly reactive load it is driving: remember E = -Ldi/dt ? The AVR is trying to rapidly vary the current in a highly inductive load - it's entirely normal for the voltage trace to be a bit lively.

RE: Unitrol 6800 static exciter voltage fluctuation

For comparison, attached is an image of 25 MW hydro unit showing exciter fluctuations of a similar magnitude.

One kind of odd thing about your graph is that the exciter had a large step response at 9:24 with almost no variation in voltage, but then both the voltage and the excitation increased at 10:09. Does the voltage control scheme change throughout the day? Does each generation regulate it's own low side bus or is there any sort of remote bus/line drop compensation?

Scotty -Interesting. In the western USA single units more than 30 MW and plants more than 70 MW are required to have PSS.

RE: Unitrol 6800 static exciter voltage fluctuation


Possibly because the UK grid is geographically small and heavily interconnected, compared to the US grid which is geographically much larger and in some areas much more sparsely interconnected? Transmission system modelling isn't really my area, generating plants just get told what to do by the TSO. winky smile

RE: Unitrol 6800 static exciter voltage fluctuation


The new exciter was installed on 2007 and i'm not sure if this evidence was present at that time but the indication seems starting from last Agust (2017).
Regarding the Turbogenerator 1 (exactly same of TG2 with the problem) the exciter is exactly the same (ABB unitroll 6800)but there are no fluctuation in voltage either in Generator voltage.
Regarding your request and dubt about the graphs instead of real evidence, i'm agree with you some time some noise can add to signals but... why the TG1 have no signals? i Think i need to inspect...
The ABB made some check 2 weeks ago in order to verify the correct functionality of the system. Channel 1 (so driver 1) have no problem and Channel 2 (so driver 2 ) have no problem too.
In attach the graph show you differences about TG1 and TG2, can you see fluctuation on generator voltage (tensione) ?


As i can see on the graphs your voltage fluctuation seems be related to a particular Power generation set, is it?... in my case these fluctuation seems to be random

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