Fluctuation of Generator Parameters During Low Demand Period

M

Thread Starter

MakBan

Hi,

Can someone explain to me the cause of fluctuation of some of the generator parameters during low demand period?

During low demand period. 2am in the morning and power factor is leading, we experience fluctuation of generator current, megawatts, mvars, power factor, excitation voltage, current and balance meter. The generator voltage and frequency remain constant. This happen twice already. Can someone explain what maybe the cause of it.
 
Would need to know the ambient temp of the area the machines normally operate in. If you are operating in a cooler environment, leading power factors are not uncommon. this would be for higher resistive loads, such as heating.
 
MakBan,

It's very interesting that you report that the generator voltage is stable--yet the excitation voltage, current and "balance meter" (I presume you are referring to the transfer voltmeter that shows the setpoint difference between auto and manual regulator) are not stable.

When the generator and its prime mover is synchronized to a grid with other generators and their prime movers the frequency of all the generators is the same, and the voltage of the generators is also a function of grid voltage in that it's usually difficult for one generator to raise or lower the grid voltage by very much (I'm referring to larger grids with many generators). Presuming the VAr meter is near zero and the power factor meter is near 1.0, if the excitation of a single generator is increased on large, or "infinite" grid the generator terminal voltage doesn't change very much--but the VAr meter and the power factor meter change because the reactive current is changing which is changing the relationship between the current and voltage sine waves. The VAr meter will increase in the lagging direction and the power factor meter will decrease, also in the lagging direction.

Similarly, if the VAr meter is near zero and the power factor is near 1.0 (unity power factor) and the excitation is decreased then the VAr meter will increase in the leading direction and the power factor meter will decrease--also in the leading direction.

In both cases the generator terminal voltage will not appear to change by much but because the excitation is changing the reactive current is changing and shifting the phase angle between the voltage and current sine waves.

Real power, watts/KW/MW is a function of the energy flow-rate into the prime mover. Reactive power is a function of excitation. At very low loads sometimes an unstable exciter can also impact real power.

It sounds as if the exciter ("AVR") is having problems when the load is low, which is what's causing the excitation current and voltage to fluctuate as well as the transfer balance volt meter.

Some generators, and even some prime movers, are not rated for light loads--real or reactive. But, we don't know enough about the nature of the load at 2:00 am (is it leading or lagging power factor) and what the generator is rated for at low loads. And if there's some kind of switching of power fact correction capacitors nearby when the load is light that could also have some effect.

If you can tell us more about the nature of the load at 2:00 am (real and reactive loads), and how if the excitation current is very near the minimum rating (sometimes called the Under-excited Reactive Ampere Limit), we might could help more. But without more actionable data. We can't be of much more help.

My best guess--<b>based on the information provided </b> is that the exciter is at its minimum limit and is having trouble holding that limit. Especially if the grid voltage is very high, which it can be when the real load is very low. But that's just a guess. There's too much we don't know. If the grid is small that would also be important to know.

Hope this helps!!!
 
The ambient temperature is about 30 deg C. Our initial assessment is the malfunctioning of either generator AVR or turbine governing system but I don't know if I am correct.

> Would need to know the ambient temp of the area the
> machines normally operate in. If you are operating in a
> cooler environment, leading power factors are not uncommon.
> this would be for higher resistive loads, such as heating.
 
CSA,

Thank you for the response, it is very helpful. As for your your question about the nature of the load at 2AM, the power factor is leading and the real power is low because some of the industrial and commercial establishment is not running during those hours.

Also, the grid has a capacity of approximately 8 GigaWatts. Our grid voltage is rated 230 KV but the grid voltage during the said fluctuation is 235 KV. Our turbine is rated 55MW and our load during the fluctuation is 47MW.

You are right with the Under Excitation because the Under Excitation Limiter alarmed before and during the fluctuation.

What may be the actions we are going to make? Thank you.
 
MakBan,

When the excitation being applied to the generator rotor is not sufficient to "match" the grid voltage the generator is said to be under-excited. And when the generator is under-exciter the power factor at the generator terminals will be leading. This is kind of a dangerous condition, because low excitation can result in slipping a pole, which can be very destructive to the generator- and turbine rotor and the coupling between then.

The same condition can happen when the grid voltage is very high, relative to the generator terminal voltage-- resulting in a leading power factor.

Some sites have transformer tap changers which can be changed when the generator is synchronized to the grid, called on-line, or on-load, tap changers. If your site has these, then it might be possible to change the relationship of the generator terminal voltage to grid voltage by changing taps.

You can also increase the excitation to raise the generator terminal voltage relative to the grid voltage. This should move the power factor closer to unity (1.0).

The question here is: What is driving the AVR to lower voltage to the point that the URAL is alarming? Is the generator being operated in some kind of power factor control mode, or VAr control mode? Probably not, but then it doesn't make sense that it's being operated in a voltage control mode, either--unless the operators are being told to maintain a certain generator terminal voltage and because the grid voltage is so high they are reducing and reducing and reducing the excitation to try to reduce the generator terminal voltage.

Again, when you're connected to the grid there's not a lot you can do to affect grid voltage. It sounds as if the grid operators are not controlling grid voltage very well during low demand periods. I've was at a site where it was unusually hot during the day resulting in a very high load demand and a low grid voltage because of the high air conditioning load (lots of induction motors), and then a cooling sea breeze came in at night and dropped the ambient temperature very quickly and the grid load dropped very quickly and grid voltage went very high for a few minutes until the grid regulators took some corrective action. I was trying to synchronize a 40 MW generato
 
CSA,

We have no on-load tap changer in our transformer. We have 10 turbine-generator units on our site all synchronized to the grid. My concern is why is it thay only one unit fluctuates during off-peak and the other nine remains stable. Maybe there is a trouble on the AVR?

Until now, that one unit continuous to fluctuate during off-peak and I don't know how to troubleshoot it. Our temporary action when fluctuating occurs is to increase the voltage setter (to increase the excitation) and then the fluctuation stops. But we don't want that, because on the other nine units, fluctuation does not occur and I know there is something wrong on our control sytem maybe the AVR.

I am thinking to replace the Under Excitation Limiter card on the AVR unit and hopefully the fluctuation will not reoccur.

What more can you suggest me on other remedies to this trouble?
 
I think CSA has best determined the most likely cause.

Just to add to his, every generator has a "D Curve" or capability curve that defines the Real Power and reactive power limitations of a generator. Assuming you have a static exciter and a digital AVR, the exciter will have some internal limiters that are set based on this capability curve (possibly even more restrictive to be conservative) preventing you from operating too low and as CSA explained slipping a pole.

You should be able to see what the Under Exciation Limit is set in the exciter (typically a multipoint curve) and determine if you are hitting this limit when the instability starts. If this is the case the simplest solution is to have the operators keep the operating point away from the limiter during high voltage conditions.

Some other options are:

1) Change the exciter limiters to be less restrictive (if margin to capability curve exists)

2) Use a higher level control (e.g. AGC) that is programmed so that the generator operating point is kept away from the limiter line by changing the AVR setpoint or MW load

3) I have seen where having a Power System Stabilizer (PSS) function enabled reduces or stops the instability. NOte: In the US this requires regulatory and planning coordination, in addition to a tuning study to determine parameters.

What type of exciter/AVR do you have?
 
bohdand,

The type of exciter we have is not static. It is a brushless exciter and also our AVR is not digital so I could not see the capabilty curve and if the excitation is truly on the under excitation limit.

You are right with the increasing of the voltage setter when the fluctuating is happening becasue it is what our operators are doing then the fluctuation stops after that.

But I don't want that scheme because on our other units, fluctuating does not happen so there is no need to raise the voltage setter.

I know there is something wrong with the AVR but I could not say what really the problem is. Now I am thinking to replace the under excitation limiter card inside our AVR unit and hopefully it will solve the problem.
 
Makban...

in order to reduce the amount of verbage, thus far committed to this thread, why not forward some measured operating values.

For example, I suggest V, A, MW, MVAr, MVA, Field V&I, temp, etc, comparing the troublesome-unit to a properly operating-unit.

Regards,
Phil Corso
 
MakBan,

It's very interesting that you report that the generator voltage is stable--yet the excitation voltage, current and "balance meter" (I presume you are referring to the transfer voltmeter that shows the setpoint difference between auto and manual regulator) are not stable.

When the generator and its prime mover is synchronized to a grid with other generators and their prime movers the frequency of all the generators is the same, and the voltage of the generators is also a function of grid voltage in that it's usually difficult for one generator to raise or lower the grid voltage by very much (I'm referring to larger grids with many generators). Presuming the VAr meter is near zero and the power factor meter is near 1.0, if the excitation of a single generator is increased on large, or "infinite" grid the generator terminal voltage doesn't change very much--but the VAr meter and the power factor meter change because the reactive current is changing which is changing the relationship between the current and voltage sine waves. The VAr meter will increase in the lagging direction and the power factor meter will decrease, also in the lagging direction.

Similarly, if the VAr meter is near zero and the power factor is near 1.0 (unity power factor) and the excitation is decreased then the VAr meter will increase in the leading direction and the power factor meter will decrease--also in the leading direction.

In both cases the generator terminal voltage will not appear to change by much but because the excitation is changing the reactive current is changing and shifting the phase angle between the voltage and current sine waves.

Real power, watts/KW/MW is a function of the energy flow-rate into the prime mover. Reactive power is a function of excitation. At very low loads sometimes an unstable exciter can also impact real power.

It sounds as if the exciter ("AVR") is having problems when the load is low, which is what's causing the excitation current and voltage to fluctuate as well as the transfer balance volt meter.

Some generators, and even some prime movers, are not rated for light loads--real or reactive. But, we don't know enough about the nature of the load at 2:00 am (is it leading or lagging power factor) and what the generator is rated for at low loads. And if there's some kind of switching of power fact correction capacitors nearby when the load is light that could also have some effect.

If you can tell us more about the nature of the load at 2:00 am (real and reactive loads), and how if the excitation current is very near the minimum rating (sometimes called the Under-excited Reactive Ampere Limit), we might could help more. But without more actionable data. We can't be of much more help.

My best guess--<b>based on the information provided </b> is that the exciter is at its minimum limit and is having trouble holding that limit. Especially if the grid voltage is very high, which it can be when the real load is very low. But that's just a guess. There's too much we don't know. If the grid is small that would also be important to know.

Hope this helps!!!

Hi,
I have a doubt ,

If generators are running in synchronization and excitation voltage have some issue with one generator so according to you it will not change the voltage of generator?
we have load unbalance issue at our plant with one generator, i have checked every thing except voltage control because there is no fluctuations in voltage.
 
Hi

Would be glad to give you some assistance if you can provide followings items:
A single line diagram
Event record /Sequence of event or trends...

Also is there kind of PMS(Power management system) installed..

OEM of the devices /equipments.. Like AVR generator protection relay if applicable..

Regards
James
 
Hi,
I have a doubt ,

If generators are running in synchronization and excitation voltage have some issue with one generator so according to you it will not change the voltage of generator?
we have load unbalance issue at our plant with one generator, i have checked every thing except voltage control because there is no fluctuations in voltage.
Hi can you quick reply to keep us updated on that thread
 
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