AVR won't decrease MVAR's when pulse is sent. It gets stuck and appears to be trying to move down on the vectormeter. What could be the possible likely causes?

Thanks for any help. I'm not an engineer, just interested in the above situation with our generator.

 

boosterkitty,

What is the manufacturer/model number of your exciter regulator (AVR)?

What's a vectormeter? (Power factor meter? VAr meter? Dynamic reactive capability curve on an HMI display?)

What kind of prime mover is driving the generator (steam turbine; gas turbine; hydro turbine; reciprocating engine; etc.)? What's the rating of the unit (in MW, I presume)?

Is the system voltage higher or lower than normal (the "grid" or "bus" voltage)?

Some exciter regulators (AVRs) have minimum limits (sometimes called under-excited reactive ampere limiters (URALs)). If the grid voltage the unit is synchronized to is much lower than normal then sometimes the excitation can't be reduced any further depending on the type of limiter. Usually when an exciter regulator (AVR) hits a limit there is some kind of alarm to alert the operator; are there any alarms like URAL?

If the pulse goes to a motor-operated potentiometer is the motor working properly in both directions? Can you increase the MVARs by 10 or so and then reduce them again until they just stop decreasing?

Can you be specific about the MVArs at the time of the problem? What is the magnitude of the MVArs (20, 30, more? Less?) Are they lagging MVArs? Or leading MVArs? (Sometimes lagging MVARs are displayed as positive, and leading MVArs are displayed as negative--if that helps).

Can you tell us what the power factor is when you can reduce the MMVArs--and if it's lagging or leading (the same polarity convention applies to a digital power factor indication).

Does this happen during a unit STOP sequence, or when it's on-line and running normally?

Are there any alarms which might be related to the problem?

 

Hi CSA,

<i>What is the manufacturer/model number of your exciter regulator (AVR)?</i>

TSG2 Brushless with Permanent Magnet Auxiliary exciter.

<i>What's a vectormeter? (Power factor meter? VAr meter? Dynamic reactive capability curve on an HMI display?)</i>

Not sure what you call it. I've only known it as a vectormeter. It shows MVArs on the X axis, MW on the Y axis and shows the stability limits. The hairlines for each axis show the generator position within the stability curve and the PF.

<i>What kind of prime mover is driving the generator (steam turbine; gas turbine; hydro turbine; reciprocating engine; etc.)? What's the rating of the unit (in MW, I presume)?</i>

25 MW Backpressure Steam Turbine at 8000rpm to an AEG generator at 1500.

<i>Is the system voltage higher or lower than normal (the "grid" or "bus" voltage)?</i>

11kV out the back of the generator stepped up to 22kV and then 132kv to the grid.

<i>Some exciter regulators (AVRs) have minimum limits (sometimes called under-excited reactive ampere limiters (URALs)). If the grid voltage the unit is synchronized to is much lower than normal then sometimes the excitation can't be reduced any further depending on the type of limiter. Usually when an exciter regulator (AVR) hits a limit there is some kind of alarm to alert the operator; are there any alarms like URAL?</i>

There's no alarms that I can see. We usually run the generator at a PF of .8 leading into a 22.5kV bus. Neither the bus voltage or grid voltage move far from 22.5 kV. We rarely require a tap change.

<i>If the pulse goes to a motor-operated potentiometer is the motor working properly in both directions? Can you increase the MVARs by 10 or so and then reduce them again until they just stop decreasing?</i>

The generator usually puts out between 10 and 23MW. Yes, I can increase the VARs successfully, but as you stated, as I bring them back they hit a wall. At this moment that wall is just below .8 PF @ 17MW. I have tried increasing and decreasing the load to get the PF further into lead, but the same behaviour occurs. At some point it will not move the PF higher.

<i>Can you be specific about the MVArs at the time of the problem? What is the magnitude of the MVArs (20, 30, more? Less?) Are they lagging MVArs? Or leading MVArs? (Sometimes lagging MVARs are displayed as positive, and leading MVArs are displayed as negative--if that helps).</i>

Lagging MVArs. For example at the moment 12 MVArs for 17MW. Optimum PF for the generator is .8.

<i>Can you tell us what the power factor is when you can reduce the MMVArs--and if it's lagging or leading (the same polarity convention applies to a digital power factor indication).</i>

The power factor that it sticks at varies with the MW load. It doesn't seem to be uniform either.

<i>Does this happen during a unit STOP sequence, or when it's on-line and running normally?</i>

We haven't had the turbine off since the problem arose.

<i>Are there any alarms which might be related to the problem?</i>

Not that I can see. Is there a chance that the Rotor Angle Limiter is operating in the wrong range, stopping the MVArs from dropping to a value that it thinks is unsafe? If so, how could that drift occur?

I appreciate the help CSA. It's pretty amazing what you do around here.

 

Boosterkitty,

Thank YOU for taking the time to answer all of the questions; that's quite a rarity and it's much appreciated.

Okay; so the vectormeter is some kind of dynamic reactive capability curve that helps operators not to exceed the rating of the machine. I presume (hope) that there are lines representing the limits of operation for MW, lagging VArs, and leading VArs. And, I presume that you are within those limit lines at part load--so the generator, presuming the excitation voltage/current is normal, isn't going to be damaged.

And that's something I forgot to ask--what IS the generator excitation voltage/current now, compared to previous operation at the same MW/MVAr output? Because if you have a brushless exciter I'm wondering if something's not wrong with the rotating diode wheel, or maybe there's a developing generator rotor ground--though that doesn't really seem likely. Perhaps there's something wrong with the rotating armature of the brushless exciter. (I'm not familiar with TSG2, and couldn't find anything about them on the World Wide Web, so I'm kind of at a loss at the moment to be or more help with that.)

At this point, presuming the exciter voltage and current are within reasonable range of previous similar operating points I would have to presume that there's something amiss with the settings of the exciter, or some potentiometer has drifted, or something like that. If there are no alarms from the exciter and/or the rotating diode sensor/field ground detector I'd have to guess that some analog component or setting has changed.

You mentioned taps and that you don't need to change taps very often. The relationship of generator terminal voltage to the grid voltage (at the low side of the step-up transformer) has an effect on VArs/power factor. If the grid voltage is normal and you haven't had to change taps, then a higher-than-normal grid voltage likely isn't the problem--unless you always run at very high or very low tap positions, in which case you just might be near an operating limit of the exciter most of the time and grid conditions just might have made it become apparent.

I wish I could be of more help, but that's about all I can offer. I do really appreciate that you took the time to answer all of the questions, and I do wish I could offer more in the way of help. But, I'm not familiar with the exciter and couldn't find anything on the Web. So, I'm just offering general ideas at this time.

Hope this helps--and please write back when you do resolve the issue!