Can someone explain to me what is the AVR's response when the excitation hits the Over Excitation Limiter or Under Excitation Limiter? When the excitation hits the OEL/UEL, will the AVR "locks" or will it regulate upto the normal range?

We experienced abnormalities on our AVR. The grid system voltage is 235KV thus the power factor become leading. We increase the excitation for the power factor to become unity then suddenly the OEL alarmed and after some seconds the Generator UnderVoltage Relay tripped.

Can someone explain to me what just happened?

Thank you in advance.

 

MakBan,

It's virtually impossible to say how your AVR operates without knowing the manufacturer and model of the AVR, but, in general, when an AVR is adjusted so that it's output reaches an overexcitation limit (OEL) or underexcitation limit (UEL) it's output cannot be raised or lowered beyond the limit.

If the VArs/Power Factor of your machine was leading and the AVR was in OEL, the likelihood is that the generator protective sensing was detecting a very "low" Power Factor (low as in "negative" (leading)) and made a determination that the generator voltage <i>with respect to the <b>nominal</b> system (bus) voltage</i> was low enough that there was a possiblity of pole slippage and/or excessive generator overheating, so it tripped the generator breaker (and possibly the turbine.?.?.?).

We also don't know enough about the configuration of the protective relays and how they operate to be able say much more than this. The sequence of events as described sounds like it could be reasonable if the system (bus) voltage was excessively high and the AVR couldn't be adjusted to correct the low leading power factor.

You should be observing that if the generator terminal voltage and excitation is constant (VAr/Power Factor Control is OFF; Automatic Voltage Control is ON) that as the system (bus) voltage changes so will the VArs/Power Factor. That's because VArs/Power Factor is a function of the relationship of the generator terminal voltage to system voltage.

When the generator terminal voltage is equal to system (bus) voltage, then the VArs will be zero, and the Power Factor will be unity (1.0). If the generator excitation is held constant (which keeps the generator terminal voltage constant) and the system (bus) voltage increases then the VArs will go into the leading direction, and the Power Factor will decrease (be less than 1.0) in the leading direction.

When the generator terminal voltage is equal to system (bus) voltage, then the VArs will be zero, and the Power Factor will be unity (1.0). If the generator excitation is held constant (which keeps the generator terminal voltage constant) and the system (bus) voltage decreases then the VArs will go into the lagging direction, and the Power Factor will decrease (be less than 1.0) in the lagging direction.

System (bus; grid) voltage changes throughout the day as load changes. Better-regulated grids don't see much of a variation, while others see more variation. Also, the distance between generation plants and substations and loads also plays a large role in system voltage variation at different points along the grid (transmission and distribution system).

But, <b>in general,</b> the fundamentals above should be observable on a normal system experiencing normal fluctuations. This is why even though the watts remain relatively constant during operation the operators may have to keep manually adjusting excitation to keep the VArs or the Power Factor to some desired setpoint (when automatic VAr or Power Factor Control is NOT being used). But, there are limits--to protect the generator--of the AVR's ability to be able to respond to very large system voltage swings. In general, most AVRs are capable of increasing generator terminal voltage and decreasing generator terminal voltage by 5% above or below rated generator terminal voltage, and that's presuming the system voltage is also at or near rated. If the system voltage is very high or very low then the AVR's ability to hold a certain VAr or Power Factor setpoint may be limited by the OEL or UEL functions--this to protect the generator (rotor and stator and load coupling/prime mover). The presumption is that system voltage is relatively stable, and when it's not then the presumption isn't valid. It would take a very sophisticated AVR to be able to deal with all the system vs. generator terminal voltage variations, and even then the generator rotor can only handle so many amps without excessive heating, and the stator windings can only handle so much leading reactive current without overheating, and if the excitation is allowed to drop too low then the generator rotor will "slip a pole" which is disastrous. So, limiters are very important--even if the system voltage is unstable. Even if the AVR was capable of high current outputs and low current outputs--the generator rotor has a fixed limit, as does the ability of the generator to remain in synchronism without slipping a pole.

Hope this helps!

 

Thank you for the response. It was very helpful.

Our excitation system is Mitsubishi MEC-3200. It is an old analog brushless excitation system. The setting of our UnderVoltage Relay is 11.52KV with our nominal generator terminal voltage of 13.8KV.

You are correct with our AVR is having trouble regulating when it hits OEL/UEL. But what component of the AVR is malfunctioning? Is it the sensing circuit? Regulating circuit? Or other circuits?

 

MakBan,

> You are correct with our AVR is having trouble regulating when it hits OEL/UEL.
> But what component of the AVR is malfunctioning? Is it the sensing circuit? Regulating
> circuit? Or other circuits?

Er, ..., Um, ..., I don't recall saying a regulator has or will have trouble when reaching OEL or UEL. Generally, every AVR I've ever worked on had no issues with output of the rectifier when reaching a OEL/UEL--the reference was limited and prevented from increasing or decreasing, but the rectifier output was stable so the current/voltage being applied to the generator rotor was stable. (This is also true of the "brushless" systems I've worked on--the reference was limited when a limit was reached, but the output of the AVR was stable.)

I wonder if the AVR at your site uses a motor-operated potentiometer to change the reference and that if your operator or DCS is continually calling for a RAISE (in your example) that the pot gets driven to the limit and then some logic backs it down slightly and then it gets driven back up to the limit and then back down, and so on. I did see one system like that many years ago, and the solution was to just stop issuing a RAISE when it hit the limit. There's no point in continuing to issue a RAISE when the AVR is at the OEL--it won't be able to put out any more excitation. And if there's a poorly designed control circuit that allows for up/down/up/down if a continuous RAISE (or LOWER) is being issued then that's going create an unstable output. Most of the higher-end AVRs have some kind of indication to the operator/control system that a limit has been reached (an indicating lamp or a process alarm), so that should be the signal to stop issuing a RAISE or LOWER.

I'm not familiar with the exciter you listed; perhaps someone else reading this post is. But, I would say if the exciter hits a limit that the operator should immediately stop issuing a RAISE or LOWER (if they aren't already). A limit is a limit--and most excitation systems aren't designed to squeeze out just a little more if one lays on the RAISE or LOWER when a limit is encountered.

Rather, I'd say there seems to be a problem with the regulation of the system voltage. If this is occurring regularly, then something is definitely amiss with the system voltage regulation. And, if you're regularly hitting UEL and OEL, then there's really something amiss with system voltage regulation--and you can't control that. You should be able to express your concerns about system voltage regulation to the system operator and show them that if the system voltage is unstable that your generator may trip (they should understand that) and that can be even more difficult for them to deal with (the loss of generation) than system voltage regulation. If the system voltage regulation is unstable that's not your problem and there's only so much you can do. Maybe they will pay you for an upgrade to your excitation system--maybe they won't; but they have an obligation to maintain a stable system for those synchronized to it. There may even be something in your contract with the system operator that says they will maintain voltage and/or frequency within a certain range, and if they're violating their contract you can ask them to comply with the contract they signed.

If the step-up transformer has taps then it might be time to consider consulting with the system regulator about changing transformer taps. (Some tap changers must be changed with no current flowing through the transformer; others can be change while on-line. Some transformers don't have any adjustable taps.) Changing taps may allow the generator not to see such a high voltage.

Anyway, you likely need to get someone to site to help with troubleshooting the AVR if you feel it becomes unstable when reaching a limit, because that shouldn't happen--the output should be stable, limited, but stable when it reaches OEL or UEL. And, there are too many possibilities for why it might not be stable to say for sure.

Hope this helps!