AVR - Slow Response

We have a GE Frame 9E machine. Recently we have a problem controlling the VARs of the machine. The response towards the command takes about 30 minutes. The excitation system is Potential Source Static Excitation. I would like to know what can cause this problem. Can it be due to the URAL card settings drifting from the calibration value? I have 2 GTs with the same system. Only one is facing this problem.

Thank you.

What have you done to try to troubleshoot the problem? We can suggest all manner of things to try, but if you've already done them you're going to think you're not getting the help you though you might. But, that would be your fault for not telling us what you've done to try to troubleshoot the problem (if anything, and if it was nothing then just say so).

Did this just start to occur? Was there a maintenance outage recently where some work was done on the exciter regulator? Was a card in the exciter regulator recently replaced for some reason? If the main step-up transformer has a tap-changer, was it recently changed? If it's an automatic tap changer, is it working properly? What we're asking is: Has there been any change after which this problem started occurring?

You haven't told us if you're trying to raise or lower VArs/generator terminal voltage, or how you're trying to raise or lower voltage.

The URAL (Under-excited Reactive Ampere Limit) card should only limit the lower signals. It's been 20-odd years since I've seen a URAL, but I think there might be an LED on the card to indicate it's "in limit" (that the limit is active).

And the above only applies to attempts to lower VArs/generator terminal voltage.

If you're trying to raise VArs/generator terminal voltage there is an upper limit to the raise signals, to try to prevent over-excitation. It's usually set on the Automatic Regulator card during adjustment/calibration.

Are you trying to send the exciter regulator signals from a Raise/Lower switch on the generator control panel, or from relays in the turbine control panel, or from some other source (DCS, etc.)?

There are LEDs on the Auto Regulator card that blink when a Raise or a Lower is active; have you observed them during the attempts to change the VArs/generator terminal voltage? If the respective light is constantly lit during an attempt to Raise or or Lower VArs/generator terminal voltage, then it has likely hit a limit on that card.

Quite often, there were interposing relays in the panel next to the exciter regulator which picked up when Raise or Lower was initiated from the Control Compartment or DCS. If your system has them, have you monitored them to see if they are picking up when the respective signals are active?

The manuals for the Potential Source Excitation System are actually pretty good, with very good procedures for adjusting and calibrating the various cards and the system. The Potential Source Excitation System should be *the* basic teaching system for understanding how exciters work; every function in an exciter is basically reduced to a single card (Field Voltage; Inner Loop Regulator; etc.). Almost all of the basic parameters are reduced to calibrated voltages by the sensing circuits (either 0-5 VDC or 1-5 VDC, I can't recall). And, one can use the probe tip of a meter lead inserted in the test connections on the fronts of the cards to check things like generator terminal voltage and field voltage, etc. It is so easy to learn and understand the inner workings of a basic exciter regulator by learning this system that, except for some of the overly complicated and extremely technical regulators produced by a couple of European manufacturers, almost every other exciter regulator can be quickly understood after learning this one. It really is a very simple and uncomplicated exciter regulator.

We can try to help you, but we need some more information. And, there is a lot of good information in the manuals and they're not that complicated; unless the rectifier bridge is really making a lot of unusual noise, you can skip the parts about phase angles of the rectifer gate circuits and such, but if you want to learn about exciters, have a look at the manual. If you already know exciters, the manual should be a very easy read, even the part about the URAL card.

Thank you for the reply. For your information, I didn't take any step to resolve the problem yet. There was also no maintenance activities carried out recently and the tap-changer is working fine.

The last calibration done on the URAL and all other excitation cards was on Jan 08. It was working fine except that the setting for limiting the VARS was not very accurate as per the reactive capability curve that was established (i.e. I tested the VAR for 100MW where it was supposed to be blocked at -18MVAR, however the VAR went down to -22MVAR). I was planning to re-calibrate and fine tune the settings during the coming outage in August.

Back to the recent problem, the problem occured when the operator was trying to reduce the VAR. The command was given first from the HMI, and following no response, the operator tried using the raise/lower switch. It took almost half an hour for them to see the changes in the VAR.

I didn't manage to look at the cards when this problem happened. Whatever I'm trying to figure out is according to the established curve where at 110MW of the machine, the VAR should be able to go down to 16MVAR. At the time the problem occurred, the lower limit is still not reached yet.

I'm not sure about one thing, is there anything that is controlling the respond time of the AVR? Time delay relay or anything like that? At this moment, I'm not aware of one.

Dear,

Can you send me that manual? I am trying to upgrade my AVR.

Thanks.