generator exciter system

[moseramiehiii]

If you are familiar with a current boost generator exciter system. We have a 5MW, 4160 volt generator that will run at 2.5MW and 0.9 PF for about 8 minutes and then the field becomes over excited and the generator begins producing excessive VARS. After a breif restart process the machine will once again run fine for an 8 minute period of time then begin to over excite. We have exhausted our troubleshooting ideas and are looking for someone that may have seen a similar situation

 

[potteryshard]

Do you have drawings available for the excitation system?

 

[nawao]

Is this happening when running in parallel with other generators in an islanded system?

 

[waross]

This is a symptom of a dropping system voltage. Your cross compensation system may not be working properly.
With an islanded set, the excitation controls the voltage. With a paralleled set, the excitation controls the VAR production.
Is it possible that you are feeding into a long line with voltage regulation at a distance? If so, as you pick up 2.5 MW of load, the feed end voltage regulator or onload tap changer may slowly drop the voltage to compensate for the lesser portion of the load. If your AVR is on voltage control instead of PF control, the result will be excess VARs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

What kind of generator and voltage regulator?

Current boost systems are usually used on SE tailends. Usually tail ends of your size have PM's, at least for most of the units I see.

When did the problem start occuring? Is this a new unit to you and this appears to happen all the time? Or is this an existing unit that used to perform normally and now has a problem?

Some more details about your application and unit would be helpful.

Mike L.

 

[powerjunx]

cross compensation, i agree to waross.. with genset being parallel, I bet there's something wrong with voltage regulator, that is; cross-current compensation circuit is somewhat to be faulty..i'm referring to reactive droop differential compensation function of AVR. try checking the possibility of CT's loosen terminals.


"..the more, the merrier" Genghis Khan

 

[ScottyUK]

Off chance: do you or your utility have an on-load tapchanger which could be responding to the voltage increase caused by your machine's reactive export? Tapping down to restore system voltage will result in an increase in the machine reactive loading.


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If we learn from our mistakes I'm getting a great education!

 

[rasevskii]

More info is needed, as catserveng and others have said. Are you parallel with a utility, are there other gensets in parallel on the same bus, is this a diesel set or what...etc. Is it a old problem that was always there or is it an old unit with a new problem, a new unit just put in...? What sort of exciter and AVR...brushless, static...? Drawings? OEM?

The fact that it takes 8 minutes to go haywire is odd. Usually excitation systems go bonkers immediately, by the time one realizes there is a problem, all the meters have gone offscale...If the cross current compensation is backwards, not connected or wrongly set, the system will go unstable in seconds, not minutes, unless something else as said, is shifting at a slow rate.

Check the obvious: loose crimps, PT connections and HV fuses, sliding drawer contacts...what is your environment..clean..wet, corrosive..paper mill...?

just some off the cuff thoughts..

rasevskii

 

[moseramiehiii]

Here is more info to support all the input so far.

1. Problem occurs when the machine is parallel with Utility.
2. Voltage regulator (VR) has been swapped between a sister unit that is working properly. Symptoms remain the same.
3. VAR/PF controller was replaced.
4. Droop CT input to the VR was monitored and didn't change at the point that the machine became over excited.

I'm new to this site and appreciate the feedback and will work on a way to upload drawings.

 

[rasevskii]

Disconnect or disable the VAR/PF control function. You have still not said what kind of regulator this is or what kind of genset. Is this a new unit just put in service or an old? If it is a diesel genset then "catserveng", our expert on such units, can help with specific questions about specific regulators and controls.

You cannot measure the droop CT output as such. It a current proportional to the generator stator current, always. Unlikely not the problem. Likely the PF control which can be slow acting.

Run it in droop mode only, on AVR. 4 or 5% negative droop is sufficient, if the units are directly on the same busbar.
Negative droop means falling voltage setting as the VARS to the bus increase, giving stability. No droop or positive droop will be unstable and the unit will trip on overcurrent as it will try to grab all the VARs from the other unit if it has a slightly higher voltage setting. In other words a reactive circulating current will flow between the units increasingly until something trips. Therefore the so called cross-current compensation, or in other words, negative droop.

Not to be confused with governor droop, another matter. That only affects KW, not VARS.

helpful?

rasevskii

 

[rasevskii]

Probably you will have to disconnect or disable VAR/PF control on both units. They may be fighting each other, each trying to maintain a set PF, but one may be working backwards, resulting in circulating current between the units that increases slowly, as the PF control is usually slow acting to maintain an average value.

rasevskii

 

[catserveng]

If you could provide just a few more details, like make of the generator, voltage regulator and the VAR/PF controller, then I maybe can give you some further troubleshooting advice.

As mentioned above, you could go into droop, and see if you have proper control. Because you have a consistant time it appears before the excitation goes high, what external imputs are coming into the voltage control system?

I just got done with a similar problem on a KATO tail end with Basler control components.

 

[powerjunx]

add more details to supplement your scenario, otherwise; post's might derailed and missing the track..though relates to the point as wild guess?



"..the more, the merrier" Genghis Khan

 

[dpmac]

Did we ever learn the outcome of this problem?

 

[Denan]

Maybe the problem occurs at 2,5MW, if it takes 8 mins to reach that load.
Maybe with load less than 2,5MW there is no problem?
Did you try to keep the load constant but lower than 2,5MW, say 2MW?

 

[ladytechnician]

hi guys have just joined not only this forum but this tread so am still on training as a generator technician and am in my third month so would you kindly give the fullnames of wat you are reffering to as RAV/PF and other terms Im constantly seeing being used.Cud you kindly bear with me really want to learn.

 

[moseramiehiii]

Thanks for all the input. The problem was finally identified as a bad resisitor in the control circuit. As the circuit heated up the resistance would increase to the point that the system could no longer regulate.