generator exciter system
generator exciter system
(OP)
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





RE: generator exciter system
RE: generator exciter system
RE: generator exciter system
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
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"Why not the best?"
Jimmy Carter
RE: generator exciter system
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.
RE: generator exciter system
"..the more, the merrier" Genghis Khan
RE: generator exciter system
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If we learn from our mistakes I'm getting a great education!
RE: generator exciter system
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
RE: generator exciter system
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.
RE: generator exciter system
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
RE: generator exciter system
rasevskii
RE: generator exciter system
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.
RE: generator exciter system
"..the more, the merrier" Genghis Khan
RE: generator exciter system
RE: generator exciter system
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?
RE: generator exciter system
RE: generator exciter system