PMG and AVR issue

[todd066]

During testing of our AVR, we reach about 58% output to the exciter and get a misfiring on one of our SCRs. The misfiring causes noise in our system, and because the noise is on the output of the exicter, the generator runs away (goes high) until our protection relay trips the exciter field breaker. Our AVR is using a PMG for input power. When we increase the output to 60% output to our unit, the noise goes away. We believe that the issue is due to EMI induced on the cabling going to the control circuit. Could the issue be our PMG? Could the switching noise of the SCRs be causing our PMG reference to be noisey and therefore impact our ouptut?

I am typically a small wire guy(I&C), so I am a bit out of my elelment on this issue.

Thanks,
Todd

 

[rbulsara]

You need someone on site. Start with the generator vendor/service department. Depending upon the critical nature of application, you can spend time and money investigating something for "engineering" satisfaction or make a business decision to replace faulty equipment.


Rafiq Bulsara

http://www.srengineersct.com

 

[rasevskii]

for todd066:

Of course we need more information than what you provide:
How big is the unit. who is the OEM, how old the unit, what do you mean by "testing the unit", was there a problem and therefore why the testing...

I previously commissioned a lot of AVRs and excitation systems also with PMGs (typical of smaller units).

If this is more modern digital system on a CAT unit we have "Catserveng" who can answer specific questions on such units professionally.

Do not be "blinded by science" and read a lot of sophisticated problems into it. It can be basic things like loose PT connections, bad hv contacts or fuses on the PTs, bad setting pots, etc. Is this a new problem on an old unit or what??

See rbulsara´s comment also. Call the OEM if available.

rasevskii

 

[waross]

Sounds like a bad AVR or loose connection.
But maybe there is no problem. When the set is unloaded the output of the AVR may be less than 50%. If you are driving the voltage up on the AVR, you may be seeing normal operation. Drive the AVR output voltage up much more on an unloaded set and you may expect an "Over-voltage" trip.
Are you testing for the sake of testing?

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

 

[ScottyUK]

Check rasevskii's list, and look for other easy fixes like maintaining good segregation between power and control cores, and making sure the gate drive wiring is twisted fairly tightly. Keep sensor cables well away from noisy conductors - the DC shifts potential relative to ground, for example - and check the shield connections are good. Make sure anything which should be connected to frame earth actually is.


----------------------------------

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