AVR Droop CT Harmonic Current Response
AVR Droop CT Harmonic Current Response
(OP)
I have situation of when a 280kVA standby generator is supplying the site load (mains fail) the voltage oscillates by approximately 5V at around 2Hz. The site load is about 135kW @ 0.98 leading, and consists of a UPS, air conditioning plant (VSD driven) and the usual basic building load. The generator is a typical diesel driven Stamford 4pole alternator with an MX321 AVR. The site has two generators (N+1 redundancy) and it doesn't mater which generator is supplying the load the voltage will oscillate. If the two generators are run in parallel supplying the isolated site load the voltage oscillation is not apparent.
The UPS and the VSD's have enough filtering to keep the voltage THD to 3% when running on the generator, but the current THD is around 20%.
Do you think it is possible that the AVR is responding to the current harmonics?
As far as I can tell the generator is operating with its capability area. Could there be a resonance interaction with the harmonic filters on VSD's?
Your thoughts please
Cheers Niall
The UPS and the VSD's have enough filtering to keep the voltage THD to 3% when running on the generator, but the current THD is around 20%.
Do you think it is possible that the AVR is responding to the current harmonics?
As far as I can tell the generator is operating with its capability area. Could there be a resonance interaction with the harmonic filters on VSD's?
Your thoughts please
Cheers Niall





RE: AVR Droop CT Harmonic Current Response
Then try switching off the UPS and the AC in turn to see if one of then is responsible for the instability.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AVR Droop CT Harmonic Current Response
Turning off the UPS (bypass mode) is not the clients prefered test option due to the sensitivity of the load. Minimizing the AC load didn't make any difference either.
Cheers Niall
RE: AVR Droop CT Harmonic Current Response
* Not my name for it, before anyone starts arguing!
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: AVR Droop CT Harmonic Current Response
Cheers Niall
RE: AVR Droop CT Harmonic Current Response
With the unit running standalone you could also disconnect the voltage bias wiring and see if the problem goes away or stays.
Also see how much of a change in the field output from rated voltage no load to when the load with a leading power factor is applied. If it's really a leading power factor the field output will drop slightly. Does the generator voltage stay at rated or rise up a bit? Is the voltage trim in the EGCP2 active? Some Cummins packages have the loss of field function active and set pretty tight, have seen stability issues because the AVR or controller thinks the leading PF is a loss of field and shuts off excitation and doesn't latch, casuing an oscillation.
Hope that helps,
Mike L.