Parallel generators at different size
- Thread starter anbm
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I don't know, but I do know how I would go about it.
I'm assuming you're talking big units, maybe for redundancy in a big standby bank
First I would consult the manufacturer's engineer and they should be able to tell you pretty quickly if it will or won't work, and why, what to look out for.
Don't quote me as I'm not the expert but: voltage regulators ...
One of the units VR will be set to "isochronous" or something like that. It will try to maintain frequency setpoint regardless of load. The other units VR will be set to " % of droop ", usually 5% droop. I believe, my understanding of how this works, is the VR will allow 5% voltage droop over the load range of 0% to 100% load, and this is how the units load share.
The lead unit set to isochronous maintains a fixed speed of rotation, and the droop units maintain a difference between the rotor angle and rotating field that determines unit loading.
There are enough variables and special expertise required that it may or may not work. Set up a caution tape and keep the clueless fakers away from it.
Edit: should have checked wikipedia. Droop control varies the rotation speed from 100% to 105% over the load range. and google "droop control".
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zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Possible? Yes, power plants do it all the time, as do co-gen plants/etc. Worth the expense of the paralleling gear with the assorted controls (and engineering time to configure)? My gut feel says no unless you're doing small-scale grid-tied hydro or have generators >3-400kw.
rcwilson
Senior Member
- Location
- Redmond, WA
Do the generators have paralleling switchgear complete with a synchroscope or autosynchronzier to make sure the unit's are in synch with a reasonable voltage match at the instant the breaker closes? If not, something might blow up or someone get seriously injured when the breaker closes.
If the paralleling gear is OK then it is up to the voltage regulators and speed governor settings to make the generators share load in proportion to their ratings. Some units have built in controls that can be cross connected to do this. Or the governor droop control can be adjusted to do the same thing.
Bottom line - paralleling generators of different sizes is done everyday, that's how our lights stay on. But we need to study the details and specifics for each generator installation to make sure it will work and be safe.
If the paralleling gear is OK then it is up to the voltage regulators and speed governor settings to make the generators share load in proportion to their ratings. Some units have built in controls that can be cross connected to do this. Or the governor droop control can be adjusted to do the same thing.
Bottom line - paralleling generators of different sizes is done everyday, that's how our lights stay on. But we need to study the details and specifics for each generator installation to make sure it will work and be safe.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Issues? Yes. Can the issues be overcome with proper set-up and controls? Yes, done all the time.
You want to have the gen supplier also be responsible for furnishing the controls and associated switchgear. You will need to have a meeting with them to go over how you want them to operate, i.e. control logic. They will do the rest.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
I have done this before also, the customer was changing out the POCO pad mount transformer and upgrading the wiring. The permanent generator on site was a 1 meg, the rental unit was around a 2 meg if I remember correctly. The generator tech said he could set it up to take as much or as little of the load off the permanent generator as needed, with it also being capable of taking the full load immediately if the permanent generator failed. The computer room in this building kept up with the payroll of all of the state of Florida employees. They also had the UPS offline for upgrading at this time also.
JoeStillman
Senior Member
- Location
- West Chester, PA
Remember to use good judgemnt in situations like this. How do you get good judgemnt? Through experience. How do you get experience? Bad judgement. Take it from me - I have lots of experience.
Be sure the new generator has exactly the same pitch as the existing one(s). Newer alternators are often wound with 2/3 pitch to reduce triplen harmonics. That wasn't always the case years ago. If two wye-connected generators with different pitch are paralleled, harmonic line-to-ground currents will circulate between them. It will look like a ground fault.
Be sure the new generator has exactly the same pitch as the existing one(s). Newer alternators are often wound with 2/3 pitch to reduce triplen harmonics. That wasn't always the case years ago. If two wye-connected generators with different pitch are paralleled, harmonic line-to-ground currents will circulate between them. It will look like a ground fault.
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