240/120 series delta 3ph generator for residential use - derating high leg imbalance?

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JHZR2

Member
Location
New Jersey
Occupation
Power Systems Engineer
This is a question related to some hurricane relief work. In one home (really will be connected to two homes next to each other via lockable disconnects), a 20kW genset was provided. It's not on the mainland and so power won't be restored for months, nor are a wide range of gensets easily found. It's a beggars can't be choosers scenario. There are no manuals and I cannot find one online!

Apparently its set up as a 3ph delta configuration, so I'm assuming there is a setup to provide ancenter point neutral the a-c phase and provide a high leg setup to provide split phase 120/240ac for home. Since its residential, and there are no three phase loads, the imbalance will naturally be high, correct?

If the controls only permit 20% unbalanced load capability, will the output be limited to around 40% of the genset rating? Since the b phase will be completely unloaded, it seems to me that the genset can only support up to 20% loading on the a and c phases, and assuming that there is some imbalance in the load on either side of the split phase, the first one to hit 20% load would trigger a fault.

Is my thought process on this correct?

thanks!
 

MAC702

Senior Member
Location
Clark County, NV
...Apparently its set up as a 3ph delta configuration, so I'm assuming there is a setup to provide a center point neutral ...

I'd verify that before I'd spend any more brain power on it.

Generators dedicated to 3-phase are common for remote powering of large pumps and other large 3-phase loads, with no provision for a neutral. I'm especially suspicious because it's a Delta.

While you are at it, verify the voltage. Many of these are 480 V.

If you had provided a model number, we could help on the research.
 
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JHZR2

Member
Location
New Jersey
Occupation
Power Systems Engineer
Im skeptical. Precisely why I'm asking on here :) high leg configuration is the only way I figure they'll get this to work for home use. I haven't been able to look at anything on the electrical business end of this unit yet.
 

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MAC702

Senior Member
Location
Clark County, NV
Okay, now I'm with you! Forget all that stuff I said earlier.

I agree in that I think you are going to have some serious limitations with the majority of your loads being 120 V.

With no load on one phase, you will be limited to 20% of the total generator capacity.

Now, the homes likely have some 240V loads: AC's usually (just in time for winter), and maybe other things depending on if gas is supplied (oftentimes it isn't on an island.)

Make sure all the 240V loads are split between the high leg and the others, not between the two non-high legs, leaving more of their capacity available for your 120V loads.

But you won't be able to plan on any additional 120V capacity because you'll only have it while some 240V loads are running.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I would try to get more information from a manual before going any further.
The 120/240 spec combined with three phase makes me think of a configurable series versus parallel coil delta, which could mean that all 12 leads are accessible.
That would give you the option of a reduced capacity single phase configuration.
If it is a wye, you might not have all 12 leads, but you could use 208Y/120 to feed two legs to one house and a different pair to the second house, reducing the imbalance between phases.

Sent from my XT1585 using Tapatalk
 

MAC702

Senior Member
Location
Clark County, NV
...If it is a wye, you might not have all 12 leads, but you could use 208Y/120 to feed two legs to one house and a different pair to the second house, reducing the imbalance between phases.

If this works out, take the unused phase combination and power some big load right there at the generator. It's free power (pretty much) and will bring up the balance. Maybe some big azz incandescent halogen security lights or something.

Assuming the 2-pole loads in the homes can operate on 208V.
 

JHZR2

Member
Location
New Jersey
Occupation
Power Systems Engineer
Okay, now I'm with you! Forget all that stuff I said earlier.

I agree in that I think you are going to have some serious limitations with the majority of your loads being 120 V.

With no load on one phase, you will be limited to 20% of the total generator capacity.

Now, the homes likely have some 240V loads: AC's usually (just in time for winter), and maybe other things depending on if gas is supplied (oftentimes it isn't on an island.)

Make sure all the 240V loads are split between the high leg and the others, not between the two non-high legs, leaving more of their capacity available for your 120V loads.

But you won't be able to plan on any additional 120V capacity because you'll only have it while some 240V loads are running.

so this is where the real challenge lies. This connects to the one (main) home with a transfer switch. Utility power on one side, genset power on the other. One set of mains goes from that transfer switch into the home - standard two hots and a neutral. There's no way to allocate 240v loads onto the third phase separately (that I can tell) and keep a standardized configuration at the transfer switch and main panel in the home.

Doesnt seem like there's a ton of utility from this...
 

JHZR2

Member
Location
New Jersey
Occupation
Power Systems Engineer
I would try to get more information from a manual before going any further.
The 120/240 spec combined with three phase makes me think of a configurable series versus parallel coil delta, which could mean that all 12 leads are accessible.
That would give you the option of a reduced capacity single phase configuration.
If it is a wye, you might not have all 12 leads, but you could use 208Y/120 to feed two legs to one house and a different pair to the second house, reducing the imbalance between phases.

Sent from my XT1585 using Tapatalk

I do think you're on to something there. I hope to have more info before long. But your comment is what drove me to first post this - you said "reduced"... any idea what the typical defeating basis is? Is it related to ampacity in the windings or something else?

thanks!
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
My google fu is not working in this case. The closest I can find is:
http://www.pmtech.org/document_center/IDLC_Operators_Manual.pdf

Unfortunately this does not list the IDLC 20 (only IDLC 30 and above) and does not give alternator connection diagrams.

_If_ the alternator has 12 leads, then you can use the 'double delta' connection for 120/240V split phase loads. You don't get full alternator capacity but you get a reasonably large fraction. See:
https://www.schneider-electric.us/en/faqs/FA229253/

Good luck!

-Jon
 

kwired

Electron manager
Location
NE Nebraska
If this works out, take the unused phase combination and power some big load right there at the generator. It's free power (pretty much) and will bring up the balance. Maybe some big azz incandescent halogen security lights or something.

Assuming the 2-pole loads in the homes can operate on 208V.
More load means prime mover uses more fuel, which probably isn't free.
 

Smart $

Esteemed Member
Location
Ohio
As I read through this thread I was wondering if someone was going to mention this configuration. :cool:
PS: There is no gain in split-phase power output capability by reconfiguring to double delta. The gain is that the generator will handle an unbalanced load much better.
 
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