Large Single-Phase Load on High-Leg Delta

[machodelmonte]

Hi all,

New to the forum, thanks in advance for any help!

I am running a diesel micro-grid selling pre-paid power, with smart meters, to a remote town in Haiti. (Check out the project here: www.sigorahaiti.com or a video of what we do here) The grid serves 700 small customers with lines we have run throughout (we have a concession from the Haitian government to operate as a private utility). While we plan to eventually move to a 3-phase grid with medium (13.2kV) distribution, we are currently powering the town with 120/240 since the distances are short and loads are low.

We are currently powered by two generators, a 25kW and a 35kW, both putting out 120/240 split phase. However, the 35kW is getting too small to handle the large loads at night, which sometimes peak at 37kW, and growing all the time. We are about 2 months away from ordering two 3-phase 100kW, installing transformers and high-voltage lines. To tide us over I would like to rent a genset from the local Caterpillar dealer. They can offer me either a 40kW monophase genset, or a 75kW tri-phase genset. They tell me that over 40kW they do not have any monophase gensets.

The CAT people tell me that they can offer the 75kW "wired 120-240", which I am assuming means high-leg delta (no one seems to know for certain), but say that that would restrict the load to 50kW. My initial reaction is that loading a high-leg delta connection up that high, with no 240-loads whatsoever, would be bad for the alternator.

So my question is: is there a way to wire 3-phase as '120/240' other than high-leg delta? And secondly, can it be healthy for the generator to run exclusively 120V single-phase loads close to the full rated capacity of the genset?

Thanks much!

 

[ggunn]

So my question is: is there a way to wire 3-phase as '120/240' other than high-leg delta?

I'm pretty sure that the high leg wiring is the only way to get 240/120V from 240V three phase. If you wire it wye the phase to neutral voltage would be 139V. You could, of course, transform the 240V 3P to 208V 3P, which would make all your phase to neutral voltages 120V but you'd have no 240V available.

 

[Smart $]

I believe they a referring to wiring the genset output as 120/240 1Ø 3W by way of a double-delta or zig-zag winding configuration... assuming a 12-lead generator.

http://manuals.chudov.com/12-Lead-Zigzag-Generator-Connection.pdf
https://www.google.com/search?q=double-delta+or+zig-zag

67% of the generator 3Ø rating will not hurt the generator.

 

[kwired]

What do you have for 240 volt loads? If you can get by with 208 on those loads then going with a 208/120 would likely be the best way to go. Otherwise maybe use transformers where needed to be able to utilize the high leg? If doing that though maybe consider higher voltage for voltage drop reasons as your transformers possibly can do 240 or 480 primary anyway.

Or make that leap to the 13.2 KV sooner instead of later, and save on purchasing equipment that won't be needed after that change is made.

 

[machodelmonte]

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Followup info

Thanks y'all for the quick replies!

@Smart $: Those links were interesting - I think you are right they may have done one of these non-traditional windings - but really I need to open it up and see for myself before moving forward.

@kwired: We have no 220 or 240 loads whatsoever, all 120 single phase. That is what worries me about loading up the single phase without using any of the line-to-line voltages, since I am afraid it isn't good for the alternator. But if it's zig-zag or double delta, sounds like it is OK, but just means I cannot use the fully rated capacity.

@ggunn: Yes we definitely could split the grid into 3-phase 120/208, and power 3 separate zones of town, since the genset is near the center of town. However, I prefer not to do this since our backup genset is still putting out 120/240, meaning I can't just throw the double-throw and connect it, instead I have to do some re-wiring.

 

[kwired]

So look at cost difference between a third of capacity of a oversized Delta generator vs. being able to use all of a smaller wye generator plus the needed three phase switch.

Then also consider the (most likely lower) efficiency of the larger but not fully utilized prime mover on the delta generator.