Loading 3 phase transformer with unbalanced load

[jmomic]

We have to supply 120/240 VAC distribution panel inside the trailers from 3 phase 600V 3 wire (no neutral system). Power requirement for each trailer is estimated to be around 50 to max 60 A during winter. The loads are space heaters, air conditioning units in summer and other regular "office" loads.
Is it possible to use three phase 600/ 120-208 (120-240) transformer and supply 4 or 5 trailers from one 75 kVA 3 phase transformer. We have Primary D and Secondary Y transformers.
What would be the effect of the unbalced load to the trasnformer?
What will happen with the voltages on secondary side of the transformer due to the extremely unbalanced load (more then 30% current unbalance)
How would the unbalnce effect the AC (air Conditioning) unit, computers and other office load? I do not see that baseboard heaters would have any issue because of the unbalances.

If our idea is not right please suggest the better one.

Thanks in advance for help

 

[SceneryDriver]

Install 3-ph breaker panels and supply 3-ph, 4-wire 120/208 power to each trailer. Balance the loads across the three phases as best you can in each trailer's panel during construction.

Your "240v" loads will run at 208v, and so things like baseboard heaters will run approx. 13% cooler. Office loads won't care, as they're practically all 120v. AC motor loads ~should~ be ok; check with the manufacturer to ensure that the units will be happy running off 20v instead of 240v.

If, once in operation, you amp readings show a phase is too heavily loaded / the transformer is too unbalanced, "roll" the phases supplying one or more trailers to help distribute the loads more evenly among phases.

You're never going to get perfect balance; someone may like it cooler in one trailer than another. Just try to get reasonably close.

This sounds very similar to temp power hookups we do in the entertainment industry to supply power for lighting and sound (among other things) at concert venues. Try balancing loads across phases when the majority of you loads are non-linear lighting dimmers, HID ballasts, and high-wattage SMPS power supplies running PA power amps


-SceneryDriver

 

[jmomic]

Thanks SD
I like your idea but I have a small problem as we are renting the trailers and they are already equiped with the panel (120/240).
It was even requested that the trailers should be aproved by ESA (Authority in Ontario) so we are not able to touch the panels.

We are going to have 10 or 11 trailers and I am wondering what would happen if I choose 2 out of 3 phases per trailer. I can supply 6 out of one distribution panel which would be supplied from 3 phase 600/120-208 Dy 75 kVA transformer (Maybe we can get 600/120-240 transformers). I am wondering what will happen with another transformer and load on it as I am not able to balalnce the load. Having 4 or 5 trailers supplied I am introducing more then 30% current inbalance by design. Who knows how it will be on site and who will open the doors and when as you said.

I feel that something is not right there but I am not able to confirm either way. All my loads inside the trailers are single phase 120 V or 240 V. Due to the unbalance voltages will be off to neutral and between phases. I am not sure that neutral will move that much in order to prevent equipment to operate or to be damaged in case of overvoltage. Not sure for this as well.

I am also not sure how will the unbalance effect the transformer. It will heat up more. How much more is the question. I am not loading it to the max so this would help too. When I am thinking more I am getting more confused and worried how it is going to work.

 

[waross]

Balance the loads as well as you can and don't worry. It is common practice in apartment buildings to wire each apartment with a 120:240 volt single phase panel and supply the panels from 120:208 Volt three phase services. The vast majority of occupants don't even see a difference. Check the name plates on the A/C's. they are probably rated for 200-230 Volts or 208:230 Volts.

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

 

[stevenal]

Assuming a constant resistance, power varies with the square of the voltage. Resistance heat will run 25% cooler, or take 25% longer to get to set point temperature. I agree this is common practice. If the services are individually metered, you will need five lug meter bases. Voltage should stay balanced because the primary delta connection provides a path for the unbalanced current.

 

[rcw retired EE]

On a construction project in Toronto, under the ESA inspection authority, we supplied the rented trailers' 120/240V panels with 120/208V from common 3-phase 600V- 208/120V transformers. First trailer was connected to phase A,B&N, second to B,C & N, third to C,A & N and so on. The load was balanced well enough. It worked for two years feeding 20-25 trailers. Transformer sizes were 75 kVA and 300 kVA. Each transformer had a distribution panel or group of fused switches to protect the cables feeding the trailer panels. The AC units in the trailer were dual rated for 208 or 240V.


Unbalanced loading should not be a problem.

 

[jmomic]

Thanks for help.
rcwilson can you please tell me how many trailers you supplied from one 75 kVA transformer?