Sizing 120/240 Delta Transformer (Mixed Three Phase and Single Phase Load)

[jim dungar]

In Philly lots of customers got high leg delta services because you could run your old 2 phase equipment from it. One phase was a-c and the other was n-b. Motors were 220v. I guess if you wanted to do it right you could use a boost/buck to even out the voltages, but I never saw it that way. Never really saw the voltage imbalance be a problem either, but then again most of those motors were nowhere near continuously loaded

Are you sure they were using a center tapped coil on a 240/120V delta? The A-C voltage would be lower than the B-N voltage with an angular displacement less than 90°. I would expect some severe motor performance reduction with this connection.
A Scott-Tee transformer connection easily converts a three-phase system into two-phase without the motor performance issues.

 

[Joethemechanic]

Yeah I'm sure. A-C is 240 and N-B is 208. Nobody wanted a Scott - T because then you couldn't run your 3 phase equipment. This way you could still run your 2 or 3 old pieces of 2 phase equipment and the majority of your load that was 3 phase.

Not to mention if it was a closed delta, the neutral- B phase load was shared on the center tapped transformer lessening the transformer impedance problem of having a N-B load

 

[Sberry]

. As long as the terminal voltage provided stays in the green, they'd continue using undersized distribution transformers. I dare not hazard a guess.

This and then some.
This isnt the exact same scale but my 400 serve on a 15K. When they did it I said,, wow but they said if it doesnt work they will be back with a 25.

 

[Joethemechanic]

Using small transformers keeps low lagging power factor from becoming as much of a problem. It also lessens the available fault current. And really think about it, for example my home and office use about 30 kilowatt hours per day. That's less than what a continuous 15A 120V load would use in a day

 

[winnie]

Yeah I'm sure. A-C is 240 and N-B is 208. Nobody wanted a Scott - T because then you couldn't run your 3 phase equipment. This way you could still run your 2 or 3 old pieces of 2 phase equipment and the majority of your load that was 3 phase.

Not to mention if it was a closed delta, the neutral- B phase load was shared on the center tapped transformer lessening the transformer impedance problem of having a N-B load

Wow. I knew Philly had some strange electrical, but that takes the cake. Thanks for expanding my knowledge.

My guess is the motors were 4 wire 2 phase, where the two coil sets were completely independent.

3 wire 2 phase would mean the motors were being driven 3 phase (not using the neutral) or were being operated 120 and 208V.

If the motors were internally 5 wire my guess the neutral connection would really mess up the high leg source.

'T' transformers can be built with appropriate taps to provide both balanced 3 phase and balanced 2 phase. Did you ever see that?

Jon

 

[Joethemechanic]

'T' transformers can be built with appropriate taps to provide both balanced 3 phase and balanced 2 phase. Did you ever see that?

Yeah there are still a lot of 2 phase buildings in philly served by Scott-T transformers. One transformer has a primary tap at 86.6% the other one is standard.

All the 2 phase motors I saw were 4 wire. Even if they were hooked up to 3 wire 2 phase. BTW the 3 wire had one wire (the common) that had to be 1.41 times the ampacity of the other two

 

[JoeStillman]

Definitely a rare bird here..utility 240/120 3 phase padmount... any chance there is a misunderstanding ?

Probably two pad mounts.

The only time I couldn't get a 208Y/120V 3Ø 4W service was when the utility only had two phase legs available for the primary. Check Google Earth and see if there are 3 wires overhead. Then tell them you don't want their 4WD service.

 

[JoeStillman]

The Need:
1000 AMP SES (600 Three Phase and 400 Single Phase)
Based on my calculation, I am thinking that I have demand load of (343.8kVA) (This total three and single phase load combined) which means I need 300kVA

You can't get a 4WD service that size in PECO territory. They limit services like that to 15 kVA max on one phase leg.

 

[Sberry]

Yes, I understand why they want a small tranny. My demand is about like a larger household maybe, I have a 3hp well and my biggest walk in cooler is 3 hp although not currently using it as I have a 1 also and while I have 5 hp comp to meet demand or backup rarely use that as I have 3 with 200 gallons on it. Same foir welding, have a 300 synch I never use and the rest is factory cord common 50A service with about 40 peak with that and very intermit with 1/2 that on machine uses 23 or less. Just no big cruncher loads,
It would be interesting to see what the peak was, pressure wash at 5 hp when the cooler was running, well and air comp on a small ac in office, wonder if it ever hit 100 and for how long?
Max,,, maybe 3 or 4 minutes under extreme circumstances a time or 2 over decade.
The assumption about demand is wayyy off with the speculators. The diy forum has many like,,,, should I upsize to 10 for my loooooooooooong run,,, almost 25 ft and I might use a 4 1/2 grinder and a light bulb at the same time and what if my bro in law is standing there with a beer,,, well then my load will rocket. Its really hard concept especially with general circuit to grasp because it has a 20 breaker and 100 ft of wire that isnt actual load, first recept 15 ft away is doing 90% of the work so we worry about v drop on the other end for 20A and 1 lamp or zing with a circ saw.
In the end 200 would have been dandy and would have simplified any backup power, 2 panels 1 meter base is a problem.
Given the nature of my buildings etc would have figured 25k if I was doing the calc but its never been a problem.
I did make a move with the pole that required a wire change and I actually negotiated with the engineer at plan review over the conductor size and the lineman made a comment about it,,, the old wire would have been fine but was now 10 short and they bring a new wire in,, and was 500 mcm maybe where the old might have been 2/0, its been a while now.
I figured efficiency but hadnt occurred to me especially at the time about fault current, its a long rural line single phase, 14000 I believe. They had 10hp limit till recent soft starts, not sure what it is now.

 

[Engser18]

Greetings All,
I am trying to size/validate the rating on the medium voltage transformer for a project. For some reason I can't find many resources that indicate how to properly do this when you have mixed loads (Three Phase and Single Phase). What Size transformer will be needed?

The Need:
1000 AMP SES (600 Three Phase and 400 Single Phase)
Based on my calculation, I am thinking that I have demand load of (343.8kVA) (This total three and single phase load combined) which means I need 300kVA

I am planning to use 120/240 Padmount Delta Transformer

Any guidance would be greatly appreciated!

You should check with power company, they're phase out 240/120 V 3 phase, open delta in my area. You may end up re-design.

 

[topgone]

I dont think I have never seen this prohibition in writing.
The typical reason is the problem with finding 1pole 208V rated breakers.
This connecion would also put loading one of the 240V legs as well as one of the 120V windings thereby increasing the apparent load on the transformer with a resultant impact on voltage regulation.
In the end a 240:208V buck- boost transformer is usually more cost effective.

O/T:
The idea of limiting the single-phase loads on delta-connected three-phase supplies is to limit the circulating current inside the delta. I played around with figures to determine the sweet spot where the circulating current is "acceptable" (various organizations have varied "acceptable limits"). I saw that the percentage of circulating current wrt to the rated phase current is 1/2 of the percentage single-phase load wrt to the total system capacity.
Here are the figures I was playing with:

kVA = 150 kVA​

Secondary voltage = 240V delta​

%Z = 2%, X/R = 1.5​

Additional load (Single-phase load) one of the phases = 6 kVA (4% of total capacity)​

Circulating current = 4.2 A (2% of rated phase current)​

The value of the three-phase loads were reduced to make sure the phase current of the windings with the single-phase loads do not exceed the rated value (hence the total capacity is not exceeded) .
If one considers a 5% circulating current acceptable, the single-phase load limit should be around 10% of rated capacity. Please check if my numbers are correct.