5% Single Phase Capacity on Center Tapped Delta Transformer secondary

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mull982

Senior Member
Can someone explain or point me to a good reference as to why the 120V single phase loads on a center tapped delta transformer secondary are limited to 5%? From what I have seen it based on circulating currents in the delta winding caused by unbalanced loads on the neutral of the transformer.

I'm looking at a particular application where this type of transformer is going to be used and have aprox 20kVA of single phase loads that will be added to this 480-240/120V transformer. Does this mean that the minimum transformer size that needs to be used is 400kVA? This seems really large for only 20kVA of single phase load. Total kVA connected to transformer is only 45kVA with about 20kVA on the center tapped winding (10kVA on each side of tap) Originally I thought a 75kVA transformer would cut it with about 12.5kVA on the untapped phases and 20kVA on the center tapped phase but now with the 5% rule its looking like I need a much large transformer.

Should I stick with a center tapped delta, or use some other winding configuration where circulating currents are not an issue.
 
M

mull982

Senior Member
Smart $ said:
Are all your loads single phase? Any three phase?

Numbers please...
Click to expand...

Ok so this particular application is to replace a transformer that feeds auxiliary loads in a diesel emergency generator (lube pumps, fuel pumps, heaters, lighting, etc...).

There are a couple of motors that are all three phase and are rated either 3/4hp or 1hp. (2) of the 1 HP motors have a nameplate rating of 208-220/440V the other 1HP motor has a nameplate of 220/440V while the 3/4HP motor has a nameplate of 230/460V. The heater has a nameplate rating of 240-277V while the lighting and receptacles are obviously rated at 120V.

There are four units that have these loads so the quantity of loads above would be multipled by four to determine the total load on the transformer. The original transformer is a 225kVA Center Tapped Delta Secondary in order to provide the 240V and 120V voltages for the equipment listed above. When we performed the load calculation for the loads mentioned above we calculated approximately 26.3kVA of motor load, 9.6kVA of lighting and rect load, and 8.4kVA of heating load for a total of 44.3kVA.

I originally thought based on this load calculation that a 75kVA transformer would cut it and leave plenty of room for future addition. When I Look again however I notice that the centertapped winding will have 21.2kVA connected to it with the motors and single phase loads and thus wont have much spare room at all with the winding being rated 25kVA of the 75kVA transformer. I also then came across some stuff that talked about having the single phase loading not more than 5% of transformer kVA in order to avoid circulating currents. In this case with 9.6kVA of single phase L-N loads the transformer would have to be aprox 192kVA in order for the single phase loads to only be 5% so maybe that's why 225kvA transformer was chosen in the first place and its not overkill.

Based on what I discussed above I was looking for some transformer size guidance in relation to the single phase loads and the 5% rule and any information that could explain why 5% is the limit.

Of course the other option is to provide a 480-208/120V transformer with a wye secondary winding. The 208V would be fine for the 1HP motor which has a nameplate of 208-220/240V but I'm not sure about the ones listed at 220/240V and 230/460V? Would these be o.k. to operate at 208V? 208V is still within 10% of the motor nameplate rating for both of these motors so maybe this is not an issue since I believe 10% is the minimum per NEMA standards. I don't think the heater would be a problem operating at 208V either, and would simply just supply a little less heat. Obviously a wye secondary would be an easier solution but wanted to make sure it would work with these motors.

Thanks
 
kwired

kwired

Electron manager
Location
NE Nebraska
What is the unbalanced neutral load? You can supply 240 volt single phase loads, you can supply 120 volt loads balanced across the neutral and they are seen no differently then 240 volt loads to the system.

I think if you have a 75 kVA transformer that 5% means the neutral needs to be carrying 3750 VA (5% of 75kVA) or less which is 31.25 amps @ 120 volts. Arrange your 120V circuits to have no more then 31.25 amps of imbalance, if you don't think you can do it - then consider a larger transformer.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150922-1204 EDT

mull982:

Consider an open delta and size the two transformers as needed. Or consider a closed delta with three transformers sized as needed.

.
 
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mull982

Senior Member
gar said:
150922-1204 EDT

mull982:

Consider an open delta and size the two transformers as needed. Or consider a closed delta with three transformers sized as needed.

.
Click to expand...

Dry type transformer is being used. Are the configurations that you mentioned above available in dry type transformers?
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150922-1622 EDT

mull982:

When you use single phase transformers in a three phase system to provide a three phase output you can use any kind of transformer that will work. You could use an air-core transformer if it would do what you needed to do, but for power transfer it won't. Depending upon the loading you can individually size the transformers based upon the loading.

.
 
M

mull982

Senior Member
Having to re-visit this one again after I was unable to convince plant to operate motors at a 120/208voltage. So I'm back to a 120/240 delta center tap.

Smart $ said:
The 5% center-tapped load limit has a foundation in older 3ph single-core transformers... but newer transformer designs have got the limit up to 30%.

Schneider agrees :angel:

http://www.schneider-electric.us/si...ang=en&locale=en_IN&id=FA101914&redirect=true
Click to expand...

The particular manufactures transformer we are using still lists 5% as the limit. Does this mean that if we use this particular transformer the total single phase kVA can only be 5% of the total transformer kVA?

I want to make sure this transformer is adequately sized, but also want to avoid overkill with the size. A 225kVA transformer for a calculated load of 45kVA just seems like overkill at face value.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
mull982 said:
Having to re-visit this one again after I was unable to convince plant to operate motors at a 120/208voltage. So I'm back to a 120/240 delta center tap.



The particular manufactures transformer we are using still lists 5% as the limit. Does this mean that if we use this particular transformer the total single phase kVA can only be 5% of the total transformer kVA?

I want to make sure this transformer is adequately sized, but also want to avoid overkill with the size. A 225kVA transformer for a calculated load of 45kVA just seems like overkill at face value.
Click to expand...
As noted earlier, the 5% limit is the current on the tap, i.e. unbalanced neutral current. It is not the total single phase load limit.
 
Besoeker

Besoeker

Senior Member
Location
UK
mull982 said:
Can someone explain or point me to a good reference as to why the 120V single phase loads on a center tapped delta transformer secondary are limited to 5%? From what I have seen it based on circulating currents in the delta winding caused by unbalanced loads on the neutral of the transformer.
Click to expand...
Delta with a neutral?
 
M

mull982

Senior Member
Smart $ said:
As noted earlier, the 5% limit is the current on the tap, i.e. unbalanced neutral current. It is not the total single phase load limit.
Click to expand...

Ok so If my 120V L-N loads on C1 and C2 (assuming C winding is center tapped) are completely balanced (4800VA on each) then the neutral current would be 0A and therefore I do not need to increase the kVA rating of the transformer above any beyond what was calculated with load calcs? I only need to consider the L-N loads for and not any 3-Phase or L-L single phase loads correct?

If the C1 and C2 windings have un-equal loading resulting in a neutral current, this seems to be when that neutral current converted to a kVA value cannot be greater than 5% of total kVA rating. Is this correct?

Even though our design calculation shows zero neutral current, what is typically done to prevent the customer from adding single phase loading that will later cause the neutral unbalance to be greater than 5%? Is the transformer typically oversized as part of the design to account for this, or is a note provided that the customer shall limit unbalanced load current to less than 5%?

Thanks.
 
kwired

kwired

Electron manager
Location
NE Nebraska
I would assume that 5% is 5% of total transformer kVA rating so 5% of a 45kVA unit would be 2250VA which would be 18.75 amps @ 120 volts.

Might be a little tricky to assure you don't go over that much neutral load unless the load is pretty constant in nature.

Would be easier to stay within a newer unit that has 30% neutral load limit though that puts you to 112 amps - the full load rating on the grounded conductors of a 45kVA unit is only 108 amps to begin with
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151315-1615 EST

If you have an open delta, then there is clearly no problem. Around here two transformer open delta systems usually have a large single phase transformer, and possibly a transformer 1/2 that size for the wild leg. If the three phase load increases DTE simply adds a third transformer making it a closed delta. There is still a very large single phase load.

So is the 5% criteria primarily related to a true three phase common core transformer?

.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
mull982 said:
Ok so...
...
...unbalanced load current to less than 5%?
Click to expand...
I cannot answer any of your questions with certainty. I have only been on the consumer end of the matter, same as you. The more you read, the more likely you will find the limit stated somewhere as 5-10%. I have never found a treatise on the matter... only the 5% limitation and circulating current explanation.
 
M

mull982

Senior Member
kwired said:
What is the unbalanced neutral load? You can supply 240 volt single phase loads, you can supply 120 volt loads balanced across the neutral and they are seen no differently then 240 volt loads to the system.

I think if you have a 75 kVA transformer that 5% means the neutral needs to be carrying 3750 VA (5% of 75kVA) or less which is 31.25 amps @ 120 volts. Arrange your 120V circuits to have no more then 31.25 amps of imbalance, if you don't think you can do it - then consider a larger transformer.
Click to expand...

I went back and looked at my calculations again. I have a total of about 35kVA in three phase loads and a total of 9.6kVA of single phase loads which are balanced evenly (4800VA each) on the C1 and C2 windings so that there is no neutral current. Obviously in reality there will be some neutral current as the load draw on the L-N loads will not be perfectly balanced.

We are also allowing about 35kVA capacity worth of spare room on the transformer for the addition of future circuits a lot of which will likely be 120V L-N loads. My concern is how to address the possibility of these future loads causing the unbalance in the neutral to exceed 5% or 31.25A. I'm curious to hear how others would approach this. Would you simply tell the end user that all future L-N loading must be balanced as to not exceed 31.2A of neutral current and put this burden on them, or would you select a larger transformer on the front end to allow a little more margin for this unbalance not knowing what the future loading will be? If a good option is to choose slightly larger transformer, I'm curious to hear other's logic on what a good size would be for an adequate safety margin again not knowing the future.

kwired said:
I would assume that 5% is 5% of total transformer kVA rating so 5% of a 45kVA unit would be 2250VA which would be 18.75 amps @ 120 volts.

Might be a little tricky to assure you don't go over that much neutral load unless the load is pretty constant in nature.

Would be easier to stay within a newer unit that has 30% neutral load limit though that puts you to 112 amps - the full load rating on the grounded conductors of a 45kVA unit is only 108 amps to begin with
Click to expand...
Not sure I follow how you arrived at a full load rating of 108A for the grounded conductor on a 45kVA unit?
 
kwired

kwired

Electron manager
Location
NE Nebraska
mull982 said:
Not sure I follow how you arrived at a full load rating of 108A for the grounded conductor on a 45kVA unit?
Click to expand...
Typo error - I meant to say ungrounded. Full load three phase amps of a 45kVA unit is 108. I guess actual full load across one side of the delta if you have a single 240 volt load only would only be 15kVA or 62.5 amps.
 
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