Single Phase Transformers in Parallel

[philly]

If I have (3) 15kVA 480V to 120/240V single phase transformers connected in parallel would I have a total capacity of 45kVA. I am almost positive I would but just wanted to double check this parallel connection.

 

[LarryFine]

If I have (3) 15kVA 480V to 120/240V single phase transformers connected in parallel would I have a total capacity of 45kVA. I am almost positive I would but just wanted to double check this parallel connection.

They must be identical, and even then, there will be differences that cause a difference in load sharing, so you won't have a reliable 3 x 15Kva available.

You should definitely get whatever blessings you can from the manufacturer for this. I take it you've already looked into separating the loads to use the secondaries separately?

 

[philly]

They must be identical, and even then, there will be differences that cause a difference in load sharing, so you won't have a reliable 3 x 15Kva available.

You should definitely get whatever blessings you can from the manufacturer for this. I take it you've already looked into separating the loads to use the secondaries separately?

This is actually an existing installation here at our plant. Each unit has a namplate on it stating it is a 15kVA unit. Three of these are connected in parallel. I was just wondering theoretically (ignoring any differences) if I have 45kVA with the three in parallel?

All the markings on the gear as well as he primary breaker are indicate a 15kVA unit, however these feed a UPS so I believe that everything is based on 15kVA and the other two units in parallel are backup. This is a SOLA hevi duty transformer. I dont think the parallel operation has anything to do with the signal conditioning these SOLA units provide?

 

[weressl]

This is actually an existing installation here at our plant. Each unit has a namplate on it stating it is a 15kVA unit. Three of these are connected in parallel. I was just wondering theoretically (ignoring any differences) if I have 45kVA with the three in parallel?

All the markings on the gear as well as he primary breaker are indicate a 15kVA unit, however these feed a UPS so I believe that everything is based on 15kVA and the other two units in parallel are backup. This is a SOLA hevi duty transformer. I dont think the parallel operation has anything to do with the signal conditioning these SOLA units provide?

Yes, you have (a) 45kVA three phase transformation. (I am presumptious here with the three pahse connection though, you may be talking about single pahse, in which case I don't think there is an issue either.)

Small impedance differences will cause circulating currents between the different units. As long as they have the same kVA, voltage ratings both on primary and secondary, and tap settings, you should be OK. If they are special transformers, like isolation and regulating transformers, you better veify it with the manufacturer and they may advise you what features the interconnection may degrade or jeopardize.

If it is conneeted as a three phase transformer single three phase OCPD's can be utilized on both primary and secondary sides as needed.

 

[philly]

Yes, you have (a) 45kVA three phase transformation. (I am presumptious here with the three pahse connection though, you may be talking about single pahse, in which case I don't think there is an issue either.)

This is a single phase connection. All 480V primary with 120/240V secondaries. All (3) units are connected in parallel with 120V/240V output to a panel.

 

[LarryFine]

I was just wondering theoretically (ignoring any differences) if I have 45kVA with the three in parallel?

As you asked the question, yes.

 

[weressl]

Yes, you have (a) 45kVA three phase transformation. (I am presumptious here with the three pahse connection though, you may be talking about single pahse, in which case I don't think there is an issue either.)

This is a single phase connection. All 480V primary with 120/240V secondaries. All (3) units are connected in parallel with 120V/240V output to a panel.

I don't think that will work. I think that you need to provide protection for the transformers individually and if you loose one then you can potentially overload the others.

 

[philly]

Yes, you have (a) 45kVA three phase transformation. (I am presumptious here with the three pahse connection though, you may be talking about single pahse, in which case I don't think there is an issue either.)



I don't think that will work. I think that you need to provide protection for the transformers individually and if you loose one then you can potentially overload the others.

These three units are actually provided with a single 2-pole 40A 480V breaker on the primary. This 40 breaker tells me that that we are protecting each 15kVA unit in the bank since 15kVA at 480V is 31.25A.

So looking at this now I'm thinking that although we have 45kVA avaliable we are protecting down to 15kVA (to protect each unit) and the other two units are just acting as stand-by units for backup.

 

[weressl]

These three units are actually provided with a single 2-pole 40A 480V breaker on the primary. This 40 breaker tells me that that we are protecting each 15kVA unit in the bank since 15kVA at 480V is 31.25A.

So looking at this now I'm thinking that although we have 45kVA avaliable we are protecting down to 15kVA (to protect each unit) and the other two units are just acting as stand-by units for backup.

That would be rather silly.

If there is a fault all three transformers are out of service, the three tranformers will increase the fault level vs. a single and since they are on-linetogether, one's fault can affect the others. So now you have to troubleshoot three transformers instead of one, to make sure you isolated ALL faults.

 

[philly]

That would be rather silly.

If there is a fault all three transformers are out of service, the three tranformers will increase the fault level vs. a single and since they are on-linetogether, one's fault can affect the others. So now you have to troubleshoot three transformers instead of one, to make sure you isolated ALL faults.

This is how they are indeed connected in the field. The output of the 3 units feeds a UPS which in turn feeds a 120/240V panel.

We actaully had a fault this weekend that was caused by lightining and damaged (smoked) one of these 15kVA units. (I guess that leaves us with 30kVA) We un-wired damaged unit and the other two units are operating fine.

Like I mentioned these are SOLA Hevi-Duty CVS transformers, so I dont know if the parallel nature of the three units has any effect on the operation of the transformers.

 

[philly]

Just for clarification purposes theoroetically 3 single phase 15kVA transformers connected in parallel will give a total of 45kVA single phase.

I'm assuming the same goes for three phase transformers as well. If three 15kVA 3-phase transformers (5 kVA windings) are connected in parallel it will give a total of 45 kVA three phase.

 

[iMuse97]

If I have (3) 15kVA 480V to 120/240V single phase transformers connected in parallel would I have a total capacity of 45kVA. I am almost positive I would but just wanted to double check this parallel connection.

I was thinking other posts have already covered the issue, but just now I was on the SquareD website and noticed that they permit their equipment to be paralleled if the impedances and ratios are equal, even when the kVAs are not equal. See http://ecatalog.squared.com/pubs/Electrical Distribution/Medium Voltage Transformers/7400DB0701.pdf

 

[weressl]

This is how they are indeed connected in the field. The output of the 3 units feeds a UPS which in turn feeds a 120/240V panel.

We actaully had a fault this weekend that was caused by lightining and damaged (smoked) one of these 15kVA units. (I guess that leaves us with 30kVA) We un-wired damaged unit and the other two units are operating fine.

Like I mentioned these are SOLA Hevi-Duty CVS transformers, so I dont know if the parallel nature of the three units has any effect on the operation of the transformers.

Bad design, actually increases the probability of failure, so in effect defeats the purpose of the UPS.

 

[philly]

Bad design, actually increases the probability of failure, so in effect defeats the purpose of the UPS.

Are you basising this off of the reasons you gave above or are there others?