Paralleling of 2 400 KVA transformer

[kikay0115]

I was asked if it is possible to parallel the 2-400KVA transformer(460/400V, YNAO) to supply a load of 500 KVA. Each transformer was supplying before a load of the same rating(400KVA). Is it possible to reuse the same cables both on the primary and secondary(200mm2) sides? If not, what cable should I use?

 

[Julius Right]

At first if you'll keep on both transformer the old load of 400 kVA, then the new one-if will be even divided between the transformers- will charge another 250 kVA [250/400*100=62.5%!!!!!].It depends of the regime-how much time will be the overload in action and how will be the actual load before overloading.
The continuous load burden[100% load, infinite time] depends on many factors as ambient cable run medium-air, earth-temperature of the ambient and many other.
I suppose the existent cable was well chosen-for 400 kVA load, short-circuit and voltage drop] and if the equivalent load[see: intermittent load -equivalent continuous load] is the same you have to check only the voltage drop-DV[will be old DV/2*1.625=0.82*DV- approx.].However a 10-15% unbalance I think has to be taken into consideration.

 

[vfd]

If the primaries of the 2 transformers are both connected on the same primary bus and their secondaries are connected on the same common secondary bus, then means parallel.

 

[charlie b]

You will have to take into consideration the impact of paralleling the secondaries on the available fault current. You will be essentially cutting the resistance in half, and doubling the fault current. So you need to know if the downstream equipment is rating for the new fault current.

 

[bob]

Prior to making the connection, you should verify that the %Z of both transformers is the same or close to the same value.

You should also verify that the phase rotation of the transformer is the same. This information is on the name plate. It looks like this

Group 1: Zero phase displacement(Yy0, Dd0, Dz0)
Group 2: 180? phase displacement (Yy6, Dd6, Dz6)
Group 3: -30? phase displacement (Yd1, Dy1, Yz1)
Group 4: +30? phase displacement (Yd11, Dy11, Yz11)

This is a good article that should help you http://electrical-engineering-portal.com/principles-of-transformers-in-parallel-connection-1

 

[GoldDigger]

Prior to making the connection, you should verify that the %Z of both transformers is the same or close to the same value.

You should also verify that the phase rotation of the transformer is the same. This information is on the name plate. It looks like this

Group 1: Zero phase displacement(Yy0, Dd0, Dz0)
Group 2: 180? phase displacement (Yy6, Dd6, Dz6)
Group 3: -30? phase displacement (Yd1, Dy1, Yz1)
Group 4: +30? phase displacement (Yd11, Dy11, Yz11)

This is a good article that should help you http://electrical-engineering-portal.com/principles-of-transformers-in-parallel-connection-1

I was trying to assign a meaning to the "YNAO" designation listed by the OP. Based on a memory jolt from your post, the best I can come up with is that it is actually Y na 0 where NA indicates "not applicable" meaning that they are wye autotransformers (460/400) and that as a result there can only be a 0 degree phase displacement.
Chances are good that if they were purchased at the same time for two different separate loads they are actually identical xformers.

 

[bob]

I was not sure what the YNAO meant. I don't think I have seen the NA on a name plate before.

 

[GoldDigger]

I was not sure what the YNAO meant. I don't think I have seen the NA on a name plate before.

Have you looked at the nameplates on any autotransformers? That would be the only place I would expect to see it, since there would be no secondary winding for the second letter in the string to refer to. Pretty clearly the O was really a zero, 0.

 

[kikay0115]

I was trying to assign a meaning to the "YNAO" designation listed by the OP. Based on a memory jolt from your post, the best I can come up with is that it is actually Y na 0 where NA indicates "not applicable" meaning that they are wye autotransformers (460/400) and that as a result there can only be a 0 degree phase displacement. Chances are good that if they were purchased at the same time for two different separate loads they are actually identical xformers.

It is an auto transformer according to my colleague(460/365-435), and we will be using the 400V tap. Can these autotransformer operate in parallel if the basic requirements are met?

 

[GoldDigger]

It is an auto transformer according to my colleague(460/365-435), and we will be using the 400V tap. Can these autotransformer operate in parallel if the basic requirements are met?

Yes, as long as they are set to the same tap it will no different from paralleling isolation transformers. So the same conditions mentioned by others will apply.

Are they currently fed from the same branch circuit right now? If you will be wiring them to the same branch circuit, connecting the two existing wires to the joined primary terminals of the two-transformer group could be considered running conductors in parallel, so those rules would apply.
If you use separate home-run connections without adding a bond between the two sets of primary terminals that would not be an issue but you may have some additional concerns about how well the load is balanced.

One the secondary side, if the pair will be feeding one single new load, the same concerns about parallel conductors would be present on that side too. Your conductors are large enough that paralleling is allowed, just make sure you follow the rules.

 

[kikay0115]

Yes, as long as they are set to the same tap it will no different from paralleling isolation transformers. So the same conditions mentioned by others will apply.

Are they currently fed from the same branch circuit right now? If you will be wiring them to the same branch circuit, connecting the two existing wires to the joined primary terminals of the two-transformer group could be considered running conductors in parallel, so those rules would apply.
If you use separate home-run connections without adding a bond between the two sets of primary terminals that would not be an issue but you may have some additional concerns about how well the load is balanced.

One the secondary side, if the pair will be feeding one single new load, the same concerns about parallel conductors would be present on that side too. Your conductors are large enough that paralleling is allowed, just make sure you follow the rules.

The transformers will be fed from the same circuit. In fact sir, I am planning to use only one circuit breaker for both transformer. I will just double the ampere rating of the CB from 600A to 1200A. What if my autotransformer is 3 phase? Can I still cannot it in parallel?