Parralleling transformers

[chaterpilar]

I have a 5 Mva 33kv>13.8 kv 60 hz ( 8.03% impedance) stepdown transformer with Dyn11 vector group used in main incomer supply, with a peak load of 3.8mva.

We are expanding the unit and hence expected load will be 5.5 Mva.

Now, there was proposal to buy a 7Mva transformer of similar specs.

The other option is to use a 5 Mva ( same specs but 8.23% impedance, lying in stock it is used,and is in good working condition) in parrallel to existing 5Mva transformer.

Which one would be a better option ?

 

[mayanees]

split bus

split bus

I think I would split the downstream 13.8kV distribution and add the 5 MVA xf to the system, with a NO tie breaker between the 2 secondaries, kirk-key interlocked so they could not be paralleled.
JM

 

[wasasparky]

How long would you have 5.5MVA on the 5MVA transformer?
I would think short overloading would not be a problem.

Is there an option for additional cooling for the transformer? i.e. fans...

The %Z's of the transformers are close enough that operating them in parallel you can handle 9.9MVA (not that capacity is an issue). Your pair of transformers would be half loaded (at peak), so might want to look at the economic impact of light loading...

 

[charlie b]

I see three possible problems. It is possible that none is significant. However, I believe strongly that an effort must be made to determine if any is significant. This is an engineering task; that is, I believe an engineer familiar with fault studies and network connections (I don't know if you have that skill, but it's been a while since I have dealt with this type of design) needs to evaluate the situation, before you put the two transformers in parallel.

Issue one is that with a second transformer in parallel with the first, the amount of fault current that can be generated is much higher (nearly double) the amount of fault current that is available now. Can the existing gear, including the breakers, handle the higher fault current?

Issue two is that if there is a fault upstream of one of the transformers, there will be fault current supplied to the fault point from the other. You will need to install a “network protection scheme,” essentially a reverse power relay at the secondary of each transformer, to protect each transformer from excessive fault current coming from the other.

Issue three is that there will be current circulating between the two transformers, given especially that they are not identical. Point in fact, no two transformers are really identical and the loads will never be perfectly balanced between the two, so that any two in parallel will exchange current between themselves, in addition to the current they supply to the loads. Since these two are known to be different, it is possible that the circulating current may cause additional heating, which in turn may or may not result in damage.

 

[chaterpilar]

Thanks for all who put in the views.

Wasa, the peak load will be for almost 8 hours. Fans are not there but can be mounted if desired.

I am thinking of going for a 7 mva transformer.

Thanks again.