Transformer Secondary Breaker Sizing

[kvramesh]

Hello, I am wondering whether we size the breaker on the secondary side of the transformer based on the loading or the full load capacity of the transformers.

I have a 75/100/125 MVA and the secondary is 26.4KV. yes, these are huge transformers. I do not know the cable ratings. So, is it a practise to size the breaker based on the base rating of the transformer (75 MVA in this case) or based on the fan rating (125MVA is the worst case here)? I know sizing for 125MVA is a safe bet but again it costs more and some times looks over commital.

Thanks for any help

 

[chaterpilar]

You have to size for 125 Mva.

 

[don_resqcapt19]

The secondary conductors and OCPD are sized based on the load. There is no reason to match the capacity of the transformer if you do not have that much load.

 

[kvramesh]

Thanks for the responses. Unfortunately it is some else who is sizing the conductors and the information is unavailable to me at this time. I think I should either know the conductor sizes or the running loads to make a decision. Thanks

 

[jatrottpe]

You also have to deal with the downstream equipment rating. Switchgear bus rating, short circuit rating, etc.

Assuming these transformers are the gateway to a fairly large facility, you do not want to come back and have to change out equipment that can not handle the future loads.

You mention transformers - Are there two each with 50% of plant loading. Is it possible that if one transformer comes down for preventative maintenance the other transfomer picks up the difference, If so you need to consider this loading on the transformer and downstream equipment

If this is a large scale infrastructure project the last thing you will want to do is explain to management why it needs to be replaced in a few years due to it being undersized. A wise person once told me, when you are value engineering a project do not include infrastructure in the value engineering as there seems to never be enough.

PS -Our facility has three 100MVA transformers (230kv-27kv) and our site loading is growing by about 2MW per year. All of our equipment is based on future forced oil cooling of the transformers.

 

[kvramesh]

You are correct. I have two transformers supplying a huge facility. One transformer will run on the fans in case of supplying the entire load by itself when the other one is under maintenance.

As you said the future loading is definitely an issue and thanks for your existing plant information and sizing details. I better size my OCPD per the trafo capacity.