MV Transformer Primary Protection

[Richard S]

We are looking at installing a dry type 3000/3990 AA/FA kVA 21kV primary x 277/480V secondary medium voltage transformer. The drawings show load break switches without protection on the primary side of transformers. These switches/transformers are fed from 21kV switchboard with MV VCB's and protective relays. Based on NEC Table 450.3(A) the maximum setting for a Circuit Breaker for primary protection is 600% of the transformer rated current. My question is do I base this 600% on the base rating of the transformer (3000kVA) or can it be based on Fan Assisted rating of the transformer (3990kVA)? The 21kV VCB's and protective relays are set for 600A (they feed multiple transformers). Based on 3000kVA rating, the transformer input rating would be 82A and 600A breakers would exceed 600% of rating. But if the FA rating of 3990kVA can be used, the input rating would be 110A and then the 600A breakers would comply with 450.3(A) for primary overcurrent.

Thanks

[Wire-Smith]

Fan assisted rating if it is fan assisted.
Question, are you going to size conductors for the 600% rating. And why would you use 600% when you have a relay? Is it a Schweitzer? Why not just leave room for in rush and then protect everything tighter?

[Richard S]

Fan assisted rating if it is fan assisted.
Question, are you going to size conductors for the 600% rating. And why would you use 600% when you have a relay? Is it a Schweitzer? Why not just leave room for in rush and then protect everything tighter?

The relays are SEL751A and each relay has the potential to feed (3) transformers. We are not the EOR for this project, but are trying to provide some guidance. The drawings actually show 1200A circuits (3x500kcmil AL MV-105 25kV per phase) if you can believe that. I think they did that because the VCB's are 1200A rated and not knowing that the relay settings will dictate the overcurrent values. Thats the only think I can think of. That's 43MVA of conductors for each circuit when you have a maximum of 12MVA of connected load.

Do you know where it may state about using the fan assisted rating? It make sense to me, but I would like to have some back-up. I looked through the RED book (IEEE-141) and a few application guides, but could find anything specific to that.

[Wire-Smith]

The relays are SEL751A and each relay has the potential to feed (3) transformers. We are not the EOR for this project, but are trying to provide some guidance. The drawings actually show 1200A circuits (3x500kcmil AL MV-105 25kV per phase) if you can believe that. I think they did that because the VCB's are 1200A rated and not knowing that the relay settings will dictate the overcurrent values. Thats the only think I can think of. That's 43MVA of conductors for each circuit when you have a maximum of 12MVA of connected load.

Do you know where it may state about using the fan assisted rating? It make sense to me, but I would like to have some back-up. I looked through the RED book (IEEE-141) and a few application guides, but could find anything specific to that.

SEL is schweitzer, i find it ridiculous to run 600% ocpd, just turn up instantaneous if anything, is there an EOR?

Your transformer fan rating will be in the transformer manufacturer literature, i don't know of anything in the code book, IEEE C57.91 might talk about it. At the end of the day if your using the manufactures fan assistance, that is the rating. just like a vented transformer has to have proper ventilation to run at its rating. although designing for fan assist is busch league, i recommend only using them to extend transformer lifespan or in the case of unexpected high loads. but you are allowed to use them for continuous load rating.

here's an article that might offer you a better view.

https://www.ecmweb.com/content/basic...e-transformers

do you know what the actual expected load is? either way the scwheitzer will let you raise the instantaneous trip setting for inrush and then you can protect the transformer tighter for overload, that's very good sophisticated equipment, love em'