Breakers in Outdoor Sub

[bwat]

For a customer owned outdoor sub (~80MVA) where there's HV transmission coming in, and then the secondary voltage is 34.5kV and several (3?) 34.5kV feeders are going to run underground from the sub, would it make more sense to have a metal enclosed switchgear lineup that had these MV feeder breakers, so the underground feeders stubbed up into the switchgear, or would it make more sense to do more of the classical outdoor substation breakers with open air bushings on top and built on a custom structure.

I would think outdoor substation breakers make sense if you're going from overhead, and then staying overhead, but if you're transitioning to underground, it would seem like a switchgear lineup would be better. Perhaps there are outdoor sub breakers specifically made for transitioning to underground though...

We're just doing some early stage prelim designs and I want to see if there's a direction that is abundantly better than the other.

I'll take any insight I can get from the crew here.

 

[jim dungar]

Open air equipment takes up more land space for each breaker.

 

[David Castor]

Outdoor breakers will likely be less expensive if you have the space. 34.5 kV metal-clad switchgear is not all that common in the US and will be expensive. If the fault currents are not too high, you can use reclosers instead of outdoor circuit breakers. That will be less expensive (in general). You will need some type of substation anyway if you are taking service at a transmission voltage. Likely the stepdown transformer will have a primary circuit breaker or circuit switcher.

The transition to UG would be made at a bus structure - bus to disconnects & arresters, then into a riser conduit supported by the structure. It's done routinely.

The outdoor substation would also be easier to modify in the future. Normally an outdoor substation would imply some type of control building for relaying and metering.

 

[bwat]

Thank you both

The transition to UG would be made at a bus structure - bus to disconnects & arresters, then into a riser conduit supported by the structure. It's done routinely.

Do you have any type of picture for that? I have some, but the arrangement is slightly different from what I think you're describing.