Station Service Transformer Feed from MV Switchgear

[rockman7892]

I've got an application on 34.5kV M-T-M switchgear where I need to feed external 150kVA transformers for supplying 120/208V AC station service power to switchgear.

With the M-T-M Switchgear arrangement there will be (1) 150kVA SST fed from each switchgear bus 1 & 2 supplying AC station service power to the respective switchgear building. In previous applications the SST/CPT was usually small enough where it could be contained within switchgear usually fed with an internal fuse drawer. When SST/CPT is located external to switchgear however what is normal practice for feeding these transformers? Addition of a dedicate breaker and relay seems like an expensive solution to feed such a small transformer so was curious if there is another standard practice for feeding external CPT from switchgear?

Furthermore the Switchgear lineup is GIS gear so likely further limits what may be able to be contained within gear itself.

In most applications I've seen in the past the CPT/SST was tapped from line side of main breaker but given redundancy with bus tie it will be fed from bus side of gear in this application.

Thanks

 

[cranky108]

So what is wrong with fuses? Most distribution transformers of that size are protected with fuses.
There is also the possibility of using a pad mounted transformer with an internal breaker. That said they usually have an internal current limiting fuse.

 

[jghrist]

You could use a cubicle with fuses to feed the external transformer. An alternate would be to feed the station service from one of the 34.5 kV feeders; maybe one from each side with a LV transfer switch so loss of one feeder would not lose station service.

 

[rockman7892]

Yes I agree with fuses being adequate. My main question though is how to accommodate fuses internal to switchgear to allow external cable connection to transformer? For transformer internal to switchgear I've used an internal fuse truck but with external cable connection I'm struggling to see how fuses would be arranged in gear to allow connection of external cable? Is it common to provide a fuse cubicle for this application in switchgear that allows external cable connection (not an MVS section)? This application is also GIS gear so struggling to see even more how fuses internal to gear would be accommodated.

Another option would be to just use a bus tap with fuses/breaker located out at transformer however with a cable fault on external cable to transformer you'd take down the entire lineup for a fault on CPT circuit.

A dedicated feeder breaker to feed station service is an option but a costly one considering cost of breaker and relay.

Thanks for the help.

 

[davidbeach]

I don't know the exact details of how it was accomplished, but we have various 34.5kV switchgear with a fuse on the bus feeding a station service transformer external to the gear. It's probably very similar to how a set of bus VTs are connected. One thing to consider for an external station service transformer is whether or not to include CTs so that that cable to the station service transformer isn't in the bus diff zone. Really bad to trip a bus lock out when energizing a bus because the unmonitored station service transformer inrush current was too much for the diff.

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