13.8kV System BILs (Dry-Type Tx)

[mbk2k3]

Hey folks, this is for a small cogen plant:

I have a 13.8kV switchgear with a 1200A, 95kV BIL vacuum breaker that feeds a 750kVA dry-type 13.8kV/600V transformer via a short 20' cable run. Vacuum breaker has surge arrestors on the load side.
The upstream supply is two large 10MVA transformers connected to the utility ~500ft away.

I'm trying to make the dry-type transformer as small as possible to fit in our e-house. I noticed I have the option of specifying a 60kV BIL instead of 95kV BIL for the HV winding of the dry-type transformer.
What information do I need to make a reasonable assumption whether the HV winding of the dry-type transformer can be lowered to 60kV (to reduce footprint)?

Short of doing a full-blown insulation coordination study (never seen one done for a small cogen CHP plant like ours), can I make some reasonable assumptions here?

I can provide lots of additional info as needed.

 

[Palletjack]

What’s the bil of all the other devices? I wouldn’t want this XF to be the lowest.
Look at the sparkover characteristics of the arrestors. They may be pushing the 60kvbil at that voltage.

I thought 60 was more for a 5-8Kv installation.

 

[mbk2k3]

the 10MVA transformers have a LV BIL of 110kV
the 13.8kV swgr has a BIL of 95kV

But there was no insulation coordination study done (not typically done for projects of this scale).

IEEE C57.12.01 is where I noticed that 13.8kV (15kV class) transformers are allowed to have a BIL of 60kV / 95kV / 110kV.

 

[cuky2000]

The options are limited to gain space for a 750 kVA, 13.8 kV/600V class standard dry-type transformers.
Small improvement could be obtained if reduced the capacity to 500 kVA with fans cooled to obtain up to 667 kVA with lower BIL relying on surge arrester for surge protection.

Although feasible, the risk/size-reduction of ~6" in lend is hard to justify. (your call).
A suggested alternative is to explore with an outdoor unit with a NEMA 3R enclosure to be located in the vicinity of the e-room.

Check also with a couple of transformer manufacturer to search for a non-standard application using cast-coil with a custom made enclosure.
The sketch below provides a brief illustration. Hope this help