Primary Fuse Voltage rating on Wye-Wye vs Delta-Wye XFMRS

[majesus]

All,

I'm working on a project where the client has already purchased two 33kV/480V Wye-Wye transformers, one at 1.5MVA the other at 1MVA. Both Wye neutrals are solidly grounded.

These transformers come with current limiting, primary fuses mounted within the transformer's cable box enclosure. Interestingly, both the Client and XFMR vendor are insisting that for these Wye-Wye transformers, the fuse voltage rating can be VLN which is 19kV. If it was a Delta-Wye XFMR, then fuses must be rated to VLL.

I understand that phase insulation rating on a Wye-Wye windings are less stressed and levels of insulation required at the grounded neutral end can be reduced. Whereas Dela must have full voltage insulation throughout. But having a VLN rated primary fuse on a Wye-Wye says we only care about internal line to ground faults. Why are we ignoring any possibility of line to line faults? I asked the vendor and client this questions, but the didn't have any convincing rational. Am I missing something? Why shouldn't these fuses be rated to VLL as Delta configuration to account for internal phase to phase faults scenarios?

 

[majesus]

All, to further add to my question. On Cooper Eaton online fuse program, it also gives me VLN rated primary fuses for Wye Wye and VLL fuses for Delta-Wye.

Not sure why for Wye-Wye you only need VLN rated fuses, how about internal phase to phase short circuit scenarios? Hopefully someone can give me some insight.

Here is the Eaton link:

http://www.coopertransfusion.com/Calc

I inputted for Wye-Wye:
Transformer KVA Rating: 1MVA
Primary kV Voltage: 33kV
Number of phases: 3
Impedance: 5.75%
Primary: Wye
Secondary: Wye
Protection type: Current sensing Bay-o-net
Reset of settings is left as default.

 

[LionelHutz]

Internal phase to phase shorts would have 2 fuses in series?

 

[stevenal]

Look at the open circuit voltage after the fuse opens. Phase to phase fault will open two fuses, with each having line to ground voltage across the open link.