Corner tapped Delta

[rpetrovski]

I have a corner tapped Delta feeding a 480 to 240/120v single phase control transformer. On the Delta it is 480 to ground on all phases but the center tapped phase which is 0. Can this cause problems with the the control xmfr? I keep blowing fuses on the line side of the xmfr. The voltage and aps look ok when I replace the fuses. It only happens when the power to the xmfr is cycled. Could this be caused by inrush to the xmfr?

 

[Smart $]

I have a corner tapped Delta feeding a 480 to 240/120v single phase control transformer. On the Delta it is 480 to ground on all phases but the center tapped phase which is 0. Can this cause problems with the the control xmfr? I keep blowing fuses on the line side of the xmfr. The voltage and aps look ok when I replace the fuses. It only happens when the power to the xmfr is cycled. Could this be caused by inrush to the xmfr?

Don't you mean corner-grounded delta? Corners are not tapped, per se (though voltage taps would end up connected to the corner on delta-configured windings... but that's usually on the primary side).

When delta-configured secondary windings are corner grounded, the center-tap of a winding would read 240V to ground, not 0.

As for this configuration causing the problem that you mention, I can't think of any reason it would when connected correctly and the control transformer operating properly. What are your xfmr ratings, fuse ratings/type, and measured amperes?

 

[augie47]

The corner grounded 480 system should not be causing the problem, IMO.
Can you provide the particulars... transformer size and fuse size & type.
(you may need a fuse with some delay to address inrush)

 

[__dan]

yep, what they said. If you're blowing the control xfmr fuses you may have 'fast acting' type fuses at the input and need a time delay type fuse, for the inrush.

A corner grounded delta has the 480v, 480v, 0v to ground you described. A red leg delta has one phase winding center tapped and grounded, giving, for a 240v red leg delta, 120v, 120v, 183v to ground.

 

[hurk27]

I have a corner tapped Delta feeding a 480 to 240/120v single phase control transformer. On the Delta it is 480 to ground on all phases but the center tapped phase which is 0. Can this cause problems with the the control xmfr? I keep blowing fuses on the line side of the xmfr. The voltage and aps look ok when I replace the fuses. It only happens when the power to the xmfr is cycled. Could this be caused by inrush to the xmfr?

To the above in red;
If you have a 480 delta corner grounded (B phase) you will have 480 to ground on two phases and 0 volts on the grounded phase, if you have a center tap between A and C phases you have a high leg delta (which I doubt, very rare) that will have 240 volt single phase between A or C to ground, but B will show 415 volts to ground.

which in this case you would have seen this in your voltage readings, which indicate you have a grounded B.

But I agree with the others, as I just went through this on a lift station that has a 3kva transformer, and the panel builders installed 15 amp fast blow fuses, we changed to 25 amp dual element fuses to over come the inrush, it would even blow 20 amp dual element, these small transformers can have a high inrush.

 

[rpetrovski]

Sorry I ment corner grounded Delta. The Control xmfr is a 5kva with LP-CC-20 fuses. I wanted to see if any one came across this before. It sounds like I need to increese fuse size to over come the inrush. Thanks.