Delta-Wye Transformer load distribution panel connections

[blazerbob1]

We have a 12.5KV Delta to 480V Wye 3 phase transformer in the MCC. The secondary is wired 4 wire to a Wye type 480V 3 phase distribution panel. We have connected three loads through breakers to the panel: a 480V 3 phase Delta to 120V Wye stepdown transformer for 120V loads; and two (2) small VFD's, both with 480V 3 phase Delta inputs.
When we closed the breaker for the transformer, it blew in less than 2 days. When we closed the breaker on each of the VFD's they blew in less than 10 seconds, each time taking out the 225A main on the distribution panel. We are now $30K into this disaster. Can anyone tell me what we have overlooked and done wrong?

 

[itsmoked]

Need more information. We get the picture but now need some specifics.

What model VFDs?
What HP motors?
How far are the motors from the VFDs?

Are you saying either VFD breaker closing will blow the main, or only both?

Got ratings on the transformers?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gepman]

In addition to the information itsmoked requested:

When we closed the breaker for the transformer, it blew in less than 2 days.

I assume that you mean the 12.5kV primary transformer.

Have you measured your voltage phase to phase, phase to neutral, and phase to ground on the 480V?

What other kinds of loads do you have in the 12.5kV MCC? Are you certain it is a 12.5kV MCC?

What is your protection scheme on the 12.5kV transformer. What are the three breaker sizes on the 480V distribution panel?

 

[davidbeach]

What are the device settings? What does the coordination study say the settings should be? What does the load flow study say the loading will be? Is there a problem or is equipment just misapplied?

 

[waross]

Is your MCC running at 12.5kV or is the 12.5:480/277 transformer feeding the MCC.
Have you grounded the wye point of your 480/277 volt system?
An ungrounded system fed from a 12.5 kV transformer is a good candidate for suspicion.
respectfully