Transformer circuit tripping

[larmee]

Thanks for any response to my post.

Today at a local plant we performed a shut down on an existing switch-gear to install a new 400 ampere breaker for a new bus duct we had installed. The installation went well for this part but on the re-start-up we had 2 circuits in the gear tripping.

Here is the scenario:

Breaker one 200 ampere 3 phase feeding a 150 kVA transformer on primary side with one set of 3/0 conductors.
Secondary feeds 400 A MCB panel-board with parallel 3/0.

Running through the sequence of trouble-shooting. Took primary wires off and breaker held. Took A Phase off and transformer circuit held. Re-installed A phase conductors and xfmr tripped. Found no problem in transformer on secondary lugs for A phase but also could not find any problems with the A phase conductors.

Long story short I turned off all the branch circuit breakers in the 400 A panel and turned the 400 A main on in the panel and the circuit held.

There may be a simple explanation but I have never encountered a situation like this before.

Was inrush or maybe core saturation the cause.

Please explain.

Thanks again

 

[ptonsparky]

What is the primary voltage?

At 480v I would be surprised that you could ever get it to hold on start. Inrush would be my SEWAG.

 

[ptonsparky]

Primary protection can be up to 250% IIRC.

 

[jusme123]

Primary breaker undersized

 

[larmee]

Tom yes the primary voltage is 480 volts and the secondary 208/120

Thanks

 

[iwire]

Breaker one 200 ampere 3 phase feeding a 150 kVA transformer on primary side with one set of 3/0 conductors.
Secondary feeds 400 A MCB panel-board with parallel 3/0.

This is a code compliant design but it is a bit unusual.

Here is how I would look at it.

The rated primary current of this transformer is 180 amps. The code allows up to 250% overcurrent protection which is 450 amps.

I would expect at least 300 amp or larger breaker on the primary side to make sure the transformer can be fired up without tripping the breaker.

 

[ptonsparky]

Is there anything in the NEC that says something has to work?

 

[Cow]

I've had this issue in the past as well. Engineer spec'ed a breaker just slightly bigger than the primary FLA of the transformer and it worked fine for a couple months. The customer then had a power outage, the transformer breaker tripped and they were never able to get the breaker reset after that. The transformer was sent in for diagnosis, to which the factory replied, "it's good, use a bigger breaker!"