600 V transformer protection

[Vincentb1298]

HI Guys,

I am an electrical engineer in Canada working on a 600V distribution system. CSA standards are applicable.

I have to size/set breakers that protect a 600V/208V transformer Dyn1 50kVA (In=48A at 600V)

For now, my predecesor choose a primary side molded case breaker of 125% In so 60A breaker with no ajustement. At secondary side, i have a 225A Low voltage panel loaded at 27.772kW (77A at 208V) protected by a 300A LSI breaker ajust to 125%In=173A.

As both breakers are adjust at 125%In, i have a bad protection coordination (TCC curve).

Do you any suggestions ?

 

[dpc]

If you are talking about miscoordination between the primary and secondary breakers, ask yourself: What difference does it make? The outage is going to be basically the same regardless of which breaker trips.

Molded case circuit breakers in series are generally not going to fully coordinate because of the instantaneous trip elements.

 

[waross]

Welcome to Eng-Tips Vincentb1298.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Vincentb1298]

The outage will be the same but we lose protection selectivity you will not know if the problem is in the transformer or in the panel...

 

[DiscoP]

While having some coordination between HV and LV is nice to have for the reasons stated, often this extra grading step cannot fit under the protection settings further up the network.

To fit all the curves in, there sometimes needs to be a compromise, and it is normally at the transformer.

It is difficult to say if this is the case with your situation - but it is possible.

Plotting the tcc curves for the protection above the 600V breaker may indicate why the protection is set the way it is.