Trasformer Protection

[lsuengineer]

I am in the process of cordinating time-overcurrent curves for a 115/4.16kV(15MVA-55c, 28MVA-65c) transformer. I have a GE F35 Relay on the high side of my transformer and I have a GE F35 Relay on the low side breaker.

At full load amps the XFMR will put out 3886 amps on 4.16kV side but the 4.16kV breaker is only rated for 3000amps. I am wondering if I should coordinate the TOC's based on the full load amps of the XFMR or the breaker rating of 3000 amps.

Any help would be appreciated.

 

[DanDel]

Your system is limited to the 3000A rating of the C/B.

 

[RRaghunath]

lsuengineer,

Are you trying to provide thermal protection or protection against phase faults.

It is usual to set the relay pick up to even much higher than the full load rating of the transformer when it is phase fault protection that is intended.

The phase fault protection is supposed not to impede the overloading capability of the transformer (as per IEC guidelines).

Sizing (or rather undersizing) of the LV side breaker in your case cannot be a reason for setting the phase fault protection pickup low. This can be handled only by limiting the loads being connected to the 4.16kV switchboard.

Trust the above is helpful.

 

[slavag]

Hi.
For this size of xfr,must used 87T protection or not?
Are we have some requerements in IEEE or IEC?
Regards.
Slava

 

[rcw retired EE]

Slavag- there is no "requirement" to use transformer differential protection. 15 MVA is in the range where most owners would add differential protection. Based on my experience (prejudiced by working for too many low-budget contractors) 10 MVA and smaller use overcurrent only, 20 MVA and larger add differential, 10-20 MVA flip a coin.

With all of the interest in arc flash, I am adding differential on anything larger than 2.5 MVA.

 

[slavag]

Hi Rcwilson.
It's my standard also , but from 5MVA.
From your post, I'm understand, we don't have any "requirement" for 87T, only experience.
Thank you.
Slava