short circuit contribution of overloaded utility transformer

[egf]

Does a 12kv-.48kv 3Ph transformer loaded beyond it's rating, i.e 2500KVA transformer loaded to 3000KVA, have any impact on the available short circuit values during a fault on the secondary side of the transformer?

 

[slavag]

Hi.
Very shortly: NO.
Actually you calculate SC according to Uk% of trafo.
Regards.
Slava

 

[dpc]

Slava is correct - the loading has very little impact on the fault current through a transformer.

A couple of secondary factors:

Pre-fault voltage at the secondary could be somewhat changed, due to the loading as well as action of upstream LTC or voltage regulator trying to compensate.

Heavily loaded motor may indicated a lot of motors on the secondary side that will cause an increase in the fault current related to motor contributions.

 

[cranky108]

In the classical method of calculating short circuit, the voltage level dosen't matter. And sence the source impedance will rarely be a maximum case, the added voltage differential won't matter that much.

Motor load will affect the asymetrical fault level, but unless you have syncronious motors they will have no effect on the symetrical short circuit level. And sence most fault studies are for symetrical short circuit, the load shouden't matter.

 

[Zogzog]

OK, I will bite, why do you have 3000kVA on a 2500kVA transformer? What temp rise are you running at? You do realize this wont last long right?

 

[burnt2x]

Maybe a case of short foresight during the design? 120% overload can only be allowed for a short time! You better specify/ acquire a bigger trafo fast!