Transformer CER Rating BS171

[fb599]

I have a 11/33kV 12/24MW CER Network transformer which I wish to set up back up over current protection. What I would like is the failure characteristics or maximum overload ratings for the transformer. Not just 1.2x FLC but I need to squeeze a couple of these in case one trips for about 10 minutes before each/ they trip.

Any thoughts or experience please. 1000:1 CT's on the LV and 500:1 CT's on the HV.

Thanks

 

[RRaghunath]

fb599,

Overloading curve for the transformer you may get from the manufacturer. In case the transformer is manufactured as per IEC60076, the transformer would conform to the loading guide IEC60354 and can be applied.

However, the requirement in the present case appears (to me) the back-up overcurrent protection setting for the transformer and not overload protection for the transformer.
Considering the fact that the transformer is grid connected one and should be able to take substantial overload for a short duration during outage of any parallel connected transformer, the recommended current setting is 175%. The TMS / inverse current type shall be selected as demended by the protection coordination study.
The Network Protection and Application Guide (NPAG) by AREVA as well as the Art & Science of Protective Relaying by A. Mason provide good guidance on the subject.