CTs for REF Protection for Distribution Transformers

Alstom will provide you the CT details. Or the LV switchgear manufacturer too

You did read page 10 of the Technical Manual didn't you?

Good morning,

The minimum value of the Knee Point Voltage for a CT (Vk) is usually defined in its nameplate, under the denomination of Special Accuracy Class (Class X). It also imposes a maximum value of Rct (CT secondary winding resistance). Sometimes, it specifies the maximum value of the magnetising current Io at knee point voltage. Remember that the knee point voltage Vk is defined as the point on the CT's magnetising curve from which a 10% increase in voltage causes a 50% increase in the magnetising current Io.

Once you have this information, there are some calculations you can perform (according to your relay's technical specs), for you to verify that the Knee Point Voltage is not exceeded under load, short circuit and impulse conditions. I did some similar calculations for a Schneider Electric's overcurrent relay (SEPAM 100 LD), installed with interposed CTs in a 2 MVA Power Transformer. Let me know if you'd like me to send you these sheets.

Best regards from Venezuela.

Dear Sir,

Thanks for your message. I would welcome any calculations you might have done for the ref protection. Emailed Removed CR4 Admin

Regards,

S.K.Lamba