About CT and relay performance

[lume7006]

Hello guys,

A circuit has a small load of 75 kVA at 4.16 kV, and a very high shortcircuit level, around 50 kA.
If we choose the CT in order to "see" the load current it will be a very low ratio (let's say 150/5) but, it will not held 100 A secondary neither saturation criteria.
However, taking into account saturation and less than 100 A sec, the ratio needs to be so high that relay will not see any load, I mean, the secondary current has a magnitude so small that the sensibility of the relay does not see it and then, it will consider as the circuit were open.

What options should we consider to solve this?
Any idea is welcome!

Best Regards

 

[marks1080]

To bring home the point even further... It is very common for a CT to saturate duringa fault. If your calculations show saturation of your CT this is nothing to be worried about. As long as the fault is interrupted everything will be fine.

 

[jghrist]

Highly saturated CTs can prevent microprocessor instantaneous overcurrent relays from operating properly even if the pickup is below the saturation point.

See

http://www.selinc.com/WorkArea/DownloadAsset.aspx?id=3168

 

[marks1080]

Very interesting paper - thanks jghrist!

Mark