Zero sequence current transformer - sizing

[bozzah]

Referring to a single line diagram, I'd like to confirm the size of the Zero sequence current transfromer.
Full load amps for this 4.16kV motor is about 800A. In addtion to 1600:5 CT(3), motor protection relay has a set of zero sequence CTs .
Zero sequence current transforer (a set with single CT for all phases) is rated 50:5. I am trying to confirm the size of this CT, i.e. 50:5. My assumption is that the reason for this CT size is due to the transformer feeding this 4.16kV bus that feeds the motor. The transformer is resistor grounded at 25A 10sec. Is this correct?

 

[davidbeach]

50:5 is a very common size for this application. Full load 3I0 should be well below 50A.

 

[scottf]

50:5A is a decent balance between the pick up current needed for 3IO current detection and physical size limits/acceptable burden ratings, since 50:5A is only 10 turns on the secondary of the CT.

 

[bozzah]

So, in this application, the determining factor for the size of the CT is resistance grounded transfomer(25A) or this sizing is just a standard practice at this voltage?
If transformer grounding resistor is, for example, 100A, an appropriate size would be say 200:5 for zero sequence CT?

 

[davidbeach]

No, 50:5 is standard for this application, even on solidly grounded transformers. Neutral current only flows during faults. By your logic we'd use fault rated phase CTs rather than load rated CTs. As scottf points out a 50:5 CT is about as low as you can go with a window CT, any lower and you need a wound CT to get more primary turns.

 

[bozzah]

understood. thanks a lot!