Motor Diff. CT Wiring: 4-wire Vs 6-wire

[elmatador]

Hello all,

We have a few medium voltage motors with a 50:5 core balance current transformer for each phase. Are there any issues with connecting the neutral of each CT together at the motor end so that you only run 4-wires back to the relay, where the fourth wire is the common neutral wire? Are there advantages to running 4-wire vs 6-wire? Wouldn't the 6-wire only add more overall length? Also does the location of the CT neutral ground affect the selection of the type of cable being used: i.e. 4 conductor or 6 conductor cable?

Also, I have read several threads about the grounding location and the general consensus appears to be grounding it at the building where the relay is located. I understand that the CT needs to be grounded at one location but is the location of the ground an important factor to consider with respect to voltage rise and shock hazard possibility?

 

[davidbeach]

The output of those CTs can never be expected to be a balanced three-phase circuit, so no cancellation in the common conductor. Six wires seems to be the appropriate number. Our practice is that the grounding happens at the first set of terminal blocks inside the control house.

 

[elmatador]

Thanks for your post David. Is this mainly because this is a differential application where you wouldn't necessarily see a balanced output if there was an internal fault on the motor? Why is it in Switchgear's that the neutral on the phase ct's are commoned up together and grounded?

Wouldn't this still work because the differential function is an instantaneous function?

 

[davidbeach]

Your phase CTs are connected in wye, they are parts of a single circuit, and the fourth wire is normally only carrying the minor unbalance current. Your motor differential CTs form three unrelated circuits and should remain independent.

 

[elmatador]

Thank you David for your comments on this subject. I see your point. We will be running a 6 conductor cable for this. We will end up having to add additional terminals on the MCC end as the vendor has configured the field connection as a 4-wire.