Use of current transformers on shielded cable

[moeszu]

Does anyone know of reference material to indicate the ability to use a conventional CT on a 25 kV shielded cable on a transformer primary cable. I would like to use a 600 volt insulated CT on the shielded cable of a 3750 kVA transformer and an electrical has indicated that it will work but I think the shielding will be a problem with current measurement.

Gene

 

[electricpete]

If you loop the shield wire termination lead back through the ct, then the shield current will flow through twice in opposite directions and as a consequence will not significantly affect the reading.

If shield wire happens to be grounded at both ends of a power cable and you have not looped back, ct would read much lower than actual due to the the shield current.

If shield wire is only grounded at one end of the power cable, then I believe there can still be capacitive current which needs to be eliminated by looping the shield ground wire back thru ct.

 

[TheBlacksmith]

That is my experience, you should only ground the shield at one end and run the shield lead back through he CT to cancel the shield current. Grounding the shield at both ends may produce some interesting effects besides CT error.

Blacksmith

 

[electricpete]

You're right blacksmith... the shield should only be grounded at one end. I was trying to think of why current would ever flow in the shield... it must be capacitive. (right?)

 

[peterb]

Electricpete -
Yes, the shield current would be capacitive under norma; conditions. If grounded at one end, the cable charging current would flow in the shield ground connection. If grounded at both ends, then ground fault current could flow in the shield and confuse the relaying if the shield ground lead isn't brought back through the CT.

 

[gjones33]

I assume we talking about a single shielded cable
G

 

[moeszu]

First, thank you for the responses on my question. Also, yes it is a single conductor (one per phase) on the primary for transformer differential metering.

Thanks,

Gene

 

[scottf]

Moeszu-

If your application is for protection only and the sheild is only grounded at one end, then the shield should not have to be routed back through the CT to "cancel" itself out.

The reason is that the current through the conductor during a fault will be much greater than the current through the shield. The CT will read the net current (i.e. conductor current minus sheild current, assuming they will be instantaneously in opposite direction). For over-current protection it will work just fine. Even for general differential protection it will work, as long as the relay is not set too sensitively.