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Topic Title: CT Wiring - Limitations on Secondary Length
Topic Summary: How long can a secondary circuit be without suffering corruption?
Created On: 09 July 2012 10:25 AM
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 09 July 2012 10:25 AM
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Avatar for JonathanHill.
JonathanHill

Posts: 225
Joined: 09 September 2002

I'm looking to raid the collective experience, please.

I'm well aware of limitations in CT secondary circuit length due to the burden drive capability of the CT when taking the load of the measuring equipment and resistance of the secondary wiring. However, I don't have a good feel for acceptable distance from the perspective of signal integrity and haven't located any suitable references. I'm aware that metering CT circuits in open busbar HV substations can exceed 150m, but I haven't come across any longer lengths.

A particular scenario I'm considering would require approx 500m route length of 2.5mm2 multicore cabling on a 1A secondary. I'm planning to specify either a 12.5 or 15VA CT rating, feeding an equipment burden of 0.2VA. My calculations show that the overall burden will be less than 10VA, so no issue there.

The multicore cable will be dedicated to the 3-phase CT circuits (with a pair of cores dedicated to each phase) and the armouring will be earthed at one end only, to avoid circulating currents. The route will be shared with other signal multicores and voice communications, with short lengths running >300m from a 3-core 11kV cable.

Colleagues are querying the wisdom of my proposals, but have yet to come up with any real argument other than they haven't experienced such long runs, and that "it doesn't feel right"!

Thanks in advance

Jonno

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Jonno
 10 July 2012 11:46 AM
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ArthurHall

Posts: 735
Joined: 25 July 2008

Jonno
I agree that what you are proposing is perfectly feasable, but long CT circuits make me nervous as well.
My only sugestion would be to use wedding ring CT's or a transducer, this would reduce to cost of the 500m of cable and eliminate the risk of open circuits.
 11 July 2012 12:36 AM
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stableford

Posts: 64
Joined: 04 April 2006

I have worked on a 13.2kV/600V transformer install, where the transformer was 320m from the feed, but one set of the differential CTs, was mounted another 100-125m away in the LV side load switchboard.
The CT's were cabled in #10AWG, so somewhere between 4mm and 6mm.
We have had no problems with the system.

Regards
Derrick
 17 July 2012 10:19 AM
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Avatar for JonathanHill.
JonathanHill

Posts: 225
Joined: 09 September 2002

Thanks for your help Arthur & Derrick - it seems I'm not losing it .... yet!

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Jonno
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