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Topic Title: Medium Switchger CT
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Created On: 05 March 2013 01:52 AM
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 05 March 2013 01:52 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

How many CT for medium Switchgear per phase?, please describe the function for each CT?
 05 March 2013 10:12 AM
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alancapon

Posts: 5761
Joined: 27 December 2005

The number of CTs per phases will depend on the protection design for that particular circuit breaker, as well as whether busbar protection is required for the board. It will also depend on the company policy for sharing CTs.

Regards,

Alan.

Edited: 05 March 2013 at 11:34 AM by alancapon
 06 March 2013 12:49 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: alancapon

The number of CTs per phases will depend on the protection design for that particular circuit breaker, as well as whether busbar protection is required for the board. It will also depend on the company policy for sharing CTs.



Regards,



Alan.



It is possible to share the CT for overcurrent protection and earth fault protection?
 06 March 2013 08:48 PM
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cookers

Posts: 205
Joined: 10 February 2012

yes if you use a combined overcurrent and earth fault protection relay it normal practice.

http://www05.abb.com/global/sc...PAJ140C%20Brochure.pdf
 07 March 2013 12:13 AM
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jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: cookers

yes if you use a combined overcurrent and earth fault protection relay it normal practice.



http://www05.abb.com/global/sc...PAJ140C%20Brochure.pdf


Nope, they use separate CT for this model, i hv seen this model by myself, as you can see from the drawing,

Anyway, thanks for ur info,
 07 March 2013 08:41 AM
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alancapon

Posts: 5761
Joined: 27 December 2005

As I said, it depends on the protection design and operating company policy. It would also be possible by altering the connections to the relay to achieve the same arrangement using only the three phase CTs and not use a separate one for earth. You need to ensure that you order the correct specification CTs for your protection application. It would be possible (but most companys would think unwise) to use the same CTs for backup overcurrent / earth fault as well as a high speed protection scheme such as unit and / or accelerated distance as well.

Regrads,

Alan.
 07 March 2013 09:43 PM
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dlane

Posts: 690
Joined: 28 September 2007

We utilise both types of setup on our protection systems.

Generally our HV transformer feeders utilise the phase CTs for earth protection.

Our HV incomers and LV incomers generally have seperate CTs for phase and earth protection circuits and a further CT for restricted earth fault protection for transformer secondaries.

Kind regards

Donald Lane
 08 March 2013 01:03 AM
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alancapon

Posts: 5761
Joined: 27 December 2005

In terms of the greatest number of phase CTs, I think we have got to five sets of CTs. For a transmission feeder, these would be:
1. First Main Protection (Usually a unit scheme)
2. Second Main Protection (Usually an accelerated distance scheme)
3. Overcurrent / Earth Fault / Sensitive Earth Fault
4. Busbar Protection Zone (a high-impedance scheme)
5. Busbar Protection Check Zone (a high-impedance scheme)

Regards,

Alan.
IET » Energy » Medium Switchger CT

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