CT Internal Resistance

[purplepete]

Gents,
good morning/evening. We are in the process of specifying CTs for a 22 kV 50 Hz switchboard protection system (Incomers, feeders and bus-zones) and are trying to validate the calcs for CT ratios, burdens etc.

We were using the ABB publication:
Calculation of the Current Transformer Accuracy Limit Factor

but have come up against the need to know the internal resistances of the CTs. Can anyone suggest an easy method to evaluate short of asking the manufacturer?
Any guidance is welcome, thanks and Happy New Year.

 

[davidbeach]

Why not ask the manufacturer?

 

[purplepete]

David,
at the moment our local guys are on leave. We are hoping for something sooner.

 

[scottf]

There is not a reliable way to estimate the internal resistance of a CT, especially MV or HV CTs, since the number of primary turns vary by ratio and design.

 

[dpc]

If these are toroidal CTs (no wound primary), the secondary resistance can be ROUGHLY approximated by looking at the physical size of the CT, determine the number of turns and making a guess at the size of the magnet wire. Making guesses on this allows the length to be estimated and then the dc resistance can be estimated.

This will be an order of magnitude value only.

If you search the SEL website, you will find several white papers by Stan Zocholl regarding CT performance in switchgear.



David Castor

http://www.cvoes.com

 

[MHEA]

My general approach is to give the CT manufacturer all the information regarding ratio, fault level, accuracy class, relevant relay requirements, etc, and let them play between the KPV and Rs to get the most economical solution.