Choice of CT Ratio

[UKEng]

How would one decide on the correct choice of CT ratio for a protection CT? For example, If the normal current in the circuit was say 200A full load, would you select a primary rating of 200A or would some other factor come into play.

Many thanks

 

[buzzp]

CT's are generally most accurate when the current they are sensing is 60-80% of the primary current. After that the accuracy starts to decline (not a whole lot though). I would use a 300:5.

 

[tgott]

Check out Schweitzer's technical paper, "Selecting CTs to Optimize Relay Performance"

http://www.selinc.com/techpprs/6027.pdf

 

[scottf]

buzzp

I have to take issue with your comment. CT accuracy generally gets worse as the current falls below the rated current. The CT accuracy get better above the rated current, up to a point where core saturation starts to take effect.

However, for most protection application, the concern is not the accuracy at or around the rated current, but the performance of the CT during transient conditions.

The SEL paper tgott suggests is a good starting point. However, there are a few assumptions they make that I take some issue with, in particular when they discuss the internal burden of the CT and give rules of thumb for ohms/turn. While this generally holds true for bushing/slip-over CTs (i.e. 1 primary turn), the user has no way of knowing for sure the number of secondary turns on CTs with the possibility of multiple-turn primaries. This is esspecially true for insulated CTs where the ratio is 800:5A or below. Bottom line is that if you want to pick CT ratios based on this method, you either need to specify the maximum ohms/turn, or ask manufacturer for a an excitation curve and secondary resistance figure during the proposal stage.

 

[buzzp]

Thanks for mentioning the other things. I thought he was just asking about CT ratios.

 

[busbar]

It's old and not very "slick", but there is supporting info at:

http://www.geindustrial.com/products/applications/GET-97D.pdf

 

[jbartos]

Suggestion: The selection may also depend on the industry standards you are adhering to, specific application, and manufacturers. Visit

http://www.geindustrial.com/pm/notes/ger3973.pdf

http://www.selinc.com/techpprs/6107.pdf

etc. for more info

 

[Bung]

The only reason you would worry about the load current is to ensure that the CT is okay thermally during the 99.9%+ of its life it is sitting waiting on the real action to start. (I'm talking protection CTs here, not that white-shoe brigade revenue chasing stuff ;-)). I would be more interested in the burden it has to drive, the maximum short circuit current, how accurately the max Isc has to be measured (no good looking for good accuracy from a 200/5 CT on a 20kA fault level circuit when the leads are 100m long and the you have an old high burden CO relay!), all the sorts of things it is being installed to do.

Bung