Choice of CT Ratio
Choice of CT Ratio
(OP)
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
Many thanks






RE: Choice of CT Ratio
RE: Choice of CT Ratio
RE: Choice of CT Ratio
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.
RE: Choice of CT Ratio
RE: Choice of CT Ratio
RE: Choice of CT Ratio
http://www.geindustrial.com/pm/notes/ger3973.pdf
http://www.selinc.com/techpprs/6107.pdf
etc. for more info
RE: Choice of CT Ratio
Bung