Current Transformer Costs

In the case of ring CTs,where the primary is one turn ( the copper bar that goes through it ) one has to increase the magnetic core material ( to "capture" more magnetic lines of force or flux,ampere-turn is constant) to maitain the same VA output of the CT and for the same class of accuracy.The axial length thus increases and the insulating material such as epoxy resin and frame size increase as well.For example, at a thresh hold ratio of 150/5A,15va/ph,class 0.2 Ring CTs,a number of problems arises.The quality of the silicon steel ,the annealing process,has to be very good before you can acchieve the specifications.The same process with the same batch of core material will easily make say, 200/5A 15VA or 150/5A 10VA class 0.2 CT.

When the ratio gets smaller,ring CT cannot meet such specifications e.g. 75/5A,15VA,class 0.2, Primary wound CTs are used.There are 2 turns in the primary side,employing Thick copper bars to carry the rated max primary current plus the KA breaking current for 1 or 3 seconds.This is more difficult to make and there is an increase of the height and material of the wound CT when one cannot increase the phase to phase spacing in a switchgear.

Therefore the cost of CTs increases as the ratio goes smaller.

Regards,

Khor

Number of coils in secondary = cost of copper = cost of CT