It is known that the volt amps output; amplitude of short circuit current; type of transient and errors to be allowed are the main features determining the iron; copper and general cost of the Current Transformer suitable for protection purpose. This question of economics is reviewed in this paper and it is shown that the economy of a Current Transformer is dependent on V.A. per A.T. input or volts per turn but not on actual VA output, Generalised theory and equations are presented for the study of economics. It is proved that the primary and secondary ratings or number of turns have no influence on the volume of copper to be used.
The influence of secondary resistance on the area of iron is presented in the forms of graphs. The generalised curves are applicable to any Current Transformer with any ratings; short circuit ratio and any flux density permissible in the core.
All through the analysis an ideal Current Transformer with β=0 (Infinite secondary time constant) is assumed. A method of obtaining similar graphs for any value of β is also presented.
A new parameter “P” to assess the capacity of a Current Transformer to withstand transient is introduced.
