Module 10 :
Differential Protection of Bus, Transformer and Generator
Lecture 39 :
Transformer Protection
Selection of CT Turns Ratio (contd..)
i.e, or
(1)
If the transformer (to be protected ) is working on tap T as shown in fig 39.2, then the above equality has to be modified as follows:
(2)
Example
Let the primary of the transformer winding has 1000 turns while secondary has 500 turns. If the primary CT ratio is 100:5, find the CT ratio required in the secondary side to establish circulatory current scheme.
Ans:
=1000, and =20
From equation (1), we get
Thus, a suitable secondary CT ratio would be 200:5.
or 
=1000, 
and 
=20
