Page 856 - Basic Electrical Engineering

For example, if the primary winding current limit is upto 1000 A, the turns
ratio of the CT is 200. For a reading on the ammeter of 2 A, we will calculate
the primary current as 2 A × 200 = 400 A. The reading of the ammeter has to

be multiplied by the turns ratio factor to get the value of line current.
It may be noted that turns ratio is the ratio of turns of the secondary and

primary windings and the transformation ratio is the ratio of current of the
primary and secondary windings, If I N = I N 2
2
1
1
then

i.e., Turns ratio = Transformation ratio.

If, however, the turns ratio is not equal to the transformation ratio, it will

introduce considerable error in the measurement. The difference between the

nominal ratio, i.e., and the actual transformation ratio is called ratio

error.
Again, if the secondary winding current and primary winding current are
not opposing each other at 180°, an error is introduced, which is called phase

angle error.
The ratio error and phase angle error in a CT, their causes, and means to

reduce these errors are explained now.
For this, let us draw the complete phasor diagram of a CT which is similar

to a two-winding transformer.