Protection Engineering Basics and Schemes Protection Engineering | Page 14

Ratio error is difference between the primary and secondary current, in figure it is shown by I r Ratio Error

= I primary( N Primary / N Secondary) − I secondary

I secondary

Let take one CT of 200 / xA, Turn ratio is 5 / 200,5VA, Class 0.5 with ratio error of 0.5, Now we need to find I secondary

0.5 = 200( 5

200) − x x x = 3.333A I Secondary = 3.333A

1.1.4 Exitation or magnetization Current: Excitation current is used to magnetize the Transformer core, our intention as manufacturer is to used to best quality core material, by best core material we means to use core which can magnetically couple primary and secondary on least amount of excitation current. When excitation current increased it increased the error in current transformers

I excitation = I primary( N secondary

N Primary

) − I Secondary pg. 14