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