Protection Engineering Basics and Schemes Protection Engineering | Page 23
1.1.7 Polarity of CT:
Polarity is very important while connecting CT in circuit, wrong polarity resulted in mal function of protection.
Usually we use Primary conductor dot in circuit diagram to show the current entrance path and dot at CT show the Current leaving path of the CT. in some drawings arrows use in place of DOTs. In any balance system,
I a + I b + I c = 0 Positive sign shows the directions of all current, let suppose when we change polarity of one CT let say B- Phase, now equation become
= I a − I b + I c-ve sign show the current direction of B is now different from A and C phase current
= I 0 + I 1 + I 2 − I 0 − a 2 I 1 − aI 2 + I 0 + aI 1 + a 2 I 2 = I 0 + I 1( 1 − a 2 + a) + I 2( 1 − a + a 2) I a − I b + I c = I 0 − 2a 2 I 1 − 2aI 2