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