Protection Engineering Basics and Schemes Protection Engineering | Page 279

Insert these values in( A),

I c = I o + aI 1 + a 2 I 2 since 1 + a + a 2 = 0

= I o + I 1 + I 2 + I o + a 2 I 1 + aI 2 + I o + aI 1 + a 2 I 2 = 3I o + I 1( 1 + a 2 + a) + I 2( 1 + a + a 2) I e = 3I o

In balance conditionI o = 0, but as system is now unbalance so there will be certain value of I o

9.7 Negative sequence Protection Unbalancing in motor causes negative sequence current in motor, unbalancing can be due to

� Phase missing due to opening of any one or two pole of associated circuit breakers

� Phase missing due to blown up fuses or fuse failure of one or two phases

� Due to faults in system( phase to ground, phase to phase, phase to phase to ground, phase to phase to phase to ground etc.) pg. 279