Previous Year Question
The line currents of a 3-phase power supply are I = 3 + j5 A, I = 2 + j2 A, I = – 2 – j1 A R Y B The zero sequence current will be:
Correct answer: A
Explanation
Zero-sequence current is one-third of the phasor sum of line currents: I0=(IR+IY+IB)/3. Here (3+j5)+(2+j2)+(−2−j1)=3+j6, so I0=1+j2 A. Thus option A.
Formula
I0=(IR+IY+IB)/3=(3+j6)/3=1+j2A
Step-by-step solution
Correct Answer
A — 1 + j2 A
Concept and Reasoning
Zero-sequence current is one-third of the phasor sum of line currents: I0=(IR+IY+IB)/3. Here (3+j5)+(2+j2)+(−2−j1)=3+j6, so I0=1+j2 A. Thus option A.
Calculation / Key Relation
I0=(IR+IY+IB)/3=(3+j6)/3=1+j2A
Exam Takeaway
Zero sequence = average of the three phase-current phasors.
Common Mistake
Do not forget the factor 1/3 after summing the line currents.
Common mistake
Do not forget the factor 1/3 after summing the line currents.
Exam tip
Zero sequence = average of the three phase-current phasors.
Quick method
Zero sequence = average of the three phase-current phasors.