# Power Systems Questions and Answers – Comparison of Faults in Three Phase System – 2

This set of Power Systems Question Bank focuses on “Comparison of Faults in Three Phase System – 2”.

Explanation: The zero sequence components will not have any phase displacement and they are co- phasal in nature.

2. Complete the given phasor diagram by assuming that operator ‘a’ is unity magnitude and counter clockwise rotation of 120 degrees. a) 1-a
b) a-1
c) 1
d) –a3

Explanation: Based on the phasor calculations the unknown vector is found using balanced condition of the three vectors A+B+C = 0 and 1+a+a2=0. Then the third vector is 1-a.

3. The unknown vector in the given figure is ______ if we assume the system is balanced with a as unity magnitude and counter clockwise rotation of 120 degrees. a) a
b) 0
c) a3
d) a4

Explanation: 1+a+ a2 = 0.

4. For a balanced three phase system having a vector representation as mentioned in the figure, complete the vector X and Y. a) X=1-a2 ; Y=a2-a
b) bX=a2 -a; Y=1-a2
c) X=a2 -1; Y=1-a
d) X=a2 -1; Y=a-1

Explanation: This question can be attempted by using the options. For a balanced system the net voltage will be zero. So option a and b are the shortlisted options. Out of these applying the rotation criteria, option a is the best match.

5. For a balanced three phase system having a vector representation as mentioned in the figure, complete the vector X and Y. a) X=√3∠30° ; Y = √3∠-90°
b) X=√3∠-30° ; Y = √3∠90°
c) X=√3∠-30° ; Y = √3∠-150°
d) X=√3∠30° ; Y = √3∠150°

Explanation: This question can be attempted by using the options. For a balanced system the net voltage will be zero. So option a and b are the shortlisted options. Out of these applying the rotation criteria, option a is the best match in polar form.
Sanfoundry Certification Contest of the Month is Live. 100+ Subjects. Participate Now!

6. The unknown vector in the given figure is ______ if we assume the system is balanced with a as unity magnitude and counter clockwise rotation of 120 degrees. a) 1∠120°
b) 1∠30°
c) 1∠0°
d) 1∠-120°

Explanation: 1+a+ a2 = 0; a= 1∠120°.

7. Complete the given phasor diagram by assuming that operator ‘a’ is unity magnitude and counter clockwise rotation of 120 degrees. a) √3∠-30°
b) √3∠30°
c) √3∠-60°
d) √3∠90°

Explanation: Based on the phasor calculations the unknown vector is found using balanced condition of the three vectors A+B+C = 0 and 1+a+a2=0. Then the third vector is 1-a. Converting it in the polar form, it is √3∠-30°.

8. In the cylindrical rotor alternator, the sub transient and negative sequence reactances are same.
a) True
b) False

Explanation: It is true because the saliency offered is zero in the cylindrical pole machines.

9. The zero sequence impedance of a synchronous machine is independent of the pitch of the armature coils.
a) True
b) False

Explanation: The zero sequence impedance does depend on the pitch of the armature coils of the synchronous machine.

Sanfoundry Global Education & Learning Series – Power Systems.

To practice Power Systems Question Bank, here is complete set of 1000+ Multiple Choice Questions and Answers. 