Power Systems Question Bank on “Comparison of Faults in Three Phase System – 2”.
1. The zero sequence depiction of the unbalancing occurring at the terminals of the induction motor will be most likely _____________
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: A
Clarification: 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°
Answer: A
Clarification: 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.
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°
Answer: A
Clarification: 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°
Answer: A
Clarification: 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
Answer: A
Clarification: 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
Answer: B
Clarification: The zero sequence impedance does depend on the pitch of the armature coils of the synchronous machine.