This set of Power Systems Question Bank focuses 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 _____________

a)

b)

c)

d)

View Answer

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) –a^{3}

View Answer

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+a

^{2}=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) a^{3}

d) a^{4}

View Answer

Explanation: 1+a+ a

^{2}= 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-a^{2} ; Y=a^{2}-a

b) bX=a^{2} -a; Y=1-a^{2}

c) X=a^{2} -1; Y=1-a

d) X=a^{2} -1; Y=a-1

View Answer

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°

View Answer

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.

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°

View Answer

Explanation: 1+a+ a

^{2}= 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°

View Answer

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+a

^{2}=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

View Answer

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

View Answer

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.**

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