Part3 — Electrical Engineering Interview Questions & Answers (MCQs)
Question 1. Why are single line to ground and double line to ground faults categorized as unbalanced faults?
a. Because they do not occur in all three lines
b. Because they give rise to unequal currents in lines
c. Because they are grounded
Correct Answer: b. Because they give rise to unequal currents in lines (✓)
Explanation: One or two of the three phases have zero current in both faults. As a result, the system is unbalanced. For example, if phase A experiences a single line to ground fault, phase B and C will have zero current while phase A will have maximum current, resulting in an unbalanced current flow in three lines. Similarly, if there is a double line to ground fault in phases A and B, phase C will have no current.
Question 2. A symmetrical fault should have?
a. Equal phase
b. Equal increment in current value
c. Both
Correct Answer: c. Both (✓)
Explanation: Balanced 3 phase faults are symmetrical faults. It occurs when all three phases are shorted at the same time, resulting in an equal current value in all three phases.
Question 3. Why do we use per unit system in fault calculations?
a. To bring the system on a single value, based on a single base
b. To eliminate differences in values of parameters in different zones of system
c. No specific reason
Correct Answer: b. To eliminate differences in values of parameters in different zones of system (✓)
Explanation: The per-unit system simplifies fault calculation by removing the need for information about transformer delta or star connections or adjusting transformer parameters based on primary or secondary referencing. Also, regardless of zone, voltages have the same range.
Question 4. What is the cause of negative sequence current in power system?
a. Ground current
b. Unbalanced current
c. Flow of current in opposite direction
Correct Answer: b. Unbalanced current (✓)
Explanation: Negative and zero sequence currents occur when there is an unsymmetrical or unbalanced current flowing through the system.
Question 5. Zero sequence current can be prevented
a. When the system has no ground connection
b. When the system has shunt reactors
c. Can never be prevented
Correct Answer: a. When the system has no ground connection (✓)
Explanation: Zero sequence current only flows in ground, so absence of ground means no zero-sequence current.
Question 6. Do per unit values have any units?
a. Same as normal values
b. There are other units
c. No units
Correct Answer: c. No units (✓)
Explanation: As per unit value is ratio of original value to base value, it cancels out the units.
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Question 7. How many bases do we need to proceed with the calculations?
a. All
b. At least one
c. At least two
Correct Answer: c. At least two (✓)
Explanation: We need at least two base values so that other base values can be calculated using them.
Question 8. Which parameter should have same base value in all zones?
a. Power
b. Voltage
c. Current
Correct Answer: a. Power (✓)
Explanation: Because the power of the system remains same.
Question 9. Generator only feeds
a. Positive sequence current
b. Negative sequence current
c. Zero sequence current
Correct Answer: a. Positive sequence current (✓)
Explanation: Only positive sequence current is fed to the system, other sequence components only arise in case of faults.
Question 10. Why do we represent delta side of transformer as open circuit in case of zero sequence network?
a. Because it has no ground
b. Because it does not let zero sequence current flow through it
c. Both
Correct Answer: c. Both (✓)
Explanation: When there is an unbalance current and ground is available in the system, zero sequence current flows. Because the delta connection has no ground, zero-sequence current cannot flow through it.
Question 11. Which connection of transformer allows the zero-sequence current to flow?
a. Wye connection with solid or reactance grounding
b. Wye connection with no grounding
c. Delta connection with no grounding
Correct Answer: a. Wye connection with solid or reactance grounding (✓)
Explanation: Any connection of transformer with any type of ground would allow zero sequence current to flow.
Question 12. Why do we only have positive sequence current in a three phase to ground fault?
a. Because it has ground
b. Because it is symmetrical fault
c. Because it is unsymmetrical fault
Correct Answer: b. Because it is symmetrical fault (✓)
Explanation: Negative and zero sequence current occur in case of an unbalance fault while three-phase to ground fault is symmetrical (balanced fault).
Question 13. What is the relation between sequence currents in case of single line to ground fault?
a. All sequence currents have equal magnitude
b. Zero sequence has different magnitude
c. All sequence currents have different magnitudes
Correct Answer: a. All sequence currents have equal magnitude (✓)
Explanation: One of the characteristics of sequence currents is that they all are equal in magnitude.
Question 14. How are the sequence networks (positive, negative and zero) modelled in a sequence circuit in case of a single line to ground fault?
a. All sequence networks are in parallel
b. All sequence networks are in series
c. Positive and negative sequence networks are in parallel
Correct Answer: b. All sequence networks are in series (✓)
Explanation: In single line to ground fault:
zero sequence current=positive sequence current=negative sequence current
As all the three sequence current are equal, the sequence networks are represented as a series circuit.
Question 15. Why are the voltages in unfaulted lines are out of phase in case of single line to ground fault?
a. Because the voltage of phase with no fault is zero
b. To maintain the phase balance
c. Both
Correct Answer: c. Both (✓)
Explanation: One of the lines in a single line to ground fault is shorted with ground, resulting in zero potential. As a result, the other two line voltages become out of phase in order to preserve phase balance.
Question 16. What are the values of voltages in case of single line to ground fault?
a. Same volts for unfaulted phases
b. Zero volts for faulted phase
c. Both
Correct Answer: c. Both (✓)
Explanation: When one of the lines is shorted with the ground, the voltage at that line becomes zero. But the un-faulted lines have the same magnitude but are out of phase with each other.
Question 17. Why do we only have positive and negative sequence in line-to-line fault?
a. Because the phases are short circuited and current circulates
b. The fault is not grounded
c. No specific reason
Correct Answer: b. The fault is not grounded (✓)
Explanation: In faults involving the ground, zero sequence current occurs (example: single line to ground or double line to ground). As in the case of line to line fault, there is no ground involved, so it does not have a zero-sequence current.
Question 18. Fault current in both the lines in a line-to-line fault have
a. Same magnitude, opposite phase
b. Same magnitude, same phase
c. Different magnitude, same phase
Correct Answer: a. Same magnitude, opposite phase(✓)
Explanation: As a result of a line to line fault, two lines become shorted. As a result, the current’s shortest path is to enter one line and depart the other, resulting in fault currents that are identical in amplitude but out of phase on both lines.
Question 19. How are the sequence networks connected in case of line-to-line fault?
a. All in parallel
b. All in series
c. Positive and negative in parallel
Correct Answer: c. Positive and negative in parallel (✓)
Explanation: The positive sequence and negative sequence voltages are equal in line to line fault, represented as a parallel combination in sequence network.
Question 20. Why are positive and negative sequence modelled in parallel in a line-to-line fault?
a. Because of a closed path.
b. Because both the currents have same magnitude but are in opposite direction
c. None
Correct Answer: b. Because both the currents have same magnitude but are in opposite direction (✓)
Explanation: In line to line fault, there is no zero sequence current, and positive & negative sequence current is in opposite direction, also the positive sequence and negative sequence voltages are equal, which satisfies the condition of parallel connection of the two sequence model.
Question 21. How are the sequence networks connected in case of double line to ground fault?
a. All in parallel
b. All in series
c. Positive and negative in parallel
Correct Answer: a. All in parallel (✓)
Explanation: In double line to ground fault positive, negative and zero sequence are equal in voltages, which satisfies the condition of parallel connection of sequence network.
Question 22. Fault current in both the phases in a double line to ground fault have
a. Same magnitude, different phase
b. Same magnitude, same phase
c. Different magnitude, same phase
Correct Answer: a. Same magnitude, different phase (✓)
Explanation: In any three-phase system, the sum of ground currents is zero. As a result, in double line to ground fault, the two faulty lines carry equal magnitude but out of phase current, resulting in a zero sum at ground.
Question 23. In double line to ground fault, the sequence voltages are such that
a. All are equal
b. All are unequal
c. Only two are equal
Correct Answer: a. All are equal (✓)
Explanation: The sequence network is connected in parallel which shows equal sequence voltages.
Question 24. To account for the change in magnitude of voltage from LV side to HV side, which parameter is changed?
a. The formula
b. Base value of power
c. Base value of voltage
Correct Answer: c. Base value of voltage (✓)
Explanation: Base value of voltages is taken as the rated voltage of transformer. As transformer voltages changes from LV to HV side so the base value of voltages also changes.
Question 25. In which case do we have to incorporate phase change in voltage when moving from LV side to HV side?
a. When transformer connections are delta-wye
b. When transformer connections are wye-wye
c. When transformer connections are delta-delta
Correct Answer: a. When transformer connections are delta-wye (✓)
Explanation: In delta, line voltage is equal to phase voltages, but in wye, line voltage is root 3 times of phase voltage. So, phase change is required in delta-wye transformer connection.
Question 26. Why do we have the negative sequence in front or ahead of a reference point, when the positive sequence is behind that reference point?
a. Because negative sequence leads
b. Because negative sequence rotates in opposite direction
c. None
Correct Answer: b. Because negative sequence rotates in opposite direction (✓)
Explanation: Positive and negative sequences are equal in magnitude but their sequence of rotation is opposite showing negative sequence ahead of reference point while positive sequence behind the reference point.
Question 27. Which quantities do not change by changing the zone in the system?
a. Base power (✓)
b. Base voltage
c. Base Impedance
Correct Answer: a. Base power (✓)
Explanation: In per unit system, we consider a common base power. So it does not change in any of the zone.
Question 28. How do we account for the phase change in positive sequence current when moving from LV side to HV side?
a. Multiplying ‘j’
b. Multiplying ‘a’
c. Depends on transformer connection (✓)
Correct Answer: c. Depends on transformer connection (✓)
Explanation: In case of wye-wye or delta-delta there is no phase shift, but in case of delta-wye connection there is a 30-degree phase difference.
Question 29. By moving from LV side to HV side, we multiply by 1<-30° in case of
a. Positive sequence current
b. Negative sequence current
c. Zero sequence current
Correct Answer b. Negative sequence current (✓)
Explanation: because negative sequence rotates in different direction.
Question 30. Why do we observe a voltage depression in one of the unfaulted phases on HV side in case of delta connection?
a. Because of nature of fault
b. Because of transformer connections
c. None
Correct Answer b. Because of transformer connections (✓)
Explanation: because the faulted phase of transformer is connected to the unfaulted phase on the other side of transformer in case of delta.
Question 31. Why is the zero-sequence current nonexistent in line-to-line fault?
a. Because it is unsymmetrical
b. Because there is no ground
a. Both
Correct Answer b. Because there is no ground (✓)
Explanation: Zero sequence current occurs in faults related to the ground. For example; single line to ground or double line to ground). As in the case of line to line fault, there is no ground involved, so it does not have a zero-sequence current.
Question 32. Why are all the line voltages on HV side differently effected in case of line-to-line fault?
a. Because of nature of fault
b. Because of transformer connections
c. None
Correct Answer b. Because of transformer connections (✓)
Explanation: In delta line voltage is equal to phase voltages but in wye line voltage is root 3 times of phase voltage. So, magnitude change in phase voltages depends directly on transformer connection.
Question 33. In a given waveform, which parameters can be used to identify the type of fault?
a. Current and voltage waveforms
b. Sequence current waveforms
c. Both
Correct Answer c. Both (✓)
Explanation: As each fault current has different current, voltage, and sequence current wave form, so it is easily to distinguish between the type of fault using these waveforms.
Question 34. What is the phase difference between line currents of the faulted phases in case of line-to-line fault?
a. 120 degrees
b. 180 degrees
c. 90 degrees
Correct Answer b. 180 degrees (✓)
Explanation: In line to line fault, both line currents are equal in magnitude but opposite in direction which is represented by a phase difference between them of 180.
Question 35. Why are the fault currents not symmetrical in case of double line to ground fault?
a. Because of the ground
b. Because it has zero and negative sequence current
c. Fault currents are symmetrical
Correct Answer b. Because it has zero and negative sequence current(✓)
Explanation: The presence of zero and negative sequence current shows that double line to ground faults are not symmetrical.
Question 36.How can we identify the positive sequence network from the given sequence network diagrams?
a. By identifying a source
b. By identifying an open circuit due to transformer connection
c. Have no source
Correct Answer a. By identifying a source (✓)
Explanation: An EMF source present in positive sequence network distinguish it from zero and negative sequence network as this source is not present in zero and negative sequence network diagrams.
Question 37. What type of transformer connections can help us in identifying the zero-sequence network?
a. Delta-wye
b. Delta-delta
c. Both
Correct Answer c. Both (✓)
Explanation: As zero sequence current cannot pass through the delta connection. It gets trapped within the delta system.
