Quiz: Electrical Engineering 4 July 2020

Quiz: Electrical Engineering
Exam: NLC
Topic: Switchgear and protection

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. A balanced 3-phase system consist of
(a) zero sequence current only
(b) positive sequence current only
(c) negative and zero sequence currents
(d) zero, negative and positive sequence currents

Q2. The positive sequence current of a transmission line is:
(a) always zero
(b) 1/3 of negative sequence current
(c) equal to negative sequence current
(d) 3 times negative sequence current

Q3. In case of an unbalanced star-connected load supplied from an unbalanced 3-phase, 3-wire system, load currents will consist of
(a) positive-sequence component
(b) negative-sequence component
(c) zero-sequence component
(d) only (a) and (b)

Q4. For an unbalanced fault, with paths of zero sequence currents, at the point of faults
(a) the negative and zero sequence voltage are minimum
(b) the negative and zero sequence voltage are maximum
(c) the negative sequence voltage is minimum and zero sequence voltage is maximum
(d) the negative sequence voltage is maximum and zero sequence voltage is minimum

Q5. Zero-sequence fault current is absent when fault is
(a) single-line-to-ground fault
(b) line-to-line-ground fault
(c) double-line-ground fault
(d) line-to-line

Q6. In case of single-line to ground fault
(a) all sequence networks are connected in parallel
(b) all sequence networks are connected in series
(c) positive and negative sequence networks are connected in parallel
(d) zero and negative sequence networks are connected in series

Q7. Zero sequence current can flow from a line to transformer bank if the windings are in
(a) grounded star/delta
(b) delta/star
(c) star/grounded star
(d) delta/delta

Q8. Negative-sequence reactance of a transformer is
(a) equal to the positive sequence reactance
(b) larger than the positive sequence reactance
(c) smaller than the positive sequence reactance
(d) None of the above

Q9. If all the sequence voltages at the fault point in a power system are equal, then the fault is a
(a) three phase faults
(b) line to ground fault
(c) line to line fault
(d) double line to ground fault

Q10. In an un-transposed three-phase transmission line
(a) the sequence network does not have mutual coupling
(b) a positive sequence currents may cause a negative sequence voltage drop
(c) the sequence impedance matrix is diagonal
(d) none of the above

Solutions
S1. Ans. (b)
Sol. positive sequence current only

S2. Ans. (c)
Sol. equal to negative sequence current

S3. Ans. (d)
Sol. only (a) and (b)

S4. Ans. (b)
Sol. the negative and zero sequence voltage are maximum

S5. Ans. (d)
Sol. Zero-sequence fault current is absent when fault is line-to-line.

S6. Ans. (b)
Sol. all sequence networks are connected in series

S7. Ans. (a)
Sol. grounded star/delta

S8. Ans. (a)
Sol. equal to the positive sequence reactance

S9. Ans. (d)
Sol. double line to ground fault

S10. Ans. (b)
Sol. a positive sequence currents may cause a negative sequence voltage drop

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