Quiz: Electrical Engineering 22 Apr 2021

Quiz: Electrical Engineering
Exam: AE/JE
Topic: Miscellaneous

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

Q1. If the secondary winding of the ideal transformer shown below has 40 turns, the number of turns in the primary winding for maximum power transfer to the 2 Ω resistor will be:

(a) 40
(b) 160
(c) 20
(d) 80

Q2. Parallel branch elements can be found out in a 1-phase transformer by conducting
(a) Open circuit test
(b) Short circuit test
(c) Both SC and OC test
(d) None of the above

Q3. 100 % string efficiency means
(a) Zero potential across each disc
(b) One of the insulator discs shorted
(c) Equal potential across each insulator disc
(d) None of the above

Q4. What is the approximate efficiency of a normal thermal power station?
(a) 20-25 %
(b) 30-40 %
(c) 45-55 %
(d) 60-70 %

Q5. To minimize the current chopping tendency, the SF6 gas is used at
(a) Low velocity and low pressure
(b) High velocity and high pressure
(c) Low velocity and high pressure
(d) High velocity and low pressure

Q6. The equivalent resistance in ohms in the circuit shown is

(a) 1/6
(b) 6
(c) 3/4
(d) 4/3

Q7. If r is the radius of the conductor and R is the radius of the sheath of the cable, the cable operates stably from the point of view of dielectric strength if:
(a) r/R>1
(b) r/R<1 (c) r/R<0.368 (d) r/R>2.718

Q8. As per Indian Electricity rule in overhead systems, the recommended span in case of RCC pole is….
(a) 40-50 meters
(b) 50-80 meters
(c) 80-100 meters
(d) 100-300 meters

Q9. Which lightning stroke is most dangerous?
(a) Direct stroke on the line conductor
(b) Direct stroke on the conductor
(c) Direct stroke on the ground wire
(d) Direct stroke on the tower top

Q10. Voltage regulation due to the Ferranti effect may be:
(a) Zero
(b) Positive
(c) Negative
(d) All of the above

SOLUTIONS
S1. Ans.(d)
Sol. For maximum transfer to the 2 Ω resistor:
8=2×(N1/N2 )^2
∴N1/N2 =√4=2
⇒N1=2N2=2×40=80

S2. Ans.(a)
Sol. For determining shunt branch elements (Rc,xm), we perform OC test at rated voltage.

S3. Ans.(c)
Sol. Due to formation of stray capacitance, unequal potential distribution occurs across discs which is expressed in terms of string efficiency.
If the string efficiency is 100 % then it means that equal potential difference occurs across each insulator disc.

S4. Ans.(b)
Sol. Efficiency of a normal thermal power station≃ 30-40 %.

S5. Ans.(a)
Sol. Due to low velocity and pressure employed in SF6 CBs, current chopping is prevented and capacitive currents are interrupted without restriking.

S6. Ans.(c)
Sol. Geff=(G1×G2)/(G1+G2 )=(2×4)/(2+4)=8/6
Req = 1/Geff =6/8=3/4 Ω.

S7. Ans.(c)
Sol. The cable operates stably from the point of view of dielectric strength if:
R/r>e or 2.718
∴r/R<1/e or 0.368

S8. Ans.(c)

Sol. RCC poles: span=80-100 meters

Line supportRecommended span
Wooden pole40-50 meters
Steel pole50-80 meters
RCC pole80-100 meters
Steel tower300 meters or above

S9. Ans.(a)
Sol. In this case, high voltage surges will damage the equipment.

S10. Ans.(c)
Sol. Voltage regulation: (VS-VR)/VR
Due to Ferranti effect: V_S<V_R
Hence, voltage regulation is negative in case of Ferranti effect.

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