Q1. The main objective of economics of power transmission is to:
A) Reduce power factor
B) Reduce losses and overall cost
C) Increase current
D) Increase conductor temperature
Correct Option: B
Explanation: Economics of power transmission aims to minimize the total annual cost, which includes capital cost and running cost due to I²R losses.
Q2. According to Kelvin’s law, the most economical size of conductor is obtained when:
A) Annual cost of energy lost = 0
B) Capital cost = Running cost
C) Annual cost of energy lost = Annual capital charges
D) Resistance of conductor is minimum
Correct Option: C
Explanation: Kelvin’s law states that total annual cost is minimum when annual energy loss cost equals annual capital charges.
Q3. If conductor size is increased beyond the economical value, the annual cost will:
A) Decrease
B) Become zero
C) Increase due to capital cost
D) Remain constant
Correct Option: C
Explanation: Increasing conductor size reduces losses slightly but increases capital cost significantly, increasing total annual cost.
Q4. Which factor does NOT affect the choice of transmission voltage?
A) Line length
B) Power transmitted
C) Power factor
D) Colour of conductor
Correct Option: D
Explanation: Transmission voltage depends on power, distance, losses, and power factor. Colour of conductor has no technical relevance.
Q5. Higher transmission voltage is preferred mainly because it:
A) Increases current
B) Increases losses
C) Reduces conductor size
D) Reduces insulation requirement
Correct Option: C
Explanation: Higher voltage reduces current for same power, thereby reducing I²R losses and conductor size.
Q6. Kelvin’s law becomes uneconomical when:
A) Load is constant
B) Load varies widely
C) Resistance is high
D) Voltage is low
Correct Option: B
Explanation: Kelvin’s law assumes constant load, which is not true in practical systems where load varies widely.
Q7. Which cost is proportional to conductor cross-sectional area?
A) Cost of insulation
B) Cost of supports
C) Capital cost of conductor
D) Cost of energy lost
Correct Option: C
Explanation: Capital cost of conductor increases directly with its cross-sectional area.
Q8. The annual cost of energy lost in a transmission line is proportional to:
A) Current
B) Voltage
C) I²R
D) Conductor diameter
Correct Option: C
Explanation: Energy loss in transmission lines depends on copper loss, which is proportional to I²R.
Q9. A satisfactory electric supply system should have:
A) High voltage fluctuation
B) Poor reliability
C) Good voltage regulation
D) High losses
Correct Option: C
Explanation: Good voltage regulation ensures reliable and efficient power supply to consumers.
Q10. Which is NOT a requirement of satisfactory electric supply?
A) Continuity
B) Reliability
C) High frequency variation
D) Proper voltage
Correct Option: C
Explanation: Frequency should remain nearly constant. High frequency variation is undesirable.
Q11. A single-phase line at 400 V delivers 100 kW at 0.8 p.f. with line resistance 10 Ω. Power loss is:
A) 9.8 kW
B) 7.8 kW
C) 10 kW
D) 12.5 kW
Correct Option: A
Explanation: Line current = 100000/(400×0.8) = 312.5 A. Copper loss = I²R ≈ 9.8 kW.
Q12. If transmission voltage is doubled for same power and p.f., copper loss will:
A) Remain same
B) Become double
C) Become half
D) Become one-fourth
Correct Option: D
Explanation: Doubling voltage halves current. Since loss ∝ I², loss becomes one-fourth.
Q13. According to Kelvin’s law, if annual energy loss cost is ₹5000, capital cost should be:
A) ₹2500
B) ₹5000
C) ₹10,000
D) ₹0
Correct Option: B
Explanation: Kelvin’s law requires annual energy loss cost equal to annual capital cost.
Q14. A 3-phase line transmits 5 MW at 11 kV, 0.9 p.f. Line current is:
A) 262 A
B) 291 A
C) 320 A
D) 350 A
Correct Option: B
Explanation: Line current = P/(√3×V×pf) ≈ 291 A.
Q15. If current in a transmission line is reduced to half, copper losses become:
A) 1/2
B) 1/4
C) 2 times
D) Same
Correct Option: B
Explanation: Copper loss ∝ I². Halving current reduces loss to one-fourth.
Q16. If capital charges = ₹8000 and energy loss cost = ₹6000, conductor size is:
A) Economical
B) Undersized
C) Oversized
D) Cannot be determined
Correct Option: C
Explanation: Capital cost greater than energy loss cost indicates oversized conductor.
Q17. Economic choice of transmission voltage mainly depends on:
A) Load only
B) Distance only
C) Load and distance
D) Resistance only
Correct Option: C
Explanation: Transmission voltage depends on both power transmitted and transmission distance.
Q18. For long-distance bulk power transmission, preferred system is:
A) DC low voltage
B) AC low voltage
C) AC high voltage
D) DC medium voltage
Correct Option: C
Explanation: High-voltage AC transmission minimizes losses and allows easy voltage transformation.
Q19. Kelvin’s law ignores which practical aspect?
A) Capital cost
B) Energy loss
C) Load variation
D) Resistance
Correct Option: C
Explanation: Kelvin’s law assumes constant load and ignores real-world load variations.
Q20. If transmitted power is increased four times at same voltage, current will:
A) Double
B) Become half
C) Become four times
D) Remain same
Correct Option: C
Explanation: From P = VI, at constant voltage current is directly proportional to power.