Q1. Which system is most commonly used for long-distance bulk power transmission?
A) DC transmission
B) Single-phase AC transmission
C) Three-phase AC transmission
D) Single-wire earth return
Correct Option: C
Explanation: Three-phase AC transmission is used because it delivers constant power, requires less conductor material, and allows easy voltage transformation.
Q2. Main advantage of high voltage transmission is:
A) Improved power factor
B) Reduced conductor cost
C) Reduced transmission losses
D) Increased line current
Correct Option: C
Explanation: High voltage reduces current for the same power, thereby reducing I²R losses significantly.
Q3. For the same power and voltage, which system requires minimum conductor material?
A) Single-phase 2-wire
B) Single-phase 3-wire
C) Three-phase 3-wire
D) DC 2-wire
Correct Option: C
Explanation: Three-phase three-wire system is the most economical in terms of conductor material.
Q4. Which conductor material is most commonly used for overhead transmission lines?
A) Copper
B) Aluminium
C) Steel
D) Bronze
Correct Option: B
Explanation: Aluminium is light, economical, and offers good conductivity per unit weight.
Q5. The main reason for using ACSR conductors is:
A) High conductivity
B) Low resistance
C) High tensile strength
D) Low cost only
Correct Option: C
Explanation: Steel core provides high tensile strength while aluminium provides conductivity.
Q6. Underground cables generally use which conductor?
A) Steel
B) Aluminium only
C) Copper
D) ACSR
Correct Option: C
Explanation: Copper has high conductivity and better thermal performance in underground conditions.
Q7. Compared to overhead system, underground system has:
A) Lower initial cost
B) Higher maintenance
C) Higher reliability
D) Higher corona loss
Correct Option: C
Explanation: Underground cables are protected from weather, giving higher reliability.
Q8. Which system has maximum reliability?
A) DC overhead
B) AC overhead
C) AC underground
D) Single-wire system
Correct Option: C
Explanation: AC underground systems are fully insulated and protected.
Q9. For same power transmission, which has least losses?
A) Low voltage DC
B) High voltage DCC
C) Low voltage AC
D) High current AC
Correct Option: B
Explanation: High voltage DC transmits power at low current, minimizing I²R losses.
Q10. Skin effect is:
A) Higher in DC
B) Higher in AC
C) Same in AC & DC
D) Zero in AC
Correct Option: B
Explanation: Skin effect occurs only in AC systems due to alternating current distribution.
Q11. A 3-phase, 50 Hz system transmits 10 MW at 33 kV (pf = 0.8). Line current is:
A) 175 A
B) 219 A
C) 262 A
D) 315 A
Correct Option: B
Explanation: Using P = √3 V I cosφ, current ≈ 219 A.
Q12. Power transmitted by single-phase system is 5 MW at 11 kV. Find current.
A) 262 A
B) 454 A
C) 568 A
D) 657 A
Correct Option: B
Explanation: I = P/V = 5×10⁶ / 11×10³ ≈ 454 A.
Q13. If transmission voltage is doubled keeping power constant, line losses will:
A) Double
B) Remain same
C) Reduce to half
D) Reduce to one-fourth
Correct Option: D
Explanation: Loss ∝ I². Doubling voltage halves current, reducing loss to one-fourth.
Q14. Which material gives minimum sag for same conductor length?
A) Copper
B) Aluminium
C) ACSR
D) Steel
Correct Option: C
Explanation: ACSR has high tensile strength and lower effective weight.
Q15. A DC 2-wire system transmits power P at voltage V. If same power is transmitted using 3-phase AC at same voltage, ratio of conductor material required is:
A) 1 : 1
B) 1.5 : 1
C) 2 : 1
D) 3 : 1
Correct Option: B
Explanation: DC system requires about 1.5 times conductor material compared to 3-phase AC.
Q16. For same power and same losses, ratio of conductor material required for single-phase AC to three-phase AC is:
A) 1
B) 1.33
C) 1.5
D) 2
Correct Option: B
Explanation: Theoretical ratio is 4/3 ≈ 1.33.
Q17. A 3-phase system transmits power P. If converted to DC with same losses and voltage, power transmitted will:
A) Increase
B) Decrease
C) Remain same
D) Become zero
Correct Option: A
Explanation: DC transmission has no skin effect and no reactive power.
Q18. Which conductor is preferred for underground cables and why?
A) Aluminium – low cost
B) Copper – high conductivity
C) Steel – high strength
D) ACSR – low losses
Correct Option: B
Explanation: Copper offers high conductivity and compact size for underground use.
Q19. Which transmission system requires minimum insulation cost?
A) High voltage AC
B) Low voltage AC
C) High voltage DC
D) Single-phase AC
Correct Option: C
Explanation: HVDC requires less insulation due to absence of alternating stress.
Q20. Corona loss is:
A) Higher in underground cables
B) Higher in DC
C) Higher in AC overhead lines
D) Zero in overhead
Correct Option: C
Explanation: Corona occurs mainly in high-voltage AC overhead lines.