1. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency:
- will increase
- will decrease
- remains unchanged
- may increase or decrease depending on the line parameters
Explanation
String efficiency is inversely proportional to frequency. At higher frequencies, the insulator string becomes more capacitive, reducing voltage across the string and thereby decreasing string efficiency.
2. Sheaths are used in underground cables to:
- Reduce current
- Reduce dielectric losses
- Protect against moisture and gases
- Act as a neutral conductor
Explanation
Lead or aluminum sheaths protect underground cables from environmental threats like moisture, acids, and gases. They also reduce dielectric stress and can act as metallic screens.
3. In suspension type insulators, the potential drop is:
- Uneven across the discs
- Equal across each disc
- Zero
- Maximum at the bottom disc
Explanation
Due to shunt capacitance, voltage across each disc varies, resulting in unequal voltage distribution.
4. Voltage drop is the main consideration while designing a:
- Feeder
- Distributor
- Service mains
- Earth wire
Explanation
While designing distributors, voltage drop is crucial to maintain proper voltage at consumer ends.
5. Skin effect exists only in:
- DC systems
- Low voltage systems
- High resistance conductors
- AC systems
Explanation
In AC, current flows more on the conductor surface as frequency increases, reducing effective cross-sectional area.
6. The conductor material used for transmission and distribution should NOT have:
- High specific gravity
- Low cost
- High electrical conductivity
- High tensile strength
Explanation
Conductors should be lightweight; high specific gravity increases the cost and weight of transmission lines.
7. The capacitance per km of a 3-phase belted cable is 0.09 F. Core-to-neutral capacitance for 5 km is:
Explanation
Core-to-neutral capacitance for 5 km = 0.09 × 5 = 0.45 F. As the configuration is equivalent to two phases, effective capacitance becomes ~0.9 F.
8. In nominal T-configuration of a medium transmission line, the shunt capacitance is:
- Equally distributed
- Infinite
- Zero
- Concentrated at the middle
Explanation
Shunt capacitance is assumed to be concentrated at the midpoint with resistance and reactance lumped equally on either side.
9. If maximum and minimum dielectric stress are 80 kV/cm and 20 kV/cm and conductor diameter is 4 cm, find insulation thickness:
Explanation
Using gmax/gmin = D/d, D = 4×(80/20) = 16 cm, so insulation thickness = (16 - 4)/2 = 6 cm.
10. The formula for sag (S) in a conductor is:
- S = WL² / 8T
- S = WL² / 2T
- S = TL² / 8W
- None of the above
Explanation
For supports at the same level, sag S is given by S = WL² / 8T where W = weight per unit length, L = span length, T = tension.
11. Which of the following statements is/are true?
- (b) and (c) only
- (a) and (b) only
- (a), (b) and (c)
- (a) only
Explanation
Feeders are designed based on current-carrying capacity. Distributors are designed based on permissible voltage drop. Statement (c) is false.
12. Which statements are correct regarding AC and DC distribution systems?
- Only (a)
- (a), (b) and (c)
- Only (a) and (c)
- Only (a) and (b)
Explanation
In DC systems, voltage drop is due only to resistance. In AC systems, both resistance and inductance cause voltage drops.
13. A short transmission line has length:
- 250 km to 400 km
- Up to 80 km
- 80 km to 250 km
- More than 400 km
Explanation
Short lines are less than 80 km long; shunt capacitance is usually neglected.
14. Stringing chart represents a graph of:
- Sag and tension vs temperature
- Sag vs supply frequency
- Sag vs conductor size
- Tension vs sag
Explanation
Stringing charts help determine sag and tension of conductors under different temperature conditions.
15. Corona loss in DC transmission supply is:
- 10 times more than AC
- Infinite
- Zero
- Less than AC
Explanation
Corona loss is proportional to frequency. DC (0 Hz) has less corona loss than AC (50 Hz).
16. Grading of cable is done to:
- Increase insulation quantity
- Achieve uniform stress
- Reduce insulation quality
- Achieve non-uniform stress
Explanation
Grading (capacitance/intersheath) ensures uniform electric stress, increasing cable life and efficiency.
17. Line support used in transmission/distribution should:
- (a) and (c) only
- (a) and (b) only
- (a), (b) and (c)
- (b) and (c) only
Explanation
Supports must be strong, long-lasting, and lightweight for structural and economic efficiency.
18. ABCD parameters for a short transmission line (Z = 20 Ω) are:
- 20, 1, 0, 20
- 1, 1, 1, 1
- 1, 1, 20, 0
- 1, 20, 0, 1
Explanation
For a short line, A = D = 1, B = Z, C = 0 → A = 1, B = 20, C = 0, D = 1
19. Cable rating to connect 3 kW load to 230 V single-phase supply:
Explanation
P = VI → I = 3000 / 230 ≈ 13 A. Closest standard rating = 15 A.
20. Extra high-tension cables are used for voltages:
- 0 to 1 kV
- Beyond 400 kV
- 33 kV to 66 kV
- 132 kV to 400 kV
Explanation
Extra High Tension (EHT) cables serve 33 kV to 66 kV systems.
21. Which statement is INCORRECT? (Distribution Design)
- Tapings are taken from the distributor
- Voltage drop not considered
- Voltage drop is main design factor
- Service mains connect to consumers
Explanation
Voltage drop is a critical parameter and must be considered when designing distributors.
22. Which statement is INCORRECT? (Substations)
- Future expansion easier in outdoor
- Outdoor takes less erection time
- Outdoor needs more space
- Outdoor is costlier than indoor
Explanation
Indoor substations are more costly due to their complex construction.
23. System used for secondary transmission of electric power:
- 33-kV, 3-phase, 4-wire
- 66-kV, 3-phase, 4-wire
- 33-kV, 3-phase, 3-wire
- 110-kV, 3-phase, 3-wire
Explanation
33 kV 3-phase 3-wire system is standard for secondary transmission.
24. Motor used for power factor correction:
- DC series motor
- Stepper motor
- Synchronous motor
- Induction motor
Explanation
Synchronous motors can operate at leading power factor, improving overall PF of the system.
25. Choose the INCORRECT statement regarding DC transmission:
- Less corona loss
- Less insulation stress
- High DC voltage generation
- No skin effect
Explanation
High DC voltages are not generated directly; instead, AC is converted to DC using converters.
26. A three-phase four-wire system is used for:
- Secondary distribution
- Generation
- Transmission
- Primary distribution
Explanation
Three-phase 4-wire systems allow both 3-phase and single-phase loads, ideal for secondary distribution.
27. Inductance per conductor in 3-phase line is ___ times that of single-phase loop:
Explanation
In a 3-phase line, per-conductor inductance is half the loop inductance of a single-phase line.
