Structure of Power System
1. Generating Station
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The place where electric power is produced.
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Examples: Thermal, Hydro, Nuclear, Solar power plants.
2. Transmission Network
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Carries electricity from generating stations over long distances at high voltage (220 kV, 400 kV, 765 kV).
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Includes transmission lines, towers, and substations.
3. Distribution Network
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Delivers electricity from the transmission system to homes, shops, and industries.
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Operates at low voltage (11 kV, 415 V, 230 V).
A Single Line Diagram (SLD) is a simplified drawing that shows how electrical power flows from the generating station to the consumer using only one line to represent all three phases.
It does not show detailed wiring — only major electrical components and their connections.
Main Parts in a Transmission SLD
1. Generating Station
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Produces electrical power (3-phase AC).
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Voltage is usually 11 kV or 33 kV at the generator terminals.
2. Step-Up Transformer
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Increases voltage from 11/33 kV to high voltage
(132 kV / 220 kV / 400 kV / 765 kV). -
High voltage is used to reduce losses during long-distance transmission.
3. Transmission Line
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Carries the high-voltage power over long distances.
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Shown as a single line in SLD, even though it is actually 3-phase.
4. Step-Down Substation
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Reduces voltage from high transmission level to medium voltage
(132 kV → 33 kV OR 220 kV → 66 kV etc.). -
Contains circuit breakers, isolators, CTs, PTs, relays.
5. Distribution Transformer
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Further reduces voltage to low voltage:
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33 kV → 11 kV
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11 kV → 415 V (3-phase)
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415 V → 230 V (single-phase)
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6. Consumers
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Homes: 230 V
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Shops/Small industries: 415 V
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Large industries: 11 kV or above
Types of Transmission
Methods of Arrangement of Conductors
MCQs on Overhead Transmission Line & Underground Cable
1. Which of the following is used for long-distance electric power transmission?
(a) Overhead transmission line
(b) Underground cable
(c) Both are same
(d) None
Answer: (a) Overhead transmission line
Explanation:
Overhead lines are cheaper, easy to maintain, and suitable for high-voltage long-distance transmission (220 kV, 400 kV, 765 kV). Underground cables are too costly for long distances.
2. Which system has more chances of faults due to weather conditions?
(a) Underground system
(b) Overhead system
(c) Both have same
(d) Depends on voltage
Answer: (b) Overhead system
Explanation:
Overhead lines are exposed to environment → affected by wind, lightning, storms, falling trees, pollution, etc. Underground cables are protected from weather.
3. Which transmission system has higher installation cost?
(a) Overhead line
(b) Underground cable
(c) Both same
(d) Can’t be determined
Answer: (b) Underground cable
Explanation:
Underground cables require trenching, ducts, insulation, jointing work → cost is 5 to 10 times higher than overhead lines.
4. Heat dissipation is better in which system?
(a) Underground cables
(b) Overhead transmission lines
(c) Both same
(d) Depends on material
Answer: (b) Overhead transmission lines
Explanation:
Overhead conductors are exposed to open air → cooling is more effective. Underground cables are surrounded by soil → poor cooling → lower current rating.
5. Which system is preferred in densely populated city areas?
(a) Overhead lines
(b) Underground cables
(c) Both
(d) None
Answer: (b) Underground cables
Explanation:
Overhead lines are unsafe in crowded areas and look messy. Underground cables improve safety and appearance.
6. Which of the following is NOT an advantage of overhead transmission?
(a) Lower installation cost
(b) Easy fault detection
(c) Aesthetic appearance
(d) Easy maintenance
Answer: (c) Aesthetic appearance
Explanation:
Overhead lines look bad and cause visual pollution. Other three points are genuine advantages.
7. Which transmission system has the highest reliability?
(a) Underground system
(b) Overhead system
(c) Both equal
(d) Depends on voltage
Answer: (a) Underground system
Explanation:
Underground cables are protected from wind, lightning, trees, and birds → fewer faults → higher reliability.
8. Overhead transmission lines mostly use which type of conductor?
(a) PVC
(b) ACSR
(c) Rubber
(d) Armoured cable
Answer: (b) ACSR (Aluminium Conductor Steel Reinforced)
Explanation:
ACSR is strong, lightweight, and economical → widely used in long-distance transmission.
9. Which system requires more maintenance?
(a) Overhead lines
(b) Underground cables
(c) Both same
(d) None
Answer: (a) Overhead lines
Explanation:
Birds, trees, storms, pollution, insulator cracks → overhead lines need frequent patrol and maintenance compared to underground cables.
10. In underground cables, the majority of losses occur due to—
(a) Corona
(b) Dielectric loss
(c) Skin effect
(d) Air breakdown
Answer: (b) Dielectric loss
Explanation:
UG cables use insulation like XLPE, paper, PVC → dielectric heating causes losses.
Corona loss occurs mainly in overhead lines at high voltage.
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