11. Transmission line connects
(a) generating station -to a switching station/step-down transformer station.
(b) step-down transformer station to service transformer banks.
(c) distribution transformer to consumer premises.
(d) service points to consumer premises.
Answer: (a) generating station -to a switching station/step-down transformer station.
Explanation:
- Transmission lines are used for power transmission.
- These connect generating station to a switching station or step-down transformer station.
- In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner.
- Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas (they are then called feed lines or feeders), distributing cable television signals, trunklines routing calls between telephone switching centers, computer network connections and high-speed computer data buses.
- RF engineers commonly use short pieces of transmission line, usually in the form of printed planar transmission lines, arranged in certain patterns to build circuits such as filters.
- These circuits, known as distributed-element circuits, are an alternative to traditional circuits using discrete capacitors and inductors.
12. A 3 -phase, the 4-wire system is commonly used for
(a) primary distribution.
(b) secondary distribution.
(c) primary transmission.
(d) secondary transmission.
Answer: (b) secondary distribution.
Explanation:
1-phase 2-wire: In this, one of the two wires is earthed, or the mid-point of phase winding is earthed. This system is used for very short distances
1-phase 3-wire: This system is identical in principle to the 3-wire dc distribution system. The neutral wire is center-tapped from the secondary winding of the transformer and earthed. This system is also called a split-phase electricity distribution system
2-phase 3-wire: In this system, the neutral wire is taken from the junction of two-phase windings whose voltages are in quadrature with each other. The voltage between the neutral wire and either of the outer phase wires is V whereas, the voltage between outer phase wires is √2V. As compared to a two-phase 4-wire system, this system suffers from voltage imbalance due to unsymmetrical voltage in the neutral
3-phase 4-wire: This system uses star-connected phase windings, and the fourth wire or neutral wire is taken from the star point. If the voltage of each winding is V, then the line-to-line voltage (line voltage) is √3V and the line-to-neutral voltage (phase voltage) is V. This type of distribution system is widely used in India for secondary distribution.
13. The rated voltage of a 3-phase power system is given as
(a) RMS phase voltage.
(b) peak phase voltage.
(c) RMS line to line voltage.
(d) peak line to line voltage.
Answer:
Explanation:
- The rated voltage of a single-phase system is given as rms value.
- The rated voltage of a three-phase power system is given as rms line to line voltage.
- The root-mean-square (rms) voltage of a sinusoidal source of electromotive force (Vrms) is used to characterize the source.
- It is the square root of the time average of the voltage squared.
14. Which of the following is usually not the generating voltage?
(a) 6.6 kV.
(b) 9.9 kV.
(c) 11 kV.
(d) 13.2 kV.
Answer: (b) 9.9 kV.
Explanation:
- The standard voltages of transmission in India are 11 kV, 22 kV, 33 kV, 66 kV, 132 kV, 220 kV, 400 kV, 765 kV.
- The standard voltages of generation in India are 6.6 kV, 11 kV, 13.2 kV.
- There is no universal standard for voltages; for example, India and much of South Asia uses 33 kV, 11 kV and 400 volt (V) as distribution voltages.
- Thailand uses 22 kV and 380 V as distribution voltages.
15. In a transmission system the feeder supplies power to
(a) transformer substations (step-up).
(b) service mains.
(c) distributors.
(d) all of the above
Answer: (c) distributors.
Explanation:
- Feeders are the conductors which connect the generating stations or substations to where the power supply requires or power to be distributed.
- There is no tapping are taken to the consumer from the feeders. So current loading of the feeders is remaining the same along its entire length.
- The current at sending end remains same as the current at receiving end.
- The feeders are designed from the point of view of its current carrying capacity.
- The cross-sectional area of the feeder conductor depends upon the current passes through them because feeders are designed from the point of view of its current carrying capacity.
- The feeders transport the power from generating station or substation to distributors.
- The high current carrying capacity of the conductor is very important in the design of the feeders.
16. Feeder is designed mainly from the point of view of
(a) its current carrying capacity.
(b) voltage drop in it.
(c) operating voltage.
(d) operating frequency.
Answer: (a) its current carrying capacity.
Explanation:
- A feeder is a conductor which connects the sub-station (or localized generating station) to the area where power is to be distributed.
- Generally, no tapings are taken from the feeder so that current in it remains the same throughout.
- The main consideration in the design of a feeder is the current carrying capacity.
17. Distributors are designed from the point of view of
(a) its current carrying capacity. -
(b) operating voltage.
(c) voltage drop in it.
(d) operating frequency.
Answer: (c) voltage drop in it.
Explanation:
- The distributor are the conductors from where a number of the tapping are taken to supply the power to the consumer.
- The current loading of the distributors are varies on its entire length. So distributors conductors are designed from the point of view of the voltage drop in it.
- The distributors are designed by considering the voltage variations or voltage regulations.
- The voltage at the consumer's terminal should be maintained within the (-+6). The distributor supply power to the service mains.
18. Transmission and distribution of electric power by the underground system is superior to the overhead system in respect of
(a) appearance and public safety.
(b) maintenance cost.
(c) frequency of faults, power failure, and accidents.
(d) all of the above.
Answer: (d) all of the above.
Explanation:
- Lesser transmission losses
- Less affected by extreme weather conditions and hence increases the reliability of supply of power
- Reduced visual impact due to being below the ground
- No interference with telephone lines
- Reduced EMFs (Electric and Magnetic Fields) and hence eliminates potential health issues
- Can be used in highly congested areas
- appearance and public safety.
19. The main drawback(s) of the underground system over, the overhead system is/are
(a) exposure to lightning.
(b) heavy initial cost.
(c) exposure to atmospheric hazards such as smoke, ice, wind etc.
(d) inductive interference between power and communication circuits.
Answer: (b) heavy initial cost.
Explanation:
- The cost of underground cables are higher compare to overhead lines
- Laying or burying costs of underground lines are greater compare to overhead lines
- Less flexible compared to overhead lines
- Difficult to find and repair the wire breaks in case of failure of the system
- Unlike overhead lines which can easily be updated to carry more power, underground lines cannot be updated to increase the capacity
20. The main drawback(s) of the overhead system over underground system is/are
(a) the underground system is more flexible than overhead system.
(b) higher charging current.
(c) surge problem.
(d) high initial cost.
Answer: (c) surge problem.
Explanation:
- For a particular amount of power transferred at a given voltage the underground cable system costs almost double the cost of the overhead system. Hence the underground system is more expensive.
- The underground cable system is safer compared to the overhead transmission system.
- The maintenance work of underground cables is very complex while it is very simple in the case of overhead lines.
- Because the cables are laid underground hence the failure or faults are less as compared to the overhead system.
- The chances of accidents in the underground systems are very low as compared to overhead systems.
- The appearance of the underground cable system is good and neat because no wiring is visible as compared to overhead lines.
- Fault location and repairs are difficult and expensive in the case of underground cables as compared to the overhead lines.
- It is difficult to join underground cables in case of conductor breakage, whereas the conductors in the overhead system can be easily joined.
- In the case of an underground cabling system, there is no interference to communication lines as compared to overhead transmission lines.
- The underground cable requires a very high degree of insulation, hence it can not be operated above 132 kV. Whereas, the overhead transmission lines have capable of transmitting power at 765 kV or even higher.
- The underground cable system is free from lighting and thunderstorm as compared to the overhead transmission system.
- The voltage drop is low in the case of underground cables as compared to an overhead line.
- Because of less spacing between conductors in the cable, it causes much capacitance, hence it has more charging current as compared to overhead lines.
- In underground cables, the surge effect is smoothened down as surge energy is absorbed by the sheath.
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