Transmission and Distribution Objective Questions with Easy Explanation

1. ……………. are the conductors, which connect the consumer’s

A. Terminals to the distribution
B. Distributors
C. Service mains
D. Feeders

Answer: (C) Service mains

Explanation:

Feeder:

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.

Distributors:

A distributor is a conductor from which tapings are taken for supply to the consumers.
The current through a distributor is not constant because tapings are taken at various places along its length.
While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the consumers’ terminals.

Service mains:

A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
Service mains of the consumers may be either connected to the distributors or sub-distributors

2. Distributors 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:

Distributors are the conductors from which numerous tapping's are taken for providing power supply to the consumers.
Therefore the current loading on the distributors varies along the length.
So distributors are designed from the point of view of voltage drop in them.

3. Which of the following are connected by the service mains?

A. Transformer and earth

B. Distributor and relay system

C. Distributor and consumer terminals

D. Distributor and transformer

Answer: C. Distributor and consumer terminals

Explanation:

Service mains:

A service mains is generally a small cable which connects the distributor to the consumers’ terminals.
Service mains of the consumers may be either connected to the distributors or sub distributors.
Distributor and consumer terminals are connected by the service mains.

4. The 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:

Feeder:

A feeder is a conductor having constant current density.
The size of the feeder designed based on current-carrying capacity.
For V ≤ 220 kV, selection of conductor done based on the current-carrying capacity.
For V > 220 kV, the selection of conductor is done based on the concept of corona or electric field intensity.
In both cases, our concern is about power loss which leads to temperature rise in the conductor.
Hence, the selection of the size of the conductor for the feeder is designed based on temperature rise.
Size of the distributor designed based on voltage drop.

5. The underground system cannot be operated above
(A) 440 V
(B) 11 kV
(C) 33 kV
(D) 66 kV

Answer: (D) 66 kV

Explanation:

The underground system cannot be operated above 66 kV because of insulation difficulties but overhead system can be designed for operation up to 400 kV or higher even.

6. The usual spans with R.C.C. poles are
(A) 40 – 60 meters
(B) 80 – 100 meters
(C) 80 – 100 meters
(D) 300 – 500 meters

Answer: (C) 80 – 100 meters

Explanation:

Wooden poles:

These are made of seasoned wood (sal or chir) and are suitable for lines of the moderate X-sectional area and of relatively shorter spans, say up to 50 meters
Such supports are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economic proposition
The wooden poles generally tend to rot below the ground level, causing foundation failure
In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil
They have a comparatively smaller life (20-25 years) and cannot be used for voltages higher than 20 kV
They have less mechanical strength and require periodical inspection

Steel Poles:

They are used for system voltages up to 33 kV in low and high-voltage distribution systems
When compared to wooden poles steel poles have advantages like lightweight, long life, and greater strength
These are used for a longer span, i.e., from 50 to 80 m
These are costlier than wooden and RCC poles
All steel supports should be well-galvanized and have a life of at least 30 years

Concrete poles (RCC Poles):

Reinforced concrete poles have become very popular as line supports in recent years.
They have greater mechanical strength, longer life, and permit longer spans than steel poles.
Moreover, they give a good outlook, require little maintenance, and have good insulating properties.
The maximum permissible span for RCC poles is 80 - 100 meters.
The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation

Steel Towers:

These towers are robust in construction
They can be used for spans 300 m or above
They are used for the transmission of power above 66 kV and are more useful for valleys, railway lines, rivers, etc
They are mechanically very strong and have a longer life than steel poles
They are capable of withstanding the most severe climatic conditions and cannot be destroyed by forest fires

7. The corona is considerably affected by which of the following?
(A) Size of the conductor
(B) Shape of the conductor
(C) Surface condition of the conductor
(D) All of the above

Answer: (D) All of the above

Explanation:

A corona discharge is an electrical discharge brought on by the ionization of a fluid such as air surrounding a conductor that is electrically charged.
Spontaneous corona discharges occur naturally in high-voltage systems unless care is taken to limit the electric field strength. It is affected by the size of the conductor, shape of the conductor and surface condition of the conductor.

Effect of supply voltage:

If the supply voltage is high corona loss is higher in the lines. In low-voltage transmission lines, the corona is negligible, due to the insufficient electric field to maintain ionization.

The condition of conductor surface:

If the conductor is smooth, the electric field will be more uniform as compared to the rough surface.
The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc.
Thus, rough line decreases the corona loss in the transmission lines.

Air Density Factor:

The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air.
Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.

Effect of system voltage:

Electric field intensity in the space around the conductors depends on the potential difference between the conductors.
If the potential difference is high, electric field intensity is also very high, and hence corona is also high. Corona loss, increase with the increase in the voltage.

The spacing between conductors:

If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor.
If the distance between them is extended beyond certain limits, the dielectric medium between them get decreases and hence the corona loss also reduces.

8. Which of the following are the constants of the transmission lines?
(A) Resistance
(B) Inductance
(C) Capacitance
(D) All of the above.

Answer: (D) All of the above

Explanation:

a b c d constants of the transmission line( Primary line constants)
The line constants are parameters that describe the characteristics of conductive transmission lines.

The primary line has the following constants

R = Resistance per unit length Ω
L = Inductance per unit length (Henry) H
C = Capacitance per unit length (Henry) H
G = Conductance per unit length ℧

All these constants are independent of frequency, therefore, they are called primary constants and these constants are measured by considering both the wires of the transmission lines.
R and L elements are in series with the line (because they are properties of the conductor) and C and G are elements shunting the line (because they are properties of the dielectric material between the conductors).
G represents leakage current through the dielectric and in most cables is very small.

9. The phenomenon of rising in voltage at the receiving end of the open-circuited or lightly loaded line is called the
(A) See-back effect
(B) Ferranti effect
(C) Raman effect
(D) none of the above.

Answer: (B) Ferranti effect

Explanation:

Ferranti Effect:

At no load (or) at light load, the voltage at the receiving end of the transmission line is more than the sending voltage.
It is known as the Ferranti effect. It is due to the charging current of the line.

How to reduce Ferranti effect:

This effect can be controlled by placing the shunt reactors at the receiving end of the lines.
A shunt reactor is an inductive current element connected between line and neutral to compensate for the capacitive current from transmission lines.
Synchronous phase modifier(SPM) is used as under excitation it works similarly to shunt reactor which absorbs lagging reactive power and delivers leading reactive power.

Important Notes:

See-back effect:

The See-beck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.

Raman Effect:

Raman effect, change in the wavelength of light that occurs when a light beam is deflected by molecules. When a beam of light traverses a dust-free, transparent sample of a chemical compound, a small fraction of the light emerges in directions other than that of the incident (incoming) beam.

10. The square root of the ratio of line impedance and shunt admittance is called
(A) surge impedance of the line
(B) conductance of the line
(C) regulation of the line
(D) none of the above.

Answer: (A) surge impedance of the line

Explanation:

The transmission line generates capacitive reactive volt-amperes in its shunt capacitance and absorbing reactive volt-amperes in its series inductance.
The load at which the inductive and capacitive reactive volt-amperes are equal and opposite, such load is called surge impedance load.
It is also called natural load of the transmission line because power is not dissipated in transmission. In surge impedance loading, the voltage and current are in the same phase at all the point of the line.
When the surge impedance of the line has terminated the power delivered by it is called surge impedance loading.
Shunt capacitance charges the transmission line when the circuit breaker at the sending end of the line is close.
The series inductance of the line consumes the electrical energy when the sending and receiving end terminals are closed.
Surge impedance loading is also defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.
Surge impedance loading depends on the voltage of the transmission line. Practically surge impedance loading always less than the maximum loading capacity of the line.
If the load is less than the SIL, reactive volt-amperes are generated, and the voltage at the receiving end is greater than the sending end voltage. On the other hand, if the SIL is greater than the load, the voltage at receiving end is smaller because the line absorbs reactive power.
If the shunt conductance and resistance are neglected and SIL is equal to the load than the voltage at both the ends will be equal.
The square root of the ratio of line impedance and shunt admittance is called surge impedance of the line
The shunt admittance of a line consists of the conductance and the capacitive susceptance. The conductance is usually ignored because it is very small compared to the capacitive susceptance. The capacitance of a line is the result of the potential difference between conductors.

11. Transmission line connects

(a) generating station -to a switching station/step-down transformer station.

(b) step-down transformer station to service transformer banks.

(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.

(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.

(d) peak line to line voltage.

Answer: (c) RMS line to line voltage.

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.

(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.

(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.

(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.

(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.

(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.

(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.

(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.

21. By increasing the transmission voltage double of its original value, the same power can be dispatched keeping the line loss

(a) equal to its original value.

(b) half of original value.

(d) one-fourth of the original value.

(e) double the original value.

Answer: (d) one-fourth of the original value.

Explanation:

The line losses are inversely proportional to the square of voltage and power factor.

P_L ∝ 1/V²

Now voltage is doubled

V₂ = 2V₁

P_L1/P_L2 = (V₂/V₁)²

= (2V₁/V₁)²

P_L2 = P_L2/4

22. If a fixed amount of power is to be transmitted over certain length with fixed power loss, it can be said that volume of conductor is

(a) inversely proportional to magnitude of the voltage and that of power factor of the load.

(b) inversely proportional to square of the voltage and square of power factor of the load.

(d) proportional to magnitude of the voltage only.

Answer: (b) inversely proportional to square of the voltage and square of power factor of the load.

Explanation:

The volume of copper required is inversely proportional to the square of the transmission voltage and the power factor.
Thus greater the transmission voltage level, lesser is the volume of copper required i.e. the weight of copper used for the conductors. The conductor material required is less, for higher transmission voltage.

23. For the same voltage drop, increasing the voltage of a distributor n-times

(a) reduces the x-section of the conductor by n times.

(b) increases the x-section of the conductor by n times,

(d) increases the x-section of the conductor by n² times.

Answer: (a) reduces the x-section of the conductor by n times.

Explanation:

R=ρlA

R = resistance, l = length, A = area of cross-section and ρ = resistivity

SI unit of resistance is the ohm (Ω).

Voltage drop = I R

Active power = V I cos ϕ

Application:

Given V2 = n V1

⇒ V2 / V1 = n

Since there is no change in the connected load, to maintain constant power transfer,

P2 = P1

V2 I2 cos ϕ2 = V1 I1 cos ϕ1

Since the type of connected load is also the same, the power factor is also constant.

⇒ I2 = I1 / n

Given that the voltage drop is same,

I2 R2 = I1 R1

R2 = n R1

ρl / A2 = nρl / A1

A2 = A1 / n

Hence, reduces the x-section of the conductor by n-times

24. In a transmission system, the weight of copper used is proportional to

(a) E².

(b) E.

(d) 1/E.

(e) none of the above.

Answer: (c) 1/E².

Explanation:

If power is transmitted, length of the line and loss in the transmission line is constant then the volume of required material will be inversely proportional to the square of supply voltage and power factor.
Hence, the volume of conductor material = 1/E²

25. The volume of copper required for an ac transmission line is inversely proportional to

(a) current.

(b) voltage.

(d) both (b) and (c).

Answer: (d) both (b) and (c).

Explanation:

P = 3 (P / √3 V Cos θ)² x [ρ (l /A)]

The volume of copper required for an AC transmission line is inversely proportional to voltage, power factor and proportion to the current.

26. Improving pf

(a) reduces current for a given output.

(b) increases losses in line.

Answer: (a) reduces current for a given output.

Explanation:

Improving the power factor results in less current being drawn, therefore less electricity costs, less heat and greater longevity of the electrical system. the installation, thus reduces the maximum demand tariff and thereby reducing your power costs.

27. For a given amount of power to be transmitted over a certain distance with fixed power loss, the volume of copper required is

(a) directly proportional to voltage.

(b) inversely proportional to voltage.

(d) directly proportional to the square of the voltage and pf of the load.

Answer: (c) inversely proportional to the square of voltage and pf of the load.

Explanation:

P = 3 (P / √3 V Cos θ)² x [ρ (l /A)]

P = 3 (P² / 3 V² Cosθ²) x ρ (l /A)

P = W = P² ρ l / V² Cosθ² A

If power transmitted, length of the line, and loss in the transmission line is constant then the volume of required material will be inversely proportional to the square of supply voltage and power factor.

28. For the same conductor length, same amount of power, same losses and same maximum voltage to earth, which system requires minimum conductor area?

(a) single phase ac

(b) 3 phase ac

(d) 3 wire dc

Answer: (d) 3 wire dc

Explanation:

For the same conductor length, same amount of power, same losses and same maximum voltage to earth, 3 wire DC system requires minimum conductor area.
For transmitting the same amount of power at the same voltage, a three-phase transmission line requires less conductor material than a single-phase line. The three-phase transmission system is so cheaper.
For a given amount of power transmitted through a system, the three-phase system requires conductors with a smaller cross-sectional area. This means a saving of copper and thus the original installation costs are less.

Important Point:

Below is given the table which shows the ratio of conductor-material in any system compared with that in the corresponding 2-wire DC system. Cos φ is the power factor in an AC system.

System	Same maximum voltage to earth	Same maximum voltage between conductors
DC system: Two wire	1	1
DC: Two wire mid-point earthed	0.25	1
DC: 3 wire	0.3125	1.25
Single phase: 2 wire	2/cos2ϕ	2/cos2ϕ
Single phase: 2 wire mid-point earthed	0.5/cos2ϕ	2/cos2ϕ
Single phase: 3 wire	0.625/cos2ϕ	2.5/cos2ϕ
2-phase: 4 wire	0.5/cos2ϕ	2/cos2ϕ
2-phase: 3 wire	1.457/cos2ϕ	2.914/cos2ϕ
3 phase, 3 wire	0.5/cos2ϕ	1.5/cos2ϕ
3 phase, 3 wire	0.583/cos2ϕ	1.75/cos2ϕ

29. Which of the following distribution systems is preferred for good efficiency and high economy?

(a) Single-phase, 2-wire system.

(b) 2-phase, 3-wire system.

(d) 3-phase, 4-wire system.

Answer: (d) 3-phase, 4-wire system.

Explanation:

There is a great saving in conductor material if DC system is adopted for transmission of electric power. However, due to technical difficulties, DC system is not used for transmission.
Considering the AC system, the 3-phase AC system is most suitable for transmission as well as distribution due to two reasons. Firstly, there is a considerable saving in conductor material. Secondly, this system is convenient and efficient.

30. The approximate cost ratio of a 220 kV underground cable transmission and 220 kV overhead transmission is

(a) 50

(b) 25

(d) 5

Answer: (c) 13

Explanation:

The approximate cost ratio of a 220 kV underground cable transmission and 220 kV overhead transmission is 13.

For 11KV System

Underground network installation is more expensive than OH lines since the cost of cables includes cable charges along with road restoration charges which make the per-unit cost of the UG cabling system several times greater than the overhead system.
The estimated cost of the UG cabling system is about 3-4 times than the equivalent OH system ( like the Est cost of 11 kV OH S/C line with dog conductor is around Rs 5-6 Lakh/ km while the Est cost of 1 km of 3 x300 sq-mm 11 kV cabling system would be around Rs 20 Lakh/km).

31. With the same maximum voltage to earth, which of the following ac systems with 0.8 pf will need more copper in comparison to dc 2-wire system?

(a) Single-phase, 2-wire (midpoint earthed).

(b) Single-phase. 3-wire (neutral half of outer).

(d) Three-phase, 4-wire (neutral = outer).

Answer: (d) Three-phase, 4-wire (neutral = outer).

Explanation:

With the same maximum voltage to earth, Three-phase, 4-wire (neutral = outer) ac systems with 0.8 pf will need more copper in comparison to dc 2-wire system.
Volume of conductor material required is 0.3125 times of that required in two-wire dc system with one conductor earthed.

32. The main reason for using high voltage for long-distance power transmission is

(a) reduction in transmission losses.

(b) reduction in time of transmission.

(d) none of the above.

Answer: (a) reduction in transmission losses.

Explanation:

The primary reason that power is transmitted at high voltages is to increase efficiency.
As electricity is transmitted over long distances, there are inherent energy losses along the way.
High voltage transmission minimizes the amount of power lost as electricity flows from one location to the next.
if voltage increased then line loss will decrease and vice versa.

33. 66 kV is suitable for transmission of power over

(a) 30 km

(b) 60 km

(d) 200 km

Answer: (b) 60 km

Explanation:

Line-to-line voltage (kV)	Length of line in km
Line-to-line voltage (kV)	Minimum	Maximum
11	15	30
33	30	60
66	40	120
110	50	140
132	50	160
166	80	180
230	100	300
400	300	400

66 kV is suitable for transmission of power over 60 km.

34. If 3 MW power is to be transmitted over a distance of 30 km, the desirable transmission voltage will be

(a) 11 kV

(b) 33 kV

(d) 3.3 kV

Answer: (b) 33 kV

Explanation:

Desirable transmission Voltage V=5.5√(km/1.6+load in kvA/150)

Here total power is 3MW and 30km.

V = 5.5 √(30/1.6 + 3000/150)

V = 5.5 √(18.75 + 20)

V = 5.5 * √38.75

V = 5.5 * 6.2249

V = 34.54 kV

So approximate transmission voltage should be 33KV.

35. The highest transmission voltage used in India is

(a) 400 kV

(b) 220 kV

(d) 765 kV

Answer:

Explanation:

Transmission voltage in India (highest) is 765 kV AC and these lines are erected by Power Grid Corporation for interstate connections throughout India.
DC transmission voltage (highest) in India is 800 kV .
This has restrictions due to conversion i.e. AC/DC & inversion i.e. DC/AC for bulk power transmission.

36. If the variable part of annual cost on account of interest and depreciation on the capital outlay is equal to the annual cost of electrical energy wasted in the conductors, the total annual cost will be minimum, and the corresponding size of the conductor will be the most economical. This statement is known as

(a) Lenz's law.

(b) Faraday's law.

Answer: (a) Lenz's law.

Explanation:

If the variable part of annual cost on account of interest and depreciation on the capital outlay is equal to the annual cost of electrical energy wasted in the conductors, the total annual cost will be minimum, and the corresponding size of the conductor will be the most economical. This statement is known as Lenz's law.

37. For high-voltage transmission lines, conductors are suspended from towers so as to

(a) increase clearance from ground.

(b) reduce clearance from ground.

(d) reduce wind and snow effects.

Answer: (a) increase clearance from ground.

Explanation:

In high voltage transmission, the conductor lines are suspended from towers in order to increase clearance from the ground.
This is done due to:
Since conductors are loaded with heavy voltage & current so from the security point of view we suspend them from towers.
It helps to make suitable clearance height from the earth so that the insulation breakdown does not occur.

38. The supports used for transmission lines should have the characteristic(s) of

(a) high mechanical strength and longer life.

(b) good looking, light in weight and easily accessible for painting and erection of line conductors.

(d) all of the above.

Answer: (d) all of the above.

Explanation:

The line supports should have the high mechanical strength to withstand the load of the conductors, insulators, and wind load on the conductors and supports themselves.
The line supports should have lightweight without losing mechanical strength.
The line supports should be cheaper in cost.
The line supports should have a low maintenance cost.
The line supports should have a longer life,
The line supports should be good-looking.
The light supports should be easily accessible for painting and the erection of the conductors on it.

39. The wooden poles well impregnated with creosite oil or any preservative compound have life of

(a) 25-30 years.

(b) 20-25 years.

(d) 5-10 years.

Answer: (b) 20-25 years.

Explanation:

Wooden poles:

These are made of seasoned wood (sal or chir) and are suitable for lines of the moderate X-sectional area and of relatively shorter spans, say up to 50 meters
Such supports are cheap, easily available, provide insulating properties and, therefore, are widely used for distribution purposes in rural areas as an economic proposition
The wooden poles generally tend to rot below the ground level, causing foundation failure
In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil
They have a comparatively smaller life (20-25 years) and cannot be used for voltages higher than 20 kV
They have less mechanical strength and require periodical inspection

Steel Poles:

They are used for system voltages up to 33 kV in low and high-voltage distribution systems
When compared to wooden poles steel poles have advantages like lightweight, long life, and greater strength
These are used for a longer span, i.e., from 50 to 80 m
These are costlier than wooden and RCC poles
All steel supports should be well-galvanized and have a life of at least 30 years

Steel Towers:

These towers are robust in construction
They can be used for spans 300 m or above
They are used for the transmission of power above 66 kV and are more useful for valleys, railway lines, rivers, etc
They are mechanically very strong and have a longer life than steel poles
They are capable of withstanding the most severe climatic conditions and cannot be destroyed by forest fires

40. Steel poles for transmission lines need protection against

(a) borer.

(b) termites.

(d) all of these.

Answer: (c) corrosion.

Explanation:

Steel poles possess greater mechanical strength and thus permit the use of longer spans (60 to 80 metres).
These poles have the longer life (more than 40 years) which can further be increased by regular painting.
These poles need protection against corrosion. Hence at the bottom (the portion which is buried underground), these poles are set in concrete muffs in order to protect them from chemical reactions.

41. RCC poles usually have the spans of

(a) 250-400 m.

(b) 80-150 m.

(d) 25-50 m.

Answer: (b) 80-150 m.

Explanation:

Concrete poles:

Reinforced concrete poles have become very popular as line supports in recent years
They have greater mechanical strength, longer life and permit longer spans than steel poles
Moreover, they give a good outlook, require little maintenance and have good insulating properties.
The maximum permissible span for RCC poles is 80 - 150 meters.
The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation

42. In India for the distribution of electric power we usually use

(a) wooden poles.

(b) steel poles.

(d) both (b) and (c).

Answer: (d) both (b) and (c).

Explanation:

Steel Poles:

They are used for system voltages up to 33 kV in low and high-voltage distribution systems
When compared to wooden poles steel poles have advantages like lightweight, long life, and greater strength
These are used for a longer span, i.e., from 50 to 80 m
These are costlier than wooden and RCC poles
All steel supports should be well-galvanized and have a life of at least 30 years

Concrete poles:

Reinforced concrete poles have become very popular as line supports in recent years
They have greater mechanical strength, longer life and permit longer spans than steel poles
Moreover, they give a good outlook, require little maintenance and have good insulating properties.
The maximum permissible span for RCC poles is 80 - 150 meters.
The main difficulty with the use of these types of electric poles is the high cost of transport owing to their heavyweight
Therefore, such poles are often manufactured at the site in order to avoid the heavy cost of transportation

43. Conductors used in ht transmission lines are stranded because of

(a) increased tensile strength.

(b) ease in handling.

(d) reduced resistivity.

Answer: (b) ease in handling.

Explanation:

A stranded conductor is consisting of several thin wires of small cross-sectional area called strands.
At the center of stranded conductor, we are using steel conductor which provided the high tensile strength to conductor. In the outer layers of stranded conductor, we use aluminum conductors, which provide the conductivity to stranded conductor.

Characteristics of stranded conductors:

The stranded conductor is having sufficient flexibility, which makes stranded conductor suitable to be coiled easily to transport it over long distance.
For a stranded conductor of same cross-sectional area, the flexibility of conductor increases with increase of number of strands in conductor.
Stranded wire tends to be a better conductor than solid wire because the individual wires collectively comprise a greater surface area.
It is used when higher resistance to metal fatigue is required
The stranded conductor is formed by twisting the strands together in layers.
The strands of each layer are laid in helical fashion over the preceding layer. This process is called stranding.

Important Points:

Generally, in successive layer, the stranding is done in opposite direction to preceding layer. This mean, if the strands of one layer are twisted in clockwise direction, the strands of next layer will be twisted in anticlockwise direction and so on ‘x’ is number of layers in conductor.

Generally, the total number of strands in any conductor is given by the formulae of,

N = 3x² – 3x + 1

Where, N is total number of strands in stranded conductor.

Generally the diameter of conductor can be calculated by using the formula of,

D = (2x – 1)d

Where, D is the diameter of conductor,

‘d’ is the diameter of each strand.

44. Which of the following properties has got higher value for aluminum in comparison to that of copper?

(a) Electrical resistivity.

(b) Melting point.

(d) Specific gravity.

Answer: (a) Electrical resistivity.

Explanation:

Copper has much greater radius than aluminium, so the outer electrons are less strongly attracted as compared to aluminium
Aluminium wires expand in high temperatures and contract in low temperatures when compared to copper, which can withstand thermal changes
The conductivity of aluminium is about 60 percent of the conductivity of copper
Electrical resistivity of aluminium is higher than copper.

45. In a 7/30 ACSR conductor why is grease put between steel and aluminium conductors?

(a) To reduce corrosion by electrolytic action between zinc (galvanizing agent on steel) and aluminium

(b) To reduce friction between the strands.

(d) To eliminate air pockets.

Answer: (a) To reduce corrosion by electrolytic action between zinc (galvanising agent on steel) and aluminium

Explanation:

This conductive grease prevents the formation of oxides, sulfides and other corrosion deposits on copper and aluminum surfaces and conductors can be prevented with its use.
The purpose of a electrical contact lubricant is to prevent corrosion and lubricate the connection for easier maintenance.
Conductive greases for electronics efficiently lubricate moving parts, offer superior protection against corrosion and help ensure electrical continuity between irregular surfaces.

46. ACSR is used in place of copper in overhead lines because of

(a) higher current carrying capacity.

(b) being lighter in weight.

(d) higher tensile strength.

Answer: (a) higher current carrying capacity.

Explanation:

Aluminum conductor steel reinforced (ACSR):

It is one of the stranded conductors composed of one or more layers of hard-drawn 1350-H19 aluminum wire that is on galvanized coated steel core.
The ACSR conductors are mainly used in distribution and transmission lines due to their high tensile strength, economical design, less weight, suitable for medium and long periods with fewer supports, and good properties of sag and high voltage overhead lines.

Advantages of ACSR conductors over copper conductors:

The conductivity of the ACSR is higher than the copper conductor.
ACSR conductor can be used for medium and long spans and needs less support.
The mechanical performance and Tensile strength of the ACSR conductor are High.
It provides lesser Sag than copper conductors.

47. ACSR conductors have

(a) all conductors made of aluminium.

(b) outer conductors made of aluminium.

(d) no conductors made of aluminium.

Answer: (b) outer conductors made of aluminium.

Explanation:

Aluminium conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
The outer strands are high-purity aluminium, chosen for its good conductivity, low weight and low cost.
The center strand is galvanised steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
Steel has higher strength than aluminium which allows for increased mechanical tension to be applied to the conductor.

48. "Expanded ACSR" are conductors composed of

(a) larger diameter individual strands for a given cross-section of the aluminium strands.

(b) the larger diameter of the central steel strands for a given overall diameter of the conductor.

(d) A filter between the inner steel and the outer aluminium strands to increase the overall diameter of the conductor.

Answer: (b) the larger diameter of the central steel strands for a given overall diameter of the conductor.

Explanation:

For reducing the corona loss and radio interference at a high voltage a fibrous or plastic material is filled between the strands.
The diameter of the conductor expands due to the filling material and hence, it is called an expanded conductor.
Expanded ACSR conductors are used in extra high voltage (EHV) transmission line.
By the use of a filler such as paper or hessian between various layers of strands so as to increase the overall conductor diameter to reduce the corona loss and electrical stress at conductor surface.

49. The function of steel wire in an ACSR conductor is to

(a) compensate for the skin effect.

(b) take a cape of surges.

(d) reduce inductance.

Answer: (c) provide additional mechanical strength.

Explanation:

Aluminium conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines.
The outer strands are high-purity aluminium, chosen for its good conductivity, low weight and low cost.
The center strand is galvanised steel for additional strength to help support the weight of the conductor and provide additional mechanical strength.
Steel has higher strength than aluminium which allows for increased mechanical tension to be applied on the conductor.
Steel also has lower elastic and inelastic deformation (permanent elongation) due to mechanical loading (e.g. wind and ice) as well as a lower coefficient of thermal expansion under current loading.

50. The material used for the manufacture of the ground wire is

(a) aluminium.

(b) galvanised steel.

(e) cast iron.

(d) stainless steel.

Answer: (b) galvanised steel.

Explanation:

Earthing is used to protect from an electric shock.
It helps by providing a path for a fault current to flow to earth.
Earth wire is made of galvanized steel because of good mechanical properties.
The purpose of a ground wire is to give excess electrical charges a safe place to go.
The solid mass of earth below our feet has a negative electrical charge, which means positive electrical charges are naturally attracted to it.

51. Guy wire is employed for

(a) providing protection against surges.

(b) providing emergency earth route.

(d) all of the above.

Answer: (c) supporting the pole.

Explanation:

Guy wire or extended cable is needed on some poles to support unbalanced lateral loads due to the utility wires attached to them or to resist ground movement.
Guy wire or extended cable is particularly needed on dead-end (anchor) poles, where a long straight section of wireline ends, or angles off in another direction and hence prevent the pole from bending.
To protect the public against faults that might allow the extended cable to become electrified, utility guy cables usually either have a ceramic strain insulator ("Johnny ball"), or a fiberglass guy strain insulator inserted near the top.
To ensure that any dangerous voltages do not reach the lower end of the wire accessible to the public. The lower end where the cable enters the ground is often encased in a length of the yellow plastic reflector to make it more visible so that people or vehicles do not run into it.

52. The sag of a transmission line is least affected owing to

(a) the weight of the conduct

(b) current through the conductor.

(d) ice deposition on the conductor.

Answer: (b) current through the conductor.

Explanation:

Sag is defined as the difference in level between points of supports and the lowest point on the conductor.

Factors affecting the sag:

Conductor weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span: Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension: The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind: It increases sag in the inclined direction.
Temperature: The sag is reduced at low temperatures and is increases at higher temperatures. In summer due to the increase in average temperature tension decreases and hence sag increases in comparison to winter.
The sag of the transmission line is least affected by current passes through the conductor.

53. Effect of temperature rise in overhead lines is to

(a) increase the sag and decrease the tension.

(b) decrease the sag and increase the tension.

(d) decrease both.

Answer: (a) increase the sag and decrease the tension.

Explanation:

Effect of increase in temperature in overhead transmission line:

With the increase in temperature Sag increases.

Sag is the distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.

S=Wl²/8T

Where,

S is the sag of the conductor

W is the weight of the conductor

l is the span length of the conductor

T is the working tension on the conductor

An increase in temperature in overhead lines leads to expansion of length and causes sag thereby decreasing the stress.

54. The sag of a transmission line conductor in summer is

(a) less than that in winter.

(b) more than that in winter.

Answer: (b) more than that in winter.

Explanation:

Factors affecting the sag:

Conductor weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span: Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension: The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind: It increases sag in the inclined direction.
Temperature: The sag is reduced at low temperatures and is increases at higher temperatures.
In summer due to the increase in average temperature tension decreases and hence sag increases comparison to winter.

55. In a transmission line, sag depends upon

(a) span length.

(b) tension in conductors.

(d) all of the above.

Answer: (d) all of the above.

Explanation:

Conductor weight: Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span: Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension: The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind: It increases sag in the inclined direction.
Temperature: The sag is reduced at low temperatures and is increases at higher temperatures. In summer due to the increase in average temperature tension decreases and hence sag increases comparison to winter.
The sag of the transmission line is least affected by current passes through the conductor.

56. Which of the following statements is correct?

(a) Ice on conductors increases skin effect.

(b) Wind pressure reduces the corona effect.

(d) Ice on conductors reduces sag.

Answer: (c) Wind pressure is taken to act at perpendicular to that of ice,

Explanation:

The physical state of atmosphere: Corona is formed due to the ionization of air surrounding the conductors. The number of ions is more than normal in the stormy weather.
The irregular and rough surface cause more corona loss because unevenness of the surface decreases the value of breakdown voltage.
A larger distance between conductors reduces the electrostatic stresses at the conductor surfaces. It helps in avoiding the formation of the corona.
The operating voltage
Air density factor
Corona loss is independent of the current in the conductor.

57. Wind loading in coastal regions is in the range of

(a) 40-50 kg/ m²

(b) 150 kg/m²

Answer: (b) 150 kg/m²

Explanation:

Wind loading in coastal regions is in the range of 150 kg/m²
Conductor weight – Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading.
Span – Sag is directly proportional to the square of the span length. Longer span gives more sag.
Tension -The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.
Wind – It increases sag in the inclined direction.
Temperature – The sag is reduced at low temperatures and is increases at higher temperatures.

58. The maximum tension in a section of overhead line conductor between two supports of unequal height occurs at

(a) the higher support.

(b) the lower point.

(d) None of the above.

Answer: (a) the higher support.

Explanation:

The maximum tension in a section of overhead line conductor between two supports of unequal height occurs at the higher support.
For safety purpose, the ground clearance of the conductors at maximum temperature and minimum loading condition should be maintained.
Analysis of the sag and tension is important in the transmission line for the continuity and quality of electrical services.
If the tension of the conductor is increased beyond the limit, it may get broken, and the power transmission of the system get erupt.
The dip of the conductor between the two level supports is called sag.
In other words, the vertical distance between the highest point of the electrical pole or tower (where the conductor is connected) and the lowest point of the conductor between the two adjacent level supports is known as sag.
The horizontal distance between two electrical supports is called the span.
If the weight of a conductor is uniformly distributed along the line, then it is assumed that a freely suspended conductor shape is a parabola.
The shape of sag increases with the increase in the length of the span. For a small span ( up to 300 meters) parabolic method and large span ( like river crossings) catenary method is used for the calculation of sag and tension.

59. Stringing chart is useful

(a) for finding the sag in the conductor.

(b) in the design of tower.

(d) finding the distance between towers.

Answer: (a) for finding the sag in the conductor.

Explanation:

Stringing chart is useful in knowing the sag and tension at any temperature
Stringing chart gives the data per sag to be allowed and the tension to be allowed for a particular temperature
Stringing chart prepared by calculating the sag and tension on the conductor under worst conditions such as maximum wind pressure and minimum temperature by assuming a suitable safety factor

60. Hot template curves are plots of

(a) temperature and humidity.

(b) conductor sag and span lengths.

(d) none of the above.

Answer: (b) conductor sag and span lengths.

Explanation:

Hot Curve – The hot curve is obtained by plotting the sag at maximum temperature against span length. It shows where the supports must be located to maintain the prescribed ground clearance.
Ground Clearance Curve – The clearance curve is below the hot curve. It is drawn parallel to the hot curve and at a vertical distance equal to the ground clearance as prescribed by the regulation for the given line.
Support Foot Curve – This curve is drawn for locating the position of the supports for tower lines. It shows the height from the base of the standard support to the point of attachment of the lower conductor. For wood or concrete line, pole line this curve is not required to be drawn since they can be put in any convenient position.
Cold Curve or Uplift Curve – Uplift curve is obtained by plotting the sag at a minimum temperature without wind price against span length. This curve is drawn to determine whether uplift of conductor occurs on any support. The uplift conductor may occur at low temperature when one support is much lower than either of the adjoining ones.

61. The effect of wind pressure is more predominant on

(a) insulators.

(b) transmission lines.

(d) none of the above.

Answer: (c) supporting towers.

Explanation:

The effect of wind pressure is more predominant on supporting towers than transmission lines.
The towers do have a reasonable surface area that would receive the force generated by any constant wind.
It would also have the forces at the suspension point generated by the wind on the transmission lines.
The transmission towers have flat surfaces whereas the transmission lines are circular.
Then there is the possibility of the wind being just at the right speed to cause the towers to oscillate.

62. Galloping in transmission line conductors arises due to

(a) asymmetrical layers of ice formation.

(b) vortex phenomenon in light winds.

(d) adoption of horizontal conductor configuration.

Answer: (a) asymmetrical layers of ice formation.

Explanation:

High winds and freezing rain can cause transmission lines to gallop.
Galloping transmission or power lines can occur when freezing rain creates icicles and odd-shaped ice on transmission towers and conductors.
High winds push on the icicles and conductors and lifts them up, creating a galloping, or jumping, motion.
Galloping is a low frequency, large amplitude, wind-induced vibration of both single and bundle overhead conductors, with a single or a few loops of standing waves per span.
It is caused by moderately strong, steady crosswind acting upon an asymmetrically- iced conductor surface.

63. Which one of the following is reduced by using stock bridge dampers on power overhead transmission lines?

(a) Sag.

(b) conductor vibration.

(d) Mechanical tension.

Answer: (b) conductor vibration.

Explanation:

Stockbridge damper is used to control or suppress the wind induced vibration of overhead conductors.
The Stockbridge damper targets oscillations due to aeolian vibration; it is less effective outside this amplitude and frequency range.
Aeolian vibration is nothing but winding induced oscillation which has amplitude of millimeters to centimeters and a frequency of 3 to 150 Hz.
Aeolian vibration causes damaging stress fatigue to the cable.
It causes failure of conductor strands.
It is connected at two end of the transmission power line span. The vibration damper has a length of steel messenger cable.
Two metallic weights are attached to the ends of the messenger cable.
The center clamp, which is attached to the messenger cable, is used to install the vibration damper onto the overhead conductor.

64. The sag of the conductors of a transmission line is 2.5 m when the span is 250 m. Now if the height of the supporting tower is increased by 25%, the sag will

(a) reduce by 25%

(b) increase by 25%

(d) remain unchanged.

(e) reduce by 12.5%

Answer: (d) remain unchanged.

Explanation:

The distance between the highest point of electric poles or towers and the lowest point of a conductor connected between two poles or towers.

Span length: It is the shortest distance between two towers or poles.

S=Wl²/ 8T

Where,

S is the sag of the conductor

W is the weight of the conductor

l is the span length of the conductor

T is the working tension on the conductor

Here in the question height of the supporting towers is increased by 25 % which is independent of sag. Hence there will be no change in sag.

65. For a 400 kV line, the spacing between phase conductors is around

(a) 8 m.

(b) 11 m.

(d) 17 m

Answer: (a) 8 m.

Explanation:

As per Indian Electricity Rule 1956, Clause No 77, the minimum distance between bottom conductor and ground of a 400KV transmission line is 8.84 meter.
the ground clearance of 220KV bottom conductor would be 7 meter.
the ground clearance of 132KV bottom conductor would be 6.1 meter.
The minimum clearance of 66KV transmission line is also taken as 6.1 meters.

An empirical formula for Found Clearance between conductors

Ground clearance=5.182+(0.305∗K)

Where:

k=V−33/33 , V in kV .

According to this, if for example the Transmission line voltage is 220 kV then first calculate K :

K=220–33/33=5.66666667

Now calculating:

Ground clearance=5.182+(0.305∗5.66666667)=6.91033333 m=7 m

66. Stranded conductors usually have a central wire around which there are successive layers of 6,12,18,24 wires. For n-layers, the total number of individual wires is

(a) 3 n (n + 1)

(b) 2 n (n + 1)

(d) 2 n (n + 1) + 1

Answer: (c) 3 n (n + 1) + 1

Explanation:

The most commonly used conductor materials for overhead lines are copper, aluminum, steel-cored aluminum, steel-cored copper, galvanized steel, and cadmium copper.
The choice of a particular material will depend upon the cost, the required electrical and mechanical properties, and the local conditions.
All conductors used for overhead lines are preferably stranded* in order to increase flexibility.
In stranded conductors, there is generally one central wire and round this, successive layers of wires containing 6, 12, 18, 24 ...... wires.
If there are n layers, the total number of individual wires is [3n(n + 1) + 1].
In the manufacture of stranded conductors, the consecutive layers of wires are twisted or spiraled in opposite directions so that layers are bound together.

67. The diameter of each strand is d then the diameter of n-layer stranded conductor will be

(a) (2n + 1) d

(b) 3 (n + 1) d

(d) 3 (n - 1) d

Answer: (c) (2 n - 1) d

Explanation:

Generally, the total number of strands in any conductor is given by,

Total number of strands in a stranded conductor = 3n(n – 1) + 1

Generally the diameter of conductor can be calculated by,

D = (2n – 1)d

Where, D is the diameter of the conductor,

‘d’ is the diameter of each strand.

68. Strain-type insulators are used

(a) at dead ends.

(b) at intermediate anchor towers.

(d) any of (a) or (b).

Answer: (d) any of (a) or (b).

Explanation:

Pin, Suspension, and Strain insulators are used in medium to high voltage systems. While Stay and Shackle Insulators are mainly used in low voltage applications.
Strain type insulator used at dead ends and on a straight line as suspension type for voltage 3.3 kV and above.
A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.

69. Wavy structure of pin insulator increases its

(a) mechanical strength.

(b) puncture strength.

(d) thermal strength.

Answer: (c) flash-over voltage.

Explanation:

Wavy structure of pin insulator increases its flashover voltage. Wavy structure of pin insulator depends on the variation of voltage. So, when we increase the voltage, the structure of pin insulators also increases.
Flashover voltage: The voltage at which an electric discharge occurs between two electrodes that are separated by an insulator; the value depends on whether the insulator surface is dry or wet. Also known as sparkover voltage.

70. The voltage rating of a multiple shells (petticoat or rain shed) pin type insulator unit cannot be increased beyond a limiting value by increasing the number of shells, because

(a) the internal voltage distribution between shells becomes unequal.

(b) the leakage path resistance starts diminishing.

(d) the puncture voltage of the material of the insulator is reached.

Answer: (a) the internal voltage distribution between shells becomes unequal.

Explanation:

The pin insulator is used in power distribution for the voltage up to 33kV.
It is placed on the cross arm of the supporting tower.
The pin insulator has grooves on the upper end for keeping the conductor.
The conductor is tied to the insulator on the top groove on straight line positions and side groove in angle positions by annealed binding wire of the same material as that of the conductor.
A lead thimble is cemented into the insulator body to receive the pin.
The voltage rating of a multiple shells (petticoat or rain shed) pin type insulator unit cannot be increased beyond a limiting value by increasing the number of shells, because the internal voltage distribution between shells becomes unequal.

71. Which type of insulators are used on 132 kV transmission lines?

(a) Pin type.

(b) Disc type.

(d) Pin and Shackle type.

Answer: (b) Disc type.

Explanation:

Disc type insulators:

Pin Insulator:

The pin insulator is used in power distribution for the voltage up to 33kV
It is placed on the cross arm of the supporting tower
The pin insulator has grooves on the upper end for keeping the conductor
The conductor is tied to the insulator on the top groove on straight line positions and the side groove in angle positions by annealed binding wire of the same material as that of the conductor
A lead thimble is cemented into the insulator body to receive the pin

Suspension type insulator:

It consists of a number of porcelain discs connected in series by metal links in the form of a string.
These are the string of insulators in vertical position.
The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.
Used above 132 kV

Strain insulators:

When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension.
These are the string of insulators in horizontal position.
In order to relieve the line of excessive tension, strain insulators are used.

Shackle insulators:

They are frequently used for low voltage distribution lines.
Such insulators can be used either in a horizontal position or in a vertical position.
Used at low voltage.

72. Whenever the conductors are dead-ended or there is a change in the direction of the transmission line, the insulators used are of the

(a) Pin type

(b) Suspension type

(d) Shackle type.

Answer: (c) Strain type.

Explanation:

When suspension string is used to sustain the extraordinary tensile load of conductor it is referred as string insulator.
When there is a dead-end or there is a sharp corner in the transmission line, the line has to sustain a great tensile load of conductor or strain.
A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
Strain insulators are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level.

Rated System Voltage VS Number of disc insulator used in strain type tension insulator string

33KV----3
66KV----5
132KV---9
220KV---15

Rated System Voltage VS Number of disc insulators used in suspension insulator string

33KV-----3
66KV-----4
132KV----8
220KV----14

73. post-type insulators are generally used in lines operating

(a) above 100 kV.

(b) below 33 kV.

Answer: (c) at any voltage level, hv or ehv.

Explanation:

Pin insulators are used for holding the line conductors on the straight running of poles. These are commonly used in power networks up to 33 kV system.
Suspension insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross- arm of the tower. For high voltage (>33KV), it is a usual practice to use suspension type insulators.
When there is a dead-end of the line or there is a corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used.
For low voltage lines (<11 kV) shackle insulators are used as strain insulators.
Stay insulators are also known as strain insulators and are generally used up to 33 kV line. These insulators should not be fixed below three meters from the ground level. These insulators are also used where the lines are strained.

74. The number of discs in a string of insulators for 400 kV ac overhead transmission line lies in the range of

(a) 32 to 33

(b) 22 to 23

(d) 9 to 10

Answer (b) 22 to 23

Explanation:

Number of insulator discs n = (Operating voltage/phase)/ Maximum voltage of each disc

Operating voltage/phase = 220*10³/√3 = 127 kV

Maximum voltage of each disc = 11 kV

Number of insulator discs n = 127/11 =11.547 ≈ 12

For safety operation one extra disc is added to the string.

Therefore, total number of insulator discs = 13

The number of discs in a string of insulators for 400 kV ac overhead transmission line lies in the range of 22-23 Nos.

Number of Insulator per String

For 400KV Line ---- 22 Disc
For 220KV Line ---- 13 Disc
For 132KV Line ---- 8 Disc
For 66KV Line ----- 5-6 Disc
For 33KV Line ----- 3 Disc

75. The non-uniform distribution of voltage across the units in a string of suspension-type insulators is due to

(a) unequal self-capacitance of the units.

(b). non-uniform distance of separation of the units of the tower body.

(d) non-uniform distance between the cross-arms and the units.

Answer: (c) the existence of stray capacitance between the metallic junctions of the units and the tower body.

Explanation:

Important points regarding the voltage distribution over a string of suspension insulators:

1) Due to the presence of shunt capacitor, the voltage across the suspension insulators does not distribute itself uniformly across each disc.

2) The voltage across the nearest disc to the conductor is maximum than others disc.

3) The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.

4) In the case of D.C voltage, the voltage across each unit would be the same. It is because insulator capacitance are ineffective for D.C.

The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.

76. The voltages across the various discs of a string of suspension insulators having identical discs are different due to

(a) surface leakage currents.

(b) series capacitance.

(d) series and shunt capacitances.

Answer: (c) shunt capacitance to ground.

Explanation:

Due to the presence of a shunt capacitor, the voltage across the suspension insulators does not distribute itself uniformly across each disc.
The voltage across the nearest disc to the conductor is maximum than the other discs.
The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.
In the case of D.C voltage, the voltage across each unit would be the same. It is because insulator capacitance is ineffective for D.C
The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.

77. The string efficiency of a string of suspension insulators is dependent on

(a) the size of the insulators.

(b) a number of discs in the string.

Answer: (b) a number of discs in the string.

Explanation:

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.
String efficiency depends upon the value of shunt capacitance. Lesser the value of capacitance, the greater is the string efficiency.
As the value of shunt capacitance approaches zero, the string efficiency approaches 100%.
The greater the string efficiency, the more uniform is the voltage distribution in each disc insulator. 100% string efficiency implies that the potential across each disc is the same.
In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used.

78. 100 percent string efficiency means

(a) one of the insulator discs shorted.

(b) zero potential across each disc.

(d) none of the above.

Answer: (c) equal potential across each insulator disc.

Explanation:

String efficiency:

The voltage applied across the suspension insulator string is unequally distributed across the individual unit.
The disc near the line conductor is extremely stressed and takes the maximum voltage.
The voltage distribution on the insulator string determines the flashover voltage and the voltage at which the localized corona and radio interference is started.
The string efficiency is defined as the ratio of conductor voltage to the voltage across the disc nearest to the conductor multiplied by number of discs.
String efficiency = (conductor voltage)/(number of discs × voltage across the disc nearest to the conductor)
String efficiency depends upon the value of shunt capacitance. Lesser the value of capacitance, the greater is the string efficiency.
As the value of shunt capacitance approaches to zero, the string efficiency approaches to 100%.
In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used.
The greater the string efficiency, the more uniform is the voltage distribution in each disc insulator. 100% string efficiency implies that the potential across each disc is same.

79. In a suspension-type insulator the potential drop is

(a) maximum across the lowest disc.

(b) maximum across the topmost disc.

Answer: (a) maximum across the lowest disc.

Explanation:

The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance.
The disc nearest to the conductor has a maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing.
The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
If the voltage impressed across the string was d.c., then the voltage across each unit would be the same. It is because insulator capacitances are ineffective for d.c.
Hence, in a suspension-type insulator, the potential drop is maximum across the disc nearest to the conductor, which is nothing but the lowest disc in the string of discs.

80. If the frequency of a transmission system is changed from 50 Hz to 100 Hz, the string efficiency

(a) will increase.

(b) will decrease.

(d) may increase or decrease depending on the line parameters.

Answer: (c) remain unchanged.

Explanation:

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.
String efficiency = Operating phase voltage (Vph)n x voltage across the disc nearest to the conductor)
Hence string efficiency does not depend upon the frequency so if the frequency is increased string efficiency remains unchanged.

81. The string efficiency of a high-voltage line is around

(a) 100%

(b) 80%

(d) 10%

Answer: (b) 80%

Explanation:

The voltage applied across the suspension insulator string is unequally distributed across the individual unit.
The disc near the line conductor is extremely stressed and takes the maximum voltage.
The voltage distribution on the insulator string determines the flashover voltage and the voltage at which the localized corona and radio interference is started.
The string efficiency is defined as the ratio of conductor voltage to the voltage across the disc nearest to the conductor multiplied by a number of discs.
String efficiency = (conductor voltage)/(number of discs × voltage across the disc nearest to the conductor)
String efficiency depends upon the value of shunt capacitance. Lesser the value of capacitance, the greater the string efficiency.
As the value of shunt capacitance approaches zero, the string efficiency approaches to 100%.
In order to decrease the shunt capacitance, the distance between the insulator string and the tower should be increased, i.e. longer cross-arms should be used.
The string efficiency of a high-voltage line is around 80 %.

82. In three-unit insulator string, voltage across the lowest unit is 17.5 kV and string efficiency is 84.28%. The total voltage across the string will be equal to

(a) 8.285 kV

(b) 44.25 kV

(d) 442.5 kV

Answer: (b) 44.25 kV

Explanation:

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.

η=V/n×Vn

Where,

V is the total voltage across the string.

Vn is the voltage across the bottom disc near to conductor

n is the number of the disc or unit insulator in a string

Calculation:

Given that,

Let the total voltage = V

Voltage across the bottom-most unit, Vn = 17.5 kV

Number of insulators units (n) = 3

String efficiency = (conductor total voltage)/(number of discs × voltage across the disc nearest to the conductor)

String efficiency,

η=V/n×Vn

84.28%=V3×17.5×103×100

∴ Total voltage, V = 44.25 kV

83. Two-insulator discs of identical capacitance value C makes up a string for a 22 kV, 50 Hz, single-phase overhead line insulation system. If the pin to earth capacitance is also C, then the string efficiency is

(a) 50%

(b) 75%

(d) 86%

Answer: (b) 75%

Explanation:

The correct option is A 75
String efficiency=Voltage across the string / n× Voltage across the lower most unit

η=V1+V2/n×V2×100

V2=V1+KV1

V2=2V1

η=V1+2V1/2×2V1×100

=3V1/4V1×100=0.75×100=75%

84. In a cable the sheath radius is R and the conductor radius is r. As r changes from 0.5 R to 0.25 R the maximum voltage gradient in the dielectric

(a) decreases by about 6%.

(b) increases by about 6%.

(d) decreases by about 15%

Answer: (b) increases by about 6%.

Explanation:

The electrostatic stress in a cable isn’t uniformly distributed. The potential gradient is inversely proportional to the distance from the centre of the cable.
Hence it will be maximum (gmax) at the surface of the conductor and goes on decreasing until it becomes minimum (gmin) at the surface of the sheath.
That means electrostatic stress in the dielectric of a cable is maximum at the surface of the conductor and minimum at the surface of the sheath.
In a cable the sheath radius is R and the conductor radius is r. As r changes from 0.5 R to 0.25 R the maximum voltage gradient in the dielectric.

85. In the case of suspension-type insulators, the string efficiency can be improved by

(a). using a longer cross arm.

(b). using a guard ring,

(c). grading the insulator discs.

(d). all of above

Answer: (d). all of above

Explanation:

The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency.

String efficiency of a string of disc insulators can be improved by using following methods.

By using longer cross-arms: The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.
By grading the insulators: In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalise the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.
By using a guard ring (Static Shielding): The potential across each unit in a string can be equalised by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

86. The ratio of puncture voltage to the flash-over voltage of a line insulator is

(a) equal to 1

(b) lower than 1

Answer: (c) much greater than 1

Explanation:

The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways:

A puncture arc is a breakdown and conduction of the material of the insulator, causing an electric arc through the interior of the insulator.
Flashover voltage is the voltage that causes a flash-over arc.
The ratio of the puncture voltage to the flashover voltage of an insulator is always greater than one.

87. The insulators may fail due to

(a) flash over.

(b) short-circuits.

(d) any of the above

Answer: (d) any of the above

Explanation:

Causes of Insulator Failure: There are different causes due to which failure of insulation in the electrical power systems may occur:

Cracking of Insulator:

The porcelain insulator mainly consists of three different materials.
The main porcelain body, steel fitting arrangement, and cement to fix the steel part with porcelain.
Due to changing climate conditions, these different materials in the insulator expand and contract at different rates.
These unequal expansion and contraction of porcelain, steel, and cement are the chief cause of the cracking of insulators.

Porosity in The Insulation Materials: If the porcelain insulator is manufactured at low temperatures, it will make it porous, and due to this reason it will absorb moisture from air thus its insulation will decrease and leakage current will start to flow through the insulator which will lead to insulator failure.

Flash Over Across Insulator: If a flashover occurs, the insulator may be overheated which may ultimately result in shuttering of it.

Mechanical Stresses on Insulator:

If an insulator has any weak portion due to a manufacturing defect, it may break from that weak portion when mechanical stress is applied to it by its conductor.
These are the main causes of insulator failure. Now we will discuss the different insulator test procedures to ensure the minimum chance of failure of insulation.
Glazing on ceramic insulators is done to make it smooth and non-absorbent.

89. The purpose of guard ring in transmission lines is to

(a) reduce the earth capacitance of the lowest unit.

(b) increase the earth capacitance of the lowest unit.

(d) none of the above.

Answer: (a) reduce the earth capacitance of the lowest unit.

Explanation:

Guard Ring or Grading Ring:

It is a metal ring electrically connected to the conductor and surrounding the bottom insulator
A guard ring is used to protect high impedance nodes in a circuit from surface leakage current and The guard ring is a ring of copper
Grading Ring or Guard Ring equalizes the potential distribution across each disc in Suspension Insulator
Guard Ring nullifies the effect of shunt capacitance of string insulator
It reduces the earth capacitance of the lowest unit and provides uniform or equal distribution of voltage at each disk
When it is used with an arcing horn it protects the hole insulator string from flashover or overvoltage

90. The use of a guard ring

(a) equalizes the voltage division between insulator discs.

(b) is an unnecessary complication.

Answer: (a) equalizes the voltage division between insulator discs.

Explanation:

A guard ring is a metallic ring connected to the conductor and surrounds the bottom disc. It is countered in such a way that the voltage distribution across the string is uniform.
Due to the non-uniform distribution of voltage, the string efficiency is very less, the insulators are not utilized properly. Also, the insulator near the conductor is under more stress and is likely to be punctured. In order to avoid these drawbacks, a guard ring is used.
A guard ring is a metallic ring connected to the conductor and surrounds the bottom disc. It is countered in such a way that the voltage distribution across the string is uniform. The basic idea behind using a guard ring is to create the effect of another capacitance such that the charging current of the earth capacitance and the ring capacitance are almost equal and opposite in direction.

91. The effect of Corona in transmission line is

(a) partial breakdown of air,

(b) complete breakdown of air.

Answer: (b) complete breakdown of air.

Explanation:

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference.
If the voltage raised higher, the luminous envelope becomes larger and higher following with greater sound and the power loss.
If the applied voltage is further increased to breakdown value, a flash over will occur between the conductors due to breakdown of air insulation.

92. Which of the following statements is true regarding corona?

(a) Corona takes place at voltage lower than breakdown voltage.

(b) Corona takes place at a voltage higher than breakdown voltage.

(d) Corona increases the transmission line efficiency.

Answer: (a) Corona takes place at voltage lower than breakdown voltage.

Explanation:

Corona Effect: The phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.

The disadvantage of Corona:

Corona causes power loss, due to this power loss, the transmission efficiency reduces.
Ozone is produced due to Corona. This Ozone may cause degradation of the conductor due to chemical action.
Due to corona, unwanted signals are generated, these noise signals may interfere with the TV or Radio Signal.
The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
Corona discharge can be observed as a bluish glow.

Additional Information

Corona discharge can be reduced by the following methods:
By increasing the conductor size: The larger the diameter of the conductor, the lesser the corona discharge.
By increasing the distance between conductors: The larger the conductor spacing, the lesser the corona.
Using bundled conductors: Using a bundled conductor increases the effective diameter of the conductor. This results in the reduction of the corona discharge.
Using corona rings: The electric field is greater where the conductor curvature is sharp. Therefore, corona discharge occurs first at the sharp points, edges, and corners. To mitigate this, corona rings are employed at the terminals of very high voltage equipment such as at the bushings of a very high voltage transformer.

93. Which of the following statements regarding corona are true?

1. It causes radio interference.

2. It attenuates lightning surges.

3. It amplifies switching surges.

4. It causes power loss.

5. It is more prevalent in the middle conductor of a transmission line employing a flat conductor configuration.

Select the correct answer using the codes given below.

Codes:

(a) 1, 3, 5

(b) 2, 3, 4

(d) 2, 3, 4, 5

Answer: (c) 1, 2, 4.5

Explanation:

The phenomenon of ionization of air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.

Factors affecting the corona:

Supply voltage of transmission lines
Operating frequency of transmission lines
Spacing between the conductors
The radius of the conductor
Condition of conductor surface
Air Density Factor

Disadvantages of corona discharge:

The glow appears across the conductor which shows the power loss occurs on it.
The audio noise occurs because of the corona effect which causes the power loss on the conductor.
The vibration of the conductor occurs because of the corona effect.
The corona effect generates the ozone because of which the conductor becomes corrosive.
Corona power loss reduces the efficiency of the line.
The radio and TV interference occurs on the line because of the corona effect.
Corona effect is more prevalent in the middle conductor in a flat conductor configuration.

Important Points:

Disruptive voltage is the minimum voltage at which the breakdown of air occurs, and corona starts.
Visual critical voltage is the minimum voltage at which visible corona begins.
One of the advantages of the corona effect is that it reduces the magnitude of lightning and switching surges

94. Corona is accompanied by

(a) violet visible discharge in darkness

(b) hissing sound.

(d) power loss.

(e) radio-interference.

(f) ozone.

(g) all of the above.

Answer: (g) all of the above.

Explanation:

Corona: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line is known as the corona.

Factors Affecting Corona:

Atmosphere:

As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.

Conductor size:

The corona effect depends upon the shape and conditions of the conductors.
Corona loss varied along with the change in the diameter of the conductor.
The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
Thus, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor.

Spacing between conductors:

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect.
It is because a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

Line voltage:

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.
However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

Mistake Points

Note that, Corona loss is also dependent on the climate condition.

Additional Information

Advantages of Corona:

Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.
The increased diameter reduces the electrostatic stresses between the conductors.
Corona reduces the effects of transients produced by surges.

Disadvantages:

Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.

Important Points

Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs.
In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.).
Visual critical voltage: It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.

Methods of Reducing Corona Effect:

It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.
The corona effects can be reduced by the following methods:
By increasing conductor size: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors that have a larger cross-sectional area are used in transmission lines.
By increasing conductor spacing: By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

95. Corona effect can be detected by

(a) presence of ozone detected by odour.

(b) hissing sound.

(d) all of the above.

Answer: (d) all of the above.

Explanation:

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
It can be detected by a hissing sound, faint luminous, and presence of ozone which is detected by odour.
The presence of ozone is harmful because it corrodes the conductor.

96. Corona loss increases with

(a) decrease in conductor size and increase in supply frequency.

(b) increase in both conductor size and supply frequency.

(d) increase in conductor size and decrease in supply frequency.

Answer: (a) decrease in conductor size and increase in supply frequency.

Explanation:

Frequency of supply: Corona loss increases as the supply frequency increases
Air Pressure: In hilly areas, the corona effect is more dominant due to reduced pressure
By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

97. The good effect of corona on overhead lines is to

(a) increase the line carrying capacity due to conducting ionized air envelope around the conductor.

(b) increase the power factor due to corona loss.

(d) reduce the steepness of surge fronts.

Answer: (d) reduce the steepness of surge fronts.

Explanation:

Advantages of Corona Effects:

Due to corona across the conductor, the sheath of air surrounding the conductor becomes conductive which rises the conductor diameter virtually.
This virtual increase in the conductor diameter reduces the maximum potential gradient or maximum electrostatic stress. Thus, the probability of flashover is reduced.
Effects of transients produced by lightning or electrical surges are also reduced due to the corona effect.
As, the charges induced on the line by surge or other causes, will be partially dissipated as a corona loss.
In this way, corona protects the transmission lines by reducing the effect of transients that are produced by voltage surges.

98. Compared with a solid conductor of the same radius, corona appears on a stranded conductor at a lower voltage, because stranding

(a) assists ionization.

(b) makes the current flow spirally about the axis of the conductor.

(d) produces surfaces of smaller radius.

Answer: (d) produces surfaces of smaller radius.

Explanation:

Basic, reason of using stranded conductor is to make the conductor flexible. If we use a single solid conductor.
It does not have sufficient flexibility and it is difficult to coil a solid conductor.
Hence, it becomes difficult to transport a single solid conductor of long length over the distance.

Factors Affecting Corona:

Atmosphere:

As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.

Conductor size:

The corona effect depends upon the shape and conditions of the conductors.
Corona loss varied along with the change in the diameter of the conductor.
The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
Thus, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor.

Spacing between conductors:

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect.
It is because a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

Line voltage:

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.
However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

99. Corona loss can be reduced by the use of hollow conductors because

(a) the current density is reduced.

(b) the eddy current in the conductor is eliminated.

(d) of better ventilation in the conductor.

Answer: (c) for a given cross section, the radius of the conductor is increased.

Explanation:

The corona loss can be reduced by using:

Conductors with large diameters: The voltage at which the corona occurs can be increased by increasing the size of the conductor and hence, the corona loss can be reduced.
Hollow conductors: These are used to increase the effective diameter of the conductor without using any additional material. Since corona loss is inversely proportional to the diameter of the conductor, corona loss decreases with an increase in the diameter. For reducing corona loss, we need to increase the diameter of the conductor.
Making conductors have increased diameter reduces the intensity of the electric field at the surface of the conductor, which reduces corona.
Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminium conductors (ACSR).
Bundled conductors: These are made up of two or more sub-conductors and are used as single-phase conductors. When using two or more sub-conductors as one conductor, the effective diameter of the conductor increases, resulting in reduced corona loss.

100. Bundled conductors reduce

(a) surface electric stress of conductor.

(b) increases the line reactance.

Answer: (a) surface electric stress of conductor.

Explanation:

Bundled conductor:

Bundled conductor are those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.
By using bundle conductors instead of the single conductor in the transmission line increases the GMR of the conductors.
Hence, by using bundle conductors GMR is increased, so inductance L decreases, and capacitance C increases.

Advantages of Bundled Conductors:

Bundling of conductors leads to a reduction in line inductance.
Bundling of conductors leads to increases in capacitance.
An important advantage of bundled conductors is its ability to reduce corona discharge
Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.
Reduction in communication line interference.

101. Corona loss in a transmission line is dependent on

(a) diameter of the conductor.

(h) material of the conductor.

Answer: (a) diameter of the conductor.

Explanation:

Corona: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line is known as the corona.

Factors Affecting Corona:

Atmosphere:

As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.

Conductor size:

The corona effect depends upon the shape and conditions of the conductors.
Corona loss varied along with the change in the diameter of the conductor.
The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
Thus a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor.

Spacing between conductors:

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect.
It is because a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

Line voltage:

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.
However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

Additional Information

Advantages of Corona:

Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.
The increased diameter reduces the electrostatic stresses between the conductors.
Corona reduces the effects of transients produced by surges.

Disadvantages:

Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.

Important Points

Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs.
Consider two conductors of radius r cm and spaced d cm apart.
In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.).
Visual critical voltage: It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.

Methods of Reducing Corona Effect:

It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.

The corona effects can be reduced by the following methods:

By increasing conductor size: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors that have a larger cross-sectional area are used in transmission lines.
By increasing conductor spacing: By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

102. Corona occurs between two transmission conductors when they

(a) have high potential differences.

(b) air closely spaced.

(d) both (a) and (b).

Answer: (d) both (a) and (b).

Explanation:

This phenomenon occurs when the electrostatic field across the transmission line conductors produces the condition of potential gradient.
The air gets ionized when the potential gradient at the conductor surface reaches the value of 30kV/cm at normal pressure and temperature.
If the voltage applied between two lines separated by some distance is greater than a certain value then we can see sparks or violet glow appearing.
When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
When the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called Corona.
The phenomenon of the corona is accompanied by a hissing sound, production of ozone, power loss, and radio interference.
The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss, and the radio noise.
If the applied voltage is increased to the breakdown value, a flash-over will occur between the conductors due to the breakdown of air insulation.
The phenomenon of violet glow, hissing noise, and the production of ozone gas in an overhead transmission line is known as the corona.
If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise, the rough points will appear brighter.
With d.c. voltage, there is a difference in the appearance of the two wires.
The positive wire has a uniform glow about it, while the negative conductor has a spotty glow.

103. In humid weather, the corona occurs at a voltage

(a) much less than that needed in fair weather.

(b) much higher than that needed in fair weather.

(n) equal to that needed in fair weather.

(d) none of the above.

Answer: (a) much less than that needed in fair weather.

Explanation:

The phenomenon of violet glow, hissing noise, and the production of ozone gas in an overhead transmission line is known as the corona.
The voltage at which the self-sustained discharge will be initiated is called the critical disruptive voltage.
Effect of atmospheric factors on corona occurrence:
If the critical disruptive voltage is less that means the corona initiation takes place at low voltage. So, any factor that is affecting the critical disruptive voltage that can affect the occurrence of the corona.
From the formula of critical disruptive voltage, we can observe that it is directly proportional to the air density factor (δ ) that is if 'δ ' increases as a result Vd increases so that the occurrence of corona will take place at higher voltages, so we can reduce the occurrence corona by increasing air density factor δ.
The air density factor depends on both temperature and pressure, In the hilly area, the falling of pressure is very high when compared to the falling of temperature. So 'δ' will be less in a hilly area and the chances of occurrence of the corona are more.
Deposition of dust, ice, snow on the surface of the conductor, which will reduce the Vd.

Important Points

Humidity is the addition of water vapor to air that reduces the density of the air (δ), which may at first appear counter-intuitive.
This occurs because the molar mass of water is less than the molar mass of dry air.
And during the humid weather ion concentration in air will decrease.
As the value of the air density factor is reduced to the humid weather condition as a result there are maximum chances of the occurrence of the corona.

104. The effect of the corona in power system is

(a) increased energy loss.

(b) increased reactance.

(d) all of the above

Answer: (a) increased energy loss.

Explanation:

There is a hissing noise with violet glow phenomenon termed as corona effect which is commonly observed in high voltage transmission lines. The corona effects leads to high voltage drop and energy loss along with release of ozone gas.
When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference.
If the voltage raised higher, the luminous envelope becomes larger and higher following with greater sound and the power loss.
If the applied voltage is further increased to breakdown value, a flash over will occur between the conductors due to breakdown of air insulation.

105. The charging current in a transmission line increases due to corona effect because corona increases

(a) line current.

(b) effective line voltage

(d) the effective conductor diameter.

Answer: (d) the effective conductor diameter.

Explanation:

Ferranti Effect: At no load (or) at light load, the voltage at the receiving end of the transmission line is more than the sending voltage. It is known as the Ferranti effect. It is due to the charging current of the line.

Skin Effect: The tendency of alternating current to concentrate near the surface of the conductor is known as skin effect. The skin effect depends on the following factors.

a) Frequency

b) Diameter of the conductor

c) Shape of the conductor

The skin effect increases with the increase of cross section, permeability and supply frequency. It reduces with the increase in resistivity of the conductor material.

Proximity Effect: The alternating flux in a conductor is caused by the current of the other nearby conductor. This flux produces a circulating current or eddy current in the conductor which results in an apparent increase in the resistance of the wire. Thus, more power losses in the windings. This phenomenon is called the proximity effect.

Corona Effect:

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if the applied voltage is low.
When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
The discharging current in a transmission line increases due to corona effect because corona increases the effective diameter. So that the capacitance will be increases and inductance will be reduced.

106. The chances of occurrence of corona are maximum during

(a) humid weather.

(b) dry weather.

(d) hot summer.

Answer: (a) humid weather.

Explanation:

Effect of atmospheric factors on corona occurrence:

If the critical disruptive voltage is less that means the corona initiation takes place at low voltage. So, any factor that is affecting the critical disruptive voltage that can affect the occurrence of the corona.
From the formula of critical disruptive voltage, we can observe that it is directly proportional to the air density factor (δ ) that is if 'δ ' increases as a result Vd increases so that the occurrence of corona will take place at higher voltages, so we can reduce the occurrence corona by increasing air density factor δ.
The air density factor depends on both temperature and pressure, In the hilly area, the falling of pressure is very high when compared to the falling of temperature. So 'δ' will be less in a hilly area and the chances of occurrence of the corona are more.
Deposition of dust, ice, snow on the surface of the conductor, which will reduce the Vd.

Important Points

Humidity is the addition of water vapor to air that reduces the density of the air (δ), which may at first appear counter-intuitive.
This occurs because the molar mass of water is less than the molar mass of dry air.
And during the humid weather ion concentration in air will decrease.
As the value of the air density factor is reduced to the humid weather condition as a result there are maximum chances of the occurrence of the corona.

107. Corona is likely to occur maximum in case of

(a) distribution lines.

(b) transmission lines.

(d) service mains.

Answer: (b) transmission lines.

Explanation:

If the spacing between the conductors is small, flashover may take place between the conductors without any hissing noise and glow. It is because the distance between the conductors being smaller; there is no time for the glow to occur.

Following are the leading causes of corona effect in transmission lines:

Conductors shape: The corona effect in transmission lines depends upon the shape and conditions of the conductor. The rough and irregular surface will give rise to more corona. It is because the unevenness of the surface decreases the value of breakdown voltage. Similarly, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor having a smooth surface.
Atmosphere: Since corona is caused due to ionization of air surrounding the conductor, therefore, it is affected by the physical state of the atmosphere. In the stormy weather, the number of ions is more than normal, and corona occurs at much less voltage as compared with fair weather.
Spacing between conductors: If the spacing between the conductors is made very large as compared to their radii (r), there may not be any corona effect. It is because larger spacing reduces the electrostatic stresses at the conductor surface.
Line voltage: Line voltage significantly affects the corona effect. If it is low, there is no change in the condition of air surrounding the conductor and hence no corona. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, the corona effect appears.

108. Corona is affected by

(a) size of conductor.

(b) shape and surface condition of the conductor.

(d) all of the above.

Answer: (d) all of the above.

Explanation:

Atmosphere:

As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.

Conductor size:

The corona effect depends upon the shape and conditions of the conductors.
Corona loss varied along with the change in the diameter of the conductor.
The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
Thus a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor.

Spacing between conductors:

If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect.
It is because a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.

Line voltage:

The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.
However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.

109. The only advantage of corona is that it

(a) produces a pleasing luminous glow.

(b) makes line current sinusoidal.

(d) ozone gas is produced.

Answer: (c) works as a safety valve for surges.

Explanation:

Due to the formation of the corona, the air surrounding the conductor becomes conducting. It increases the virtual diameter of the conductor. The increased diameter decreases the electrostatic stresses between the conductors.
The corona effect reduces electrostatic stresses between the conductors. This reduces the probability of flashover and improves system performance.
Corona reduces the effects of transients produced by lightning.

The undesirable effects of the corona are:

The glow appear across the conductor which shows the power loss occur on it.
The audio noise occurs because of the corona effect which causes the power loss on the conductor.
The vibration of conductor occurs because of corona effect.
The corona effect generates the ozone because of which the conductor becomes corrosive.
The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
The corona power loss reduces the efficency of the line.
The radio and TV interference occurs on the line because of corona effect.

Minimizing corona:

Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona.

The following factors may be considered to control corona:

Conductor diameter – For reducing corona loss, this method of increasing conductor diameters is very effective. Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductors(ACSR) conductors.
The voltage of the line – Voltage of transmission lines is fixed by economic considerations. To increase the disruptive voltage the spacing of the conductors is to be increased, but this method has some limitations.
Spacing between conductors – If the space between conductors increases, then the voltage drops between them also increases due to increase in inductive reactance.

Important points:

Disruptive voltage is the minimum voltage at which the breakdown of air occurs and corona starts.
Visual critical voltage is the minimum voltage at which visible corona begins.

110. Presence of ozone owing to corona

(a) improves the pf.

(b) reduces the pf.

(d) improves regulation.

Answer: (c) corrodes the material.

Explanation:

The phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.

The following are the factors affecting the corona;

Effect of supply voltage – If the supply voltage is high corona loss is higher in the lines. In low-voltage transmission lines, the corona is negligible, due to the insufficient electric field to maintain ionization.
The condition of conductor surface – If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
Air Density Factor – The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
Effect of system voltage – Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high, and hence corona is also high. Corona loss, increase with the increase in the voltage.
The spacing between conductors – If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor. If the distance between them is extended beyond certain limits, the dielectric medium between them get decreases and hence the corona loss also reduces.

Advantages of Corona:

Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.
The increased diameter reduces the electrostatic stresses between the conductors.
Corona reduces the effects of transients produced by surges.

Disadvantages:

Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.

111. The dielectric strength of air under normal conditions is about

(a) 100 kV_p /cm

(b) 21.1 kV_p/cm

(d) 200 ky_p/cm

Answer: (c) 30 kV_p/cm

Explanation:

Dielectric breakdown occurs in air at an electric field strength of about Emax = 3 × 106 V/m.
Dielectric breakdown occurs when a charge build-up exceeds the electrical limit or dielectric strength of a material.
Dielectric strength of air = 3 × 106 V/m = 21.21 kV/cm (rms)

112. Disruptive corona begins in smooth cylindrical conductors in air at NTP if the electric field intensity at the conductor surface goes up to

(a) 21.1 kV (rms)/cm.

(b) 21.1 kV (peak)/cm.

(d) 21.1 kV (rms)/m.

Answer: (a) 21.1 kV (rms)/cm.

Explanation:

if the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
Disruptive corona begins in smooth cylindrical conductors in air at NTP if the electric field intensity at the conductor surface goes up to 21.1 kV (rms)/cm.

113. The dielectric strength of air is

(a) proportional to barometric pressure.

(b) proportional to absolute temperature.

(d) none of the above.

Answer: (a) proportional to barometric pressure.

Explanation:

Dielectric breakdown occurs in air at an electric field strength of about Emax = 3 × 10⁶ V/m. Dielectric breakdown occurs when a charge buildup exceeds the electrical limit or dielectric strength of a material.
Dielectric strength of air = 3 × 10⁶ V/m = 21.21 kV/cm (rms)

114. Visual critical voltage is

(a) lower than disruptive critical voltage.

(b) higher than disruptive critical voltage.

(d) none of the above.

Answer: (b) higher than disruptive critical voltage.

Explanation:

In the case of transmission lines with parallel conductors, the corona glow does not be at the critical disruptive voltage Vc, but it begins at a higher voltage called the Visual Critical Voltage. It is denoted by Vv.
Thus visual critical voltage is the minimum phase-neutral voltage at which the visual corona starts or appearance of the faint luminous glow of violet color all along the transmission line conductors.

115. The critical voltage limit of a transmission line is increased by

(a) increasing the radius of the conductors.

(b) increasing the spacing between conductors.

(d) reducing the radius of the conductors.

Answer: (a) increasing the radius of the conductors.

Explanation:

The critical voltage limit of a transmission line is the same as the critical disruptive voltage.
Critical Disruptive Voltage is defined as the minimum phase to the neutral voltage required for the corona discharge(corona losses) to start.
There is another type of rating also for transmission lines known as critical visual voltage.
The corona glow does not begin at the critical disruptive voltage Vc, but it begins at a higher voltage called the Visual Critical Voltage.
The critical voltage limit of a transmission line is increased by increasing the radius of the conductors.

116. The maximum permissible value of fair-weather corona loss for a corona line is.

(a) 0.6 kW/3-phase km.

(b) 1.2 kW/3-phase km.

(d) 2.4 kW/3-phase km.

Answer: (d) 2.4 kW/3-phase km.

Explanation:

Effect of system voltage – The electric field intensity around the conductor depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also high, and hence corona loss is also high.
Effect of Frequency – The corona loss is directly proportional to system frequency.
Effect of Density of Air – The corona loss is inversely proportional to air density factor. The corona loss increases with the decreases in density of air. The corona loss of the hilly area is more than that of the plains because plain have low density of air.
Effect of Conductor Radius – If the wire area has high surface area, then their surface field intensity is low, and hence corona loss is less.
The maximum permissible value of fair-weather corona loss for a corona line is 2.4 kW/3-phase km.

117. Corona losses are minimized when

(a) conductor size is reduced.

(b) smooth conductor is used.

(d) current density in conductors is reduced

Answer: (b) smooth conductor is used.

Explanation:

Corona loss:

The power dissipated in the system due to corona discharges is called corona loss.
Accurate estimation of corona loss is difficult because of its variable nature.
It has been found that the corona loss under fair weather conditions is less than under foul weather conditions.

Methods by which the corona effect can be reduced:

It has been observed that intense corona effects are seen at a working voltage of 33 kV or above. So, design should be made to avoid corona on the sub-stations or bus bars rated for 33 kV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment.

(i) By increasing conductor size.

By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.

(ii) By increasing conductor spacing.

By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.
However, spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger cross arms and supports) may increase to a considerable extent.
For the corona loss to occur the operating voltage must be higher than the critical voltage limit.
So, if we increase this limit we can reduce the corona loss.
To have a high value of the Vd the value of m must be as high as possible which is possible by the use of smooth conductors.
The corona effect can be minimized by increasing both spacings between conductors and the diameter of the conductors

118. The corona loss on a particular system at 50 Hz is 1 kW/km per phase. What is the corona loss at 60 Hz in kW/km per phase?

(a) 0.83

(b) 1.0

(d) 1.2

Answer: (c) 1.13

Explanation:

We know that corona loss is ∝(f+25)
Here, f1=50 Hz
f2=60 Hz
P2/P1=f2+25/f1+25
⇒P2/P1=60+25/50+25=85/75=1.133

Transmission and Distribution Objective Questions with Easy Explanation

Contact Form