Power System Most Asked MCQS Part-4

July 29, 2021

1. The surge impedance value of overhead lines is

A. 50 to 100 ohms

B. 400 to 600 ohms
C. 800 to 1000 ohms
D. 10 to 50 ohms
Answer: B. 400 to 600 ohms

Explanation:

An impedance that renders the line as the infinite line is known as surge impedance.
It has a value of about 400 ohms and phase angle varying from 0 to -15 degrees for overhead lines and around 40 ohms for underground cables.
For overhead line surge impedance is given by 400-600 Ω
For underground cable surge impedance is given by 40-60 Ω

2. The capacity of a 3-phase transmission line increases with which of the following?

A. Current density increases
B. Transmission voltage increases
C. Power factor decreases
D. Length of the conductor increases

Answer: B. Transmission voltage increases

Explanation:

As transmitting the voltages, the power is being constant and P = VI cos ϕ.
So at higher voltages, to keep the power constant, the current will be less and hence we can minimize the transmission losses.
By increasing the power factor, the losses will get minimized.
So, an increase in voltage level and power factor improves transmission efficiency.

Advantages of high transmission voltage:

1. Reduces the volume of conductor material
The volume of conductor material required is inversely proportional to the square of transmission voltage and power factor. The greater the transmission voltage, the lesser is the conductor material required.

2. Increases transmission efficiency
Transmission efficiency increases when the line voltage is increased.

3. Decreases percentage line drop
Percentage line drop decreases when the transmission voltage increases.

Important Points:

Limitations of high transmission voltage:
1. The increased cost of insulating the conductors
2. The increased cost of transformers, switchgear and other terminal apparatus

3. A 3-phase transmission line increases transmission efficiency when _________.

A. Power factor decreases
B. Current density increases
C. Line voltage increases
D. Length of the conductor increases

Answer: C. Line voltage increases

Explanation:

As transmitting the voltages, the power is being constant and P = VI cos ϕ.
So at higher voltages, to keep the power constant, the current will be less and hence we can minimize the transmission losses.
By increasing the power factor, the losses will get minimized.
So, an increase in voltage level and power factor improves transmission efficiency.

Advantages of high transmission voltage:

1. Reduces the volume of conductor material
The volume of conductor material required is inversely proportional to the square of transmission voltage and power factor. The greater the transmission voltage, the lesser is the conductor material required.

2. Increases transmission efficiency
Transmission efficiency increases when the line voltage is increased.

3. Decreases percentage line drop
Percentage line drop decreases when the transmission voltage increases.

Important Points:

Limitations of high transmission voltage:
1. The increased cost of insulating the conductors
2. The increased cost of transformers, switchgear and other terminal apparatus

4. SF6 circuit breaker have the advantages of

A. very much reduced electrical clearance and minimum current chopping problem
B. no reduction in dielectric strength
C. performance independent of ambient conditions
D. All of the above

Answer: D. All of the above

Explanation:

Advantages of SF6 circuit breaker:

The SF6 gas is inflammable and chemically stable
Their decomposition products are non-explosive and hence there is no risk of fire or explosion
Electric clearance is very much reduced because of the high dielectric strength of SF6
It gives the noiseless operation
There is no over-voltage problem because the arc extinguishes at natural current zero
There is no reduction in dielectric strength because no carbon particles are formed during arcing.
It requires less maintenance and no costly compressed air system is required
SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, and transformer reactor, etc. without any problem.
The performance is independent of ambient conditions

Important Points:
The voltage levels of different circuit breakers are given below.

Air-blast circuit breaker ----132 kV
Air-break circuit breaker ---400 V to 15 kV
Tank type oil circuit breaker ----up to 220 kV
SF6 circuit breaker -------up to 765 kV
Vacuum circuit breaker ----22 kV to 66 kV

5. A lightning arrester is usually located nearer to

A. Transformer
B. Isolator
C. Busbar
D. Circuit breaker

Answer: A. Transformer

Explanation:
Lightning arresters or surge diverters:

A lightning arrester or a surge diverter is a protective device that conducts the high voltage surges on the power system to the ground
It consists of a spark gap in series with a non-linear resistor
The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap, but a dangerously high voltage will break down the air insulation and form an arc
The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa
One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded.
It usually located near the Transformer in the substation.

6. Power stations and sub-stations are protected against direct strokes of lightning using:

A. Rod gap arrester
B. Overhead ground wires
C. Earthing screen
D. Horn gap arrester

Answer: C. Earthing screen

Explanation:
Earthing screen:

The power stations and sub-stations generally have expensive equipment
These stations can be protected against direct lightning strokes by providing an earthing screen
It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the substation or power station
The shield is properly connected to earth on at least two points through a low impedance
On the occurrence of a direct stroke on the station, the screen provides a low resistance path by which lightning surges are conducted to ground. In this way, station equipment is protected against damage
The limitation of this method is that it does not provide protection against the traveling waves which may reach the equipment in the station

Important Point:

Overhead ground wires:
The most effective method of providing protection to transmission lines against direct lightning strokes are using overhead ground wires
It provides considerable protection against direct lightning strokes on transmission lines
A grounding wire provides a damping effect on any disturbance traveling along the line as it acts as a short-circuited secondary
It provides a certain amount of electrostatic shielding against external fields
Thus, it reduces the voltages induced in the line conductors due to the discharge of a neighboring cloud

7. Depending upon the voltage to be handled, switchgear may be broadly classified into:
A. Outdoor type

B. None of these
C. Indoor type
D. Both Outdoor type and Indoor type

Answer: D. Both Outdoor type and Indoor type

Explanation:
Depending upon the voltage to be handled, switchgear may be broadly classified into

Outdoor type Switchgear
Indoor type Switchgear

Outdoor Switchgear:

For voltage beyond 66 kV, we install outdoor switchgear equipment.
It is because, for such voltages, the clearances between conductors and the space required for switches, circuit breakers, transformers, and other equipment become so great that it is not economical to install all such equipment indoor.

Indoor Switchgear:

For voltages below 66 kV, switchgear is generally installed indoor because of economic considerations.
It is generally of the metal-clad type
In this type of construction, all live parts are completely enclosed in an earthed metal casing
The primary object of this practice is the definite localization and restriction of any fault to its place of origin

8. A device, which detects the fault and supplies information to the breaker for circuit interruption is called:

A. Circuit breaker
B. Arrester
C. Conductor
D. Relay

Answer: D. Relay

Explanation:

A device, which detects the fault and supplies information to the breaker for circuit interruption is called relay.
Relay is a switch that controls (open and close) circuits electromechanically
The main operation of this device is to make or break contact with the help of a signal without any human involvement in order to switch it ON or OFF
It is mainly used to control a high-powered circuit using a low power signal
Generally, a DC signal is used to control circuit which is driven by high voltage like controlling AC home appliances with DC signals from microcontrollers

9. Surge diverters and Surge absorbers are the types of:

A. Relays
B. Circuit breakers
C. Overvoltage protection devices
D. Reactors

Answer: C. Overvoltage protection devices

Explanation:
Surge diverter:

A surge diverter (or arrestor) is a device attached to the main power board which gives protection against damaging power surges, either from outside or from within (such as from faulty wiring or appliances).
The device prevents excess power from damaging or destroying sensitive electrical and electronic appliances such as computers and audio gear.

Surge absorber:

A surge absorber is a protective device that reduces the steepness of wavefront of a surge by absorbing surge energy.Although both surge diverter and surge absorber remove the surge, the manner in which it is done is different in the two devices.
The surge diverter diverts the surge to earth, but the surge absorber absorbs the energy of a surge.

10. For which of the following thermal bimetal is used?

A. overload relay
B. Generator
C. in accelerometer
D. in the motor

Answer: A. overload relay

Explanation:
Thermal bimetallic strip:

A bimetallic strip is used to convert a temperature change into mechanical displacement
The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass
The strips are joined together throughout their length by riveting, brazing or welding
The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature
The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled

Applications:

Clocks
Thermostats
Thermometer
Heat engine
Electric devices: Bimetal strips are used in miniature circuit breakers to protect circuits from excess current. Bimetal strips are also used in time-delay relays, lamp flashers, and fluorescent lamp starters

11. Which of the following range of voltage is used in medium transmission lines?

A. 20 kV to 100 kV
B. 66 kV to 400 kV
C. 100 kV to 400 kV
D. 3.3 kV to 6.6 kV

Answer: A. 20 kV to 100 kV

Explanation:
Transmission lines are classified based on three criteria.
a) Length of transmission line
b) Operating voltage
c) Effect of capacitance

The table below summaries the classification of transmission lines.

Transmission Lines	Length of transmission line	Operating voltage	Effect of capacitance
Short transmission line	(0 - 80) km	(0 - 20) kV	'C' is not considered
Medium transmission line	(80 - 200) km	(20 - 100) kV	'C' is lumped.
Long transmission line	(> 200) km	(> 100) kV	'C' is distributed

12. Shunt reactors are needed:

A. to bring down receiving end voltage under heavy loads

B. to bring down receiving end voltage at light loads
C. to boost receiving end voltage under light load condition
D. to boost receiving end voltage under heavy loads

Answer: B. to bring down receiving end voltage at light loads

Explanation:
Under no-load condition or light load condition, medium and long transmission lines may operate at leading power factor due to the capacitance effect.

So that receiving end voltage becomes greater than sending end voltage.

In this case, shunt reactors are needed to bring down receiving end voltage at light loads.

The leading power factor can be changed to a lagging power factor by using a shunt reactor. By using a shunt reactor, it will compensate for the effect of capacitance and changes the power factor.

Note:

The shunt capacitor is used to improve the power factor.
A series reactor smoothens the wave shape.
A Series capacitor reduces the net reactance in a line.
Shunt inductor reduces the Ferranti effect by limiting overvoltages at the load side under lightly loaded conditions.

13. A long transmission line has a large capacitance. If such a line is open-circuited or connected to the very light load at the receiving end, the magnitude of the voltage at the receiving end becomes higher than the voltage at the sending end. Such a phenomenon is called:

A. Skin effect
B. Corona loss
C. Ferranti effect
D. Sag in transmission lines

Answer: C. Ferranti effect

Explanation:
Ferranti Effect:

A long transmission line has a large capacitance. If such a line is open-circuited or connected to the very light load at the receiving end, the magnitude of the voltage at the receiving end becomes higher than the voltage at the sending end. Such a phenomenon is known as the Ferranti effect.

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.

14. The receiving end voltage for a long line under no-load conditions is

A. less than the sending end voltage
B. more than the sending end voltage
C. equal to the sending end voltage
D. Any of the above

Answer: B. more than the sending end voltage

Explanation:
Ferranti effect:
During no load condition, the current flowing is only charging current due to line capacitance. It increases the capacitive var in the system.
Since the line is under no load the line inductance will be less. Therefore, the capacitive var becomes greater than inductive var during no load or light load condition.
Due to this phenomenon the receiving end voltage becomes greater than sending end voltage. This effect is also called Ferranti effect.
Ferranti effect in a transmission line is reduced using voltage compensation which in turn is accomplished by Shunt Reactor
The shunt reactor is just like a transformer, but it has only primary and no secondary
The shunt reactor is connected to all three phases i.e. R, Y and B phase

15. Ferranti effect states that under certain conditions, the sending end voltage is

A. Less than receiving end voltage
B. Greater than receiving end voltage
C. Equal to receiving end voltage
D. Not having any impact on the receiving end voltage

Answer: A. Less than receiving end voltage

Explanation:
During no-load condition, the current flowing is only charging current due to line capacitance. It increases the capacitive var in the system.

Since the line is under no load the line inductance will be less. Therefore, the capacitive var becomes greater than inductive var during no load or light load condition.Due to this phenomenon the receiving end voltage becomes greater than sending end voltage. This effect is also called Ferranti effect.

16. Ferranti effect on long overhead lines is experienced when:

A. Power factor is unity
B. Line is lightly loaded
C. Line is heavily loaded
D. Line is fully loaded

Answer: B. Line is lightly loaded

Explanation:
Ferranti effect:
The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of a light load or open circuit at the receiving end.

Ferranti Effect occurs when the current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line(during light or no load).
This capacitor charging current leads to a voltage drop across the line inductor of the transmission system which is in phase with the sending end voltages.

This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently, the receiving end voltage tends to get larger than the applied voltage leading to the phenomena called the Ferranti effect in the power system.

Thus both the capacitance and inductor effect of the transmission line are equally responsible for this particular phenomenon to occur, and hence Ferranti effect is negligible in the case of a short transmission line as the capacitance of such a line is practically not considered.

In general for a 300 Km line operating at a frequency of 50 Hz, the no-load receiving end voltage has been found to be 5% higher than the sending end voltage.

Note:

Ferranti effect is not a problem for short transmission lines because there is no capacitance effect
Ferranti effect means receiving end voltage is greater than sending end voltage
This phenomenon occurs in medium and long transmission lines which are at lightly loaded or no-load condition. This is because of the capacitance effect.

17. Which of the following is a solution to Ferranti's effect?

A. shunt capacitor
B. shunt reactor
C. series capacitor
D. none of the above

Correct Answer: B. shunt reactor

Explanation:
Ferranti effect:

The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the ''Ferranti effect.''
This type of effect mainly occurs because of light load or open circuit at the receiving end.
VR > Vs
Ferranti effect is due to the charging current of the line.
Ferranti effect is not a problem for short transmission lines because there is no capacitance effect
Ferranti effect means receiving end voltage is greater than sending end voltage
This phenomenon occurs in medium and long transmission lines which are at lightly loaded or no-load condition. This is because of capacitance effect

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.

18. Ferranti effect in long transmission lines is due to the effect of

A. Line reactance
B. Line capacitance
C. Line resistance
D. None of the above

Answer: B. Line capacitance

Explanation:
Ferranti effect:
The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of light load or open circuit at the receiving end.

Ferranti Effect occurs when current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line(during light or no load).
This capacitor charging current leads to a voltage drop across the line inductor of the transmission system which is in phase with the sending end voltages.

This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently, the receiving end voltage tends to get larger than applied voltage leading to the phenomena called Ferranti effect in power system.

Thus both the capacitance and inductor effect of transmission line are equally responsible for this particular phenomena to occur, and hence Ferranti effect is negligible in case of a short transmission line as the capacitance of such a line is practically not considered.

In general for a 300 Km line operating at a frequency of 50 Hz, the no-load receiving end voltage has been found to be 5% higher than the sending end voltage.

19. Shunt reactors are needed:

A. to bring down receiving end voltage under heavy loads
B. to bring down receiving end voltage at light loads
C. to boost receiving end voltage under light load condition
D. to boost receiving end voltage under heavy loads

Answer: B. to bring down receiving end voltage at light loads

Explanation:
Under no-load condition or light load condition, medium and long transmission lines may operate at leading power factor due to the capacitance effect.

So that receiving end voltage becomes greater than sending end voltage.

In this case, shunt reactors are needed to bring down receiving end voltage at light loads.

The leading power factor can be changed to a lagging power factor by using a shunt reactor. By using a shunt reactor, it will compensate for the effect of capacitance and changes the power factor.

Note:

The shunt capacitor is used to improve the power factor.
A series reactor smoothens the wave shape.
A Series capacitor reduces the net reactance in a line.
Shunt inductor reduces the Ferranti effect by limiting overvoltages at the load side under lightly loaded conditions.

20. What is the number of suspension insulators required for 132 kV transmission?

A. 6
B. 12
C. 10
D. 8

Answer: B. 12

Explanation:
In suspension insulator number of insulators is connected in series to form a string and the line conductor is carried by the bottom-most insulator.

Each insulator of a suspension string is called a disc insulator because of its disc-like shape.

Each suspension disc is designed for normal voltage rating 11 kV, so by using different numbers of discs, a suspension string can be made suitable for any voltage level.

Calculation:
Given voltage rating = 132 kV
As the rating of each disc insulator is 11 kV, the total number of disks required are

= 132/11 = 12

More MCQS With Fully Explained

1. Under over excitation synchronous phase modifier works as

2. The neutral of the power system may be connected to earth

3. The time to open the circuit breaker contact is called as

4. A 100 Ah capacity battery should deliver a current of 10 A for approximately _____ hrs.

5. For 400KV line, The Spacing Between phase conductor is around

6. Shunt capacitors in a sub-station

7. An alternator is said to be over excited when it is operating at

8. In case of transformer Buchholz relay is used to protect from

9. Series capacitors are used to

10. Disadvantage of low power factor is

11. The colour of neutral three core flexible cable is