Power System Most asked MCQS Part-2

July 25, 2021

1. Cables used for underground services from 33 kV to 60 kV are:

A. extra high-tension cables
B. high tension cables
C. super tension cables
D. extra super voltage cables

Answer: A. extra high-tension cables

Explanation:
Classification of underground cables on the basis of voltage level is given below

Low tension (L.T.) Cable -----0 – 1 kV
High tension (H.T.) Cable-----1 – 11 kV
Super tension (S.T.) Cable-----11 – 33 kV
Extra high-tension (E.H.T.) Cable-----33 – 66 kV
Extra super-tension(E.S.T.) Cable------66 kV and above

2. Dielectric strength of rubber is around

A. 3 kV/mm
B. 10 kV/mm
C. 30 kV/mm
D. 300 kV/mm

Answer: C. 30 kV/mm

Explanation:
Dielectric Strength:

It reflects the electric strength of insulating materials at various power frequencies.
It is the voltage per unit thickness at which a material will conduct electricity.

Material ans its Dielectric Strength (kV/mm)

Air----3
Oil---5-20
Rubber---30-40
Mica----118
Alumina----13.4
Diamond-----2000

3. Low voltage cable has operating voltage up to:

A. 1 kV
B. 11 kV
C. 22 kV
D. 66 kV

Answer:A. 1 kV

Explanation:
The classification of cables based on their operating voltages is as given below.

Low-tension (L.T.) cables — up to 1 kV
High-tension (H.T.) cables — up to 11 kV
Super-tension (S.T.) cables — from 22 kV to 33 kV
Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
Extra super voltage cables — beyond 132 kV

4. Sheaths are used in underground cables to

A. Provide proper insulation
B. Provide mechanical strength
C. Protect the cable form moisture
D. None of the above

Answer: C. Protect the cable form moisture

Explanation:
Lead sheath:

In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation as shown in the figure. It has minimum dielectric stress in a cable.
The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.

Bedding:

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. It is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

Armouring:

Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injuries while laying it or handling it.

5. Screened cables can be used for voltages:

A. up to 11 kV
B. 11 kV to 33 kV
C. 33 kV to 66 kV
D. 66 kV to 132 kV

Answer: C. 33 kV to 66 kV

Explanation:

In practice, underground cables are generally required to deliver 3-phase power. For the purpose, either three-core cable or three single-core cables may be used.
For voltages upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons.
However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used.

The following types of cables are generally used for 3-phase service:

Belted cables — up to 11 kV
Screened cables — from 22 kV to 66 kV
Pressure cables — beyond 66 kV.

Screened cables:

These cables are meant for use up to 33 kV, but in particular cases, their use may be extended to operating voltages up to 66 kV. Two principal types of screened cables are H type cables and S.L. type cables.

6. Which of the following protects the cable from moisture, gases and other damaging liquids such as acids or alkalis?

A. Armouring
B. Lead sheath
C. Bedding
D. None of the above

Answer: B. Lead sheath

Explanation:
Lead sheath:

In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in the figure. It has minimum dielectric stress in a cable.
The extruded Lead sheath also serves as Metallic Screen. Copper Tape/Copper sheath can also be used but Copper is costly and would rather be used as overhead conductors with less mechanical stresses.

Bedding:

Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape. It is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

Armouring:

Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injuries while laying it or handling it.

7. The cable used for high voltage applications is:

A. Vulcanized India Rubber (VIR) cables
B. Polythene insulated cable
C. Elastomer insulated cable
D. Gas-filled cable

Answer: D. Gas-filled cable

Explanation:
The following types of cables are generally used for 3-phase service:
1. Belted cables - up to 11 kV
2. Screened cables - from 22 kV to 66 kV
3. Pressure cables - beyond 66 kV

Pressure cables are of two types:

Oil-filled cables
Gas-filled pressure cables

8. As the cable length increases, there is

A. less insulation resistance and more leakage current
B. more insulation resistance and less leakage current
C. less insulation resistance and less leakage current
D. more insulation resistance and more leakage current

Answer: A. less insulation resistance and more leakage current

Explanation:
Insulation resistance:

Insulation resistance is defined as the ratio of applied direct voltage across insulation to the corresponding current flowing through it
The current flow through insulation is called leakage current
Leakage resistance or insulation is the opposition that a substance offers to the flow of leakage current

Insulation resistance of the cable is given by

The resistance of insulation is inversely proportional to its length.

Application:

As the insulation resistance is inversely proportional to the cable length, with the increase in cable length, the insulation resistance decreases.
As the insulation resistance decreases, leakage current will increase.

9. Which of the following protects a cable against mechanical injury?

A. Bedding
B. Sheath
C. Core
D. Armouring

Answer: D. Armouring

Explanation:
Lead sheath:
In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation as shown in figure.
It has a minimum dielectric stress in a cable.

Bedding:
Over the metallic sheath is applied a layer of bedding which consists of fibrous material like jute or hessian tape.
It is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

Armouring:
Over the bedding, armouring is provided which consist of one or two layers of galvanized steel wire or steel tape.
Its purpose is to protect the cable from mechanical injuries while laying it or handling it.

10. HT cables are generally used up to

A. 11 kV
B. 33 kV
C. 66 kV
D. 132 kV

Answer: A. 11 kV

Explanation:

Low tension line---Up to 1 kV
High tension line---Up to 11 kV
Super tension line---22 kV to 33 kV
Extra high tension line---33 kV to 66 kV
Extra super voltage cable---Beyond 132 kV

11. The material commonly used for insulation in high voltage cables is

A. Teflon
B. impregnated paper
C. rubber
D. PVC

Answer: B. impregnated paper

Explanation:
Generally, insulation of cables is required to have high dielectric strength, to provide the necessary withstand against normal operating and impulse voltages.

There are mainly two types of cable insulating materials.
Impregnated Paper
Polymer.

Impregnated paper is the most popular insulating material, especially for the land installations in the H.V and EHV lines.
Polymer insulation is preferably used in new installations like DC and the submarine system.
Around 83% of HV cables are paper insulated.
Solid paper insulated cables are suitable for application up to the MV range. At higher voltage, there is some operation problem with them due to high dielectric losses and pressure of voids.
In oil-filled cables at high voltage, the formation of voids is prevented by maintaining a flow of oil under pressure.
The impregnation of paper improves the dielectric properties of paper insulation.
Polymer insulation is immune to thermal degradation and must not be prone to defects such as voids, protrusions, or contaminants.
For extra-high voltage (400 kV and higher) new types of types mode up of PPLP (Poly propylite laminated paper) are used.
PPLP has the advantage of combining the technical traditional technology used for impregnated paper cables with the excellent electrical and dielectric properties of synthetic insulators.

12. Which of the following is NOT a property of good insulating material used for cables?

A. Non-inflammable
B. High dielectric strength
C. High insulation resistance
D. High permittivity

Answer: D. High permittivity

Explanation:

The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. In general, the insulating materials used in cables should have the following properties.
High insulation resistance to avoid leakage current
High dielectric strength to avoid electrical breakdown of the cable
High mechanical strength to withstand the mechanical handling of cables
Non-hygroscopic i.e., it should not absorb moisture from air or soil; The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like a lead sheath
Non-Inflammable
Low relative permittivity
Low cost so as to make the underground system a viable proposition
Unaffected by acids and alkalies to avoid any chemical action

The various insulating materials used in manufacturing of cables are rubber, VIR, paper, Polyvinyl Chloride (PVC), Varnished Cambric, Polyethylene, Gutta-Percha, Silk, Cotton, enamel etc.

13. The insulation resistance of a piece of 10 km long single core underground cable is 10 MΩ. The insulation resistance for a length of 25 km the same cable would be

A. 1.6 MΩ
B. 4 MΩ
C. 25 MΩ
D. 62.5 MΩ

Answer: B. 4 MΩ

Explanation:

14. Cable impregnation means the application of ______ compound on the paper used in cables.

A. non-insulating oil
B. insulating oil
C. insulating metal
D. non-insulating metal

Answer: B. insulating oil

Explanation:

Cable impregnation means the application of insulating oil compounds on the paper used in cables.
Impregnated paper is a sheet with superior liquid-retaining and water-absorbing properties and is made from natural cotton linter and wooden pulp.
Other insulating materials used in the manufacturing of cables are rubber, VIR, paper, Polyvinyl Chloride (PVC), Varnished Cambric, Polyethylene, Gutta-Percha, Silk, Cotton, enamel etc.

15. _________conductors are most suitable for indoor and outdoor wires and cables.

A. Aluminium
B. Hard drawn copper
C. Annealed copper
D. Silver

Answer: C. Annealed copper

Explanation:
Annealed Copper:

Due of superior conductivity, annealed copper became the international standard to which all other electrical conductors are compared.
In 1913, the International Electrotechnical Commission defined the conductivity of commercially pure copper in its International Annealed Copper Standard.
Copper is used as the electrical conductor in many categories of electrical wiring.
Copper wire is used in power generation, power transmission, power distribution, telecommunications, electronics circuitry, and countless types of electrical equipment.
Copper and its alloys are also used to make electrical contacts. Electrical wiring in buildings is the most important market for the copper industry.
Roughly half of all copper mined is used to manufacture electrical wire and cable conductors.

Aluminium:-

An aluminium wire have a 1.5 times larger cross-section to pass the same current as a copper wire, but two times lighter.
Weight is one of the most important parameters for high-voltage power lines that transmit power over long distances.
Therefore, only aluminium wires are used in main overhead power lines.

Hard drawn copper:-

Hard drawn copper has high electrical conductivity and great tensile strength.
Use in transmission lines.

Silver:-

Because of best thermal and electrical conductivity among all metals, it is best for electrical applications.
Its cost is high and less abundance on Earth so have limited applications.

Power System Most asked MCQS Part-2

1. Cables used for underground services from 33 kV to 60 kV are:

(adsbygoogle = window.adsbygoogle || []).push({}); 2. Dielectric strength of rubber is around

2. Dielectric strength of rubber is around

(adsbygoogle = window.adsbygoogle || []).push({}); 3. Low voltage cable has operating voltage up to:

3. Low voltage cable has operating voltage up to:

4. Sheaths are used in underground cables to

4. Sheaths are used in underground cables to

(adsbygoogle = window.adsbygoogle || []).push({}); 5. Screened cables can be used for voltages:

5. Screened cables can be used for voltages:

(adsbygoogle = window.adsbygoogle || []).push({}); 6. Which of the following protects the cable from moisture, gases and other damaging liquids such as acids or alkalis?

6. Which of the following protects the cable from moisture, gases and other damaging liquids such as acids or alkalis?

(adsbygoogle = window.adsbygoogle || []).push({}); 7. The cable used for high voltage applications is:

7. The cable used for high voltage applications is:

(adsbygoogle = window.adsbygoogle || []).push({}); 8. As the cable length increases, there is

8. As the cable length increases, there is

(adsbygoogle = window.adsbygoogle || []).push({}); 9. Which of the following protects a cable against mechanical injury?

9. Which of the following protects a cable against mechanical injury?

(adsbygoogle = window.adsbygoogle || []).push({}); 10. HT cables are generally used up to

10. HT cables are generally used up to

(adsbygoogle = window.adsbygoogle || []).push({}); 11. The material commonly used for insulation in high voltage cables is

11. The material commonly used for insulation in high voltage cables is

(adsbygoogle = window.adsbygoogle || []).push({}); 12. Which of the following is NOT a property of good insulating material used for cables?

12. Which of the following is NOT a property of good insulating material used for cables?

(adsbygoogle = window.adsbygoogle || []).push({}); 13. The insulation resistance of a piece of 10 km long single core underground cable is 10 MΩ. The insulation resistance for a length of 25 km the same cable would be

13. The insulation resistance of a piece of 10 km long single core underground cable is 10 MΩ. The insulation resistance for a length of 25 km the same cable would be

(adsbygoogle = window.adsbygoogle || []).push({}); 14. Cable impregnation means the application of ______ compound on the paper used in cables.

14. Cable impregnation means the application of ______ compound on the paper used in cables.

(adsbygoogle = window.adsbygoogle || []).push({}); (adsbygoogle = window.adsbygoogle || []).push({}); 15. _________conductors are most suitable for indoor and outdoor wires and cables.

15. _________conductors are most suitable for indoor and outdoor wires and cables.

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2. Dielectric strength of rubber is around

3. Low voltage cable has operating voltage up to:

5. Screened cables can be used for voltages:

6. Which of the following protects the cable from moisture, gases and other damaging liquids such as acids or alkalis?

7. The cable used for high voltage applications is:

8. As the cable length increases, there is

9. Which of the following protects a cable against mechanical injury?

10. HT cables are generally used up to

11. The material commonly used for insulation in high voltage cables is

12. Which of the following is NOT a property of good insulating material used for cables?

13. The insulation resistance of a piece of 10 km long single core underground cable is 10 MΩ. The insulation resistance for a length of 25 km the same cable would be

14. Cable impregnation means the application of ______ compound on the paper used in cables.

15. _________conductors are most suitable for indoor and outdoor wires and cables.