Underground Cables MCQ Quiz - Objective Question with Answer for Underground Cables - Download Free PDF

Methods of laying underground cables

Direct Laying:

• This method requires digging a 1.5m deep and 0.45m wide trench, which is then covered with sand.
• The cables are laid in the trench and covered with a 10 cm-thick layer of sand. To protect against mechanical injury, the trench is then covered with bricks and other materials.
• If more than one cable is required to be laid in a trench, then a horizontal or vertical inter-axial spacing of 30 cm is provided to prevent mutual heating.
• Direct Laying (Direct Buried Method) requires re-excavation when load expansion or modification is needed, making it costly and labor-intensive.

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Draw in the system:

• Ducts or conduits of cast iron or concrete or glazed stone with manholes are placed at suitable locations along the cable route. The manholes are used for pulling the cable in position.

Troughing System:

• A troughing system, also known as a cable troughing system or cable raceway, is a specialized system designed to protect, organize, and route electrical cables within a defined pathway.
• Cables are laid in pre-cast concrete or fiberglass troughs which are then covered.

Solid system:

• Underground cables are laid in open pipes or troughs along the cable route. The troughs are usually made of asphalt, stoneware or cast iron.
• Asphaltic compound is used for filling the troughs once the cable is laid in position.

Concept

The problem involves the Murray loop test, which is used to locate a ground fault in an underground cable.

The distance of the fault from the testing end is given by:

\(d= {L \over 1+{R_2\over R_1}}\)

where, d = Distance

L = Total cable length

Calculation

Given, L = 500 m

Ratio of other arms in the testing system = 4:1

\({R_2\over R_1}=4\)

\(d= {500\over 1+4}\)

d = 100 m

So, d = 100 m from the test end.

However, in the Murray loop test, the fault location is measured along one side of the looped cable, meaning the fault is actually at 2 × 100 = 200 m from the test end.

Belted Type Cables

• Belted type cables are used for medium voltage applications, typically up to 22 kV.
• They consist of three conductors, each insulated separately and covered with an overall belt insulation.
• The insulation is made of materials like impregnated paper, and a lead sheath is used to provide protection.
• For voltages above 22 kV, the insulation thickness increases, both radial and tangential stresses are developed due to the voltage gradient in the insulation.
• Radial stress acts perpendicular to the layers of insulation. Due to the potential difference between the conductor and the outer sheath, radial stress forces the electric field outward.
• Tangential stress (also called circumferential stress) acts along the curved insulation surface. Uneven voltage distribution and imperfections in insulation lead to this stress along the layers.

Insulation in High Voltage Cables:

High voltage cables are designed to carry high voltage electrical power from one point to another. These cables need to have effective insulation to ensure safety, reliability, and efficiency. The insulation material in high voltage cables serves several critical purposes:

• Preventing electrical leakage: It prevents the electrical current from leaking out of the conductor to the surrounding environment or other conductors.
• Protecting against short circuits: It helps to prevent short circuits by keeping the conductors separated.
• Ensuring safety: It protects individuals and equipment from coming into direct contact with high voltage.

Materials Used for Insulation:

Different materials can be used for insulation in high voltage cables, each with its own properties and applications. The most common materials include:

• Lead: Although it has been used in the past, lead is not commonly used for insulation in modern high voltage cables due to its toxicity and environmental concerns.
• Rubber: Rubber has good insulating properties but is generally used in low to medium voltage applications. It is not typically used for high voltage insulation because it may not provide adequate performance under high voltage stress.
• Paper: Paper insulation, specifically oil-impregnated paper, is commonly used in high voltage cables. It has excellent dielectric properties, which means it can effectively withstand high voltage without breaking down. The paper is typically impregnated with a dielectric fluid (such as mineral oil) to enhance its insulating properties and thermal stability.

Conclusion:

The correct answer is option 2: Paper. Paper insulation, especially when impregnated with oil, is widely used in high voltage cables due to its excellent dielectric properties and ability to effectively handle high voltage stress. It ensures the safe and reliable operation of high voltage power transmission systems.

Concept

The insulation resistance of a single-core cable is given by:

\(R={ρ\over 2\pi l}ln({R\over r})\)

where, ρ = Resistivity

l = Length

R = Sheath radius

r = Core radius

Calculation

Given, ρ =  4.5 × 1014 cm2 =  4.5 × 10⁴ km2

l = 1 km 

r = 1.25 cm

R = r + thickness = 2.5 cm

\(R={4.5\times 10^{4}\over 2\pi \times 1}ln({2.5\over 1.25})\)

\(R={4.5\times 10^{4}\over 2\pi \times 1}\times 0.693\)

R = 4966.81 Ω 

Note: As per the answer key, the resistivity value should be 4.5 × 1013 cm² to get 496 Ω as the correct answer.

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

Belted cables:

• These cables are used for voltages up to 11 kV but in extraordinary cases, their use may be extended up to 22 kV
• The belted type construction is suitable only for low and medium voltages as the electrostatic stresses developed in the cables for these voltages are more or less radial i.e., across the insulation
• For high voltages (beyond 22 kV), the tangential stresses also become important
• These stresses act along the layers of paper insulation
• As the insulation resistance of paper is quite small along the layers, therefore, tangential stresses set up leakage current along the layers of paper insulation
• The leakage current causes local heating, resulting in the risk of breakdown of insulation at any moment

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	Dielectric Strength (kV/mm)
Air	3
Oil	5-20
Rubber	30-40
Mica	118
Alumina	13.4
Diamond	2000

Concept:

Function	Colour code
Single-phase line	Red/Brown
Single-phase neutral	Black/Blue
Ground wire	Green
Three-phase line 1	Red
Three-phase line 2	Yellow
Three-phase line 3	Blue
Three-phase neutral	Black
Three-phase protective ground or earth	Green (or) Green - Yellow
Neutral wire (3-core flexible cable)	Blue

 

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

Type of Cable	Voltage level (kV)
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

Representation of ACSR strand Conductor:

A standard conductor is represented as A/S/D for ACSR conductor.

Where,

A is the number of aluminum strands.

S is the number of steel strands.

D is the diameter of each strand.

Example: If the ACSR conductor having 7 steel strands surrounded by

25 aluminum conductors with a diameter of 0.05 mm will be specified as

25/7/0.05.

Application:

Given: 

Cable is expressed as 3/0.029 

This implies the cable consists of 3 strands of 0.029 mm.

Direct Laying Of Underground Cables

The cables to be laid using this method must have a serving of bituminized paper and hessian tape so as to provide protection against corrosion and electrolysis.

The direct laying procedure is as follows:

• A trench of about 1.5 meters deep and 45 cm wide is dug.
• Then the trench is covered with a 10 cm thick layer of fine sand.
• The cable is laid over the sand bed. The sand bed protects the cable from moisture from the ground.
• Then the laid cable is again covered with a layer of sand about 10 cm thick.
• When multiple cables are to be laid in the same trench, a horizontal or verticle spacing of about 30 cm is provided to reduce the effect of mutual heating. Spacing between the cables also ensures a fault occurring on one cable does not damage the adjacent cable.
• The trench is then covered with bricks and soil to protect the cable from mechanical injury.

Advantages

• Simple and cheap method.
• The heat generated in cables is easily dissipated in the ground.​

Disadvantages

• ​Alterations in the cable network are not easy.
• Maintenance cost is higher.
• Identifying the location of a fault is difficult.
• This method can not be used in congested areas such as metro cities where excavation is too expensive.

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.

Concept of Voltage Rating:

•   The voltage rating of a cable refers to the maximum voltage to which it may be connected and have running through it.
•   Voltage rating specifies the safe voltage that the insulation of a cable can withstand.

Explanation:

Voltage Rating of cable gives the capacity of insulation for that cable withstand. Voltage grading is nothing but the voltage rating for which insulation of cable can withstand. so the answer is voltage grading of cable specifies the safe voltage that the insulation of a cable can withstand.

Colour of different types of Wire or Cable:

Type	Colour code
Single-phase line	Red/Brown
Single-phase neutral	Black/Blue
Ground wire	Green
Three-phase line 1	Red
Three-phase line 2	Yellow
Three-phase line 3	Blue
Three-phase neutral	Black
Three-phase protective ground or earth	Green (or) Green - Yellow
Neutral wire (3-core flexible cable)	Blue

Note:
In a 3-core cable brown(active), light blue (neutral), and green/yellow(earth).

The correct answer is option 2):(6 cm)

Concept:

Maximum dielectric stress (at the surface of the conductor) is given by

\({{g}_{mx}}=\frac{V}{rln\left( \frac{R}{r} \right)}\)

Minimum dielectric stress (at the inner sheath) is given by

\({{g}_{mn}}=\frac{V}{Rln\left( \frac{R}{r} \right)}~\)

Condition for minimizing dielectric stress (economical)

\(\frac{R}{r}=e\)

The ratio between the maximum and minimum stress in the dielectric of a single-core cable is given as

 \(g_{mx} \over g_{mn} \) = \(D \over d\)

Insulation thickness =  \(D-d \over 2\)

Where

g = electric field intensity

d = Diameter of the conductor 

D = Diameter of the inner sheath

r = conductor radius

R = inner sheath radius

V = peak value of the potential of conductor w.r.t sheath

Calculation:

Giveng_mx =  80 kV/cm

g_mn =  20 kV/cm

d = 4 cm

D = \(g_{mx} \over g_{mn} \) ×  4

=  \(80 \over 20 \) ×  4

= 16

Insulation thickness =  \(D-d \over 2\)

= \(16 -4 \over 2\)

= 6 cm