Underground Cables - Free MCQ Practice Test with solutions, GATE EE Power

• 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.

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.

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

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.

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

Different types of insulating materials can be used in underground cables depending on the specific requirements and applications.
Common insulating materials used in underground cables include:

• Oil-impregnated paper: Oil-impregnated paper insulation is a traditional and widely used insulating material for underground cables. The paper is impregnated with a special type of insulating oil to enhance its dielectric properties and provide electrical insulation.
• PVC (Polyvinyl Chloride): PVC insulation is commonly used in underground power cables. It is a thermoplastic material that offers good electrical insulation properties and resistance to moisture and environmental factors.
• Varnished cambric: Varnished cambric is a type of fabric-based insulation that is treated with varnish to enhance its electrical properties. It has been historically used in underground cables but has become less common in modern installations.
• Rubber: Rubber insulation, particularly ethylene propylene rubber (EPR) or cross-linked polyethylene (XLPE), is commonly used in medium and high-voltage underground cables. Rubber offers good electrical insulation properties and is resistant to moisture and environmental factors.

In practice, different types of insulating materials may be used in combination within an underground cable, depending on the specific requirements for voltage rating, environmental conditions, and cable design.

• Sulfur hexafluoride (SF₆) is itself an excellent insulating gas used in high-voltage applications, such as in switchgear, circuit breakers, and transmission lines. When referring to "sulfur hexafluoride cable", it is likely referring to high-voltage cables where SF₆ gas is used as the insulating medium. Therefore, the correct answer to your question would be Compressed gas
• Sulfur hexafluoride, or SF₆, is a type of gas that is commonly used as an insulator in electrical equipment, particularly in high-voltage applications such as high-voltage circuit breakers, switchgear, transformers, and transmission lines. The reason for its popularity is its excellent insulating properties and high electro-negativity, which means it is very good at preventing electrical discharge.
• The gas is typically kept under moderate pressure, thereby increasing its dielectric strength and making it an even more effective insulator. The gas is non-toxic, non-flammable, and chemically stable, which makes it safe for use in electrical systems.

Voltage stress in underground cable

The voltage stress at distance 'x' from the center of the cable is given by:

From the above expression, the stress (E) is inversely proportional to the distance (x).
The voltage stress is maximum at the surface of the conductor (x = d).
The voltage stress is minimal at the surface of the sheath (x = D)

Calculation

Given, E_max = 50 kV/cm (rms)

E_min = 10 kV/cm (rms)

d = 2.5 cm

D = 5 × 2.5 = 12.5 cm

D - d = 12.5 - 2.5 = 10 cm

Thickness = 10/2 = 5 cm