Electrical Power System Lecture 5: Underground Cables

Questions and Answers

What is the maximum voltage rating of belted cables?

22KV

33KV

11KV (correct)

44KV

Which method involves digging a trench for cable installation?

Conduit laying

Draw in System

Direct laying (correct)

Surface laying

What material is commonly used to protect the underground cable in the direct laying method?

Concrete blocks

Metal sheets

Bricks and other materials (correct)

Gravel

What is the primary function of providing spacing between multiple cables in a trench?

To reduce mutual heating and prevent damage Signup and view all the answers

What voltage rating does a screened cable support?

Up to 66KV Signup and view all the answers

Which method is preferred for short length cable routes such as road crossings?

Draw in System Signup and view all the answers

What depth is typically required when digging a trench for direct laying of cables?

1.5 meters Signup and view all the answers

Which of the following is NOT a component typically found in the Draw in System?

Fine sand Signup and view all the answers

Which material is typically NOT used for insulation in underground cables?

Fibrous bedding Signup and view all the answers

What is the primary purpose of a metallic sheath in an underground cable?

To protect the cable from moisture and damaging liquids Signup and view all the answers

Which layer is applied to protect the metallic sheath from corrosion and mechanical damage?

Bedding Signup and view all the answers

What classification of underground cable has a maximum voltage capacity of 11KV?

HT Cables Signup and view all the answers

Which of the following cable types is associated with a voltage rating beyond 132KV?

Extra Super voltage cables Signup and view all the answers

What material is used for armouring a cable?

Galvanized steel wire Signup and view all the answers

What is the main function of serving in underground cables?

Protect armouring from atmospheric conditions Signup and view all the answers

Which type of cable is classified as having a maximum capacity of 1000 V?

LT Cables Signup and view all the answers

What is the primary advantage of using the solid system for cable laying?

Good mechanical protection Signup and view all the answers

Why do underground cables generally require larger conductor sizes compared to overhead lines?

They lack natural cooling Signup and view all the answers

Which factor contributes to the higher construction complexity of underground cables?

Use of insulation materials Signup and view all the answers

What is a significant disadvantage of underground cables in terms of fault management?

Complicated fault location Signup and view all the answers

Which voltage range are underground cables primarily used for, due to their construction limitations?

Up to 33KV Signup and view all the answers

What is a notable difference regarding the cooling capability of overhead lines in comparison to underground cables?

Overhead lines have natural cooling Signup and view all the answers

In terms of costs, how do underground cables compare to overhead lines?

Underground cables are more expensive to construct Signup and view all the answers

What is a common misconception about the mechanical strength of underground cables versus overhead cables?

Underground cables provide superior protection Signup and view all the answers

What does the permittivity ε of the insulation material represent?

The ability of the insulation to store electrical energy Signup and view all the answers

How is dielectric stress defined in the context of a single-core cable?

As a combination of electric intensity and potential gradient Signup and view all the answers

What is the relationship between electric intensity Ex and potential gradient g in a single-core cable?

Ex is equal to potential gradient g Signup and view all the answers

What does the inner sheath diameter D represent in a single-core cable?

The effective radius for electric field calculations Signup and view all the answers

What happens to the dielectric stress as the distance x from the center of the cable increases?

It may decrease at higher distances Signup and view all the answers

What is the role of the lead sheath in a single-core cable?

To maintain an earth potential Signup and view all the answers

How is the surface area of a cylinder calculated in the context of a single-core cable?

$2 π x imes 1$ Signup and view all the answers

What does the charge per meter Q depend on in a single-core cable?

The potential difference V and core dimensions Signup and view all the answers

What is one of the main reasons underground cables are utilized instead of overhead lines?

They are considered reliable in certain environments. Signup and view all the answers

Which component of an underground cable is primarily responsible for protecting against moisture?

Cable sheath Signup and view all the answers

What type of material can the conductor in an underground cable be made from?

Copper or aluminum Signup and view all the answers

How many cores can an underground cable have at most?

Four Signup and view all the answers

What effect does the wire screen in an underground cable have?

It controls the electric field and discharges fault currents. Signup and view all the answers

What determines the thickness of insulation on each core of an underground cable?

The voltage to be withstood by the cable Signup and view all the answers

Which of the following is NOT a function of the cable's insulation in an underground cable system?

Conduct electricity Signup and view all the answers

In what situation will underground cables typically be employed?

In locations where overhead lines are impractical Signup and view all the answers

Study Notes

Underground Power Cables

Underground cables are used for reliable power transmission when overhead lines are impractical, such as near plants and substations.
Installation costs for underground cables are higher than overhead lines, but they provide better protection from environmental factors.

Construction of Underground Cables

Cores/Conductors: Can be made of aluminum or copper and can have multiple configurations (one to four cores).
Cable Sheath: Protects against moisture.
Wire Screen: Controls electric field and discharges fault currents.
Insulating Layer: Insulates electric conductors; common materials include impregnated paper, varnished cambric, or rubber.
Metallic Sheath: Made from lead or aluminum to protect from moisture and damaging liquids.
Bedding: A fibrous layer protecting the metallic sheath from corrosion and mechanical damage.
Armouring: Consists of galvanized steel wire or tape to prevent mechanical injury.
Serving: A protective fibrous layer similar to bedding, guards armouring from atmospheric conditions.

Classification of Underground Cables

Voltage Capacity:
- LT Cables: Up to 1000 V (Low Tension)
- HT Cables: Up to 11 kV (High Tension)
- ST Cables: 22 kV to 33 kV (Super Tension)
- EHT Cables: 33 kV to 66 kV (Extra High Tension)
- Extra Super Voltage Cables: Greater than 132 kV
Construction Type:
- Belted Cables: Up to 11 kV
- Screened Cables: Up to 66 kV
- Pressure Cables: Over 66 kV

Methods of Laying Underground Cables

Direct Laying:
- Involves digging a trench (1.5 m deep, 45 cm wide) and using sand layers for protection.
- Requires inter-axial spacing of at least 30 cm for multiple cables to mitigate mutual heating effects.
Draw-in System:
- Utilizes ducts with manholes for cable pulling, ideal for short lengths like in workshops and road crossings.
Solid System:
- Involves laying cables in open pipes or troughs, providing good mechanical protection but at a higher cost than direct laying.

Comparison: Underground Cables vs. Overhead Lines

Construction Complexity: Underground cables are more complex and costly to construct than simpler overhead lines.
Conductor Size: Underground cables require larger conductor sizes due to limited heat dissipation compared to overhead lines.
Voltage Capacity: Overhead lines support higher voltage capacities; underground cables are typically limited to 33 kV.
Fault Detection: Overhead cables offer easier fault detection and repair than underground cables, which require more time and effort.

Dielectric Stress in Cables

Dielectric stress refers to electrostatic forces acting on cable insulation under normal operating conditions.
Electric intensity and potential gradient are equivalent at any point in the cable, affecting the insulation's performance.
Variations in dielectric stress can impact the operational life and reliability of underground cables.

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Description

This quiz covers the key concepts of underground power cables as discussed in the fifth lecture of the Electrical Power System course. Explore the advantages and disadvantages of underground cables compared to overhead lines and understand their applications in power transmission.