Underground Cabling

I. Introduction

Underground cabling plays a crucial role in power systems as it provides a reliable and efficient method of transmitting electricity. Unlike overhead transmission lines, which are exposed to environmental factors and can be affected by weather conditions, underground cables are protected from these external influences. In this topic, we will explore the fundamentals of underground cabling and compare it to overhead transmission lines.

II. Comparison of Cables and Overhead Transmission Lines

A. Advantages of Underground Cabling

• Reduced visual impact: Underground cables are hidden from view, preserving the aesthetics of the surrounding area.
• Protection from weather conditions: Underground cables are not affected by weather conditions such as strong winds, ice, or lightning.
• Lower transmission losses: Underground cables have lower transmission losses compared to overhead lines.

B. Disadvantages of Underground Cabling

• Higher installation costs: The installation of underground cables is more expensive compared to overhead lines.
• Difficult maintenance: Underground cables are more challenging to maintain and repair due to their hidden location.
• Limited capacity: Underground cables have a limited capacity for transmitting electricity compared to overhead lines.

C. Comparison of Cost, Reliability, and Maintenance

Aspect	Underground Cabling	Overhead Transmission Lines
Cost	Higher	Lower
Reliability	Higher	Lower
Maintenance	Difficult	Easier

III. Classification of Cables

Cables can be classified based on voltage level, construction, and insulation material.

A. Based on Voltage Level

• Low Voltage (LV) Cables: Used for transmitting electricity at voltages up to 1 kV.
• Medium Voltage (MV) Cables: Used for transmitting electricity at voltages between 1 kV and 33 kV.
• High Voltage (HV) Cables: Used for transmitting electricity at voltages above 33 kV.

B. Based on Construction

• Single-Core Cables: Consist of a single conductor surrounded by insulation and sheathing.
• Multi-Core Cables: Consist of multiple conductors insulated from each other and surrounded by insulation and sheathing.

C. Based on Insulation Material

• Paper-Insulated Lead-Covered (PILC) Cables: Insulated with impregnated paper and covered with lead sheathing.
• Cross-Linked Polyethylene (XLPE) Cables: Insulated with cross-linked polyethylene, which provides better thermal and mechanical properties.

IV. Requirements of Cable Construction

The construction of underground cables must meet certain requirements to ensure their reliability and performance.

A. Conductor Material

The conductor material used in underground cables should have low resistance and high conductivity to minimize transmission losses. Copper and aluminum are commonly used as conductor materials.

B. Insulation Material

The insulation material should have high dielectric strength to withstand the voltage stress and prevent leakage of current. Common insulation materials include paper, polyethylene, and cross-linked polyethylene.

C. Shielding and Sheathing

Shielding and sheathing are used to protect the cable from external influences and provide mechanical strength. Metallic shields and layers of thermoplastic or thermosetting materials are used for this purpose.

V. Capacitance of Single and Multi-Core Cables

A. Definition of Capacitance

Capacitance is the ability of a cable to store electrical energy in an electric field. It is measured in farads (F) and depends on the geometry and dielectric properties of the cable.

B. Calculation of Capacitance in Single-Core Cables

The capacitance of a single-core cable can be calculated using the following formula:

$$C = \frac{{2\pi\epsilon L}}{{\ln\left(\frac{{D}}{{d}}\right)}}$$

Where:

• C is the capacitance
• (\epsilon) is the permittivity of the insulation material
• L is the length of the cable
• D is the diameter of the cable
• d is the diameter of the conductor

C. Calculation of Capacitance in Multi-Core Cables

The capacitance of a multi-core cable can be calculated by summing the capacitance of each individual core.

VI. Economic Core Diameter

A. Definition of Economic Core Diameter

The economic core diameter is the diameter of the conductor that results in the lowest total cost of the cable, considering both the cost of the conductor material and the cost of losses.

B. Factors Affecting Economic Core Diameter

The economic core diameter depends on factors such as the cost of the conductor material, the cost of losses, and the load current. It is important to find the balance between conductor cost and losses to determine the optimal economic core diameter.

VII. Dielectric Stress in Cable

A. Definition of Dielectric Stress

Dielectric stress is the electrical stress experienced by the insulation material of a cable. It is caused by the electric field generated by the voltage applied to the cable.

B. Calculation of Dielectric Stress in Cables

The dielectric stress in a cable can be calculated using the following formula:

$$E = \frac{{V}}{{\ln\left(\frac{{D}}{{d}}\right)}}$$

Where:

• E is the dielectric stress
• V is the voltage applied to the cable
• D is the diameter of the cable
• d is the diameter of the conductor

VIII. Grading of Cables

A. Purpose of Cable Grading

Cable grading is done to ensure a smooth distribution of the electric field along the length of the cable. It helps in reducing the stress on the insulation material and improving the overall performance of the cable.

B. Types of Cable Grading

• Capacitive Grading: Involves the use of capacitors to distribute the voltage stress evenly along the cable.
• Inductive Grading: Involves the use of inductors to distribute the voltage stress evenly along the cable.

IX. Ionization and Heating of Cables

A. Causes of Ionization and Heating

Ionization and heating in cables can be caused by factors such as high voltage stress, high current, and poor insulation. These factors lead to the breakdown of insulation and the generation of heat.

B. Effects of Ionization and Heating

Ionization and heating can result in insulation failure, increased losses, and reduced cable lifespan. It is important to prevent and mitigate these effects to ensure the reliable operation of underground cables.

C. Prevention and Mitigation Techniques

• Proper insulation design and material selection
• Adequate cooling and ventilation
• Regular maintenance and monitoring

X. Phenomena of Dielectric Losses and Sheath Loss in Cables

A. Definition of Dielectric Losses

Dielectric losses refer to the energy losses that occur in the insulation material of a cable due to the presence of an electric field. These losses are caused by the polarization and relaxation of the insulation material.

B. Causes of Dielectric Losses

Dielectric losses are caused by factors such as the dielectric constant of the insulation material, the frequency of the applied voltage, and the temperature of the cable.

C. Definition of Sheath Loss

Sheath loss refers to the energy losses that occur in the metallic sheath of a cable due to the presence of an electric field. These losses are caused by the resistance of the sheath material.

D. Causes of Sheath Loss

Sheath loss is primarily caused by the resistance of the metallic sheath material. It can be minimized by using materials with low resistivity and proper sheath design.

XI. Thermal Resistance of Cables

A. Definition of Thermal Resistance

Thermal resistance is a measure of the resistance to heat flow in a cable. It is determined by the thermal conductivity of the cable materials and the dimensions of the cable.

B. Calculation of Thermal Resistance in Cables

The thermal resistance of a cable can be calculated using the following formula:

$$R = \frac{{\ln\left(\frac{{D}}{{d}}\right)}}{{2\pi\lambda L}}$$

Where:

• R is the thermal resistance
• D is the diameter of the cable
• d is the diameter of the conductor
• (\lambda) is the thermal conductivity of the cable material
• L is the length of the cable

XII. Step-by-Step Walkthrough of Typical Problems and Solutions

A. Problem 1: Calculation of Capacitance in a Single-Core Cable

Given the length, diameter, and insulation material of a single-core cable, calculate its capacitance.

B. Problem 2: Calculation of Dielectric Stress in a Cable

Given the voltage, diameter, and conductor diameter of a cable, calculate its dielectric stress.

C. Problem 3: Calculation of Thermal Resistance in a Cable

Given the diameter, conductor diameter, thermal conductivity, and length of a cable, calculate its thermal resistance.

XIII. Real-World Applications and Examples

A. Underground Cabling in Urban Areas

Underground cabling is commonly used in urban areas to minimize visual impact and ensure the reliability of power supply. It is often used for residential areas, commercial buildings, and public spaces.

B. Underground Cabling in Industrial Settings

Underground cabling is also widely used in industrial settings to provide a safe and efficient method of power transmission. It is commonly used in factories, manufacturing plants, and industrial complexes.

XIV. Advantages and Disadvantages of Underground Cabling

A. Advantages

• Reduced visual impact
• Protection from weather conditions
• Lower transmission losses
• Enhanced reliability

B. Disadvantages

• Higher installation costs
• Difficult maintenance
• Limited capacity

XV. Conclusion

In conclusion, underground cabling is an important component of power systems, offering several advantages over overhead transmission lines. It provides a reliable and efficient method of transmitting electricity while minimizing visual impact and protecting the cables from weather conditions. Understanding the classification of cables, requirements of cable construction, and phenomena associated with underground cabling is crucial for designing and maintaining reliable power systems.

Summary

Underground cabling is an important component of power systems, offering several advantages over overhead transmission lines. It provides a reliable and efficient method of transmitting electricity while minimizing visual impact and protecting the cables from weather conditions. This topic covers the comparison of cables and overhead transmission lines, classification of cables, requirements of cable construction, capacitance of single and multi-core cables, economic core diameter, dielectric stress in cables, grading of cables, ionization and heating of cables, phenomena of dielectric losses and sheath loss in cables, thermal resistance of cables, and real-world applications of underground cabling. Understanding these concepts is crucial for designing and maintaining reliable power systems.

Analogy

Imagine a city with a complex network of roads and highways. Overhead transmission lines are like the highways, visible and exposed to the elements. On the other hand, underground cabling is like the underground tunnels and roads, hidden from view and protected from weather conditions. Just as the underground tunnels provide a reliable and efficient way for vehicles to travel, underground cabling provides a reliable and efficient method of transmitting electricity.