Power Cable Technology

Sushil Kumar Ganguli, Vivek Kohli

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781498709118
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Energy

Power Cable Technology provides a precise understanding of the design, manufacture, installation, and testing of a range of electric power cables—from low-voltage, 1, 000/1, 100V cables to extra-high-voltage, 400kV cables—with reference to future trends in the industry.

The authors' mantra is: know your cable. Thus, the book begins with a comprehensive overview of power cable design and manufacturing through the ages, and then:

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• Describes the characteristics of the materials currently used in the production of various power cables
• Explains how to calculate the die orifice for drawing wires, how tolerance in manufacturing affects material weight and consumption, and how and why lubricants are used
• Addresses the formation, stranding, and insulation of the electrical conductors, as well as the sheathing, armouring, and protective covering of the power cables
• Delivers an in-depth discussion of quality systems, quality control, and performance testing
• Covers the many nuances of cable installation, including laying, jointing, and terminating

Throughout, the authors emphasise consonance between design theory and practical application to ensure production of a quality power cable at a reasonable cost. They also underscore the importance of careful handling, making Power Cable Technology a must read for power cable engineers and technicians alike.

1
Introduction

An electrical power generating station is the heart of a power distribution system, and transmission lines its arteries. Without a transmission system, the generated power cannot be brought to the points of consumption and hence would be no value. It is interesting to note that the commercial sale of electricity began more than 100 years ago, attributable to Thomas Edison.

Copper was the first metal used to transmit electricity in the early 1880s. Copper conductors, however, had to face the challenge of weight. Later it has to face competition against Aluminium. Further with the price of metal being relatively high, the overall cost of the overhead transmission lines could not be made economically viable, particularly for long-distance ones. Naturally, Aluminium conductors became popular because of their low weight and cost. It was in California, in 1894, that aluminium was first allowed to be used as a conducting material. However for underground cables Copper remains as conducting material for western countries and middle-east supply systems.

The transmission systems can be divided into two categories and as follows:

1. Overhead transmission lines made of all-aluminium wire stranded conductors (AAC), aluminum wire stranded conductor steel reinforced (ACSR), all-aluminium alloy conductors (AAAC), and aluminum conductors alloy reinforced (ACAR). These are long-distance, high-voltage transmission systems installed across countries.

2. Insulated underground power cables of different voltage grades used for supplying power within urban/rural areas.

Discussion in this book will be confined to power cables and their technical features during the design, manufacturing, and performance stages. The machinery employed is also of importance. It is interesting to note that processing units are designed and manufactured to meet physical and electrical requirements. Naturally, these units may need to be customized in almost all the cases. The subject is a vast one. Efforts will be made to incorporate as much variations with the maximum possible technical discussion.

‘Cables’ in the early days were designated for flexible ropes made of jute or steel wires and used mainly for tugging and anchoring ships. The same term is now used for long and flexible insulated electric power and communication lines. The term ‘cable’, however, is attached to the prefix ‘electric’ to distinguish it from normal ropes. Even now, though the manufacturing of cables involves an intricate technology, people still see it as a simple rope-like product. One expects that it should supply power as per one’s requirement. However, there are several technical points and conditions which need to be considered while designing, manufacturing, and installing cables. Many users who do not understand its full significance tend to belittle the technological importance of the product. At times, engineers too ignore these facts and do not highlight the technical features in their paperwork. Therefore, purchasing/commercial authorities are unable to put stringent conditions when buying cables. Electrical systems should not be taken lightly, and it is in the interest of the state and utility, and the users, to ensure trouble-free power at all times.

A lot of technical input is needed for the manufacturing technology to build a cable. Due to a lack of understanding, several manufacturers push low-quality products into the market at a price that does not even meet the material cost of a quality manufacturer. Naturally, low-cost offers are considered under tight budgets and economic conditions and authorities are only concerned about low-cost products. It needs to be understood that the ultimate sufferers are the nation, the utility, and the user. All this leads to a 20%–25% power loss from just transmission lines, be them overhead or underground.

The early development of underground power cables resulted from a long and experimental research work, which was started simultaneously in different countries.

Initially, underground cable works were undertaken to develop communication systems to connect distant islands in Europe, particularly in the United Kingdom, for administrative purposes. This was later extended to develop power cable systems to supply electricity to distant lands as well. Underground electric power cable made way for a more aesthetic look to human establishments, like townships and cities, and kept people away from coming in contact with bare electric wires and cables, avoiding untoward accidents. At hazardous working sites, such as in a mine or within a shaft, where excavation, drilling and blasting are to be undertaken, the intermittent shifting of power points becomes necessary. Power lines should be moved within the work site frequently without endangering the life of workmen and ensuring that no fire hazard breaks out while work is in progress.

This is an impossible task if bare electrical conductors are installed on poles. Naturally, such lines should be insulated and made flexible to withstand all sorts of critical conditions. A major development was also called for when power was to be supplied to nearby islands of England, France, Italy, Sweden, Norway and the like from the power stations situated on the main land. The lines were to cross the sea and the ocean. At times, the system had to withstand severe tidal force and stormy weather and even attacks from wild fish.

While designing a power cable, the following properties are required to be attributed:

1. The cable has to be sufficiently flexible in order to be laid underground in a safe way, or at any place in constricted areas and bends, without being damaged.

2. It should withstand sufficient pulling strength.

3. The longer the length, the less costly the installation would be. The more the number of the the chances of failure and costs.

4. The cable should withstand jerks, shocks and impacts of falling debris or rocks in case of a natural catastrophe.

5. The cable should have the following electrical features:

a. Low conductor resistance

b. High dielectric strength of an insulating material

c. Low thermal resistivity

d. Low dielectric loss factor

e. Capacity to withstand intermittent overloading and predetermined short-circuit conditions

f. Lower dimensions to bring down the cost

g. Materials that shall last for more than 40 years

6. Cables should have the following chemical features:

a. Should be impervious to moisture, water and chemicals

b. Should be able to withstand hostile environments

c. Should have a high thermal stability against ageing and not interact with the material used in their manufacturing

d. Should contain materials that are fire resistant and low smoke in areas prone to fire accidents

Apart from these qualities, cables may be customized with additional features to meet specific purposes.

The history of the development of insulated cables started with the need for a suitable underground communication system. This required a proper conducting material, such as copper, and an insulating material that would not allow leakage of power, not absorb moisture and remain mechanically stable yet flexible. Initially, watertight gutta-percha was the insulating material used. The cable was stiff with a single core and considerable diameter. With time, the demand for multichannel system rose, resulting in the design of a multicore cable with a reduced insulated conductor diameter, twisted and laid together. This was achieved by insulating a conductor with a wax-impregnated cloth. Bunched or laid-up cores were protected by drawing them in a steel or lead pipe. Wax, however, cracked when bent sharply and melted as the temperature rose within the conductor and the surroundings. Gutta-percha when used in land had two major defects. First, it was susceptible to oxidation, and, second, it would soften and deform eve at a slightly higher temperature. Next, jute and hessian cloths impregnated with bitumen were used. The limitations here were the inability to withstand high temperatures and the problem of bending. The material was hard and stiff which limited the length of production. Jointing was also difficult. Transmission lines for telephone and telegraph need a more sophisticated insulating system. During the intermediate period, compounds of natural rubber were developed and were brought in as an insulating material. But the limitation was that a large power supply could not be undertaken, as the technology available for processing rubber cables was not adequate. For communication cables, the performance of natural rubber was unacceptable.

By this time, different types of industrial complexes came into existence, and the demand for the supply of power increased. Along with the communication system, power cable development was also initiated. Ferranti laid 10 km of gutta-percha-insulated cables in England. The cables’ disadvantage was that it was stiff. Large power transmission lines could not be constructed with this material. Accelerated ageing was another problem.

These problems were solved in 1875 with the finding that paper could be used as an insulating material provided it did not come in contact with moisture. Wheatstone and Cooks proposed lead as an alternative sheathing material. Further investigations revealed that if an insulated conductor or a bundle of conductors were protected by drawing a lead pipe over the assembly, not only the insulated conductor would remain free from ingress of moisture but the cable would also remain flexible. But the weight of the cable increased considerably. In spite of this, the suitability of lead as a protecting material became an established fact. In 1890, a lead sheath was introduced. Initially, a lead pipe was drawn over the cable core by drawing dies. Later in 1895, a regular lead sheath was introduced by employing an extrusion process. Simultaneous efforts were made to produce insulating paper with an improved quality. However, paper as such retained moisture that needed to be removed. This led to the development of drying and impregnating processes. Paper drying under vacuum was initiated for communication cables, and the processes of drying and impregnation were developed for power cables.

As for a lead sheathing, pure lead is soft and is often attacked by micro-organisms. Normally, lead alloy is used to protect the cable core. A sheath could also get damaged under mechanical forces. Naturally, a metal sheath needs to be protected by applying a moisture proof layer, called bedding, which consists of a hessian cloth soaked in bitumen. Mechanical protection can be provided by applying an armour of steel wire or steel tape. Steel wire or tapes in turn were protected against corrosion by applying a bitumencoated hessian cloth or a layer of jute. In order to prevent cable layers from sticking together, a coating of lime was applied. This kind of cable was popularly known as the PILC DSTA or PILC SWA cable.

In the course of time, during the Second World War, drastic changes occurred in the insulating material technology. Plastic in the form of PVC was introduced. Polythene came into the market as an insulating material, which could compete with paper. Its limitation was its temperature-withstanding property. Simultaneously, synthetic elastomer (synthetic rubber) was developed to manufacture flexible cables for special applications. This development continued step by step, bringing in more new materials and cables to cater to individual and specific requirements. Some of the important developments are shown chronologically:

1880	Electric cable with gutta-percha, rubber cables, paper cables and 10 kV cables were introduced by Ferranti.
1892	British Insulated Callender’s Cables introduced shaped conductors.
1914	Höchstadter Screen Cable made three-core cables up to 33 kV.
1926	Emanuelli’s pressurization with an oil field cable, up to 66 kV.
1930	Siemens introduced first PVC cable in collaboration with Bayer, Germany.
1943	First 132 kV oil-filled cable installed in service.
1949	Introduction of mass impregnated non-draining compound (MIND).
1950	a. Commercial PVC/PE cables manufactured b. Aluminium sheath cable and aluminium as a conductor material
1956	Cross-linked polyethylene introduced with low coefficient of expansion.
1960	EPR – ethylene propylene rubber entered the arena of high-voltage cables.
1970	Cables containing thermoplastic and thermoset compounds were commercialized, which took over the market rapidly.

In India, the first cable co...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Authors
• 1. Introduction
• 2. Basic Materials for Manufacturing Various Types of Electric Power Cables
• 3. Wire Drawing: Copper and Aluminium
• 4. Conductor Formation–Stranding: Theory and Practice
• 5. Insulation and Insulated Conductors
• 6. Assembling and Laying Up of Multicore Cables and Protective Metal Sheathing
• 7. Armouring and Protective Covering
• 8. Electrical Parameters for Cable Design
• 9. Quality Systems, Quality Control and Testing
• 10. Special Cables
• 11.Power Cable Laying, Jointing and Installation
• References
• Index