Assembling Of High And Low Voltage Transmission Line Accessories

ABSTRACT

Electric power transmission is the process by which large amounts of electricity produced at power plants, such as industrial-scale solar facilities, are transported over long distances for eventual use by consumers. In Nigeria, electricity is sent from power plants to the National transmission grid, a vast network of electric power lines. Due to the large amount of power involved, and the properties of electricity, transmission normally takes place at high voltage (69 kV or above). And in order to setup a working transmission line, all the accessories must be put in place both for low and high voltage transmission.

Assembling of high and low voltage transmission line accessories is the process of gathering all the power transmission wiring accessories in order to have complete power transmission system.

 TABLE OF CONTENTS

Title Page

Approval Page

Dedication

Acknowledgement

Abstract

Table of Content

CHAPTER ONE

1.0      Introduction

1.1      Objective of the study

1.2      Significance of the study

1.3      Classification by operating voltage

CHAPTER TWO

2.0     Literature review

2.1      Review of historitical background of power transmission

2.2     Review electric power transmission

2.3     Review of high-voltage cable

2.4     Construction review of high-voltage cable

CHAPTER THREE

3.0     Methodology

3.1      Electric power transmission system

3.2.0      Assembling of power transmission wiring accessories

3.2.1     Transmission towers

3.2.2     Conductors (power lines)

3.2.3     Insulators

3.2.4      Substations

3.2.5     Earthing materials

3.3   High voltage transmission advantages and disadvantages

CHAPTER FOUR

4.1      Cable construction

CHAPTER FIVE

5.1      Conclusion

5.2      References

 CHAPTER ONE

1.0                                          INTRODUCTION

Electric power transmission is the process by which large amounts of electricity produced at power plants, such as industrial-scale solar facilities, are transported over long distances for eventual use by consumers. In Nigeria, electricity is sent from power plants to the National transmission grid, a vast network of electric power lines. Due to the large amount of power involved, and the properties of electricity, transmission normally takes place at high voltage (69 kV or above). Electricity is usually shipped to a substation near a populated area. At the substation, the high voltage electricity is converted to lower voltages suitable for consumer use, and then shipped to end users through (relatively) low-voltage electric distribution lines.

For newly constructed solar energy power plants, if no existing suitable transmission facilities were available, new transmission lines and associated facilities would be required. The construction, operation, and decommissioning of high-voltage transmission lines and associated facilities would create a range of environmental impacts. The type and magnitude of the impacts associated with transmission line construction, operation, and decommissioning would vary depending on line type and size, as well as the length of the transmission line, and a variety of other site-specific factors.

1.1                               OBJECTIVE OF THE STUDY

The objective of this study is to assemble all the electrical accessories that made up a complete power transmission system of both low and high tension system.

1.2                             SIGNIFICANCE OF THE STUDY

This study make it possible for students to fully understand how electric power is been transmitted. It makes me to be able to know and understand the function of each accessory in power transmission system. Also to know the ranges of voltages that are been transmitted in overhead power transmission line.

1.3               CLASSIFICATION BY OPERATING VOLTAGE

In assembling of any electric power accessories, one must consider the supply voltage of the system. Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

• Low voltage (LV) – less than 1000 volts, used for connection between a residential or small commercial customer and the utility.
• Medium voltage (MV; distribution) – between 1000 volts (1 kV) and to about 33 kV, used for distribution in urban and rural areas.
• High voltage (HV; subtransmission less than 100 kV; subtransmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.
• Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.
• Ultra high voltage (UHV) – higher than 800 kV.

CHAPTER TWO

2.0                                                    LITERATURE REVIEW

2.1     REVIEW OF HISTORICAL BACKGROUND OF POWER TRANSMISSION

The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729 by the physicist Stephen Gray. The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time).

However the first practical use of overhead lines was in the context of telegraphy. By 1837 experimental commercial telegraph systems ran as far as 20 km (13 miles). Electric power transmission was accomplished in 1882 with the first high-voltage transmission between Munich and Miesbach (60 km). 1891 saw the construction of the first three-phase alternating current overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt.

In 1912 the first 110 kV-overhead power line entered service followed by the first 220 kV-overhead power line in 1923. In the 1920s RWE AG built the first overhead line for this voltage and in 1926 built a Rhine crossing with the pylons of Voerde, two masts 138 meters high.

In 1953, the first 345 kV line was put into service by American Electric Power in the United States. In Germany in 1957 the first 380 kV overhead power line was commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the construction of the highest overhead line pylons of the world. Earlier, in 1952, the first 400 kv line was put into service in Sweden, in 160 km (100 miles) between the more populated areas in the south and the largest hydroelectric power stations in the north. Starting from 1967 in Russia, and also in the USA and Canada, overhead lines for voltage of 765 kV were built. In 1982 overhead power lines were built in Russia between Elektrostal and the power station at Ekibastusz, this was a three-phase alternating current line at 1150 kV (Powerline Ekibastuz-Kokshetau). In 1999, in Japan the first powerline designed for 1000 kV with 2 circuits were built, the Kita-Iwaki Powerline. In 2003 the building of the highest overhead line commenced in China, the Yangtze River Crossing.

Mathematical analysis

An overhead power line is one example of a transmission line. At power system frequencies, many useful simplifications can be made for lines of typical lengths. For analysis of power systems, the distributed resistance, series inductance, shunt leakage resistance and shunt capacitance can be replaced with suitable lumped values or simplified networks.

Short and medium line model                   

A short length of a power line (less than 80 km) can be approximated with a resistance in series with an inductance and ignoring the shunt admittances. It is important to note that this value is not the total impedance of the line, but rather the series impedance per unit length of line. For a longer length of line (80–250 km), a shunt capacitance is added to the model. In this case it is common to distribute half of the total capacitance to each side of the line. As a result, the power line can be represented as a two-port network, such as ABCD parameters.

The circuit can be characterized as

where

• Z is the total series line impedance
• z is the series impedance per unit length
• l is the line length
• is the sinusoidal angular frequency

The medium line has an additional shunt admittance

where

• Y is the total shunt line admittance
• y is the shunt admittance per unit length

Short length of power line

Medium length of power line

2.2                REVIEW ELECTRIC POWER TRANSMISSION

Electric-power transmission; is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. Transmission lines, when interconnected with each other, become transmission networks. The combined transmission and distribution network is known as the “power grid” in the United States, or just “the grid”. In the United Kingdom, the network is known as the “National Grid”.

A wide area synchronous grid, also known as an “interconnection”

Historically, transmission and distribution lines were owned by the same company, but starting in the 1990s, many countries have liberalized the regulation of the electricity market in ways that have led to the separation of the electricity transmission business from the distribution business

2.3                       REVIEW OF HIGH-VOLTAGE CABLE

A high-voltage cable, also called HV cable, is used for electric power transmission at high voltage. A cable includes a conductor and insulation, and is suitable for being run underground or underwater. This is in contrast to a conductor, which does not have insulation. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, AC and DC power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent breakdown of the insulation. Often a high-voltage cable will have a metallic shield layer over the insulation, connected to earth ground and designed to equalize the dielectric stress on the insulation layer.

High-voltage cables may be any length, with relatively short cables used in apparatus, longer cables run within buildings or as buried cables in an industrial plant or for power distribution, and the longest cables are often run as submarine cables under the ocean for power transmission.

2.4       CONSTRUCTION REVIEW OF HIGH-VOLTAGE CABLE

A cross-section through a 400 kV cable, showing the stranded segmented copper conductor in the center, semiconducting and insulating layers, copper shield conductors, aluminum sheath and plastic outer jacket.

Like other power cables, high-voltage cables have the structural elements of one or more conductors, insulation, and a protective jacket. High-voltage cables differ from lower-voltage cables in that they have additional internal layers in the insulation jacket to control the electric field around the conductor.

For circuits operating at or above 2,000 volts between conductors, a conductive shield may surround each insulated conductor. This equalizes electrical stress on the cable insulation. This technique was patented by Martin Hochstadter in 1916; the shield is sometimes called a Hochstadter shield. The individual conductor shields of a cable are connected to earth ground at the ends of the shield, and at splices. Stress relief cones are applied at the shield ends.

Cables for power distribution of 10 kV or higher may be insulated with oil and paper, and are run in a rigid steel pipe, semi-rigid aluminum or lead sheath. For higher voltages the oil may be kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation.

Sebastian Ziani de Ferranti was the first to demonstrate in 1887 that carefully dried and prepared paper could form satisfactory cable insulation at 11,000 volts. Previously paper-insulated cable had only been applied for low-voltage telegraph and telephone circuits. An extruded lead sheath over the paper cable was required to ensure that the paper remained absolutely dry.

Vulcanized rubber was patented by Charles Goodyear in 1844, but it was not applied to cable insulation until the 1880s, when it was used for lighting circuits.^[1] Rubber-insulated cable was used for 11,000 volt circuits in 1897 installed for the Niagara Falls Power Generation project.

Mass-impregnated paper-insulated medium voltage cables were commercially practical by 1895. During World War II several varieties of synthetic rubber and polyethylene insulation were applied to cables.^[2] Modern high-voltage cables use polymers or polyethylene, including (XLPE) for insulation.

AC power cable

High voltage is defined as any voltage over 1000 volts. Cables for 3000 and 6000 volts exist, but the majority of cables are used from 10 kV and upward.^[3] Those of 10 to 33 kV are usually called medium voltage cables, those over 50 kV high voltage cables.

Figure 1, cross section of a high-voltage cable, (1) conductor, (3) insulation.

Modern HV cables have a simple design consisting of few parts. A conductor of copper or aluminum wires transports the current, see (1) in figure 1. (For a detailed discussion on copper cables, see main article: Copper wire and cable.)

Conductor sections up to 2000 mm² may transport currents up to 2000 amperes. The individual strands are often preshaped to provide a smoother overall circumference. The insulation (3) may consist of cross-linked polyethylene, also called XLPE. It is reasonably flexible and tolerates operating temperatures up to 120 °C. EPDM is also an insulation.

At the inner (2) and outer (4) sides of this insulation, semi-conducting layers are fused to the insulation.^[4] The function of these layers is to prevent air-filled cavities between the metal conductors and the dielectric so that little electric discharges can arise and endanger the insulation material.^[5]
The outer conductor or sheath (5) serves as an earthed layer and will conduct leakage currents if needed.

Most high-voltage cables for power transmission that are currently sold on the market are insulated by a sheath of cross-linked polyethylene (XLPE). Some cables may have a lead or aluminium jacket in conjunction with XLPE insulation to allow for fiber optics. Before 1960, underground power cables were insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of these oil-and-paper insulated cables in use worldwide. Between 1960 and 1990, polymers became more widely used at distribution voltages, mostly EPDM (ethylene propylene diene M-class); however, their relative unreliability, particularly early XLPE, resulted in a slow uptake at transmission voltages. While cables of 330 kV are commonly constructed using XLPE, this has occurred only in recent decades.

Quality

During the development of HV insulation, which has taken about half a century, two characteristics proved to be paramount. First, the introduction of the semiconducting layers. These layers must be absolutely smooth, without even protrusions as small as a few µm. Further the fusion between the insulation and these layers must be absolute;^[6] any fission, air-pocket or other defect – of the same micro-dimensions as above – is detrimental for the breakdown characteristics of the cable.

Secondly, the insulation must be free of inclusions, cavities or other defects of the same sort of size. Any defect of these types shortens the voltage life of the cable which is supposed to be in the order of 30 years or more.

Cooperation between cable-makers and manufacturers of materials has resulted in grades of XLPE with tight specifications. Most producers of XLPE-compound specify an “extra clean” grade where the number and size of foreign particles are guaranteed. Packing the raw material and unloading it within a cleanroom environment in the cable-making machines is required. The development of extruders for plastics extrusion and cross-linking has resulted in cable-making installations for making defect-free and pure insulations. The final quality control test is an elevated voltage 50 or 60 Hz partial discharge test with very high sensitivity (in the range of 5 to 10 picoCoulombs) This test is performed on every reel of cable before it is shipped.

HVDC cable

A high-voltage cable for HVDC transmission has the same construction as the AC cable shown in figure 1. The physics and the test-requirements are different. In this case the smoothness of the semiconducting layers (2) and (4) is of utmost importance. Cleanliness of the insulation remains imperative.

Many HVDC cables are used for DC submarine connections, because at distances over 30 km AC can no longer be used. The longest submarine cable today is the NorNed cable between Norway and Holland that is almost 600 km long and transports 700 megawatts, a capacity equal to a large power station.
Most of these long deep-sea cables are made in an older construction, using oil-impregnated paper as an insulator.