Introduction
Since the discovery of electromagnetic induction and the invention electromagnetic rotary devices by Michael Faraday in the 1830's, the use of electricity has grown at an exponential rate. Through this increased use of electricity worldwide, the demand for it has also increased rapidly which has made for the requirements of high power transmission cables.
This report will describe the history of power transmission cables right from the discovery of electricity through to the power cables being used today. It will describe the technical advances made in power transmission and the factors associated with distributing electrical power.
History of Power Transmission
Before there was electrical power transmission, various other methods were used to transmit power long distances. These included telodynamics (cable/rope systems), pneumatic and hydraulic transmission. The most commonly used method of telodynamics transmission was cable cars which could have lines running for several miles on a single sections. At the beginning of the 20th century pneumatic transmission was used throughout Europe for city power transmission. Hydraulic transmission was used in the 19th century to deliver power to factory motors through the use of high pressure water mains. London's system was capable of delivering 7000hp (5 Megawatts) over a 180 mile network of pipes carrying water at 800psi [6]. By the end of the 19th century it was becoming clear that there were a number of benefits associated with establishing electrical power transmission systems.
In the early days of using electrical power and transmitting electrical power, there were two clear drawbacks. The first was that there were a large number of different devices all requiring different voltage levels. Specialised generators were required for the different devices each with their own separate lines. The second drawback was that the generators had to be considerably close to the load they were supplying, typically less than 1 mile for low voltage devices. It was however known that if higher voltages were used, power could be transmitted further. It was also known that both problems could be solved if a solution could be found that could cheaply transform voltage levels from a single universal power line.
In the early days of electrical power transmission there were a number of researchers very interested. They knew that the same amount of power could be received on a cable by increasing the voltage level and decreasing the current proportionally. They also knew from Joule's Law that the capacity of a wire is proportionate to the square of the current travelling on it, regardless of the voltage. It was therefore known that by doubling the voltage the same cable was capable of transmitting the same amount of power four times the distance.
In 1878 at the Paris Exposition, electric arc lighting was installed along the Avenue de l'Opera and Place de l'Opera, using electric Yablochkov arc lamps power by Zenobe Gramme alternating current dynamos [7]. Through the use of high voltages these arc lamps were reported to be powered on a 7 mile circuit. Following this large numbers of cities began to have lighting systems installed which received their power via electrical transmission lines which fed multiple customers from a central power plant. Electrical systems at that time were in direct competition with the more dominant gaslight utilities.
Delivering electrical energy through the use of a central plant and network was not very different from the gaslight business, or the hydraulic or pneumatic power transmission systems currently in use. It was therefore a very attractive business model for investors. In 1879 the California Electric Company is San Francisco used two Charles Brush's direct current generators to supply numerous customers with electricity for their arc lights. This was the first case of electricity being sold from a central plant to multiple customers via transmission lines [8]. It was not long before the California Electric company opened a second plant with a further 4 generators. The cost per week per lamp for light was $10 which would give light from sundown to midnight. In December 1880 the Brush Electric Company set up a central power station to light a 2 mile strip of lighting on Broadway. By the end of 1881 a large number of cities across the USA had Brush arc lighting systems installed which produced public light right into the 20th century. By 1893 there were 1500 arc lights illuminating New York streets [9].
The first electricity system to supply incandescent lights was built by the Edison Electric Illuminating Company in lower Manhattan which was eventually capable of delivering power to an area one square mile using 6 "jumbo dynamos" housed at Pearl Street Station. In 1882 when the service began there were 85 customers with 400 light bulbs. Each of the dynamos could produce 100 kW which was enough power so supply 1200 incandescent lights. Transmission of the power was carried out at 110V via underground conduits. 100,000 feet of underground conduit was installed which accounted for the majority of the system cost which was around $300,000. In the initial years the operating costs were higher than the income for the first two years and in 1890 fire destroyed the plant. Edison's lights were preferred as they were cheaper, provided light that was warmer and operated at lower voltages than the arc lights. Also Edison had a 3 wire system that could supply either 110 volts or 220 volts to some motors.
Charles Parsons' production of turbo generators in 1889 saw a massive increase in the amounts of power being transmitted to various locations. Within two decades the amount of power that the turbo generators could output increased from 100kW to 25 megawatts.
George Westinghouse, an American entrepreneur and engineer, became interested in electricity and quickly and correctly realised that Edison's low voltages were not efficient enough to scale up for long distance transmission. He also realised that long distance transmission required high voltages and that there was a lack of inexpensive voltage conversion technology for direct current applications.
In 1876, the Russian engineer, Pavel Yablochkov, patented his mechanism for a step up transformer which used induction coils prior to the Paris Exposition demonstrating his arc lights. Following this Lucian Gaulard and John Dixon Gibbs developed more efficient, less expensive AC transformers.
Between 1884 and 1885, three Hungarian engineers, Zipernowsky, Blathy and Deri discovered that all previously built coreless or open-core devices were not capable of regulating voltage, and were therefore impractical. It was these engineers from the Ganz Company in Budapest that invented the efficient "Z.B.D" alternating current transformer. Their patent described two types of no pole designs. There was the "closed-core transformer" and the "shell-core transformer". The breakthrough of these designs would finally make it possible to deliver power for lighting homes, businesses and public spaces. It was also these Hungarian engineers that discovered the relationship between number of turns on each coil and the voltage on the primary and secondary sides of the transformer, Vs/Vp = Ns/Np.
In 1886 Westinghouse, Stanley and Franklin Leonard Pope implemented the concept of using step up and step down transformers. However there were still issues with the inefficiency of generators and high voltage transformers. 0n May 16th, 1888, Nikola Tesla gave a lecture entitled A New System of Alternating Current Motors and Transformers, in which he described the equipment which would allow efficient generation and the use of alternating currents. The simplicity of Tesla's inventions meant that AC generators and motors could be manufactured cheaply and would require very little maintenance. The main factor which made alternating current wildly used was the development and availability of the AC transformer used to step voltages up and down.
It was not always clear why alternating current was used over direct current and a number of direct current transmission systems were installed successfully without the use of transformers. Rene Thury, a Swiss pioneer in electrical engineering believed that the transmission of power of long distances could be achieved through the use of direct current. He was well aware of the work carried out by Marcel Deprez who avoided the use transformers by placing generators and loads in series with one another. Thury used this concept to develop the first commercial system for high voltage direct current transmission. The Thury system was used on a number of DC transmission projects which transmitted power from Hydro generators. The first was low voltage system was in 1885 in Genoa, Italy and the first high voltage system was in Genoa, Italy in 1989.
Due to the fragility of series distributions systems and the lack of reliable DC conversion, the Thury system was not very successful. It wouldn't be until the 1940s that DC technology would be great enough, through the development of mercury arc valves, that DC could really be used. The AC "universal system" therefore rapidly became the mainly used method. By careful choice of utility frequency, lighting and motors could be run off the same supply.
In 1891 at the international electricity exhibition in Frankfurt the first transmission of three-phase AC using high voltage took place. This was a 25kV line connecting Neckar and Frankfurt which was a distance of approximately 175km.
The first large scale hydroelectric generators were used at the Niagara Falls in the USA and provided power to Buffalo, New York via power transmission lines.
Throughout the 20th century voltage levels used for power transmission increased. Fifty-five transmission systems were in use by 1914 with the most of them operating at 70kV and the highest being 150kV. The first three-phase AC power transmission at 110kV was used in 1907 between Croton and Grand Rapids, Michigan. It was then about 5 years before another system of the same voltage level was installed.
In 1923 two 220kV systems went into service. These were the Pit River - Cottonwood - Vaca - Dixon line which was built for transmitting power from hydroelectric stations in the Seirra Nevada to the San Francisco Bay Area and also the Big Creek - Los Angeles line which was upgraded. In 1929 the first 220kV line in Germany was completed. This line was then upgraded and the first transmission at 380kV took place on October 5th 1957.
1952 saw the completion of the world's first 380kV line which was built in Sweden. The line was 952km long and ran between Harspranget and Hallsberg. Following this the world's first extra high voltage transmission at 735kV took place on a Hydro-Quebec transmission line in 1965 and then the world's first transmission at 1200kV in the Soviet Union in 1982.
Electrical power transmission continued to develop and with the demand for electrical power in factories during World War I governments applied pressure to make transmission lines and grids available for ammunition factories.
In 1926 all of the electrical networks in the United Kingdom became interconnected to form the National grid which now operates at a number of different voltage levels.
General Background Theory
The Basic Power System
To understand the need for power transmission it is essential to look at the main characteristics of the average power system. The average power system at minimum will consist of an energy source, a prime mover, an electrical generator, and a load, each connected so some sort of control system as shown below. The energy source will usually be in the form of coal, gas or oil. It may also be the use of the potential energy of water stored in some sort of reservoir which is at a greater height than the generator. The prime mover is usually some sort of turbine or internal combustion engine. The generator which converts the mechanical force into electrical energy is usually a three-phase ac alternator. The control system consist, at minimum, of three major parts: a speed governor which is required to maintain a constant required frequency, a magnetic field current regulator to maintain a constant voltage, and a fuel flow regulator to supply the correct amount of energy input compared to the changing load conditions.
Load
Prime mover
Generator
Energy Source
Control
Basic Power System
Transmission and Distribution Systems
Delivering power from a power station to a household is normally done in two stages. The first of these two stages is carried out by the transmission system and the second is the distribution system. There is no strict universal definition of which is which however in general the transmission system will carry power from the power generation plants to more localised substations closer to towns and villages. The distribution system will then transport power between these localised substations and households within the towns and villages.
Transmission systems will tend to have much greater power handling capabilities as they are required to transmit higher powers generated by the power plants. Transmission lines will also tend to be much longer which requires higher voltages to accommodate for the voltage drops in these longer lines.
Distribution systems are used to branch out the power from the substations to local urban and rural areas. Distribution systems tend to have relatively shorter lines with lower power levels flowing in them, and a significantly larger number of lines than on the transmission system.
As well as the two main stages stated above there is also commonly a sub transmission network. This network is used to transmit power to distribution substations throughout the system and also to supply large industrial loads.
Operating Voltage Levels
Various different voltage levels are used throughout the operation of the electrical power network. The figure below shows the difference between transmission and distribution systems and also the typical voltage levels associated with each. Note: figure shows the operating voltages for an American system.
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Electrical System - Image from United States Department of Energy
Within the generating stations, voltages produced are usually in the range of 11-25kV for large three-phase synchronous generators. These voltages must then be stepped up (via a transformer) to voltages typically of 400kV or 275kV for transmission to load-centre substations. This transmission network will also commonly feed a sub transmission network which operates at 132kV which is received via various transmission substations. The distribution networks will operate at voltages levels of 33kV, 11kV or 6.6kV and will supply the final consumer feeders at 415kV three phase which will give 240V per phase which is what each household will receive. These voltage levels change for different countries however these are the typical levels for the UK.
The different levels of voltages used within electrical power networks can be categorised as shown in the table below.
Voltages
Network
400kV
Super grid
275kV
Super grid
132kV
Sub transmission
66kV
Major distribution
33kV
Major distribution
11kV
Small scale distribution
6.6kV
Small scale distribution
415V
Low voltage distribution to customers
230V
Voltage delivered to UK households
Interconnected Power Systems
In the years when electrical distribution was fairly new, many electric power systems were operated as standalone systems. These systems were generally located in large urban areas and tended to operate at low voltage utilization or medium voltage distribution levels. This method of distribution meant that there was no backup should a power outage occur. It also limited the area being supplied to the amount of power of which the power station could supply.
Power stations nowadays are increasing greatly in size and 2000MW capacity stations are now very common. Larger power stations can produce greater amounts of energy at lower costs than smaller power stations and are also a great deal more efficient. For this reason it is much more beneficial and economical to transmit power over longer distances from one power station than to have several smaller less efficient power stations that are located closer to the load. These larger power stations must therefore meet the main base load and are interconnected so that they will feed into a large network and not just to one load.
As the electricity network is now interconnected it means should one source or line fail in one area, power could be transmitted from another system to the load. Also, in order to meet a sudden increase in demand a certain amount of generating capacity known as the spinning reserve is required. This involves having a number of generators running at normal speed and capable of supplying power instantaneously. If the machines are not already running then they require some time to start up which can take up to an hour. By having certain power stations only for this purpose it is a great deal more economical.
System A
System B
System C
System D
System E
G
G
G
Interconnected Power System
Direct Current and Alternating Current Transmission
At present the majority of high power transmissions lines transfer the power from the power stations to household, commercial and industrial consumers, using AC three-phase transmission and distribution systems. It is also necessary to perform a number of voltage transformations to reduce power losses and minimise costs.
As a number of different voltage levels are required throughout the electric power network, it is much easier to use AC transmission and distribution systems as voltage levels can be transformed easily through the use of a transformer. This voltage transformation can be directly related to Faradays law which states that "The EMF induced in a circuit is directly proportional to the time rate of change of magnetic flux through the circuit." It can be said that at higher voltages the current is reduced therefore the voltage drop and line losses are also reduced.
The argument for using DC over AC transmission is very dependent on the individual situation for where the line is being used. However, in general there are a number of advantages in using DC over AC transmission and distribution that apply in most cases.
Investment costs. It is relatively less expensive to install a high voltage DC (HVDC) transmission line than installing an AC transmission line with the same transmission capacity. It is however more expensive to build a DC substation due to the DC to AC conversion and vice versa. The cost of transmission cables however is cheaper for HVDC and it is also cheaper to acquire land for the installation of a HVDC transmission line. Furthermore the operational and maintenance costs for HVDC are also less. For the HVDC system initial loss levels are greater however this does not vary as the transmission distance increase as it does with AC transmission.
Long distance water crossing. The maximum distance that the power can be transmitted in subsea AC cables is limited by the reactive power flow due to the large capacitances. With HVDC this is not an effecting factor and for this reason HVDC transmission is required to transmit the long distances.
Lower losses. HVDC transmission lines have significantly less losses compared with AC transmission of the same capacity. It is necessary to add on the losses with the convertor stations however these are still very low and HVDC transmission still has a lot less losses.
Asynchronous connection. It is very difficult and sometimes not possible to connect two AC networks together due to stability reasons. Using a HVDC link between the two networks is often the only way of exchanging power between the two.
Control. The active power in a HVDC link can be very easily controlled and is one of the main reasons for using HVDC.
Limit short circuit currents. A HVDC transmission does not contribute to the short circuit current of the interconnected AC system.
Environment. It is more efficient to make use of existing power plants. A HVDC transmission lines requires much less land coverage and thus the right-of-way cost if considerably lower than it would be for an AC transmission line, therefore the visual impact is also smaller. It could also be possible to make use of right-of-ways already present by expanding power transmission capacity.
Although the advantages outweigh the disadvantages it is important to take these disadvantages into consideration. The disadvantages with DC power transmission are as follows.
Harder conditions for circuit breakers. This is because the current does not reduce to zero twice a cycle, therefore switching is not carried out on a DC link but effected by means of the terminal inverters and rectifiers. This makes it difficult to interconnect DC systems with T-junctions.
Voltage transformation. It is necessary for voltage transformation to take place on the AC sides of the system.
Rectifiers and inverters draw reactive power and this must be supplied locally.
DC conversion stations are much more expensive than conventional AC substations.
In general a HVDC transmission system is more efficient than AC transmission, is highly compatible with any environment and can be easily integrated into it without any major effects to the surrounding environment.
Overhead Lines
Overhead lines are used as much as possible over underground cables due to the much higher costs associated with underground cables. A major part of the insulation for these lines is the air that surrounds the cables. There are regulations in place that insure that the height of the power conductors are above a given minimum. For this reason it is necessary to calculate the sag in the line and its variations with temperature. Although the power transmitted in the line is restricted by a number of factors, the main restriction is on the amount of current that can be present as this must be at a level to insure that the cable does not exceed a maximum temperature. It is therefore stated that the current rating will depend on the ambient temperature; summer, normal and winter ratings are used. As well as the factors already mentioned thermal time-constants of the conductors are used to give short-term and long-term emergency ratings.
When planning new installations of overhead power lines it is very important to take careful consideration into the chosen routes as towers and lines can have a considerable visual impact on the surrounding area. It is often best to make use of existing structures by modifying them to give reduced weights and dimensions, and also to use guyed structures. The chosen route for a new transmission line must take into consideration the natural landscapes and the geology, flora, and wildlife of the land. Also industrial and residential constraints must be met.
Making effective use of a "right-of-way" by a given line or lines can be expressed by an index defined as follows:
(Line loading) / (width of right of way) (tower height)
There are several different types of conductor used for overhead power cables. Very often utility companies will install bare conductor for overhead lines. Hard drawn copper, all aluminium conductors (AAC), and aluminium conductor steel reinforced (ACSR) are the most commonly used types of bare conductor for overhead lines. The hard drawn copper conductor can be either a single solid copper conductor or can be made up of several strands of copper conductors wound together. The ACC conductor will usually be made up of several strands of aluminium conductors. The ACSR conductor is made up of several strands of steel conductor as a core with several strands of aluminium surrounding them. The steel strands add additional strength to the ACSR compared with AAC and copper conductors.
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Very often during overhead line installation, phase conductors will have to be installed very closely to one another or in close proximity to a tree. When this is the case, a special type of overhead cable referred to as spacer cable must be used. Spacer cable is made up of insulated aluminium or copper conductor surrounded by an insulating outer layer of which the outer surface is not considered to be at a ground potential. Plastic spacers are also used between the individual conductors in order to stop them coming in contact with one another.
Three-phase transmission and distribution circuits can also be constructed through the use of a single three-conductor cable. This single three-conductor cable can come in the form of either belted or shielded. Which of these forms is used depends on the voltage levels that will used on the cable. In general voltages of 15kV or less will use a belted cable and cables with voltages greater than 15kV will use the shielded type. The shielding method can withstand higher voltages as it reduces the electrical stresses that can occur at these voltages. As well as reducing the stresses, the shield also provides a path to earth for the outer surface of the cable for personnel protection.
Underground Cables
There are a number of factors which make underground cables the preferable over overhead lines. Although not as economic, growing public pressure is forcing electricity companies to install power cables underground in order to preserve the amenities of both town and countryside. This is even the case sparsely populated areas; in that bulk-transmission cables have been installed underground where areas of outstanding natural beauty exist.
Underground cable transmission is considerably more expensive than overhead line transmission, "at 345kV a figure of 10 times the cost of an equivalent overhead line is quoted for average suburban areas in the USA, this ratio decreasing with lower voltages."[1] One of the challenges for power engineers today is to design power cables that are capable of carrying the large powers in use and envisaged as well as being more economically attractive. As with the overhead lines the main restriction on current that can flow in the cable is due to the temperature rise on the insulating material used. However, with overhead lines this is less severe as they are required to be flexible which means that the overall diameter is limited which directly limits the size of the conductor.
Underground cables which are used in distribution circuits with voltages in the range of 4.16-34.5kV tend to be referred to as underground residential distribution (URD) cables. The conductors used within these cables are usually copper or aluminium, of which aluminium is far more commonly used in utility systems. These conductors are covered in a thin layer of semiconducting material which is used to help shape the electric fields around the centre of the conductor and also to prevent voltage spikes near the conductor surface. In order to prevent high electrical fields from breaking down the insulation and shortening the cable life, the cables must be designed to keep electrical stress low. There are a number of different types of insulation used for underground cables which include polyethylene (PE), cross-linked polyethylene (XLP), ethylene propylene rubber (EPR), polyvinyl chloride (PVC), and chlorosulfonated polyethylene (CSP). A layer of semicon is also applied to the outer surface of the insulation and a protective outer jacket covers this.
Concentric neutral conductors are placed around the insulation jacket which forms the system neutral. These conductors are usually made out of tinned copper and may be covered with an outer jacket to reduce the effects of corrosion.
In the case where underground distribution circuits consist of one or two phases of a three phase system, the cable will usually be a single conductor with a full concentric neutral. Having a full concentric neutral means that the current rating of the neutral conductor as a whole, matches the current rating of the phase conductor. When all three phases of the three-phase distribution circuit are being used, either the full size or a one third reduced neutral may be used. A reduced neutral can be used in a three phase system as the neutral current on the distribution feeder is relatively small during stable operating conditions. The reduced size neutral can limit the magnitude of circulating currents in the concentric neutral strands.
Conclusion
Many transmission and distribution systems that are being used today have been in use since around the 1970s when lower voltage levels were being used and the electricity demand was less. Due to the increase in distributed generation systems and an increase in electricity demand, transmission and distribution systems are struggling to cope with the high levels of power that the systems are required to handle. This means that transmission and distribution systems continually have to be upgraded and reinforced to cope with the higher power levels. It is the task of the power systems engineer to take into consideration a large number of factors to insure systems can cope with these high power levels and have protection equipment in place that is also able to operate properly at these higher power levels.
The increased demand for electricity has also seen an increased in direct voltage transmission as the losses can be greatly reduced over long distances. Development in the technology used for the conversion of DC voltages has made it more economical to use DC power transmission.
