 ELECTRICAL TRANSMISSIONSten Bohlin, Kjell Eriksson, Gunnar Flisberg ABB Power Systems A.B Ludvika Sweden
ABSTRACTA more effective use of the world's considerable potential of renewable energy resources is an alternative to decrease pollution. Electrical transmission has to be used since electric power must be transported.
It is not practical to store electric energy in large quantities. Furthermore, the production place is fixed for renewable energy resources such as hydro, tidal and geothermal power. As production and consumption places in most cases are separated, the only possibility to make use of the power is to build transmission lines. The use of more renewable energy resources such as hydropower can be a powerful tool in the battle for a better environment as it can reduce the green house effect and man-made carbon dioxide and other gases. The potential of renewable power resources still not used is enormous.
From a technical point of view, a considerable amount of power can be transported on ac. and dc. transmission lines over very long distances. Also between areas separated by long distances of water, the use of a dc. system gives the possibility to transmit the energy by submarine cables.
Environmental effects from transmission lines can be found only close to the line. Biological effects from electrical and magnetic fields around the line have been intensively discussed during the last couple of years.
Certain research studies show marginal adverse health effects on human beings. Other studies do not confirm these effects. Ongoing ambitious field studies in Europe and North America will give further valuable information in this area.
The active use of transmission to replace polluting energy sources by renewable energy sources could eliminate, in 10 to 15 years, about:
Transmission of electric energy is the task of bringing energy from one place to another, from the point of generation to the point of consumption. Originally, when electric energy was first used but not available at the point of consumption and a distant generation source could be used, a transmission line was built to connect the two points. Many times the available generation could produce more than the consumer needed and more consumers were connected to the transmission. Thereby the generation installation could be used more efficiently. The capital spent on the generation installation could be more efficiently utilized thus giving a faster pay-off and a lower energy price to the consumers. 1. Why transmissionTransmission of electric energy is still today a means to carry out the above two tasks -- bringing electric energy from distant generation and sharing the electric energy between several consumers. To fulfill these tasks the electrical intermeshed and interconnected networks of today were developed, thereby striving towards a maximum utilization of the available generation sources and to provide a safe supply to the consumers. As will be shown here transmission could be used to improve also the environmental effects of the electric energy production.
Running a generation source at high and stable load not only gives the best economy in relation to the resources invested but it also keeps the ambient impact at a lower level, compared with several generation sources of the same type operating at different load levels at different times depending on the need of their specific loads. These circumstances are particularly valid for thermal generation stations which are more economical and which produce lower ambient impact if operating at a constant load.
One of the challenges arises when we try to supply the electric energy required by the market at the same time as we try to protect the environment. Global warming or the greenhouse effect, is currently seen as the most significant environmental issue now facing us. Thermal power stations releasing man-made carbon dioxide and other gases are increasing this effect.
On the other hand, available potential of renewable resources such as hydro, wave, tidal and geothermal power are still enormous. A typical weakness with many of the best renewable power resources in the world is chat they are located far from the load centre and the transmission systems have to be expanded if we want to use these sources.
I am not declaring that the use of available renewable resources such as hydro and geothermal power together with an expanded transmission system will solve all environmental problems. A more intensive use of, e.g. hydro power and long transmission will help to reduce the greenhouse effect but may contribute to other environmental effects of a local nature. 2. Characteristics of transmission
The type of transmission used depends on the type of generation and distances involved. In the case of thermal generation a "fuel transmission line" could be an alternative to transmission of electric energy and the generation could be closer to loads if this gives other advantages in the specific case. As not all consumers are located in the same place transmission of electric energy is needed in almost all cases. For hydro power the generation locations are fixed and an electric transmission line is the only alternative.
Let us concentrate on transmission and take a look at the type of system available. By transmission we define today systems with voltages from 69 kV and above. At lower voltages we use the terminology distribution. Today's interconnected and meshed networks use threephase alternating current, ac, with a frequency of 50 or 60 Hz as the commonly used technique taking advantage of the easy use for transformation between voltage levels. Direct voltage, HVDC, is used especially for long transmission lines where it gives the advantage of the same power level being transmitted to/on comparatively small lines. This in turn results in lower losses and economical advantages. But, HVDC can also b* used for special applications when it is possible or difficult to connect the two networks by an ac transmission, e.g. for stability reasons.
The capacity of an 800 kV ac line is around 2000 14W and an anticipated figure for future 1200 kV lines is 5000 MW. A realistic maximum distance for an ac transmission is around 1200 km.
The most powerful HVDC transmission used today has a capacity of around 3000 kW, but an increase by a factor of 2 at least is within the existing technology. There are no practical limitations of line length for an HVDC overhead line.
Water crossings can be included and the existing technique when using HVDC gives possibilities for cable routes of several hundreds of kilometers.
The above characteristics of transmission lines for transmission of electric energy means that there is a possibility of using long transmission lines to a larger extent than is the case today. Thereby the proportion of the renewable and cleaner types of generation of electricity is increased and reduces the amount of generated green house effect gases.
Such bulk power transmissions over long distances can be built at moderate cost. A transmission of 2000 MW over 1000 km would cost less than 1 cent/kWh. 3. Environmental advantages with transmission
An example of a replacement to cover additionally needed energy or replacement of high-pollution sources has been presented by the use of Norwegian hydro power in Central Europe. Assuming that 15 TWh could be exported from Norway per year this could reduce the pollution in Central Europe by:
It has also been concluded that the transmission across Skagerrak between Norway and Denmark in the 15 years it has been in operation has given a net transmitted power of 31 TWh Norwegian hydro energy to Denmark. The corresponding environmental results for Denmark is a relief of pollution by 22.5 million tonnes of CO2, 85,000 tonnes of NOx and 185,000 tonnes of SO2. 4. Environmental effects of transmission lines
Environmental effects from transmission lines have been reported and discussed and will shortly be referred to here. It is obvious that an overhead line has visual effects on the landscape which in many cases are unwanted and the required right-of-way contributes to the deforestation. Considerations have to be taken to avoid the most sensitive areas from a visual point of view.
Environmental impact such as soil erosion in connection with transmission lines is in most cases limited, but can occur in particularly sensitive areas. The risk of damage can be avoided if planning and working methods in connection with line work have been arranged in a proper manner. Another effective method to avoid harmful influence is to use selective clearing of the vegetation in the line area. Trees that can reach tall dimensions will be cut but other vegetation in the area can remain intact.
All other side effects from transmission lines are basically due to the electric and magnetic fields and the ionization emanating from the electric field.
Although concluded and ongoing investigations have not proven that these effects can be damaging to humans, they have caused public concern, and power companies are finding it increasingly difficult to get permission to build new overhead transmission lines.
The electric field is caused by the voltage applied to the line and proportional to the voltage level. The magnetic field is caused by and proportional to the current actually passing through the line. Both types of fields decrease rapidly with the distance from the transmission line.
Not only transmission lines but also installations in buildings and e.g. household appliances such as vacuum cleaners, TV sets, electric stoves etc., generate electric and magnetic fields of power frequency, 50 or 60 Hz. While the magnetic field from a transmission line with two or more phases decreases with the square of the distance, the field from a household appliance decreases still more pronounced, approximately as the cube of the distance.
The electric field close to the high voltage conductors can give rise to corona causing ionization leading to generation of ozone and oxides of nitrogen, possible radio and TV interference, and audible noise.
Corona discharges generate small quantities of ozone (O3) and oxides of nitrogen (NO and NO2). These gases are also generated by many other sources such as industries and traffic. The natural level of ozone can vary within wide limits, e.g. concentrations in connection with thunderstorms of 0.05-0.15 ppm have been measured. Normal values outside polluted areas fall between 0.001-0.003.
Investigations show that the highest value from power lines occurring in practice can reach 0.001 ppm with the highest voltage levels and that no ozone contributions to background levels could be measured even from high voltage ac transmission lines
From an HVDC line corona, air molecules are separated into positively charged ions, O2 and N2, and free electrons. A small part of these positive and negative ions are then converted to ozone and nitrogen gases. In case of a wind, say laterally to the line, they create a plume, which can still be detected a few hundred metres from the line. According to investigations, the total production of ozone over a year, as well as the peak production, is less than for a corresponding ac line. It is concluded that no effects could be found. The production of nitrogen gases emanating from HVDC transmission lines is so small, that it cannot be distinguished from the ambient gas level.
Corona effects on transmission line conductors will result in audible noise at high voltage levels. The noise will increase under rain and smog conditions.
For ac lines and voltages above 400 kV noise levels between 50-60 dB (A) at the edge of 'right of way' can be inconvenient.
For dc lines different measurements indicate that up to +/- 600 kV, the audible noise level 40 m from the centerline will seldom go above 40 dB (A). Even if the audible noise during very heavy rain can amount to 50 dB (A) this effect is considered insignificant.
It is a widespread consensus that the electric field effects cause no adverse health effects to human beings or animals.
The magnetic field and its effects and influence on biological life has been debated since the beginning of the 1980's. In 1979, a reputed American journal published a report by a psychologist Wertheimer from Denver, USA indicating a connection between childhood cancer and high current distribution lines. A number of epidemiological studies has then examined the association of the risk for cancer to exposure of magnetic fields from power lines.
In the absence of objective, and reliable measures of exposure suitable for use in epidemiological studies, indirect measures have generally been used. Such measures included job titles for occupational exposures and the configuration of and distance from overhead wiring systems for residential exposures. These measures have been criticized because they do not take into account temporal variations, nor do they necessarily reflect the extent or intensity of past exposures.
Also few of the published studies have addressed 'confounding factors" adequately, and the differing end points and populations studied make it difficult to draw general conclusions.
Thus from the epidemiological studies published to date, it is very difficult to conclude if there is a connection between the risk of human cancer and the exposure to magnetic fields from electrical power lines, installations and equipment.
As a consequence of the above weak indications a number of research projects to try to find the physiological mechanism for the possible connection between risk of cancer and fields were carried out.
Within the research projects the effects of magnetic fields on molecules, cells, tissues and animals have been investigated in laboratory experiments. Individual investigations have indicated increases in the frequency of chromosome damage and deformation of embryos of chicken, mice, rats etc. The effects seem to be limited to certain windows both in frequency and amplitude of the exciting magnetic field.
Another effect of magnetic fields that has been indicated is the influence on secretion of pineal melatonin hormone in the brain. Melatonin affects the need of rest and sleep, perception etc. There are some indications that low nighttime production of melanin correlate with an increased frequency of otherwise rare male breast cancer.
From the aggregate of studies that has so far been presented, it is not possible to prove if there is a health risk from magnetic fields of power frequency or not. Neither has it been possible to establish a physiological mechanism by which the influence of magnetic field could pose health risks to human beings. If 50-60 Hz magnetic field exposure poses health risks, these risks seem to be small compared to many other hazards of normal life.
While waiting for a clearer picture which could explain the possible risks and alleviate the public concern or establish reasonable limit values many utilities have adopted the principle of prudent avoidance, i.e. reduce the field exposure where the cost is low.
Today several ambitious investigations are underway both on the epidemiological side and on the experimental side. Thus it is hoped that a more clear picture will emerge in a few years.
The environmental impact from HVDC transmissions is quite different from ac transmissions. Reference to results from ac investigations are therefore, in general, not relevant for dc. The electromagnetic dc field at earth level is basically determined by the conductor configuration and its distance to the earth surface. The field distribution will however, also be influenced by the ion distribution in the air. The ions are mainly produced by the corona at the surface of the conductors, and the ion flow between the conductors and the earth corresponds to a dc current of less than 0,5 mA/m2 at earth level.
The magnetic dc field, measured at the earth level under the conductor has for a line carrying 1000 A, about half the magnitude as compared with the natural earth magnetic field and decreases proportionally to the distance from the conductors.
It is therefore a consensus among experts that there is no reason to expect any harm due to the magnetic field from dc lines.
The environmental effects from transmission lines as shown in this paper are all local to the close vicinity of the line route and do not contribute in an additive way to global effects in the same way as pollution of e.g. green house effect gases. 5, Use transmission to reduce pollution
By using transmission to a larger extent the type of pollution improvement as described above for the Norwegian - Central Europe transmission could be adopted in practically all regions of the world. On the other hand in industrial countries with mature transmission systems, internal need for increased interconnection should in many cases be possible to solve without building new lines. Power flow on the existing lines could be increased and better controlled than today by use of already existing or emerging transmission technologies. This includes upgrading of lines within a certain ROW to considerably higher power levels.
Of the available global hydro energy potential, roughly 15,000 TWh/year, only around 2000 TWh/year is exploited, generating somewhat above 20% of the installed electricity. The potential hydro energy would then be sufficient to cover both to replace the existing polluting electric energy sources and provide for the necessary expansion in the world for the next couple of decades. Probably it would not be practically possible to eliminate all high pollution generation and probably some of the available hydro power is not feasible for various reasons such as flooding of large areas or necessity to relocate large populations. However, it would be possible to use a considerably larger part of potential hydro resources.
One example is Central and Western Europe which today have exploited almost all its hydro resources but which could receive hydro power from Central Africa and/or Eastern Soviet Union, two regions with abundant amount of potential hydro power.
Even with a conservative approach it could be assumed that another 2000 TWh/year of hydro power can be developed to replace existing fossil generation if transmission is more actively used thereby replacing more polluting sources. The annual reduction in greenhouse effect gases obtained through this conversion per year would then be:
By such measures taking place during a 10-15 year horizon, roughly one third of today's total pollution by electrical generation can be eliminated.
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