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Electromagnetic Fields Modeling Using GIS

Kent W. K. Lam,Sandra Z. W. Au
Department of Land Surveying and Geo-Informatics
The Hong Kong Polytechnic University,HungHorn, Kowloon,Hong Kong
email : lskent@polyu.edu.hk


Abstract
Overhead transmission lines emit both electric and magnetic fields and they are called the extremely-low-frequency electromagnetic fields (ELF-EMF). Some researches believed that these fields would induce biological effects in human. However, research results are still inconclusive and further investigations are required before a definite conclusion can be drawn. However, it is important to have some measurement on the ELF-EMF of the environment for precaution. Following The International Radiation Protection Association and The International Non-Ionizing Radiation Committee (IRPA/INIRC) Guidelines, a prototype system is set up using GIS software. Equivalent charges’ method is used to determine the strength of the radiation in the area nearby the overhead transmission lines. The results are overlaid with the zoning map. Areas with radiation higher than the values suggested in the guidelines are identified. Results are conclusive with the Electricity Networks Ordinance in Hong Kong.

Introduction
Hong Kong’s population has recently reached a record high of more than six million. As the growing rate is not going to decrease after 1997, the population density will become higher and higher. As a result, it is becoming more difficult for engineers and planners to plan the route of overhead power transmission systems to supply power for various newly developed districts in Hong Kong. Recently, there was a dispute between a power supply company and a group of village people. They complained that the overhead power transmission system under construction is too close to their village which would affect their health and their property value.

Environmentalists suggest that power transmission system is a very serious and the dominant source of ELF-EMF radiation to humans and the environment. Some districts in Hong Kong, the overhead power lines are very near to the residential areas. Some epidemiological studies have suggested that a link may exist between exposure to power-frequency electric and magnetic fields (EMFs) and certain types of cancer, primarily leukemia and brain cancer. Other studies have found no such link. Currently, laboratory researchers are still studying how such an association is biologically possible. At this point, there is no scientific consensus about this issue. A national EMF research effort is under way, and major study results are expected in the coming few years.

With the increase in public awareness of the possible biological effects caused by radiation from overhead power transmission systems, the Electrical and Mechanical Service Department (EMSD) in Hong Kong is now starting to analyze the emission of ELF-EMF from the power 230.lines and towers in urban areas. Besides, Hong Kong Advisory Environmental Council also formed a working group on EMFs to observe and respond to the changes and update of the results from the International Radiation Protection Association (IRPA) and other countries. With the availability of digital base map data in Hong Kong, a prototype system is set up to examine the possibility of using Geographic Information System (GIS) to facility the route planning of power transmission systems. The system is developed based on the guidelines produced by the International Radiation Protection Association and the International Non-Ionizing Radiation Committee (IRPA/INIPC). Modeling the ELF-EMF emission would be done by the incorporation of GIS in this project. The ELF-EMF emission model will be incorporated in a GIS and results are presented in 2.5-dimension. ELF-EMF radiation from an overhead power transmission system is analyzed together with land use and building coverages to identifj residential areas which might be in danger and allow to claim for compensation. Characteristics of Extremely-Low-Frequency Electromagnetic Field (ELF-EMF) From Overhead Power Lines

Biological and environmental effects caused by the extremely-low-frequency electromagnetic field (ELF-EMF) have been under investigation for the last twenty years (Goodland, 1973, Smith and Best, 1989, Carpenter and Ayrapetyan, 1994, Ueno, 1995). There is still no scientific consensus about this issue. The first mention of a possible association between ELF-EMF and cancer was published in Russia during 1970 (Wertheimer and Leeper, 1979). High correlation was detected between “high-current configurations”, i.e. proximity of houses to ELF-EMF sources such as transmission lines, transformers and large size wires, with cancer, especially childhood cancer (Ahlbom et al, 1987). Laboratory studies conducted by the National Institute of Environmental Health Sciences and U.S. Department of Energy in 1995 suggest that there may be a link between exposure to power-frequency electric and magnetic fields and certain types of cancer, primarily leukemia and brain cancer. However, other studies find that such link does not exist. Over the past few decades, many studies were conducted to examine the possible health effects from EMFs and they oflen produced conflicting results. A number of independent and authoritative review panels have found no concrete scientific evidence that there is a health hazard from EMFs.

Electromagnetic fields (EMFs) are a compound of two different kinds of fields : electric fields and magnetic fields. Normally, these two fields occur simultaneously anywhere electricity is in use. These two fields are usually analyzed separately. Electric field refers to the electrical charge surrounding a charged object and a measure of the electric force a charged object exerts on other objects in the region. Electric field is described with the symbol E, a vector, in units of volts per meter (V/m). Magnetic field refers to an object’s magnetic force which a moving electric charge produces on other moving objects. Magnetic field can be described with either symbol H, a vector, in units of ampere per meter (A/m) or magnetic flux density with the symbol B, a vector in units of Tesla (T) as used by the EMFs researchers.

ELF-EMF radiation exists in the range of 30 Hz to 300 Hz in the electromagnetic spectrum and is classified in the non-ionizing radiation range. The ELF-EMF radiation from overhead power lines has a wavelength of 5000 km and a frequency of 50 Hz. At the very low frequency range of 231.the spectrum, slowly changing electric and magnetic fields can be treated as if they are static or no change. In addition, a power frequency field with a wavelength of more than 5000 km has a very low energy level that does not cause heating or ionization. However, alternative current (AC) fields do create weak electric currents in conduction objects, including humans and animals. This is the reason why there is a potential for EMFs to cause biological effects. One of the major factors which affect the magnitude of ELF-EMF emitted from overhead power lines is the distance separation between a point of interest and the power lines. This is the factor which should be modeled in a GIS together with the use of a proper ELF-EMF emission model.

ELF-EMF Emission Models
There are several methods which can be used to model the radiation emitted from conductors.

These methods are:
  1. Biot-Savart Law,
  2. Maxwell Equations,
  3. equivalent charges method.
The Biot-Savart Law is used to determine the magnetic field caused by a current in a closed path C’. Hence, this method is suitable for the transmission lines in a closed loop but not for overhead transmission lines which are in alignment.

Both the Maxwell Equations and the equivalent charges’ method are suitable to model or compute the electromagnetic radiation caused by overhead transmission system. However, more assumptions have to be made when the Maxwell Equations are applied and computations are more complicated than the equivalent charges’ method. As both methods produce very similar results with the same level of accuracy, after consultation with the chief environmental engineer in a local power supply company, the equivalent charges method is selected.

Ecluivalent Cha.mes Method
When the equivalentes method is applied to compute the electric field, assumed to be infkitely long and parallel to the ground and the conductor is the conductors are considered to be a sufficiently “good conduction”. The computation of electric field emitted from power lines using the equivalent charges method consists of two stages:
  1. calculation of the equivalent charges per unit length of conductor,
  2. calculation of the electric field produced by these charges.
The charges carried by the conductors of a multi-conductor line can be computed using the following matrix equation :

[Q] = [Cl [v]

where [Q] and [V] are column matrices of the charges and potentials of the conductors and [C] is a square matrix of the characteristic and mutual capacitance coefficients.

Once the charges per unit length are known, the electric field at a point, (xi>yi),maybe calculated, the intensity of the electric field is calculated by means of Gauss’s theorem E = q / (2per) where r is the distance from the point at which the field is calculated to the conductor, carrying charge q per unit length. The E field can be resolved in X and Y direction, Ex and E y and the magnitude of electric field is give by:


In calculating the magnetic field level emitted from a power line, the following assumptions are made:
  1. the conductors are parallel to the earth plane,
  2. the earth currents are negligible,
  3. the earth has a relative permeability of 1.0.
The magnetic field vector of a point P due to a current, Ii, has a magnitude of


where r is the perpendicular distance of the conductor from P.

This vector may be resolved into vertical and horizontal components, HiX and Hi, respectively. The magnitude of the magnetic field is given by


and the magnetic flux density is given by

B=mH, where m=4px10-7 is the permeability of free space.

Methods of Anslysis
Under the Electricity Networks Ordinance in Hong Kong, owners of property within a 50-metre corridor of a high voltage are allowed to claim for compensation. Consideration for compensation would be determined according to the distance separation between the power line and the property. In many countries including Hong Kong, the guidelines published by the International Non-Ionizing Radiation Committee in 1990 are used as a control to the emission of EMFs in their territories. According to the guidelines, members of the general public should not be exposed on a continuous basis to unperturbed electric field strengths exceeding 5 kVm-l and magnetic flux densities exceeding 0.1 mT(Duchene, 1991). If the calculated values are greater than these threshold values, the area is defined to be affected.

The criterion given by the Electricity Networks Ordinance and the INIRC are used for identifing affected areas and properties in the study region. The region near Tseng Lan Shue nearby the Black Point overhead transmission lines (400kV) is selected for this study. Figure 1 shows the base map together with the route of the power line in the study region.

Three dimension terrain information is essential for the determination of E-field and B-field in the region. The strengths of E-field and B-field are a function of r which is the distance between the power line and the point of interest. With the availability of the digital terrain model, the (x, y, z) coordinates can be used to compute the strengths of E-field and B-field in the region. The locations and the heights of the towers can be obtained from the electric company. By using the equivalent charges method as described in the last section, the strengths of both E-field and B-field can be determined. Hence, for the study region, there are three TIN surface models. Other than the original terrain model, there are also one B-field surface model and one E-field surface model. The strengths of these two fields are processed through a customized program. The results are imported into a GIS to generate TIN coverages.

Since GIS is a 2D system, it will be difficult to perform analysis on surface models such as TIN models. One solution is to convert the TIN surface models to lattice surface models. Then, all the analysis would be performed in raster mode. Based on the magnitude values associated with each pixel in the models, field strengths are classified into different classes according to table 1. The shaded records on the table are the field strengths exceeding the safety level and defined to be ‘exposed’. Other than the surface coverages for the terrain, the B-field and E-field, there are also land parcels, buildings, paths and power lines coverages in the system. These coverages are also converted to raster form before performing overlay analysis.

Discussions
Figures 2 and 3 from the field guidelines statec llustrate the exposed area in the region. Exposed area means that area is suffered and the magnitude exceeds the IRPA/INIRC standard. The IRPA/INIRC that the threshold electric field is 5 kV/m and the magnetic field is 0.1 mT. It is clear that both fields diminish rapidly with increase in distance. With 23m and 11m away from the power line, the E-field and B-field would become insignificant respectively. Hence, only the areas which are very closed to the power lines would experience the fields.

The results (exposed area) as illustrated in Figure 2 and 3 are overlaid with the building coverage to identifj buildings which are inside the exposed area. The results are shown in Figures 4. From the analysis, it is found that only one house is affected by E-field and two houses are affected by B-Field. Similar analysis is performed by applying the 50-metre corridor criterion in the system. A 25m buffer zone is generated around the power line and houses inside the zone are identified. It is found that the results are the same as the previous analysis.

From the above results, one can conclude that GIS can be used for route planning of overhead transmission lines with the use of a proper ELF-EMF emission model. The strengths of both B-field and E-field in the vicinity of the proposed power lines can be determined with the availability of a digital terrain model and the proposed locations of the towers. The B-field and E-field surface models generated can be used to identifj the “exposed area” in the region. By converting the TIN surface models to lattice surface models, overlay analysis can be performed with coverages such as land use coverage, building coverage. Properties and buildings which are inside the “exposed area” can easily be identified. Owners of the properties can be notified to claim for compensation. Planners and engineers can also use the information to assess the number of possible claims and estimate cost for compensation. Alternative routes can easily be introduced into the system once the coverages are available. An optimum route can be found by altering the locations of the towers to stay away from in residential zones.

However, as both fields are very sensitive to distance between the point of interest and the location of the power line. As for the case of planning, digital terrain model(DTM) is usually acquired from small to medium scale topographic maps. The DTM generated from this type of data will definitely not be very accurate. Nevertheless, as long as the planners and the engineers are aware of the fact, the information provided from this system can still be very usefid. Of course, more reliable results can be obtained if an accurate DTM is provided.

References
  • Ahlbom A., et al., (1987) Biological effects of power line fields. In: New York State Power Lines Project, Scientific Advisory Panel Final Report. New York: New York State. pp. 67-87.
  • Carpenter D. and Ayrapetyan S., (1994) Biological Eflects of Electric and Magnetic Fields: Source and Mechanisms, Academic Press.
  • Duchene A., Lakey J. and Repacholi M., (1991) IRPA Guidelines on Protection Against Non-Ionizing Radiation, Pergamon Press, pp. 83-100.
  • Goodland R., (1973) Power Lines and the Environment, The Cary Arboretum of the New York, New York.
  • Smith C. and Best S., (1989) Electromagnetic Man : Health and Hazard in the Electrical Environment, J. M.Dent & Sons Ltd., London.
  • Wertheimer N. and Leeper E., (1979) Electrical wiring configurations and childhood cancer. Am. J. Epidemiol. Vol. 109, No. 3, pp. 273-284.
  • Ueno S., (1996) Biological Effects of Magnetic and Electromagnetic Fields, Plenum Press, New York.
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