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N? .00.. - one 5.. £8.53 5.02 8 .000 5.00: $2 000.5020 5.. 8:09.000 80:00 0:0.00N 307. 00:005. 9:80:08: 080:8: - 080 $0 .09 SN 80:09.80 8.000 5:5. 80.80.». 8.00::80 508:0: 800:0: :8 .80. -m... 0. .m. ”83:20 500.. .5 :98 88.. 2.0:... 80 00... 5...... .0:0:09.000 0.80.8: 0000... .00.”. 0:003 acoammOmm: m0 0:000:08 50:00 \ 080 0.250 N 0305 19 1.3 Cohort studies of electrical workers: Stimulated by the results of the cancer and leukemia studies, cohort studies of electrical workers have been conducted. Occupational groups were: telephone operators, telecommunication industry workers, linemen, station ‘ operators, telephone company workers, electrical utility workers, etc. Aiming for better results, sample sizes were large and observation periods extended over many years. Relevant results are presented in Figure 4 (4). We can observe that before 1990, although many risk ratios for leukemia and brain cancer were over one, there was no significant excess over the years. But since 1990, as a result of more homogeneity in the sample groups used for the studies, significant excesses were recorded. Cohort studies’ results, tend to strongly support the possibility of an association between these cancers and exposure to EMF, as suggested by previous studies. This association stood out in the case of skin melanoma case control, rather than for brain cancer or leukemia case control studies, as was anticipated. (Skin melanoma is in excess in 5/ 7 cohorts). 1.4 Skin melanoma case control: Two contradicting studies on eye melanoma among electrical workers have been conducted. [Table 3] (4). The first published by Swerdlow, reported elevated odd ratios of eye melanoma for electric and electronic workers. The second was done in four Canadian provinces by Gallagher who did not find any excesses among his cases. Although it is commonly argued that excess of skin and eye melanoma in these group of workers is more related to a higher socio- 20 RR (95% Cl) All Leukemia SMR 5‘ l 4 q 4 0" Pun o 8‘"? 2 . ’ sm PIR * ‘ PM" o PMR l PRR SIR PMR P¥R { J I 0 1 i l I 0 l J 005 4 N 3 '3 I' 92 . 3 - ‘6? 3 5.3 2 m n n .L i: 2 g ‘5 0°» 0 0 en 2 a ‘V v- a) 3 m m “ E 3’. a c 3 3 93 3 v 0° ‘9 " E a 3 Q P c o E . E 8 2 S a g 0 E 0 O 0 Q C O .0 g g .9 C 2 3 t a 3: 3 = O " = h 3 O O 0 o =3 = G .5 g 2 3 a: o z: 2 a 2 s o .J ., LEUKEMIA RISKS AMONG ELECTRIC WORKERS Figure 4. The vertical axis shows the ratio of the number of cancer cases among groups exposed to stronger fields to the number of cases among groups exposed to weaker fields. The vertical lines indicate the statistical uncertainty that results from the relatively small number of people in each case. 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In most of these studies, lung cancer cases were high while leukemia cases were negligible. However, later studies of chronic myeloid leukemia conducted by Preston (4), reported highly elevated odd ratios in welders(OR=25.4), which is remarkable when compared to the negative results found earlier. 1.6 Male breast cancer: Since male breast cancer is an extremely rare disease, any number of such cases would be of high concern. Recently, Matanowsky (9) conducted a study of telephone workers. The results, although reporting small numbers, indicate a high risk of male breast cancer among telephone central office technicians. Two cases were found among 9,561 workers, while ordinarily none would be expected. Urged by Matanowski’s findings, Demers (10) carried out a case‘ control study of 227 male breast cancers drawn from 10 registries from the Surveillance, Epidemiology and End Results (11) from the National Cancer Institute. He chose 300 controls through random digit dialing or from medicare eligibility lists. His results confirmed the 23 previously reported ratios for any exposed job(OR=l.8, CI=1.0-3.2). However, the highest odd ratios were among electricians, telephone linemen, and electric power workers (OR=6.0, CI=1.7-21.5), and radio or communication workers (OR=2.9, CI=O.8-10.2). These results are significant to cancer researchers in suggesting a proposed mechanism by which exposure to EMF interact with the melatonin hormonal system. Conclusion: No positive answer has yet been widely accepted concerning the magnetic fields-cancer correlation. The epidemiologic studies conducted for this issue have found some statistical evidence of such correlation, but because of the small sample sizes and the lack of EMF exposure data or assessment, no conclusions could be drawn. II- REPRODUCTION: Does exposure to EMF has any effect on certain reproductive concerns such as infertility, outcome of pregnancy, childhood cancer and prenatal exposure to electrical appliances?. Studies in that area can be grouped under the following subtitles: -Outcome ofpregnancy in wives ofexposed workers. -Outcome of pregnancy and use of electric blankets. -Infertility of exposed male workers. -Central nervous system of exposed fathers, -Childhood cancer and prenatal exposure to electric appliances. 24 Table 4. Studies of leukemia incidence in welding populations. (ref 6) acute leukemia . all leukemia lung cancer 0 RR 0 RR 0 RR 1) 6 0.96 4 (m) 1.71 2) - -- 0(1) -- 3) 20 0.83 13 (a) 1.04 4) 7 2.25 5) --(m) (3.8) 6) 19 0.89 6 (a) 0.67 7) 0 -- 6 0.95 8) 0 -- 17 1.5 9) 4 4.2 10 2.2 10) 4 0.35 50 1.32 11) 1 2.5 7 1.38 12) 6 (a) 1.81 27 0.99 13) 43 0.99 193 1.42 14) 15 1.14 12 1.60 15) 27 0.85 7 (m) 0.76 381 1.46 4 (l) 0.63 305 1.27 Pooled data RR 146 0.92 40 0.92 1008 1.34 25 2.1 Outcome of pregnancy and use of electric blankets: Electric blankets and heated water beds cause probably the largest and longest domestic exposure to EMF that one can get... on the order of 50 to 100 mG peak *. Manufacturers have begun marketing blankets with greater reduced magnetic field strength. Since these are in every day use, they might have an effect on pregnancy outcomes, by either acting directly on the foetus or on the gametes through some genotoxicity (4). So far, only two studies have been carried in the subject by the same authors, Wertheimer and Leeper (4) who found some correlation with house wiring configuration. 2.2 Outcomes of pregnancy of wives of exposed workers: The only study conducted in this area was done in 1983 by Nordstorm (12), who tried to assess the effect of fathers exposure to EMF at work, and the outcomes of their wives’ pregnancy. A questionnaire was addressed to active and former employees of a Swedish electrical plant. As is commonly done, exposure was assessed based on occupational history. Abnormal pregnancies were more numerous, resulting from an increase in congenital malformations among couples whose men work in high voltage switch yards. * Measurements made by the author’s advisor indicate an exposure range of 0.05 to 95 mG ion an older electric blanket. 26 2.3 Infertility of exposed male workers: In Sweden, Nordstorm (12) attempted to investigate the difficulty in attaining pregnancy in couples whose male was exposed to EMF duringfertility. The report indicated difficulty rates of 20% among 400kV switchyard workers, 19% among 380—220kV transmission line workers, and only 6% among 130kV exposed workers. Thus, couples whose men work in high voltage switchyards (400kV), experience more difficulty in attaining pregnancy. Since the danger detected was related to high voltages, this is likely to be an electric field effect if there are no confounders. 2.4 Brain cancers and paternal occupational exposure before birth: Searching for a possible relationship between childhood brain cancer and paternal exposure at birth, at least four population-based case control studies have been published. The sample sizes for these studies were large (from 157 to 499) (4), and the children’s ages range was from 0 - 14 years old. Odd ratios of these studies are high, several of them reaching statistical significance. Correlating these results with the previous one which found an excess of brain cancers among workers exposed to EMF, strengthens the importance of the issue and inquires further investigations. 2.5 Childhood cancer and use of electric appliances: Savitz (7) inquired mothers on their use of electrical appliances before and after a child’s birth. His studies were published in 1990, reporting little association between the occurrence of childhood 27 cancer and the use of electric blankets (OR=1.3,CI=0.7-2.2). An increase in leukemia and brain cancer cases was more pronounced (OR=1.7, CI=0.8-3.6) for leukemia and (OR=2.5, CI=1.1-5.5 ) for brain cancer. CONCLUSION: Some authors proposed an association between EMF exposure from electric blankets or through exposed spouses and some pregnancy outcomes, such as prolonged gestation period, low birth weight, congenital malformation, spontaneous abortion...etc. The difficulty in attaining pregnancy among couples whose men are switchyard workers may be worth pursuing, in the context of the observation made by Nordstorm (12). III- NEUROPSYCHOLOGICAL EFFECTS: One study (4) reported that no differences in psychiatric diseases or psychological abnormalities, were observed between exposed and nonexposed people. Results were not only negative, but Knave(4) reported that performance on psychological tests were better among exposed than non exposed workers. This has been attributed to a higher economic and educational levels for people working in high voltage substations. EMF effects have even been studied for the occurrence of suicides and depression cases.The relative results were negafive. 28 CONCLUSION Of the three studied effects of EMF exposure, cancer, reproduction and neuropsychological effects, cancer is the one that had raised most public and epidemiological interest. Leukemia, brain cancer and male breast cancer are three sites where excess risks have been reported, among workers engaged in " electrical occupations". Although almost all "electrical occupations" have shown an excess of one cancer or another, the results reported are inconsistent and sometimes contradictory and confounders have not been isolated. These studies suffer from the lack of a common baseline for EMF measurements, and thus their conclusions are questionable. Studies on the EMF-reproduction effects, have affirmed the effects of occupation on outcomes of pregnancy, male infertility and childhood cancer. In the case of magnetic fields, no measurements were taken nor were chemical or socioeconomic confounders eliminated. Psychological effects studies were negative. Some further study is needed to identify confounders. Using conventional scientific methods, the argument that these fields cause cancer, reproductive damage, or other heath effects falls far short of being convincing. The results of the epidemiologic studies were inconsistent and often contradictory. They do suggest the existence of complex biological phenomenon. CHAPTER THREE EXPOSURE ASSESSMENT I- INTRODUCTION: Exposure assessment is defined as the determination or estimate of the magnitude, frequency of occurrence and rate of exposure of an individual or group to an agent. Based on this science, the epidemiological studies relating exposure to magnetic fields and the occurrence of cancer were subject to a major weakness: insufficiency of data on field intensities , and absence of an exposure assessment guideline (based on health effects research. This has launched numerous measurement projects and the development of instrumentation, methodologies, guidelines, and exposure models, all directed towards exposure assessment for EMF. The intent of the guidelines is not to exclude creative approaches to field measurement problems, but to set a common baseline for measurements, and thus make comparison between studies possible. Although a systematic approach to exposure assessment was proposed by EPA (l3),(EPA, 1986) certain technical factors make exposure assessment for magnetic fields at 29 30 low frequency difficult and complex. They include: 1. EMF are not directly sensed by humans (as is for example, sound level), and thus questionnaire methods are subject to significant errors. 2. EMF exposure is not memorable, with the exception perhaps of home appliances and electric blankets. 3. The lack of an accepted definition of exposure and dose for EMF leads to the uncertainty of what to measure. Scientific research in the area has not verified mechanisms by which low intensity EMF interacts with the human body. Without such a mechanism, there is no guidance to what characteristic of the field should be measured: magnitude, frequency, maximum ..... , etc. 4. The variation of the field over time due to instantaneous current (load) variations and/or movement by the subjects makes measurements inconsistent and complicated. 5. The variation in direction: The three field components corresponding to x, y, and z perpendicular axis are different. The measurement of the three components is necessary. 6. Sensing of the magnetic field via Faraday’s law and a search coil implies that a motion voltage is added to the magnetic field-induced voltage, if the coil moves. This effect must be controlled or field strengths will be overestimated by orders of magnitude. EXPOSURE METRIC Magnetic fields are commonly expressed in tesla (T) or microtesla (0T). whereas a unit that has been used historically is the Gauss (G) or the milligauss (mG), where 1 G = 10'4 T or 1 mG = 0.1 uT Technically, the unit Tesla is used for magnetic flux density, or B-field, and not for the exciting magnetic field. The magnetic field or H- field is expressed in terms of Amperes/meter (A/m). The two are related by the permeability of the medium in which the fields are established. E = 1131 . In media containing no iron, cobalt, or nickel (or related alloys), B and H are linearly related. This includes all biological and most organic media. However, for the purpose of all the field effect studies and the following chapters, the term ‘magnetic field’ refers to the magnetic flux density, which is a vector quantity related to the direction and magnitude of the current flow, and is expressed in units of Gauss or milligauss. In order to set a metric for exposure, scientists should determine which feature of the magnetic field is directly related to health effects. Thus because of the absence of such information, the choice of a specific exposure metric remains arbitrary and unclear. Although several metrics are possible, the most commonly used has been the time integrated field exposure expressed in terms of‘mG-hr, or an average field in milligauss. In this thesis, exposure is evaluated as an average field expresses-in units of milligauss. 31 II- SOURCES OF EMF EXPOSURE AND THEIR CHARACTERIZATION: Unlike other environmental agents, everyone is exposed to low frequency magnetic fields (EMF) to some degree. Sources of electromagnetic field exposure range from home appliances, electric blankets, electric shavers, household wiring to distribution lines and 765 kV transmission lines. The relative contribution of different sources to overall individual exposure is not well documented and the maximum total likely varies 1 to 100 mG among individuals in urban areas. Figure 5 shows the approximate 60 Hz magnetic field strengths from various common sources (8). Note that some household and workplace appliances provide at least as much exposure as power lines. Some 60-Hz magnetic field measurements have been reported and characterized as below: In Rural Area: Magnetic field measured in 14 commercial and retail locations in rural Wisconsin and Michigan had a mean value of 1.1 mG (14) (ITT Research, 1984). In An Office Stuchly (15) reported a magnetic field exposure of 0.5 mG in an office in Canada. M S The same study by Stuchly reported a measured field of 12.5 m G at 0.3 m away from the screen. YQL He also measured magnetic field levels of 1.5 to 7 mG, 0.3 m in front of video display terminals. 32 33 Welding and steel production: Welding machines and induction furnaces are potential sources of high occupational magnetic field exposure. Workers in steel production using arc or induction furnaces are exposed to 50 Hz fields as high as 1-to-100 G near the work areas (15). Electric Transmission Facilities: Four Canadian generating stations exhibited localized magnetic fields as high as 2.7 Gauss while the typical levels are 10 times lower. Generating plant workers in the Federal Republic of Germany may be exposed to l-t0120 Gauss fields, as reported by Stuchly (15). He also reported estimated magnetic fields above a superconducting cable carrying 13 KA of 1.4 G and 0.45 G for burial depths of 0.75m and 1.4m respectively. 13 kA is the maximum current ever present, on a steady basis, in any power system. Proposed High School Site In Okemos, Michigan: Measurements at a proposed high school site, near 46 kV lines, showed that, 500 feet from the line, magnetic field strengths would be typically 0.08 mG. This number is much smaller than strengths used in any health effect studies. Directly under the 46 kV line, 11.4 mG fields were reported. The typical line current was predicted to be 200 amps, while the typical daily maximum current is 310 amps. These measurements are taken from Dr. Gerald Park and Khadija’s field notes, at Michigan State University. More extensive field measurements have been reported by Bracken (14) in 1988. He investigated the fields in some electric 34 WITHIN HOMES Away from appliances Next to appliances Electric blankets C DISTRIBUTION/SUBSTATION LINES Edge of right-of-way C Within right-bf-way HIGH-VOLTAGE TRANSMISSION LINES Edge of right-of—way Within right-of-way OCCUPATIONAL EN VIRONMENT 5 Office Specialized high exposure 0.1 1.0 1 10 J 60-Hz Magnetic Field Strenghs (milligauss) Figure 5. Approximate strength of produced within the body by several common sources.(ref 8) Common QRare the average magnetic field 35 worker environments. The highest magnetic field for an electrician was near an industrial power supply, 103 mG. The second highest was near underground and overhead transmission lines for power workers, 41 mG. Sources of magnetic fields in the office environments include electrical distribution transformers and wiring in vaults that are located inside large office buildings. More than 100 mG measured directly above the transformer can be caused by large heating, cooling and lighting loads. However, most office space is under a 0.1 mG magnetic field, 5000 times less than the earth’s steady magnetic field. 111- INSTRUMENTATION: There are two complementary approaches to obtaining exposure assessment: field strength measurements and computer simulations. Field measurements can take the form of point-'in-time or long term, depending on the exposure assessment proposed. Long term measurements are obtained through stationary recording systems, while for point—in -time measurements survey meters are necessary. Field measurement instrumentation must be compact, light weight, and capable of monitoring and recording data over extended periods. Alternating magnetic fields fields are usually sensed with single and multiple coils that generate a voltage, proportional to the instantaneous magnetic field. In the case of multiple (three) orthogonal coils, the resultant magnetic field is computed from the square root of the sum of the squares of the fields from the three coils, or with a one coil device, as a true rms superposition of the orthogonal field components. 36 Meters that are currently used in field effect studies, characterize exposure as the time integral of induced voltage in the coil (or coils) over the time the device is worn. They use either microprocessor-based data storage of the fields three orthogonal components (14), or a simple device that integrates the induced voltage (B-Field) over the period of time the device is worn. The reading produced is a single-value-time-integrated exposure. In order to remove the effect of the motion of the observer and that of the earth’s magnetic field, high pass filtering is necessary. In the United States, exposure assessment projects commence almost daily. Of 126 EMF-related research projects going on in the U.S. and other countries, 15 of them are directed towards occupational exposure (16). In 1989, the Electric Power Research Institute (EPRI), had developed an electric and magnetic field digital exposure (EMDEX) meter (16). This meter is capable of recording EMF exposure for extended times, and storing that data in the memory of an onboard computer. The device measures magnetic fields from 0 to 25,000. milligauss, and electric fields from 0 to 500 kV/m. It has a bandwidth of 40-400 Hz. Similar in function to the EMDEX is the “ElectroMagnetic Dosimeter” developed at the Institue de Recherche d’hydro-Quebec. which measures magnetic fields up to 4000 mG, and electric fields up to 40 kV/m. During measurements, it is recommended that the operator stands still, hold the .meter straight and read the maximum reading the meter shows. In the case of electric fields, the measurements are more subject to errors and uncertainties. Since the human body acts like a conductor, an attenuation or 37 enhancement of the electric field in question is likely to occur. Electric field survey meters are usually suspended in free space by an insulated handle, keeping them away from any conducting objects including the observer (17) (IEEE, 1979). Magnetic field measurement uncertainties come from the continuously varying current loads over time. However, the most prominent issue of exposure assessment is the uncertainty about the risk from the EMF fields, which in turn, makes the improvement of the measurement techniques and procedures impossible. I V- MAGNETIC FIELD EXPOSURE STANDARDS: As a response to the public concern about magnetic field exposure health effects, utility regulators are beginning to set magnetic field exposure standards within and along the edge of a transmission line right of way. The majority of the standards which do exist, address the magnitude of the electric field, some address the exposure to high frequency electromagnetic fields, andonly a few addressing the magnitude of the magnetic field. The table below summarizes the existing standards in 60—Hz power lines in the United States 38 Table 5. State regulations limiting field strengths (1 8) Elec Field Mag Field kV/m mG STATE VOLTAGE IN-ROW EDGE-ROW EDGE-ROW Florida 500kV 10 2 200 * <200kV 8 2 150 Minnesota - — - - Montana - - 1 - New Jersey - - 3 - New York - - 1.6 - North Dakota - 9 - - Oregon - 9 - - * 200 m6 for single circuits 250 mG for double circuits Although all the states listed have set electric field limits, Florida is the only state that has effectively taken action to limit the magnetic field intensity along the transmission line ROW. The 200 mG maximum field exposure set by Florida stemmed from existing technology constraints and from health data (19). New York has proposed magnetic field limits to 200 mG for all new lines, at the edge of the right of way. This standard was based upon the concept that all new constructions should not increase magnetic fields beyond what they already are. Other states are applying the regulatory concept of limiting exposures to their lowest achievable (known by the“ acronyn ALARA), borrowed from the field of nuclear radiation protection. Outside the United States, magnetic field exposure standards 39 have published by some countries. USSR is the only country that has an official regulation which limits magnetic field exposure at power line frequency of 50 Hz, while the United Kingdom and the Federal Republic of Germany have proposed standards, which are not yet mandatory (20). In the United States, constraints have gone as far as widening the right of way. Consumer Power company regulations forbids the construction of buildings within 18 feet of the center of a 46 kV line and within 36 feet of the center of a 138 kV transmission line. It is worth pointing out that these clearances required by the National Electrical Safety Code are based upon the electrical worker’s safety and not upon any field reduction concept (21). CHAPTER FOUR MAGNETIC FIELD CONCEPT I-.PROPERTIES: Magnetic fields in the vicinity of overhead transmission lines are caused by the current flowing in the line conductors. This current is caused by the movement of electrons in the conductor. Magnetostatics consists of the study of magnetic fields which are —) constant in time. Related to the magnetic field H , is the magnetic _) flux density B . They are related via the permeability 0 =00 u, by: B” =nxfi (4.1) Where it = The permeability which is a property of the material in which the field is located. 00 = The permeability of free space (Vacuum) and is a universal constant given by no: 4T1x10'4 Henry/m 1J1. = Relative permeability with respect to that of free space. Physical materials have a relative permeability close to unity, except for ferromagnetic materials like iron, nickel, cobalt and ,4 some ferromagnetic ceramics which have relative permeability 4O 41 sufficiently different from one, to be of concern. In this thesis, the magnetic field will be calculated in free space, and thus 11. is equal to one, meaning that the permeability ll. equals that of free space Ho- I.1 Ampere’s Lg_: The magnetic fields around transmission lines are described by a fundamental law of magnetostatics: Ampere’s Law which is expressed as: jfi-di=1 (42) It states that the circulation of the magnetic field around any closed path is equal to the free current flowing through the surface bounded by the path. Ampere’s Law is useful for magnetic field calculations for simple geometries. This can be illustrated by the following practical case of a long straight wire carrying a constant current, which is of interest in this thesis. Example (22): Magnetic field of a long straight wire of radius b and carrying a current I: Consider two circles in a plane perpendicular to the conductor, of radii I'ISb and r22b, and whose centers coincide with that of the conductor’s circular cross section as shown below. 42 (a) Inside the conductor: r15 b The current enclosed by the circle of radius r1 is 2 2 l'Ir1 r1 Therefore from Ampere’s Law: _, poxrlxl B = ——-——§—x¢ 2be Where 6 is the polar coordinate unit vector along the wire. (b) Outside the conductor: r22 b The area enclosed by the circle of radius r2 contains the total current I. Thus from Ampere’s Law, = “0 W (4.3) 2I'Ixr2 Note that the field intensity in inversly proportional to the distance _) from the conductor and that B is directed tangentially to circles centered on the conductor and on planes perpendicular to the conductor as shown in Figure 6. 1.2 Conservation of magnetic flux: The magnetic flux through a given area is determined by: «p = jB’odA‘ ' (4.4) ' MAGNETIC FLUX LINES \ \ \ \ \ ~36. ) ) / \ g // CURRENT CARRYING CONDUCTOR Figure 6. Magnetic field surrouding a current-carrying conductor. When the current flow is out of the paper, then the magnetic flux lines are directed as shown. 44 Over an arbitrary closed surface S, the magnetic flux vanishes as: A closed surface S ¢=§§od =0 (4.5) S Where the surface integral is carried out over the bounding surface of an arbitrary volume. This gives the well known result that there are no magnetic flow sources, and that magnetic flux lines are continous and always close upon themselves. Equation(4.5) is also referred to as an expression for the law of Conservation of magnetic flux, stating that the total outward magnetic flux through any closed surface is zero (22). 1.3 Magnetic forces on current-carrying conductors: _) A Charge q moving with a velocity u in a —> magnetic field with a flux density B , experiences a magnetic force —) Fm, given by: —> —) —) Fm = q u x B From the expression of magnetic force, the magnetic force per unit length on a conductor of length l, is given by: —> —) —) F/l = I x B -—) ——> Where I is the current in the conductor, and B is the magnetic flux density at the one conductor location due to all the other 45 current carrying conductors. Thus, there are forces on mechanical conductors which must be considered in the design of transmission lines. 1.4 Inductance: Suppose we have a coil with N turns, then magnetic flux linkages are denoted by X, such that for no leakage 7» = N x d) (4.6) Inductance relates current to the magnetic flux linkages by: it = L x I (4.7) Through Faraday’s Law which will be stated later on in this chapter,a voltage dh/dt < II or, v = L di/dt (4.8) is induced by time-varying fields. Thus, the material inductance is significant for the magnetic coupling problem (1). II- TIME VARYING MAGNETIC FIELDS: Power transmission lines operate at 60 Hz and other low frequencies. As far as electromagnetic phenomena in concerned, these frequencies are very small. However, these currents are not constant or direct currents, and calling the transmission line fields electrostatic and magnetostatic is actually incorrect. Electric fields in ac applications are time varying and electromagnetic fields mean fields caused by an alternating electric current. Thus, transmission line fields are time varying at 60 cycles per second, commonly represented by constant fields as the “quasi—static approximation”. 46 2.1 Quasi-static Approximation : The distances involved in transmission line environmental and health effects are usually very small compared to the 60 Hz wavelength of 5,000 km. Thus it is frequently valid to approximate the magnetic and electric fields by static formulas, especially for ground level fields (1). This is the quasi-static approximation . Above the ground planes, the magnetic field has both vertical and horizontal components which are phasors with generally different phase angles. The resulting field vectors trace out ellipses as functions of time, as will be shown in the magnetic field calculations in the next chapter. The peak field value is given by the major axis of the ellipse. This value is of concern above the ground planes, under the current carrying conductor. 2.2 Faraday’s Law: Time varying magnetic fields induce voltages in any conductor in the vicinity of the field. According to Faraday’s law, these voltages are given by : (4.9) 9.1% Since the B-field lines are circles around the current carrying conductor, the magnetic flux lines link any conductor parallel to the line. This causes a longitudinal induced voltage in the conductor. Magnetically coupled. voltage is dependent on the line current, but independent of the line voltage. CHAPTER FIVE CALCULATIONS OF MAGNETIC FIELDS OF OVERHEAD TRANSMISSION LINES I - INTRODUCTION: Magnetic field calculations are necessary for the overall design of transmission lines. Recent epidemiologic studies seem to indicate the importance of magnetic in addition to electric fields. However, in contrast to E-field measurements and calculations, there has been less work accumulated for calculating, measuring and predicting the B- fields under the transmission lines. In the meanwhile, the increasing loading of transmission lines has increased the relative importance of magnetic field effects. This in turn has resulted in activity in the following areas: Calculation and measurement techniques for magnetic fields, and measurement of induced current on objects of different shapes, for all line voltage classes and line configurations. This chapter presents the theoretical calculations of the magnetic fields in proximity to the line conductor, and the maximum field above the ground.The assumptions, coordinate system used for the calculation, as well asa list of useful definitions are first presented. 47 1.1 Definitions (2): Vector: A vector is described by a magnitude and an angle in space. It is indicated by an arrow over a capital letter (B ). Phasor: A phasor is a complex number. It is a quantity with a sinusoidal time variation described by a magnitude and an angle in time. Unless otherwise specified, it is used only within the context of steady state linear alternating systems. In polar coordinates, it can be written as Aeje where A is the amplitude or the magnitude, and 0 is the phase angle. In the following calculations, a phasor is indicated with a wave sign over a capital letter (E ) or .with sinusoidal functions of time [e(t)]. mm: A vector field of magnetic flux density ( B-field) is used to describe the magnetic field generated by currents in the conductors of transmission lines; It is defined by its space components along the three orthogonal axes. For steady state sinusoidal fields, each space component is a phasor that may be expressed by an rms value expressed in Tesla (SI) which is one weber per square meter (Wb/mz), or by another commonly used unit: The Gauss (1mG=10 0T), and a phase as in I; (t) = bx(t)i’ + by(t)y’ + bz(t) 2’ (5.1) Where J? , y’ , 2’ are the unit vectors along the x, y and z axis and bx(t), by(t), bz(t) are phasor functions of time. The x-space component is 48 49 bx(t) = Bxcos((t)t+¢x) = mecoswt + Bm-sincot (5.2) Bx, (bx, Bx.r and Bx,i are the magnitude, phase angle, real and imaginary parts of bx(t), respectively. It is also useful to visualize the vector B, expressed in equation 5.1 as a vector moving in space. It can be shown that this vector rotates in a plane and describes an ellipse (Figure 8). The length of the semi-axis represents the length of the maximum field strength. A quarter period later, the field is in the direction of the minor axis, and the length of the semi axis represents its magnitude. The field in the direction perpendicular to the plane of the ellipse is zero.’ Single Phase AC Field: A single phase magnetic (electric) field is generated by conductors, energized by a single phase source of alternating current (voltage). All the field components are in phase. The field at any point can be described in terms of its time - varying magnitudes and invariant directions. Three-Phase AC Fields: Three—phase transmission lines generate a three-phase field whose space components are not in phase. The field is described by the field ellipse, i.e, by magnitude and direction of the major and minor semi-axes. When the minor semi-axes is much smaller (less than 10%) than the major semi-axis, the field may be practically considered single phase. This occurs close to boundary surfaces such as ground, at a height of 1 m. 50 Figure 7. Example of magnetic field ellipse at a point in time. (ref2) 51 1.2 Assumptions: 1. The calculation is sufficient using two-dimensional analysis. 2. Transmission lines are parallel. 3. The earth is a flat surface. 1.3 Coordinate system: Consider the coordinate system with unit vectors a x, ii y and ii 2 along the x, y and z -axis respectively, where the Z-axis is parallel to the line. See Figure 9. (xj,yj) represents the observation point, while (xi,y,-) represents the current carrying conductor point (2). II- MAGNETIC FIELD WITH NO EARTH RETURN CURRENT: From Ampere’s law on page 41, the magnetic field strength, Hij- at point (xj,yj), at a distance ’ij from a conductor carrying a current Ii, has an amplitude Hij = ‘ (5.3) ‘6"? Where “i” is the index for the conductor coordinates while j is for the observation point coordinates, as shown in Figure 8. Hij is the magnetic field intensity at the location j, at (xj,yj), due to the conductor i, at, (xi,y,-). (In vector notation: 52 j IS THE OBSERVATION POINT _) Hj ,i (Xj,YJ-) 1 IS THE CURRENT POINT > X Figure 8 . Coordinate system for magnetic field calculation.(ref2) 53 —, Where ¢ij is the unit vector in the direction of the cross product of the vector current and the vector segment rij A ._ .A x._ . ¢ij : —yl_n_ylu + l “x" l’lj X H} (5.5) it Since there is usually more than one conductor in parallel, the total magnetic field is the sum of all the contributions from the other currents. Thus the total magnetic field at the observation point (xj,yj) is n I A._ l A” H’ ' ZZHrij¢U (5‘6) 1 The magnetic flux density is A B = 0x11 , (5-7) Where it = 411 x 10'7 H/m for both air and ground. The vertical and horizontal components of the total magnetic field are phasors which completely describe the field. Each of these components is expressed by real and imaginary parts: éx = Brx+jéix (58) +139 (5.9) III- MAGNETIC FIELD WITH EARTH RETURN CURRENT: 3.1 Earth Return Currents: In most practical calculations, the magnetic fields in proximity to balanced three-phase lines may be calculated considering only the currents in the phase conductors and the lightning or neutral ground wires, while neglecting the earth return current. Calculations show that the magnetic field is primarily produced by these currents, especially at large distances from the conductor. For balanced three- phase systems, the return currents distributed in the earth, sum to zero. However such cancellation is imperfect for lower—voltage distribution lines (120V to 35kV), because the wires are often separated in space, the phase currents are often unequal, and more importantly, because some of the return currents do not follow the distribution neutral wires as they are supposed to. Instead, they may take multiple return paths through many possible ground connections from the distribution systems as well as at customer level, such as metal water pipes, to which most urban electrical systems are grounded at each house. The intensity ofthe magnetic fields from Unbalanced currents on a distribution line tends to fall off less rapidly (l/R) than the field surrounding a set of conductors carrying balanced currents (1/R2). Magnetic fields from dispersed return currents add to those from balanced distribution currents, and in some cases can form the dominant source of background indoor fields. 54 55 3.2 Calculations: A total derivation of the magnetic field produced by each conductor and its earth return is shown in reference (2). It is expressed by the, following equation: F1~ — —‘ ¢ ———1‘ 1+l(—_.2 )4 {13' (510) ‘1 ‘ 2Hr.. if 2mg}. 3 rm]. if ' The first term is the same as equation (6.4). It gives the usual expression for magnetic fields, in proximity to the conductor (up to 100 m for a 545 kV line(2). The second term is a correction term that accounts for the earth return current. Here the parameter Y describes the earth conditions. NIH Y= [jmu(o+jme)] (5.11) 11 Where 0' = The earth conductivity (0' 0.001 to 0.002 Ohm/m) g = The earth permitivity (e . 8.85 x 10'12 ) r’ij= [ (xi-xj)2 + (yi+yj+ 1/ T )2 ]1/2 ,a complex number .+ .+2/ X--x' y—z—1——y, 7):: +——‘,,.’tt (5.12) x rt} y (M = -( ii Equation (5.12) is complex valued. This shows that when the earth resistivity is taken into account, the magnetic field H is not in phase —’ with the conductor current. Note that the magnetic field vector Hi] can be expressed by the following real and imaginary vectors: H, = I-lxrux+Hryuy 56 The ellipse traced by the magnetic field has axes, whose magnitude and direction are calculated in appendix A at the end of this chapter. IV- COMPUTER SIMULATIONS: Several PC-based computer software programs are available for calculating magnetic and electric fields from transmission lines. Some use a human activity model to study the interaction between the activity of persons and their physical environments for exposure assessment. The model is combined with engineering-based methods for calculating the electric and magnetic fields associated with the voltages and currents carried by transmission lines. Some of these PC-based programs are useful in simulating and characterizing the electric and magnetic fields for a variety of transmission line design options, and in providing some field reduction or elimination strategies. Examples include the EXPOCALC computer software package, developed by the Electrical Power Research Institute (EPRI), which provides a method for simulating exposure to transmission line electric and magnetic field during various human activities. A typical field simulation program is the TLField product of Power Technologies, Inc., which, because of the purpose of this thesis, will be described below. TLField ( Tmnsmission Line Field2(232 TLField is a special PC computer program developed by Power Technologies, Inc. of Schenectady, New York to evaluate and minimize electric and magnetic. fields from transmission and distribution lines. This program has the capability of displaying graphic plots of the field profiles, indicating the maximum fields, in both sides of the centerline, as well as the field magnitudes at both edges of the right—of-way. TLField has the special feature of providing some techniques for line design with fields reduced to their lowest possible values. Some of its other features include: (1) The capability of synthesizing the required voltages or currents for specially installed wires (cancellation or degaussing circuit) to minimize or possibly cancel the fields along the right of way. (2) Analysis of cases with unbalanced currents in underground cables. (3) The capability to reduce electric fields by erecting ground horizontal wires along the right-of—way at some critical locations. (4) Clarity of the graphic plots: phase conductors are indicated by small diamonds, shield wires are indicated by crosses, and field cancellation/degaussing wires are indicated by small triangles. Thus a physical picture of the line configuration can be seen on the printer or screen output. TL Field permits up to 40 conductors, including shield wires, phase conductors and ground horizontal ground wires. A maximum of 24 phases are allowed, while a maximum of only 9 shield wires and 10 ground horizontal wires are allowed. Magnetic field units are in 57 58 mG or Gauss, while a choice of a metric or English unit system is available. Fields are calculated at a height of 1m (metric) or 3 feet (British) above the ground. Their values are obtained as the resultant of the vertical and horizontal components of the field. In this thesis, this program will be used as a tool for predicting, calculating, and exploring methods to reduce the magnetic field at the right—of—way (ROW), at 1m above the ground. CHAPTER SIX TRANSMISSION LINE GEOMETRY DESCRIPTION I. Right Of Way: Overhead transmission lines requires strips of land to be designated as Right-Of—Way (ROW)). Outside these strips, the only interference of the transmission lines with distant areas is the aesthetic effect on the landscape and radio interference. Near the energized transmission lines, within the ROW, an area of actual danger of discharge occurs, and this area can not be occupied by trees or buildings. The prohibitions are included in electrical codes and related regulations. ’ 1.1 Definition: The land below the danger zone defines the right of way corridor, representing an area in which the presence of the line represents constraints on the use of the land. 1.2 Size of the R-O-W: The size of the right of way can be expressed as: 51:2(1) +O+C) 59 60 Where D = Interphase distance (distance between phase conductors) 5 = The maximum horizontal displacement of the midspan of the outside conductor, (and therefore related to the maxi- mum sag). C = A horizontal clearance which is a function of the voltage. Table 6 give some practical values for right-of—way, inter-phase distance, midspan sag, for a three—phase circuit with various large line-to-line voltage levels. Table 6. Characteristics of lines for various system voltages (24). System voltage(kV) 420 525 765 1000 1300 1500 Midspan sag (m) 12.0 13.5 15.0 17.0 19.0 20.0 Interphase distance (111) 7.3 9.2 12.8 16.1 19.0 20.8 Right-of-way (m) 35.5 42.3 52.0 62.5 72.0 76.5 As a result of environmental awareness and increasing demand for more transmission facilities, obtaining right-of—way for transmission lines is often difficult, and for a better utilization of the land, multiple circuit designs are becoming economically attractive. In the following chapters, we are interested in the calculations of magnetic fields in the centerline, at 'the edges of the right-of-way and at some distances from its edge, where planned construction might 61 take place. II. Height Above the Ground: A transmission lines ‘danger zone’ must be kept at a given height above the ground, to allow normal activities to take place underneath them. The minimum clearance between the lowest voltage transmission lines and the ground is not less than 5.5 m. Clearance increases with voltage. III. Conductor Spacing: Conductor spacing is the distance between conductors as measured from the pole. These spacings are usually maintained by fiberglass-core ceramic insulators at cross-arms, which are subject to substantial field exposure. Designs must withstand conductor motion 6 due to ‘wind-induced galloping” or fault currents. IV. Conductors: Conventional transmission lines use conductors such as ACSR*, AAAC, ACAR, SSAC and others depending on operating conditions such as compactness, maximum power transfer, wind gusts or ice exposure. However, the choice of a conductor size (cross section) is usually based on electric and economic requirements. * Aluminum Conductor Steel Reinforced is the most common 62 V. Shield Wires: Overhead shield wires are installed on transmission lines to provide a path to ground for lightning protection. The use of shield wires will usually require an increased pole height, an increased pole strength, and for certain configurations an increased conductor spacing. VI. Bundle,Two conductor, Three conductor, Multiconductor: A circuit phase can consist of more than one conductor. Each conductor of the phase is referred to as a subconductor. A two conductor bundle has two subconductors per phase. These may be arranged in a vertical or a horizontal configuration. Similarly, a three conductor bundle has three subconductors per phase. They are usually arranged in a triangular configuration with the vertex of the triangle up or down. A four conductor bundle has four subconductors per phase, usually arranged in a square configuration. For regular bundle conductors and for calculations of magnetic fields in chapter 5, away from the conductor surface, it is convenient to consider the equivalent single conductor having a diameter, deq, given by: Where D: The bundle diameter 11 = The number of subconductors in the bundle 63 d = The diameter of the subconductors, which is generally the same for all subconductors. For nonregular bundles, the equivalent diameter is calculated as the diameter of the single conductors with the same total charge. For such calculations, it is sufficient. to assume a single-phase energization of the bundle. CHAPTER SEVEN ROLE OF LINE CONFIGURATION IN EXPOSURE TO AND CONTROL OF THE MAGNETIC FIELD I-CONTROL OF MAGNETIC FIELDS: In June 1990, the Environmental Protection Agency was alleged to have described magnetic fields as a “possible, but not proven cause of cancer in people’ (25)*. Even though most studies show no correlation between the exposure to magnetic fields and possible health effects, public concern is already creating pressure and expectations for measures to reduce or eliminate such fields.- However, in order to provide a guidance to what elimination technique should be necessary, a mechanism of how these fields interact with the human body must be in hand. Unfortunately, such a mechanism is still unknown and subject to scientific research. Exposure to magnetic fields is still an uncertain risk for possible hazards, and it has been stated by the director of the EPA that “We * This was in a preliminary draft of a report/ leaked to the press. 64 65 don’t really understand the risk well enough to know whether one exists, and we can’t give guidance to what kind of avoidance technique really would be important” (25). Some published articles demand exposure standards and the related measurements necessary for magnetic field control. Some IEEE members have called for such standards. A working group of IEEE standards coordinating committee 28 (The Committee on Nonionizing Radiation) is considering guidelines for limiting magnetic fields. In the meanwhile, utilities are seeking and identifying methodologies for magnetic field exposure reduction. So far, possibilities for new techniques and equipment are being explored, to limit magnetic field exposure for power line maintenance workers. Transmission line designs that are in use are being reviewed from a new perspective: Field exposure reduction. 11- UNCERTAINTIES OF FIELD CONTROL TECHNIQUES: Intensity, duration of exposure, frequency, orientation, exposure spikes, are all aspects of magnetic fields in the vicinity of current carrying wires. However, which approach should be taken for magnetic field reduction is still uncertain. Even after many laboratory and epidemiologic studies, at least four uncertainties have not been clarified. 66 1) Intensity: If exposure to magnetic fields is harmful, would one associate stronger fields with higher risk of diseases?. Such an intensity-effect relationship has not been demonstrated yet. The first epidemiologic studies that focused on magnetic fields, have shown average flux densities measured indoors of only 2-3 mG, whereas many common sources of exposure are known to produce field intensities of tens or hundreds of milligauss. 2) Duration: How is the effect of chronic exposure to low level fields compared to brief exposure to more stronger fields?. This has not been known yet, and thus limiting human exposure is difficult. 3) Frequency: Magnetic fields are time varying, fed by alternating currents that change direction and intensity 60 times a second, for 60-Hz power frequency (US). It has been suggested that magnetic fields are harmful only at certain frequency bands, and certain intensity ranges, or even through the harmonics created by certain loads. 4) Orientation: There is a suggestion that ac magnetic fields may interact with biological systems on the cellular level if the field frequency is in phase with the resonant frequency of certain cellular ions within the mostly static dc magnetic field of the earth itself. Biological effect occurs if the ac fields. are parallel to the earth’s 67 dc fields. Rather than wait until these uncertainties are clarified, it may be prudent to explore field intensity minimization as a precautionary measure. III- LINE CONFIGURATION AND REDUCTION OF MAGNETIC FIELDS: 2.1 INTRODUCTION: The magnetic. field at ground level in the vicinity of a single or multiple-phase line circuit results from the contribution from every single current-carrying conductor. Thus, the line configuration is critical for changing the magnetic field under or close to the transmission lines, in particular along the edges of the right of way, which is of concern. Numerous three-phase circuit configurations are being used by utilities for high and low voltage transmission. There are three-phase horizontal circuits, vertical, delta and vertical delta configurations, single circuits, double circuits, shielded or unshielded circuits, bundled or compact circuits with different with different phase arrangements,...etc. Some parameters are very critical in line design. They include: - Line configuration - Height above the ground - Conductor sag - Conductor mechanical parameter (metal, diameter,...) - Line phase spacing -. Phase arrangement 68 - Shield wires The arrangements of the phases in a typical three-phase transmission line is designed to cancel parts of the magnetic field, more than a few meters away. Design constraints range from natural factors such as the wind, snow, maximum temperature and ground resistivity, to economic factors such as construction and maintenance cost limitations. In chapter 5, it has been shown that the resistivity of the ground has an effect only at large distances from the phase conductors (> 100m for 550kV lines). Different mechanical and electrical line configurations generating magnetic fields have been investigated by the Electrical Power Research Institute (EPRI), at EPRI’s High Voltage Transmission Research Center (HVTRC), operated by General Electric Company at Lenox, Massachusetts (25). The reader must remember that the magnetic field is produced by current, and the greater the current, the greater the field. 2.2 VOLTAGE INCREASE: Magnetic fields vary in intensity as a function of the amount of current flowing in the conductor. Consider the simplified power transmission model shown below: 69 Figure 9. Simplified power transmission model (26). For this circuit, the power transfer over the line is given by: E Es P = fix [)2] sin(8) = J3>brekiwZeuiucocim G) §>623E>L;aniwtnu)a}>0102»KlbloE>L REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) REFERENCES The Institute of Electrical and Electronics Engineers. “ The Electrostatic amd Electromagnetic Effects of AC Transmission Lines”. 79 EH0145-3- PWR, IEEE, New York, 1979. Electric Power Research Institute. “Transmission Line Reference Book / 345 kV and Above / Second Edition.” EPRI, Palo Alto, California, 1988. Weedy, B.W. “Environmental Aspects of Route Selection for Overhead Lines in the U.S.A.”. Electric Power Systems Research, 16: 217-226, 1988. Theriault, G. “Health Effects of Electromagnetic Radiation on Workers. Epidemiologic Studies”. School of Occupational Health. McGill Univ, Montreal, Quebec, Canada, 1991. Miettinen, Olli S. “Theoretical Epidemiology: Principles of Occurrence Research in Medicine”. John Wiley & Sons, 1985. Wertheimer N, Leeper E.” Electric wiring configurations and childhood cancer.” American Journal of Epidemiology. 109: 293-284, 1979. Savitz D.A. Watchel, F.A. Barnes, E.M. John, J.G. Tvrdik, “Case-control study of childhood cancer and exposure to 60-Hz magnetic fields”, American Journal of Epidemiology, 128: 21-38, 1988. Electric Power Research Institute. “EMF: The Debate on Health Effects”. EPRI Journal, vol 12, Oct/Nov, 1987. Matanowski G. “The Hopkinsv- telephone workers study”. Health and Safety Report, 7:3-4, 1989. Demers, RA, D.B. Thomas, K.A. Rosenblatt et :11. “Occupational 105 (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) 106 Exposure to Electromagnetic Radiation and Breast Cancer in Males”.Conference Abstract, American Journfl of Epidemiology, 132: 775-776, 1990. Nordstorm S, E.Birke and L. Gustavson. “Reproductive hazards among workers at high voltage substations”. Bioelectromagnetics 1983; 4:91-101. EPA. “Guidelines For EStimating Exposures”. Fed. Reg 51: 34042-34054, 1986. Bracken, T. Dan. “Occupational Exposure Assessment for Electric and Magnetic Fields in the 10-1000 Hz Frequency Range”. Prepared for Scientific Workshop on the Health Effects of EMF, NIOSH, January 1991. Stuchly, M.A, D.W. Lecuyer and RD. Mann,(1983): “Extremely Low Frequency Electromagnetic Emissions from Video Display Terminals and Other Devices”. Health Phys. 45: 713-722, 1983. Sussman, Stanley S, “Electric and Magnetic Field Exposure Assessment”. EPRI Journa_1, March, 1990. The Institute of Electrical and Electronics Engineers. “IEEE Recommended Practices for Measurements of Electric and Magnetic Fields from AC Power Lines”. 1311;, New York, New York, 1979.Now superceded by IEEE 644 - 1987 on the same topic. Nair, 1. Morgan, K.H. Florig, Biological effects of power frequency electric&d magnetic fields, U.S. Congress, Office of Technology Assessment Background paper, OTA -BP- E- 53, Washington, DC, U.S. Government Printing Office, May, 1989. Fitzgerald, K.I, Nair, Morgan, M.G. “Electromagnetic Fields: The Jury’s Still Out”. IEEE Spectrum, 22-35, Aug, 1990. World Health Organization, Environmental Health Criteria 69, Magnetic Fields, Geneva, 1987. itional Electric Safety Code - 1990.; ANSI C2-1990. (22) (23) (24) (25) (26) 107 Cheng, David K. “Field and Wave Electromagnetics”. Addison-Wesley Publishing company, March 1985. Power Technology. “Preliminary Users Manual: A program to evaluate and minimize electric and magnetic fields from transmission lines”. Power Technologies Inc, Schenectady, NY, Revised Oct, 1991. Mongelluzo, R. Piccolo, A. Rossi F, “Modeling Environmental effects of EHV and UHV Transmission lines”, Electric Power Systems Research, 4:235 - 245,1981. Electric Power Research Institute. “Looking Ahead on EMF”. EPRI Journal, Oct / Nov 1990. Arthur R.Bergen.-Power System Analysis. Prentice-Hall, Englewood Cliffs, New Jersey, 1986. MICHIGAN STATE UNIV. LIBRARIES V - 3: l 1| NW1Wll"W”WIWHIWW1| » 31293007929221 ,