Yüksek gerilim hatlarının elektromagnetik etkileri

Abstract

Evde, okulda, işyerinde, sanayide, kısaca yaşamın her alanında gün geçtikçe artan bir şekilde kullanılmakta olan elektrik enerjisinin yararlan yanında acaba zararlı etkileri söz konusu mu? En basitinden, evlerde yeri geldiğinde saatlerce televizyon karşısında oturulmakta, insanlar işleri gereği elektrik alanlarıyla sürekli olarak karşı karşıya kalmakta ve ülkemizde, özellikle büyük şehirlerimizde bazı yerleşim alanlarıyla enerji iletim hatları adeta içice geçmiş durumda bir görüntü sergilemektedir. Yoğun yerleşim merkezlerinin enerji gereksinimini karşılamak için zorunkı olarak uzak mesafelerden yüksek gerilim hatlarıyla enerji iletimi yapılmakta, böylece hatlar çevresinde yüksek elektrik ve magnetik alanlar oluşmaktadır. Nüfüsun büyük şehirlerde toplanma eğiliminde olması, şehir merkezlerinden çevreye doğru genişleyen bir görüntünün ortaya çıkmasına sebep olmakta, bu durum da, esas olarak şehir dışı için tasarlanmış yüksek gerilim hatlarının sanki şehir içindeymiş gibi görünmesine yol açmaktadır. Bu tezde, yüksek gerilim hatlarının elektromagnetik etkileri konusu ele alınmış, bir yandan bu konuyla ilgili bugüne kadar yapılmış araştırmalar taranarak bazılarından örnekler verilirken, bir yandan da hatlar çevresinde deneysel çalışma olarak yapılan elektromagnetik alan ölçümleri aktarılmış, ayrıca sayısal hesaplama örneği de sunulmuştur. Öncelikle metinlerin daha iyi anlaşılabilmesi için bazı temel kavramlar verilmiş, bu arada çok sayıda tıbbi ve biyolojik terimin bulunması nedeniyle bunlar da bir alt başlık altında açıklanmıştır. Üçüncü bölümde "Örnek Araştırmalar" adı altında bugüne kadar konu ile ilgili olarak yapılan geçmiş araştırmalardan örnekler ve sonuçlan sunulmuştur. Esas olarak bu konudaki bilgilerin çeşitliliği ve sonuçların değişikliği sebebiyle, örnek arastama sayısı fazla tutulmuş, sonuçlar arasındaki farklılıklara ve benzerliklere dikkat çekömeye çalışılmıştır. Dördüncü bölümde, laboratuvarda hat modeli çevresinde ve açık havada gerçek hava hattı çevresinde bizzat yapılan deneyler sunulmuş, beşinci bölümde ise sayısal hesap örneği verilmiştir. Son bölümde eldeki sonuçlar toplu olarak sunulmuş ve öneriler sıralanmıştır.

Electromagnetic fields with high frequencies and great intensities are today reckoned among the environment factors of human being, because they have a proven effect on living organisms. The problems arising from the influence of even low- frequency fields on the human organisms have been more vehemently discussed in the public during the last years, all the more as the demand and the use of electric energy has still an increasing tendency. The universal increase of the number of electric appliances and the increase in performance of the energy supplying installations has inevitably the consequence of greater intensity of the electric and magnetic stray fields. The electric energy for high voltages must be transmitted mainly by overhead lines, with the main interest being in three-phase current transmission systems. Temptation for high voltage system is based on economic and engineering motivations. As, according to the energy demand, they must be installed increasingly in densely populated areas, more and more people will stay close to overhead lines and work in the neighbourhood of energy plants. For example, a single 765-kV line can carry as much power as 30 138-kV lines at 1/10 the construction cost and with only 1/13 the amount of right-of-way land required per kilowatt. Electromagnetic fields produced by the 765 kV line are much more intense than those in the 138 kV line, and this begs the question about field effects on public health, such as subtle biologic effects. Some experiments indicated that even low level fields may influence the biochemical activity of some body tissue. With growing transmitted voltage the electric field strength underneath the conducting wires increases considerably. Underneath a 400 kV conduction a maximum field strength of 8 kV/m appears on the ground, and underneath a 1300 kV conduction even 17 kV/m must be taken into account. In the switchgears, of the corresponding levels of voltage, the operating personnel may be exposed at certain points to a higher field strength. Magnetic stray fields underneath overhead lines reach flow densities, relative to the conductor current, of 14 /xT/kA, with constant loading of the three-phase current systems. Natural magnetic fields appearing on the ground may have maximum values of about 0.5 nT in the a.c. field (with frequency below 100 Hz), and about 50 [İT in the d.c. field. In close vicinity to electric household appliances the appearing stray fields have 0.5 mT at most. The flow densities assessed in high-power installations, for example in the chemical industry, amount to about 0. 1 mT. Also in the recently developed magnetic suspension railways, living beings may come into vn contact with strong stray fields which are without shielding in the order of magnitude between 1 and 10 mT and in the frequency range between 0 and 100 Hz. Height of line = 15m Fig. 1. Field due to a 765 kV line. The experimental difficulties in the investigation of the effect of external fields arise primarily from the fact that it's unknown in which way the field can exert its influence on the organism in the body. Due to the high electric conductivity of the body of living organisms the external electric field is strongly distorted. Such a conducting medium distorts the original electric field so as to reduce it by enormous (typically, a hundred million at power line frequencies) factors inside the body. At the same time, the field external to the body can be increased by factors of 10 to 100 by the body itself acting as a conductor. The result is that only estimates of the internal fields are available, and these estimates vary from about one millionth to one hundred millionth of the original, unperturbed external electric field. Because of this enormous attenuation, the electric fields are reduced to levels that are negligibly small compared to the normal background electric fields in the body generated by thermal fluctuations. For a representative cell diameter of 20 microns (micrometers) having a resistivity typical of body tissue (about 2 flm), these unavoidable thermal fields amount to about 0.02 V/m at body temperature in an bandwidth of 100 Hz. Hence, electric fields of the order of a million V/m would be required outside the body to produce fields across a cell that are of the same order of magnitude as the unavoidable thermal fields. In contrast to external electric field situation, magnetic fields are unperturbed by the human body and penetrate without attenuation. This transparency of the body to magnetic field comes about because the body contains almost no magnetic material. Internal electric fields are generated within the body by these time-varying magnetic fields through a well-known mechanism called the Faraday effect. These induced electric fields are also very much smaller than the unavoidable thermal electric fields across the cell. vm WITHIN HOMES Away from aconances Next lo accuances Electric cıankets DISTRIBUTION SUdTRANSMISSION UNES Edge ot nçnı-o»*ay {" Wiinm rıçnı 5 kV/m. Research work conducted in other European countries, however, disputed the Soviet findings and indicated that other factors that exist in the switch yard may have contributed to complaints such as noise, oil fumes, and exposure to a variety of chemicals by the Soviet utility workers. 100x25 20 c CO Q. tn © E 15 10 x102 0 farmer's exposure to domestic sources E vo!t/m x100 Lab. rat - 150 days at P 65 kV/m ; I I I / / / / > farm worker Î annual ; 345-kV : line y Mx102 30400 B J nAmp/cma Fig. 3. Electric effects on farm workers and rats. A paper published in 1979 in the American Journal of Epidemiology, authored by Nancy Wertheimer and Ed Leeper, indicated that the number of children who died from cancer was around 24% higher because they lived in homes subjected to high current configuration, as opposed to the control group. However, when a group of researchers from Brown University attempted a similar study, they found no evidence to support the report by Wertheimer and Leeper. The methodology was analyzed which indicated that Wertheimer and Leeper failed to consider other potential causes of cancer such as pollution, genetics, and diet; that the selection of controls may have been biased; the researchers did not consider other environmental variables such as neighbourhood and traffic; and no adjustment was made for the effect of family cancer pattern. Another study that centered on research in cell biology using tissue cultures exposed to electromagnetic fields provided useful information on the effects of electromagnetic fields on living organisms. The primary interaction between electromagnetic fields and a biological cell appears to occur mainly at the cell membrane rather than inside the cell. The function of the membrane is the control of ion transport and its interaction with electromagnetic fields may disrupt that function. Some studies indicated that while noticeable effects on biological cells have been observed, no effect was found for the whole organism. Research was conducted on animals subjected to electromagnetic fields over a sustained period of time under a source of 30 kV/m whose surface effect on the head of female swine is the same as 10 kV/m at the head of a human. Currents induced internally were, of course, different - the swine felt the electromagnetic field effect while the human could not. 1000 a E 500-kV transmission line c o ?a 2 1 10 100 1000 Distance From Source (mettrs) 10.000 Fig. 4. Distribution of magnetic induction over distance. In recent years the general public has become increasingly aware of factors affecting the environment The impact of this awareness has been felt in many areas of the electric power industry. In high voltage transmission, recent research conducted to investigate biological effects of electric and magnetic fields is a result of this public concern and the concern of the utilities which serve the public. As the result it can be said that more extensive research based on definitive, sustained data is needed to arrive at a satisfactory and meaningful conclusion about the problem of electromagnetic field impact from HV power lines and the XI complementary researches about the biological effects of these fields seem to be necessary. 9. fc IQujj Ajisusq xny oijajbeft Ö 0. Fig. 5. Distribution of magnetic induction over time. In this thesis, the subject of the electromagnetic effects of high voltage transmission lines is considered and some examples are given. In the first chapter a general introduction is made, after that in the second chapter some basic concepts are presented. In the third chapter, it is tried to explain the main problem by giving nine examples. Then, the experimental studies which have been done in the laboratory and outside conditions are described. At the last, the charge simulation method which is a often used method to calculate the electric fields around the high voltage transmission xu lines is explained and an example is given. The conclusions and the recommendations are given in the sixth chapter.