15-06-2012, 05:24 PM
transmission of electrical power by high voltage A.C overhead lines
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INTRODUCTION:
The transmission of electrical power by high voltage A.C overhead lines is well established and some of the associated problems such as noise, interference with radio and television transmission, and the danger of flashover are well controlled. Reports of the occurrence of subjective complaints such as increased headache, lassitude, nausea and loss of libido amongst Russian substation workers were published in 1966 by Asanova & Rakov, and were followed by other reports from the USSR (Sazonova 1967, Korobkova et al. 1972). It was suggested that these symptoms were due to occupational exposure to electric fields in 500 kV and 765 kV substations. Some of the Soviet reports suggested that the workers experienced frequent small shocks and that these accounted for the symptoms. Despite the reassurance given in reports by Western European and North American investigators (Kouwenhoven et al. 1967, Strumza 1970, Michaelson 1979) the suspicion remains in some quarters that currents or changes induced by high electric fields, even though they are imperceptible, can be damaging to the health of exposed persons. In this paper reference will be made to previous reviews of the subject, concentrating particularly on those aspects which refer essentially to studies on humans.
Because of the widespread use of electricity in the modern domestic and industrial environment, any or all reports purporting to demonstrate that electric fields from power lines cause or aggravate ill health must be given serious consideration and be critically assessed. For this reason two recent reports of non-occupational exposure merit attention, namely that by Wertheimer & Leeper (1979) in which it was suggested that exposure to electromagnetic fields, as assessed by levels of domestic electric current concentration, was related to an increase in childhood cancer. The other by Perry et al. (1981) claimed to demonstrate an association between high voltage power lines and an increased incidence of suicide.
In addition to these reports, studies are under way in Sweden to investigate whether electric fields either in vivo or in vitro can cause chromosomal abnormalities in exposed persons or their off spring. The rationale for such an investigation and the study design itself are opens to question and will be discussed.
Since last few decades electricity has become a vital part of our life and it is proving to be an integral part of modern life. The electricity system produces extremely low frequency electromagnetic field which comes under Non ionic radiations which can cause health effects. Apart from human effect, the electrostatic coupling & electromagnetic interference of high voltage transmission lines have impact on plants and telecommunication equipments mainly operating in frequency range below UHF.
Exposure of humans to electromagnetic fields from high voltage transmission lines is a great concern about humans since transmission lines are found everywhere in our environment. Each country has its own standard for exposure to EMFs based on guidelines set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The biological effect of these exposures is a subject of great interest to researchers. Some possibilities of leukemia have been reported along with other health effects such as reproduction systems variation in blood sugar contents, neuronal disorders such as alzheimer, depression etc. the main reason of all these effects is alteration of Melatonin Secretation as a result of EMF exposure. Melatonin Secretion is involved in aging process, sleep orders & disorders, breast carcinogenesis, tumor genesis. With growing population the human is approaching more towards transmission line, the distant transmission lines are now in close proximity to humans. he result studies related to proximity effects have taken two opposite ends resulting in ambiguity. Some conclusions had shown no harmful effects on humans whereas the recent one are agreeing on the bad effects on manmade electrical sources like HV transmission lines. Some scientist have identified & studied the workplaces of possible risks& some had recommended to study other appliances along with transmission lines such as monitors, televisions, hairdryer copying machines etc. An extensive literature survey was done on the above topic & authors observed a gap in relevant literature especially in India on hot line workers. The aim of proposed research is to make aware the government, government organizations & people coming in close proximity of TLS about possible health risks & ways to minimize the effects (if any) in near future.
With the population growth around the world, towns are expanding many buildings become near high voltage overhead power transmission lines and many will be there very soon. This matter increases the investigations about the health effects of electromagnetic fields near the transmission lines. For example, exposure to electromagnetic fields (EMFs) at power frequency (50/60 Hz) may increase the risk of many diseases [1, 2], due to the currents induced in the human tissues, and the strength of these induced currents depends on the intensity of the outside magnetic field.
It was the study of complaints by Russian substation workers in the mid 1960s which triggered off the worldwide interest in high voltage field effects (Asanova & Rakov 1966). The symptoms complained of were nonspecific and included headache, fatigue and general gastrointestinal disturbances. These authors state that they gained the impression that changes in the cardiovascular system were encountered more frequently and were more marked in persons systematically subjected to electric fields (e.g. maintenance personnel), than those exposed sporadically (e.g. signalmen, substation attendants).
There is no detailed account of the incidence of disorders in relation to the duration of work in electric fields, nor is the incidence of similar symptoms in unexposed workers discussed. No objective basis is given for asserting that the observed disorders were in any way related to electric fields (Michaelson 1979).
HISTORY:
Extremely high voltages in EHV lines cause electrostatic effects, where as short circuit currents & line loading currents are responsible for electromagnetic effects. The effect of these electrostatic fields is seen prominent with living things like humans, plants, animals along with vehicles, fences & buried pipes under & close to these lines. The following section will explain the electrostatic effects on
Human beings:
When a person who is isolated from ground by some insulating material comes in close proximity to an overhead transmission line, an electrostatic field is set in the body of human being, having a resistance of about 2000 ohms. When the same person touches a grounded object, it will discharge through his body causing a large amount of discharge current to flow through the body. Discharge currents from 50-60 Hz electromagnetic fields are weaker than natural currents in the body, such as those from the electrical activity of the brain and heart. For human beings the limit for undisturbed field is 15 kV/m, R.M.S., to experience possible shock. When designing a transmission lines this limit is not crossed, in addition to this proper care has been taken in order to keep minimum clearance between transmission lines.
A number of papers have been published to study the effects of high electrostatic field on human beings, as number of cases have already been registered indicating the effect of electrostatic field from transmission lines on human beings.
Animals:
Many researchers are studying the effect of electrostatic field on animals. In order to do so they keeps the cages of animals under high electro static field of about 30 kV/m. The results of these experiments are shocking as animals (are kept below high electrostatic field their body acquires a charge & when they try to drink water, a spark usually jumps from their nose to the grounded pipe) like hens are unable to pick up grain because of chattering of their beaks which also affects their growth.
Plant Life:
Plants which are in proximity of electrostatic field due to transmission lines suffer from various types of damage depending upon the field strength. The main damage due to electrostatic field is it prevents the full growth of the plant and grain.
Vehicles:
When a vehicle is parked under high voltage transmission line an electrostatic field is developed in it. When a person who is grounded touches it a discharge current flows through the human being. In order to avoid this parking lots are located below the transmission lines the recommended clearance is 17 m for 345 kV and 20 m for 400 kV lines.
Others:
For fences, buried cables, and pipe lines proper care has been taken to prevent them from charging due to electrostatic field. When using pipelines which are more than 3 km in length & 15 cm in diameter they must be buried at least 30 laterally from the line center.
Effect of Power-Frequency Magnetic fields on human health:
The magnetic field emanating from a electronic system has influence on tissues in the human body. These influences may be beneficial or harmful depending upon its nature, that they may be used for MRI, Citiscan etc. or may cause several types of cancers like: leukemia or blood cancer, lymphoma which weakens the immune system of body etc. It has been found that the magnetic radiation emanating from Video Display Terminals may cause skin rash, tissue cancer, eye problems and in many situations cause abortions in pregnant women’s. Scientists around the world are now working on studying the health effects associated with magnetic field emanating from high voltage transmission lines.
It is believed that, the magnetic field might induce a voltage in the tissue of human body which causes a current to flow through it due to its conductivity of around 0.1 to 0.2 Siemens/meter. This is not the case for certain workers as the cell walls are made from protein acting as insulation barriers to the flow of current. In order to limit the magnetic field in homes or in occupations such as line workers, organizations like WHO, IRCA have given guidelines. In 1966, two Russian Scientists published their report on electricians working with electrical distribution lines (both males & females) experience breast cancer. They suspected that it may be due to magnetic field. Based on their report the Russian government recommended the following guidelines to limit the exposure of workers to the magnetic field.