Electrical facilities produce power frequency electromagnetic fields. It is reported that these power frequency fields may disturb working of sensitive electronic equipment. Even a human’s biological system is reported to be sensitive to these non-ionizing Extremely Low Frequency Electromagnetic Fields (ELF EMF). Concerns for technical as well as health effects of ELF EMF around power frequency sources were evaluated in history and they are still continuing. Even after extensive efforts common and uniform opinion is not drawn technically or clinically. However, professional organisations, like EPRI, have started their efforts towards field management research. Several research programmes were initiated on long run to identify exact association of power frequency fields with instruments and humans. These programmes have the objective to examine and identify ways to reduce the level of exposure to electric and magnetic fields around transmission and distribution lines, substations and other electrical utilities. EPRI or WHO’s research programme provides common platform to all the efforts concentrated towards effects of ELF EMF. It avoids the duplication of work as well as helps to test and evaluate the most promising ideas in this concern so that they can be adopted technically. According to EPRI, ‘EMF Management’ is much more than simply reducing the fields. It may include a variety of activities like communication programs, EMF measurement and assessment, research programs, design of power system components with low EMFs, modification of power system to reduce EMF, tracking ground and net current flows, shielding the sources of EMF, shielding the areas occupied by people or sensitive equipment and reduction of exposure to EMF by selective use of space and time. This article explores all the issues in concern to power frequency ELF EMF management.
EMF measurements
EMFs are generated by generation plants, equipments, T&D lines, and almost all the appliances that work on electric power. Electric field is produced by voltage, whereas current is the source of magnetic field. Major sources of electrical fields are overhead high voltage transmission lines, high voltage equipments in generating stations, overhead distribution lines, special laboratory or industrial equipment and electrical appliances. Magnetic fields are produced by conductors and buses in generating stations and electrical substations, substation equipments, overhead and underground transmission lines, water lines, water mains when carrying ground currents of the power system, electrical wiring of residences, commercial and industrial buildings, electrical appliances and industrial equipment that contains motors or transformers or high current conductors.
The IEEE has set a detailed set up for measurement of electric and magnetic fields.
EMF calculation
In the last few decades, concern about the health effects of EMFs has increased rapidly worldwide. Therefore, the main purpose of calculating the electric and magnetic field sources, such as distribution lines, overhead transmission lines, transmission cables, substations, and residential as well as commercial and industrial sector is to minimise the impact of their dangerous effects.
The electric field can be calculated near the power line at 1 metre above from ground is essentially vertical along the route of the power line. On the other hand, magnetic fields may be calculated 1 metre above from ground is essentially having both vertical and horizontal components. The calculation shall be done at the location of maximum line sag and shall provide MF values at 1 metre above ground level. For more accurate result of magnetic fields from sets of conductors in 3D BIOT- SAVART law is used,
Mathematically it can represent as
Where,
B = magnetic field
dl = differential element length
ŕ = vector dl to the field point
I = current flows in the conductor
The electric field is determined by the following parameters, such as:
- Working voltage of equipment or line
- Diameter of conductors
- Number of conductors per bundle
- Distance between phases
- Distance of conductors from the ground etc.
Distribution line EMF MGT
Overhead distribution lines generate an electric field around the line that influences the human being and the nearby objects located at ground surfaces. Normally, distribution lines electric fields are so small that they do not produce any effects like spark discharge. But in case of magnetic field management or reducing the magnetic field produced by the distribution system created two special challenges. First one is associated with the large societal cost of an overall change in distribution system. The 2nd one is related to the different techniques that are required to reduce the magnetic field caused by phase and net current.
EMF MGT for transmission lines
Among all the sources of EMF, Overhead Transmission Line (OHTL) produced the greatest EMF. Nowadays, Extra High Voltage (EHV) is transmitted through OHTL. The installations and distribution of EHV transmission line produce the highest electric fields near the ground surface. OHTL produces Extremely Low Frequency (ELF) EMF, which requires reducing to minimum level. In the age of electrification, the evolution of EHV, people are to stay around ELF field sources, which may further be held responsible for biological effects of human as well as living beings. Electric and magnetic field management of overhead transmission lines required in different factors of line design and technique for reduction of EMF have low value. Electric field is reducing to extremely low values using grounded grids of densely spaced wires. And also it’s possible to significantly reduce the magnetic field of lines with voltage up to 115 kV or by using loops cancellation method. Underground cabling is the alternative solution to minimise the electric field.
EMF MGT for transmission cables
EMF is an issue that concerns overhead transmission lines. Concern about possible health effects due to EMF overhead transmission line is replaced by Underground Transmission Line (UGTL) as the solution of EMF problem. In UGTL, above the surface of the ground, the electric field is eliminated, but the magnetic field is not, because the soil around the underground cable has practically the same permeability as air. Thus, near the surface of the ground MF from underground transmission line is higher than that of overhead transmission line as shown below.
Fig 1: Magnetic field profiles for overhead and underground (Solid Dielectric Cables) transmission lines…
EMF management for substations
Substation is a key component of electrification of any region for smooth operation and reliability of transmission and distribution system. The substation having mainly major components as transmission line, step up/step down transformers, substation buses, sub transmission feeders, and generating circuits.
The EMF produced by substations may expand outside the substation perimeter, where residential and commercial activity takes place. Management of EMF near the substation is very complex issues. Sources of Electric Field (EF) in the substation are the substation buses and HV equipment. Electric field can be controlled at minimum value by design of buses and their geometry and also be reduced by increasing the space between ground and buses.
On the other hand, sources of magnetic field around the substation are substation buses, incoming and outgoing cables, capacitor banks used for reactive power etc. Management of MF around a substation has mainly two purposes: the first is associated with the field level inside the substation to ensure safety for workers, and second one is to minimise the MF outside the perimeters of the substation where residential and commercial settlements are found. This can also be reduced by modeling of conductors and buses.
Residential EMF management
EMF is also produced near the residential area due to the HV transmission line cross over in case of urban areas. People spent most of the time in the home, over the contact of EMF issues. Electric field in the home is relatively small and insignificant as compared to the magnetic field.
Sources of magnetic fields in residential areas are as follows:
- Grounding system of the residence
- Overhead power distribution line
- Electrical appliances
- Electrical wiring
- Overhead transmission lines
- Ground connection at electrical subpanels
Most of the magnetic fields generate in the residential area due to the electrical appliances such as electric heater, air-conditioning, computer etc. placed in the residential environment. It has been a concern because a number of epidemiological studies have indicated to some dangerous diseases – such as leukemia, breast cancer, miscarriage etc.
School, commercial and industrial EMF management
Electric and magnetic fields are not only produced in generation and transmission systems – but they are also produced in the distribution system. The main purpose of management of electric and magnetic fields in schools, colleges, commercial and industrial premises is lowering the exposure to EMF. Sources of MF in industrial areas are mainly the electrical machinery, and equipment that are used in electrical distribution and transmission. According to the survey of EPRI, magnetic field sources in school, commercial are given below.
Sources in school
- Net currents in electrical wiring
- Fluorescent lights
- Electrical typewriters
- Air conditioners and computers
- Aquariums etc.
Sources in office
- Net currents in electrical wiring
- Fluorescent lights
- Currents in cables
- Portable heaters
- Switches and relays in electrical distribution
- Fans, computers etc.
EMF values in the industrial areas are higher than the residential or commercial areas. Therefore, need for management of their EMFs is very important. Their harmful effects include breaking of the bones and tissues.
Conclusions
- Exact association of power frequency fields with instruments and humans is still being evaluated – and efforts are going on worldwide.
- Electric and magnetic fields around transmission and distribution lines, substations and other electrical utilities can be reduced by various technical approaches.
- Communication programmes, EMF measurement and assessment, research programmes, design of power system components with low EMFs, modification of power system to reduce EMF, tracking ground and net current flows, shielding the sources of EMF, shielding the areas occupied by people or sensitive equipment and reduction of exposure to EMF by selective use of space & time are the steps that are used for EMF management.
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