Electromagnetic Fields Modeling Using GIS
ELF-EMF Emission Models
There are several methods which can be used to model the radiation emitted from conductors.
These methods are:
- Biot-Savart Law,
- Maxwell Equations,
- 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:
- calculation of the equivalent charges per unit length of conductor,
- 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, (x
i>y
i),maybe calculated,
the intensity of the electric field is calculated by means of Gauss’s theorem E = q / (2
per)
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, E
x 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:
- the conductors are parallel to the earth plane,
- the earth currents are negligible,
- the earth has a relative permeability of 1.0.
The magnetic field vector of a point P due to a current, I
i, 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, H
iX and H
i, respectively.
The magnitude of the magnetic field is given by
and the magnetic flux density is given by
B=
mH, where
m=4
px10
-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.
