Electric substations produce electric
and magnetic fields. In a substation, the strongest fields around the
perimeter fence come from the transmission and distribution lines
entering and leaving the substation.
The strength of fields from equipment
inside the fence decreases rapidly with distance, reaching very low
levels at relatively short distances beyond substation fences. In
response to the public concerns with respect to EMF levels, whether
perceived or real, and to governmental regulations, the substation
designer may consider design measures to lower EMF levels or public
exposure to fields while maintaining safe and reliable electric
service.
Electric and Magnetic Field Sources
in a Substation
Typical sources of electric and
magnetic fields in substations include the following:
1. Transmission and distribution lines
entering and exiting the substation
2. Buswork
3. Transformers
4. Air core reactors
5. Switchgear and cabling
6. Line traps
7. Circuit breakers
8. Ground grid
9. Capacitors
10. Battery chargers
11. Computers
Electric Fields
Electric fields are present whenever
voltage exists on a conductor. Electric fields are not dependent on
the current. The magnitude of the electric field is a function of the
operating voltage and decreases with the square of the distance from
the source. The strength of an electric field is measured in volts
per meter.
The most common unit for this
application is kilovolts per meter. The electric field can be easily
shielded (the strength can be reduced) by any conducting surface such
as trees, fences, walls, buildings, and most structures. In
substations, the electric field is extremely variable due to the
screening effect provided by the presence of the grounded steel
structures used for electric bus and equipment support.
Although the level of the electric
fields could reach magnitudes of approximately 13 kV/m in the
immediate vicinity of high-voltage apparatus, such as near 500-kV
circuit breakers, the level of the electric field decreases
significantly toward the fence line. At the fence line, which is at
least 6.4 m (21 ft) from the nearest live 500-kV conductor (see the
NESC), the level of the electric field approaches zero kV/m. If the
incoming or outgoing lines are underground, the level of the electric
field at the point of crossing the fence is negligible.
Magnetic Fields
Magnetic fields are present whenever
current flows in a conductor, and are not voltage dependent. The
level of these fields also decreases with distance from the source
but these fields are not easily shielded. Unlike electric fields,
conducting materials such as the earth, or most metals, have little
shielding effect on magnetic fields. Magnetic fields are measured in
Webers per square meter (Tesla) or Maxwells per square centimeter
(Gauss). One Gauss = 10^–4 Tesla. The most common unit for this
application is milliGauss (10^–3 Gauss).
Various factors affect the levels of
the fields, including the following:
1. Current magnitude
2. Phase spacing
3. Bus height
4. Phase configurations
5. Distance from the source
6. Phase unbalance (magnitude and
angle)
Magnetic fields decrease with
increasing distance (r) from the source. The rate is an inverse
function and is dependent on the type of source. For point sources
such as motors and reactors, the function is 1/ r^2; and for
single-phase sources such as neutral or ground conductors the
function is 1/r.
Besides distance, conductor spacing and
phase balance have the largest effect on the magnetic field level
because they control the rate at which the field changes. Magnetic
fields can sometimes be shielded by specially engineered enclosures.
The application of these shielding techniques in a power system
environment is minimal because of the substantial costs involved and
the difficulty of obtaining practical designs.
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