Most environmental stress is caused by weather and by the
surrounding environment, such as industry, sea, or dust in rural areas. The
environmental stresses affect both mechanical and electrical performance of the
line.
Temperature
The temperature in an outdoor station or line may fluctuate
between –50°C and +50°C, depending upon the climate. The temperature change has
no effect on the electrical performance of outdoor insulation. It is believed
that high temperatures may accelerate aging. Temperature fluctuation causes an
increase of mechanical stresses, however it is negligible when well-designed
insulators are used.
UV Radiation
UV radiation accelerates the aging of nonceramic composite
insulators, but has no effect on porcelain and glass insulators. Manufacturers
use fillers and modified chemical structures of the insulating material to
minimize the UV sensitivity.
Rain
Rain wets porcelain insulator surfaces and produces a thin
conducting layer most of the time. This reduces the flashover voltage of the
insulators. As an example, a 230-kV line may use an insulator string with 12
standard ball-and-socket-type insulators.
Dry flashover voltage of this string is 665 kV and the wet
flashover voltage is 502 kV. The percentage reduction is about 25%.
Nonceramic polymer insulators have a water-repellent
hydrophobic surface that reduces the effects of rain. As an example, with a
230-kV composite insulator, dry flashover voltage is 735 kV and wet flashover
voltage is 630 kV.
The percentage reduction is about 15%. The insulator’s wet
flashover voltage must be higher than the maximum temporary overvoltage.
Icing
In industrialized areas, conducting water may form ice due
to water-dissolved industrial pollution. An example is the ice formed from acid
rain water. Ice deposits form bridges across the gaps in an insulator string
that result in a solid surface.
When the sun melts the ice, a conducting water layer will
bridge the insulator and cause flashover at low voltages. Melting ice-caused
flashover has been reported in the Quebec and Montreal areas.
Pollution
Wind drives contaminant particles into insulators.
Insulators produce turbulence in airflow, which results in the deposition of
particles on their surfaces. The continuous depositing of the particles
increases the thickness of these deposits.
However, the natural cleaning effect of wind, which blows
loose particles away, limits the growth of deposits. Occasionally, rain washes
part of the pollution away. The continuous depositing and cleaning produces a
seasonal variation of the pollution on the insulator surfaces.
However, after a long time (months, years), the deposits are
stabilized and a thin layer of solid deposit will cover the insulator.
Because of the cleaning effects of rain, deposits are
lighter on the top of the insulators and heavier on the bottom. The development
of a continuous pollution layer is compounded by chemical changes.
As an example, in the vicinity of a cement factory, the
interaction between the cement and water produces a tough, very sticky layer.
Around highways, the wear of car tires produces a slick, tar-like carbon
deposit on the insulator’s surface.
Moisture, fog, and dew wet the pollution layer, dissolve the
salt, and produce a conducting layer, which in turn reduces the flashover
voltage. The pollution can reduce the flashover voltage of a standard insulator
string by about 20–25%.
Near the ocean, wind drives salt water onto insulator
surfaces, forming a conducting salt-water layer which reduces the flashover
voltage. The sun dries the pollution during the day and forms a white salt
layer. This layer is washed off even by light rain and produces a wide
fluctuation in pollution levels.
The Equivalent Salt Deposit Density (ESDD) describes the
level of contamination in an area. Equivalent Salt Deposit Density is measured
by periodically washing down the pollution from selected insulators using
distilled water.
The resistivity of the water is measured and the amount of
salt that produces the same resistivity is calculated. The obtained mg value of
salt is divided by the surface area of the insulator. This number is the ESDD.
The pollution severity of a site is described by the average ESDD value, which
is determined by several measurements.
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