T & D - 27) Environmental Stresses On External Insulation

Environmental Stresses On External Insulation (on photo: Workers on transmission tower 1953. Two workers standing without the use of safety lines atop a transmission tower in the R line during its construction)

Introduction To External Insulation

The external insulation (transmission line or substation) is exposed to electrical,mechanical, and environmental stresses. The applied voltage of an operating power system produces electrical stresses. The weather and the surroundings (industry, rural dust, oceans, etc.) produce additional environmental stresses.

The conductor weight, wind, and ice can generate mechanical stresses.

The insulators must withstand these stresses for long periods of time. It is anticipated that a line or substation will operate for more than 20–30 years without changing the insulators.

However, regular maintenance is needed to minimize the number of faults per year. A typical number of insulation failure-caused faults is 0.5–10 per year, per 100 mi of line.

Environmental Stresses

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.

1. 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.

2. 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.

3. Rain

Beads of rainwater on the silicone rubber surface of a high voltage insulator

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.

4. Icing

A power employee hammers at the ice covering a pylon, insulators and conductors

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.

5. 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.

Insulator pollution

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.

Table 1 shows the criteria for defining site severity.

The contamination level is light or very light in most parts of the U.S. and Canada. Only the seashores and heavily industrialized regions experience heavy pollution.

Table 1 – Site Severity (IEEE Definitions)

Description	ESDD (mg/cm2)
Very light	0–0.03
Light	0.03–0.06
Moderate	0.06–0.1
Heavy	<0.1

Table 2 – Typical Sources of Pollution

Pollution Type	Source of Pollutant	Deposit Characteristics	Area
Rural areas	Soil dust	High resitivity layer, effective rain washing	Large areas
Desert	Sand	Low resistivity	Large areas
Coastal area	Sea salt	Very low resistivity, easily washed by rain	10–20 km from the sea
Industrial	Steel mill, coke plants, chemical plants, generating stations, quarries	High conductivity, extremely difficult to remove, insoluble	Localized to the plant area
Mixed	Industry, highway, desert	Very adhesive, medium resistivity	Localized to the plant area

Typically, the pollution level is very high in Florida and on the southern coast of California. Heavy industrial pollution occurs in the industrialized areas and near large highways.

Table 2 gives a summary of the different sources of pollution. The flashover voltage of polluted insulators has been measured in laboratories. The correlation between the laboratory results and field experience is weak. The test results provide guidance, but insulators are selected using practical experience.

6. Altitude

A closeup shows the damage caused by the heat of the flashover, it actually shattered the ceramic all around the edges of the insulator:

The insulator’s flashover voltage is reduced as altitude increases. Above 1500 feet, an increase in the number of insulators should be considered. A practical rule is a 3% increase of clearance or insulator strings’ length per 1000 ft as the elevation increases.

Maintenance Of The Insulators

In zones where there is pollution, besides a good election of the insulator, is advisable to have a maintenance plan.

In other words, we need to wash or clean the insulator.

Cant see this video? Click here to watch it on Youtube.

This is more important in areas with severe environments of pollution or low rain probability, being necessary the elimination of the pollutant layer placed on the insulator.

This maintenance can be carried out with the system energized, wash in hot, orde-energized. The later method is used when cannot be applied another method by technical reasons or when the adhesive characteristics of the pollutant, require the use of wash with chemical solutions to recover the insulation level.

Many times, the wash is carried out by hand. In general the most employed methods are: the wash by water to high, average or low pressure, with dry air compressed or with spurts of abrasive materials and more recently the use of ultrasonic.

Any of the techniques used has to guarantee that the insulator will not suffer damage, neither that we are going to get worse the present situation.

The wash with spurts of water is the most effective and economic method, if the contaminant is dust, salt or land, or if these pollutants are not much adhered to the surface.

If the contaminant element has a high adhesion, (for example the cement or pollutant originating from chemical businesses or by-products of the petroleum) we have to wash the insulator with abrasive elements. They can be smooth elements, as shattered shell of cobs of corn or shells of nut, fine dust of lime, or more abrasive elements as the fine sand.

Always the opinion of the manufacturer will be kept in mind for not damaging the surface of the insulator.

To prevent the flashover during the wash, the following observations have to be considered:

1. The wash of the insulator will begin from lowest phase conductive.
2. When we wash, the water should not fall directly on a dirty insulator.
3. We will begin to wash from the lower part of the insulator until finishing in the upper part.
4. Is very important to keep in mind the direction of the wind.