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where airborne contaminants may accumulate on insulators.

8.8.1 Contamination Flashover Mechanism: When a layer of contaminants on an insulator is moistened by fog, dew, light rain or snow, it will become more conductive and the leakage current along the surface of the insulator will greatly increase. Where the current density is the greatest (for suspension insulators near the pin, and for post insulators at the points of least diameter), heat caused by the increased leakage current will evaporate the moisture causing the formation of a dry band. This band usually has an higher resistance than the adjacent moistened area which means that the band will support almost all the voltage across it. This will result in the breakdown of the air and the formation of an arc across the dry band. The arc will cause the moisture film at the dry band edges to dry out, enlarging the dry band, eventually to the point where the voltage across the band is just below the air breakdown value. If an increase in precipitation occurs causing a lowering of contaminant resistance, a second breakdown can occur. If conditions are right, a cycle of repeated and ever-increasing surges will be set up which

will result in several discharges joining, elongating and bridging the entire insulator and resulting in a power arc. See Figure 8-5 for a graphic description.

FIGURE 8-5: CONTAMINATION BREAKDOWN PROCESS OF A SINGLE PORCELAIN INSULATOR UNIT

8.8.2 Effect of Insulator Orientation: The orientation of insulators has an effect on

contamination performance. Vertical strings of suspension insulators or vertical post insulators do not wash well in the rain because of the sheltering effects of the insulator skirts.

Contaminants will tend to remain on the underside of the insulator which is not immune from the moistening effects of fog or wind blown rain and snow. Horizontally oriented suspension

insulators and post insulators have their undersides more thoroughly washed by the rain and therefore tend to fare better than vertical insulators in contaminated areas. Another advantage of insulators in nonvertical positions is that any ionized gases caused by arcing will not contribute to setting up conditions where an arc could jump from one bell to another or along the skirts of a vertical post.

M O IS T U R E L A Y E R F O R M S O N C O N D U C T IN G M A T E R IA L

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IN IT IA L C O N D U C T IN G S T A T E

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L E A K A G E C U R R E N T D R IE S O U T M O IS T U R E N E A R P IN

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A R C B R ID G E S O V E R D R Y A R E A H E A T IN G A N D E N L A R G IN G IT

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E N L A R G E D D R Y A R E A H O L D S E N T IR E U N IT V O L T A G E A N D A R C E X T IN G U IS H E S

A R C R E S T R IK E S A S M O R E M O IS T U R E A P P E A R S O N D R Y A R E A

(E )

A R C B R ID G E S E N T IR E IN S U L A T O R

(F ) D R Y A R E A

D R Y A R E A

C O N T A M IN A T IO N L A Y E R

A R C A R C

8.8.3 Designing for Adverse Contamination Conditions: There are several means available for improving line insulation performance in a contaminated atmosphere.

One way to compensate for contaminated conditions is to increase the leakage distance of the insulation. The leakage distance is the distance along the surface of the insulators from the top of the string (or post) to the energized hardware, not including any metal such as insulator caps and pins.

Table 8-4 gives recommended leakage distances for various levels of contamination. The increased leakage distance can be obtained by adding additional standard insulator bells (using a longer post insulator) or by using fog insulators, which have more leakage distance for the same overall insulator length. The additional leakage distance on fog insulators is obtained by having more and/or deeper skirts on the underside of the insulator bell. In addition to the leakage distance, the shape of the insulator has an effect on contamination performance, especially when fog units are being used.

Research into the performance of existing lines with similar contamination should play an important part in the final determination of insulating for atmospheric contamination.

An alternative to increasing the total leakage distance of the insulator string is to use a resistance graded insulators. These insulators have a glaze that permits a small but steady leakage current to flow over their surface. This leakage current gives the insulator much better contamination performance without having to increase leakage distance. The base of a resistance graded insulator should be solidly bonded to the structure ground wire to permit the leakage current to flow easily to the ground. To aid in determining whether to use this type of insulator, its advantages and disadvantages are listed below.

Advantages and Disadvantages of Resistance Graded Insulators Advantages Disadvantages

• No extra leakage distance required. • Higher initial costs.

• Longer intervals between insulator washings.

• Small but continuous power loss.

• No radio noise (due to a more uniform voltage distribution across string).

• Not entirely successful in very heavily contaminated areas.

TABLE 8-4

SUGGESTED LEAKAGE DISTANCES FOR CONTAMINATED AREAS

Contaminate

Level Environment

Equivalent Amount NaCl

mg/cm2

Suggested Leakage Distance rms L-G*

in/kV Very Light Areas without industries and with low

density of houses equipped with heating plants. Areas with some density of industries or houses but subject to frequent winds and/or rainfall. Areas not exposed to sea winds.

0-.03 NA-1.0

Light Areas with industries not producing particularly polluting smoke and/or areas with average density of houses equipped with heating plants. Areas with high density of houses and/or rainfall. Areas exposed to winds from the sea but not less than 10 miles from the coast

.03-.06 1.0-1.25

Moderate Areas with high density of industries and suburbs of large cities with high density of heating plants producing pollution. Areas close to the sea or in any case exposed to relatively strong winds from the sea (within 10 miles of the sea).

.06-.1 1.5-1.75

Heavy Areas subjected to industrial smoke producing particularly thick conductive deposits. Areas with very strong and polluting winds from the sea. Desert areas, characterized by no rain for long periods, exposed to strong winds carrying sand and salt, and subjected to regular condensation

.1-.25 2.0-2.5

*rms L-G is root mean square line to ground voltage

Washing of the insulators should not be used in place of properly designing for contamination but rather should be used in addition to the other steps where it is felt to be necessary.

Insulator performance in a contaminated environment can be improved by coating the surface with suitable silicone grease. The grease absorbs the contamination and repels water. It is necessary, however, to remove and replace the grease at intervals determined by the degree of contamination. As with washing, the use of grease should only be considered as a remedial step.

Resistance graded insulators should not be greased.