Effect of various aspects of the insulator on its pollution accumulation

2.3.6 Critical wetting conditions

There is a critical amount of water required on the insulator surface to produce the minimum flashover voltage. The most severe conditions require sufficient quantity and time-duration of wetting to dissolve the majority of the conductive contaminants without removing them from the insulator surface⁴.

Depending on the types of pollutant present and the insulator characteristics, the critical wetting conditions most commonly occur during fog, dew or light misty rain.

2.3.7 Effect of various aspects of the insulator on its pollution accumulation

2.3.7.1 Profile

The contamination-collection processes on insulators in-service are very complex. Observations of pollution distribution on insulators installed in a desert area are illustrated in Figure 2-23 ⁷⁷. In such areas, it has been found that the insulators having an aerodynamic profile are less contaminated than are those with a more convoluted profile. However, this is not always the case - as Figure 2-24 shows. Some field observations have shown the opposite situation⁸⁰⁷⁸⁴⁰. Further, such differences are not unique to desert environments ⁷⁹.

A mechanism whereby antifog insulators collect less pollution than do aerodynamically shaped insulators in certain desert areas may be as follows. Near the coast, where the humidity during the night is generally high, the insulators may be wetted so that the bond between its surface and pollution is increased. Due to the relatively larger exposed surface of the aerodynamic insulator - which allows it to cool more effectively than that of other insulator types, this insulator will be wetted more than the antifog insulator with a more convoluted surface. Hence, the aerodynamic insulators may then collect more pollution. Another factor that may play a role is the area of the exposed top surface. This is especially so in regions where pollution fallout may be considerable. Also, there is the difference in the cleaning by the wind of the pollution particles for the different profiles⁸⁰.

Figure 2-23: Distribution of salt on the surfaces of insulators of two greatly different profiles after field exposure in a desert area⁷⁷.

In areas with regular monthly precipitation, insulators with an aerodynamic profile are less contaminated in both the short-term (monthly) and the long-short-term (a year or more) exposure ³⁸. Some areas receive rain only for a few months while the rest of the year is very dry. In such areas, aerodynamic sheds may collect less contamination during the dry months than do those with more complex profiles. After the rainy months, aerodynamic sheds are certainly less contaminated than are those of the convoluted-shed design⁸¹. If maintenance is performed, an open profile is much easier to handle than a profile with a convoluted underside. The top/bottom ratio of the pollution on the insulator sheds can be different in different areas and for different times of the year. Sometimes, the bottom surface of a shed is more polluted than the top surface and sometimes the opposite occurs⁸¹.

20 40 60 80 100 120 140

Ghazlan Yanbu Dhahran Riyadh Tabouk Arar Bisha Abha

ESDD measured (as a % of that collected on a standard profile)

Figure 2-24: A comparison of the amount of pollution collected on different shapes of insulator at eight desert-pollution stations ⁸⁰.

2.3.7.2 Orientation

Results obtained in Mexico - in 23 insulator testing stations installed under various climatic and pollution conditions - have provided correction factors for chemical composition and uneven distribution of salts for different regions. Also, long-term patterns of pollution-accumulation show that cap-and-pin insulator strings with an inclined orientation tend to collect less contaminants than do vertically mounted ones - the ratio being 0.9. Horizontally installed insulators collect even less - the ratio being 0.15. However, orientation effects vary depending on the region (rural, marine, industrial or a combination of them)⁸².

Tension insulators may also be subject to a direction effect if the major source of contamination is from a well-defined source

125. In this case, there can be an influence of orientation and direction in determining the insulator performance under natural pollution for a particular location or type of location. For other locations where contamination can accumulate rapidly, or the frequency of natural cleaning by rainfall is very low, the influence of orientation may be significantly altered - from that stated above - for the same insulator type.

2.3.7.3 Diameter

Field experience indicates that - for cylindrical insulators - the larger the diameter of an insulator, the smaller the ESDD level it accumulates over a given time as compared to that on the bottom surface of a 250 mm suspension insulator⁸³. The results of the measurement of ESDD on a series of cylindrical insulators with different diameters, which were exposed - under de-energised conditions - to typical coastal contamination, are shown in Figure 2-25 ⁸⁵.

The relationship between the level of relative ESDD and the average diameter, D, of the insulator was found to be:

55 .

9

. 13 •

= D

ESDD

where ESDD_r = 1 for the cylindrical insulator with an average diameter of 115 mm. However, it has been recommended by Ozaki et al⁸⁴ that, for design purposes, it would be more appropriate to use a more conservative relationship - such as:

ESDD

= 2 6 . • D

Note: this latter function takes into account the rather large scatter of the measured values.

Ratio of ESDD (1.0 at D = 115mm)

1/2

1

ESDD = 13.9D^-0.55 ESDD = 0.5+ 6.9D^-0.55 = 2.6D^-0.21 2.0

1.0

0.5

0.3

0.2 0.7

115 200 400 700 1000

Average Diameter, D, mm

Figure 2-25: Relationship between the diameter of a porcelain insulator and the contaminant-deposit density under de-energised and natural service conditions⁸⁵.

2.3.7.4 Material

Another factor that influences the pollution deposit on insulators is the housing material. Figure 2-26, which is based on that reported by Imagawa et al ⁸⁶, shows comparative ESDD measurements taken on silicone rubber and porcelain insulators at both inland and coastal sites. These results indicate that silicone rubber insulators tend to accumulate more pollution than do the porcelain ones. Measurements performed in Tunisia⁶¹ have indicated that this trend is also true for desert-type environments.

0,0001 0,001 0,01 0,1

0,0001 0,001 0,01 0,1

ESDD (Porcelain), mg/cm²

ESDD (Polymer) mg/cm2

Site "A" - After Typhoon Site "B" - 3 month Site "B" - 1 year Site "C" - 3 month Site "C" - 6 month Site "D" - 3 month

Figure 2-26: Comparison of ESDD for porcelain and polymer insulators at 4 different sites⁸⁶.

2.3.7.5 d.c. Energisation

There are differences in the contamination accumulation between energised insulators (under d.c. voltage) and un-energised insulators because of the effect of the electric field. The amount of pollution collected is a function of the magnitude of the applied d.c. voltage, as well as that of the electric stress at the point of measurement⁸⁷⁸⁸. More information about pollution accumulation under d.c. voltage is included in Section 7.4.2

2.3.7.6 Conclusion

All of the aforementioned effects culminate in the build-up of contaminants on the insulator surface. In particular, it is dependent on the product of pollution deposit-rate and the time interval between the washing events. An equilibrium condition may take some years to occur between the deposit-rate and insulator cleaning-rate. This is illustrated in Figure 2-27.

Figure 2-27: Schematic history of polluted ceramic insulator ⁵⁴.