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1. 2. 3.

Pollution Evaluation 3.1.

Pollution types

The in-service withstand voltage of insulator strings can be influenced by different types of pollution : active pollution and inert pollution. Active pollution This pollution is directly at the origin of leakage currents and resulting phenomenas (arcing activity, flashover) which lead to the reduction of the insulator string whithstand voltage. Active pollution can itself be classified in two types : - conductive pollution : metallic deposits, bird droppings, acid rain, salt fog, - pollution that needs to dissolve : salt deposit near the sea, salt contained in desert sand, gypsum coming from the soil or quarries, cement, fly ash, chemical pollution due to industrial activity or using of fertilizers and treatments in agriculture ... As for conductive pollution, the global conductance of the pollution layer is the principal element in the severity level. But in the case of soluble salts, the global conductance depends on the amount of pollution in a dissolved state and therefore on the amount of water spread on the insulator surface. Two salt characteristics, the solubility and the dissolving rapidity, are important (see table 1, the classification of salts according to their solution properties). For example, more a pollution is soluble and fast dissolving, less the pollution layer needs water (rain, fog...) and time to form a highly conductive layer and eventually time to lead to the flashover. In the other hand, this type of pollution is generally easily purged. For a same severity level, the insulator string withstand voltage can then depend on salt properties and on the wetting process characteristics. In Table 2, active pollution is characterized at means of the Equivalent Salt Deposit Density (ESDD) value. For soluble pollution, these value is given for a completly dissolved state. Low dissolving salts Fast dissolving salts Slow dissolving salts

MgSO4, Na2SO4, CaSO4 Table 1

High solubility salts MgCl2, NaCl NaNO3, Ca(NO3)2, ZnCl2

Inert pollution This type of pollution is not conductive but can indirectly influence the withstand voltage of an insulator string. If the material constituting inert pollution is hydrophilic, as for example kaolin and tonoko used in artificial pollution tests, water did’not stay in the shape of droplets but forms a film. In addition, a thicker water film is retained on the insulator surface. During wetting periods, more soluble salts are dissolved in a continuous film of solution and therefore the global conductance is higher. Some laboratory tests show that for a same amount of active pollution, an insulator string withstand voltage can decrease of 50%, when the inert pollution amount varies from 0.1 mg/cm2 to 30 mg/cm2. 1

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In table 2, inert pollution is characterized at means of the Non Soluble Deposit Density (NSDD) value.

3.2.

Evaluation of pollution severity

The application of this guide is directly related to the knowledge of the pollution severity of the site where the insulators are to be installed. The evaluation of the pollution severity can be made with an increasing degree of confidence : - qualitativeley from indications given in Table 2 (2nd column), - from information on the behavior of insulators from lines and substations already in service on that site (see Annex B), - from measurements in situ. For measurements in situ, differents methods are generally used. They are : 1) volume conductivity for the pollutant collected by means of directional gauges ; 2) Equivalent Salt Deposit Density (ESDD) and Non Soluble Deposit Density (NSDD) on the insulator surface (see annexe A) ; 3) total number of flashovers of insulators strings of various lengths ; 4) surface conductance of sample insulators ; 5) leakage current of insulators subjected to service voltage (highest current values during subsequent time intervals, Ih. The first two methods do not require expensive equipment and can be easily performed. The volume conductivity method gives no direct information by itself on the frequency and on the severity of the contamination events on a natural site. The ESDD/NSDD method characterizes the pollution severity of the site. Information on wetting shall be separatly obtained. The accuracy of these methods depends upon the frequency of measurement. However, for the ESDD method, an automatic measuring system has been developed (NSDD is not measured) and therefore pollution severity can be measured continuously, in order to find, for example, the suitable timing of washing. The method based on total flashovers need expensive test facilities. Reliable information can be obtained only for insulators having a length close to the actual length and flashing over at a voltage near the operating voltage. The last two methods which need a power source and special recording equipment have the advantage that the effects of pollution are continuously monitored. These techniques have been developed for assessing the pollution rate and the results , when related to test data, are used to indicate that the pollution is still at a level known to be safe for operationnal service or whether washing or re-greasing is required.

Note. - For more information on these different methods see « Electra « N0. 64 - May 1979 and CIGRE WG 33-04 TF-01 Review ? ? ?

3.3.

Pollution severity levels

For the purposes of standardization, four levels of pollution are qualitatively defined, from light pollution to very heavy pollution.

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Table 2 gives, for each level of pollution, an approximate description of some typical corresponding environments (2nd colum). Other extreme environmental conditions exists which merit further consideration, e.g. snow and ice in heavy pollution, heavy rain, arid areas. In addition, the last column gives typical ESDD/NSDD values for standard cap and pin insulators. These values are deduced from results of some artificial pollution tests (performed according to solid layer methods described in IEC publication 507) and from field measurements. Some insulator characteristics, for example profile, have an important influence on the pollution quantity deposed on insulators themselves. Therefore, these typical values are only available for standard glass or ceramic cap and pin insulators. For field measurements and in order to determine a pollution severity level of a site, the ESDD/NSDD values are maximum values obtained during a measurement period. The accuracy of the obtained pollution severity level value depends on the duration of the measurement period (to be sure to take exceptional pollution events into account) and on the frequency of measurments (particularly for fast dissolving and low occurrence pollution type)

Pollution level

Examples of typical environments

ESDD/NSDD mg/cm2

- Areas without industries and with low density of houses equipped with heating plants I - Light - Areas with low density of industries or houses but subjected to frequent 0.03< ESDD < 0.06 winds and/or rainfall and - Agricultural areas 1) NSDD < 0.3 - Mountainous areas All these areas shall be situated at least 10 km to 20 km from the sea and shall not be exposed to winds directly from the sea 2) - Areas with industries not producing particularly polluting smoke and/or 0.1 < ESDD < 0.2 with average density of houses equipped with heating plants and NSDD < 0.3 II - Medium - Areas with high density of houses and/or industries but subjected to frequent winds and/or rainfall 0.03< ESDD < 0.06 - Areas exposed to wind from the sea but not too close to the coast (at and least several kilometers distant) 2) 0.3 < NSDD < 3 - Areas with high density of industries and suburbs of large cities with 0.3< ESDD < 0.6 high density of heating plants producing pollution and NSDD < 0.3 III - Heavy - Areas close to the sea or in any case exposed to relatively strong winds 0.1< ESDD < 0.2 from the sea 2) and 0.3 < NSDD < 3 0.03< ESDD