With the rising electricity demand and increase in the bulk transfer of power, it has become imperative for transcos to improve the efficiency of networks, including lines and substations. System efficiency is measured mainly in terms of the continuity of service and the avoidance of faults that cause economical losses. Flashovers, caused by pollution, are a major reason for service disruption. Insulator performance begins to deteriorate when pollutants in the air settle on its surface and combine with the humidity caused by fog, rain or dew. The mixture of pollutants and moisture forms a layer on the insulator, which can conduct electricity and lead to a short circuit. Unless the insulator is naturally cleaned or adequately maintained, power transfer through transmission lines will be affected by possible flashovers.
Sources of pollution
Various kinds of pollution can affect insulator performance, with typical contaminants being the smog created by automobile emissions and the smoke released through wildfires, and industrial and agricultural burning. Salt industries release salts, which get deposited on insulator surfaces and form a contaminant layer. Industrial pollution from a wide variety of chemical industries and oil refineries leads to severe emissions that affect insulators in heavily industrialised areas. Other causes are cement deposits from cement plants, construction sites and rock quarries; and the ash generated from coal mining, coal handling plants, brick kilns and volcanic activity. Metallic deposits from mining processes, mineral handling processes and the use of fertilisers in fields are also known to contaminate insulator surfaces.
There are three ways to offset the effects of pollution on insulators: correctly selecting the insulator type, undertaking proper maintenance, and eliminating or reducing pollution. Composite insulators, composed of materials like rubber or rubber of silicone and propylene ethylene rubbers, are much more competitive and efficient in comparison with traditional glass and porcelain ones. These insulators have various advantages over the traditional kind, such as greater resistance to damage due to the elasticity of material; lesser cleaning requirements in polluted areas; and minimal maintenance costs.
In severely polluted zones, in addition to the correct selection of insulators, a firm maintenance plan should be in place. The insulator should be washed and cleaned regularly, particularly in areas with extreme pollution levels or less rain.
Insulator maintenance can be carried out with the system energised or de-energised. The de-energising method is used when the pollutant’s adhesive characteristics necessitate a wash with chemical solutions to recover the insulation level. In general, the most frequently employed methods are washing with water at a high, average or low pressure; washing with compressed dry air; or washing with spurts of abrasive materials. The technique should not further damage the insulator. Washing with water is the most effective and economical method if the contaminant is dust, salt or sand or a pollutant that does not stick to the surface. If the contaminant has a high level of adhesion, like in the case of cement, or pollutants originating from chemical businesses or by-products of petroleum, the insulator should be washed with abrasive elements.
To prevent flashovers during washing, certain precautions should be taken. The insulator should be washed from the lowest phase conductor onwards, starting from its lowest part; water should not fall directly on it; and the wind direction should be kept in mind.
Eliminating the pollution source is another effective method for maintaining and prolonging the life of an insulator. However, this is possible only when the contaminant is of an industrial type, as it is very difficult to eliminate natural sources.
Hydrophobicity is an important insulator property that improves its ability to withstand pollution. It reduces the magnitude of the leakage current on the insulator surface as well as the probability of the appearance of dry bands. In a surface lacking hydrophobicity, water dampens the entire surface and forms a conductive layer along with the contaminants. In a hydrophobic surface, the layer of pollution is broken, which makes the creation of dry bands difficult and prevents flashovers. Hydrophobicity is reduced over time due to pollutant layers, electric discharge, electric fields in the union zones of different materials making up the insulator, and environmental and chemical effects. Hydrophobic greases like the grease of silicon and silicon room temperature vulcanisation can be used to correct this situation.
Net, net, the proper maintenance of insulators is an important aspect in ensuring the smooth functioning and safety of transmission lines.