Porcelain has enjoyed a virtual monopoly; as an insulating material for electrical equipment such as transformers, switchgears etc and in transmission lines. The advantages of porcelain are its high insulation strength. But it has a low strength to weight ratio and are prone to fragmentation under stress.
Polymer materials, on the other hand, have high insulation value at par with porcelain and acceptable strength both under compression and tension. It has a better water and sleet shedding properties and is therefore more useful in contaminated and polluted environment. Although the cost of both porcelain and polymer material are the same but polymer is more favored because of its better handling. It has a better strength to weight ratio i.e. it is much lighter in weight than porcelain. The weight of suspension type polymer insulator for the 138 kV class is only 8% of the porcelain insulator of the same voltage level. This vital property of polymers permits the use of lighter supporting structures, more compact design, narrow Right of Way (ROW) requirements and thus significant reduction in cost.
In the polymer insulator a fiberglass insulation rod or shaft serves as the internal structure and around which the polymer insulator is attached usually in the shape of petticoats or rainsheds. The fiberglass rod has a high compression and tensile strength. The metal fittings at both the ends are crimped directly to the fiberglass rod. Proper sealing is done to avoid ingress of moisture or contamination into the fiberglass rod. The petticoats, as in porcelain insulators, provide a longer leakage path between conductor and the support, so as to keep the leakage current to a minimum value. A variety of polymer insulators are shown in figure.
The rate at which the petticoats or rainsheds dry up is crucial and depends on a number of factors such as the contamination level of the area, temperature, humidity and wind velocity following the cessation of the rough weather. In areas with extreme contamination; for example near an industrial area or power plant, insulators with various petticoat sizes are used, so as to obtain a greater distance between outer edges of petticoats, eventually to avoid a flashover.
The polymer suitable for High Voltage (H.V) applications are Ethylene Propylene (EP) and Silicon Rubber (SR). Ethylene Propylene polymer has high resistance to corrosion and better physical properties whereas Silicon Rubber shows better performance under contamination and offers higher resistance to Ultra Violet (UV) sun rays. A combined EP and SR polymer has better hydrophobic (water repellent), electro-mechanical properties and high resistance to industrial pollution. These insulating materials are also used in bushings of transformers, reactors, switchgears, capacitors, instrument transformers, lightning arresters etc.