ELECTRICAL EVALUATION OF SILICONE RUBBER FOR HIGH VOLATGE COMPOSITE INSULATORS MAYA .Y.PAI (1VA09EE024) PAVITHRA N. (1VA09EE031) RAMYA C.D. (1VA09EE034) SASHANK S. (1VA09EE040)

BHARATH HEAVY ELECTRICALS LIMITED Electroporcelains Division Bangalore EXTERNAL GUIDE - Mr. Sheshapati Manager Insulation Engineering (Ph-9449831408) INTERNAL GUIDE - Prof. T.G. Manjunath Department of EEE S.V.I.T, Bangalore

OBJECTIVE OF THE PROJECT The main objective of this project is to evaluate special properties of silicone rubber used for manufacturing composite insulators. This includes analysis of silicone rubber of different manufacturers for ‘Hydrophobicity’ and ‘Resistance to Tracking and Erosion’. Composite insulators are used in Transmission Lines and Railway Traction and are replacing conventional Porcelain and Glass insulators. Composite designs typically use engineered polymers which offer higher mechanical strength, greater design flexibility, reduced weight, lower breakage rates, superior dielectric strength, low leakage current and power loss, and excellent pollution performance than ceramic components.

What is an Insulator ? An electrical insulator is a material whose internal electric charges do not flow freely, and which therefore does not conduct an electric current, under the influence of an electric field. A perfect insulator does not exist, but some materials such as glass and paper, which have high resistivity, are very good electrical insulators. Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. The term Insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers.

Types of Insulator Types of Insulators based on materials used: Porcelain Insulators Glass Insulators Composite Insulators PORCELAIN INSULATORS – Porcelain insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed water. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of about 4-10 kV/mm.  

GLASS INSULATORS – Glass insulators were used to insulate lightning rods and cables from structures. Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains. Glass of special compositions is used as an insulating material and annealing process to obtain high compressive strength toughens it. 10kV porcelain insulator Glass insulator

COMPOSITE INSULATORS - Polymer insulators essentially consist of a fibre glass core covered by weather sheds of silicone rubber and equipped with metal end fittings. It is also called Composite Insulators, which means it is made of at least two insulating parts – a core and housing equipped with end fittings. Composite insulators have many advantages over the ceramic and glass insulators such as good performance in contaminated environment, light weight, easy handling, maintenance free, and considerably low cost etc. Because of these properties it is gaining popularity worldwide and replacing the conventional ceramic and glass insulators.

A composite insulator consists of a core material, end fitting, and a rubber insulating housing. The core is of FRP (Fibre Reinforced Plastic) to distribute the tensile load. The reinforced fibres used in FRP are glass (E or ECR – Epoxy Corrosion resistant) and epoxy resin is used for the matrix. The portions of the end-fitting to transmit tension to the cable and towers are of forged steel, malleable cast iron, aluminium etc. The rubber housing provides electrical insulation. It covers the FRP rod thereby protecting it from corrosion due to atmospheric exposure. Composite insulators are also known as polymeric or non-ceramic insulators. Silicone rubber has superior electrical characteristics and weather resistance properties over a wide range of temperatures, for use in the housing. It is resistant to oxidation, has low surface energy, and resists degradation from ultraviolet radiation. These properties make silicone rubber a good choice for electrical insulators.

Studies have shown that the specific creepage distance required in polymer insulator is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators is approximately 12% to 15% of the equivalent weight of ceramic or glass insulator. Because of better pollution and wet performance there has been an increased use of these insulators over the years.

Advantages of using Composite Insulators Over the past few decades, silicone composite insulators have been increasingly used for different outdoor applications as the housing/shed material for a wide variety of products. The progressive increase in usage of silicone insulators is attributed to several advantages, as cited below: 1. Reduced Weight 2.Hydrophobicity 3. Superior Pollution Performance 4. Superior Ultra Violet Resistance 5. Shorter Processing Time 6. Complex Shapes 7.Flexibility 8.Resistance to Tracking and Arcing 9.Shatter Resistance 10.Economy of Operation

Need for testing Composite Insulators Due to contaminants on the surface of the Silicone rubber and due to water accumulation there is a possibility of increase in surface discharge currents. Electrical arcing across a dry gap called dry band arcing, generates ozone, high temperatures and UV radiation exposure on the polymer surface. The tracking and erosion resistance of a compound is a measure of its ability to withstand intense leakage currents combined with dry band arcing.

Tests performed to analyze the properties of Silicone Rubber Hydrophobicity Test - The superior electrical performance of composite insulators and coated insulators stems from the hydrophobicity (water-repellency) of their surfaces which changes with time due to exposure to outdoor environment and partial discharges (corona). Hydrophobicity is the property that prevents water from forming a sheet on the surface of a polymer. When water readily beads and runs off a hydrophobic surface, it can remove loose contamination from the insulator surface. Removing contamination can result in lower leakage currents.

Inclined Plane Tracking and Erosion Test - The inclined plane test is a severe test to evaluate the tracking performance of insulation materials. A track is defined as a conducting carbonaceous path caused by sparking and arcing on insulating components. Tracking is initiated by surface discharges in regions of high electric field strength and it is usually promoted by dampness and pollutants on the insulation surface. Tracking: This is the process that produces tracks as a result of the action of electric discharges on or close to the insulation surface. Erosion: The progressive wearing away of electrical insulation by the action of electrical discharges.

Tensile Strength Test- Tensile strength test is used to evaluate the tensile (tension) properties of vulcanized thermo-set rubbers and thermoplastic elastomers. Through this test the ultimate tensile strength and percentage elongation are calculated to evaluate the flexibility of the Silicone Rubber used in the Composite Insulator.

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