HV_Assignment1.pdf - Rushi Maadhav

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SASTRA Deemed to be University - BEEDEE613 High Voltage Engineering Assignment 1 A Rushi Maadhav 121005010 1. Solid Dielectrics Used in Practise The majority of the insulating systems used in practice are solids. They can be broadly classified into three groups: organic materials, inorganic materials and synthetic polymers. Organic materials are those which are produced from vegetable or animal matter and all of them have similar characteristics. They are good insulators and can be easily adopted for practical applications. However, their mechanical and electrical properties always de- teriorate rapidly when the temperature exceeds 100◦ C. Therefore, they are generally used after treating with a varnish or impregnation with an oil. Inorganic materials, unlike organic materials, do not show any appreciable reduction in their electrical and mechanical properties almost up to 250◦ C. Important inorganic mate- rials used for electric applications are glasses and ceramics. They are widely used for the manufacturer of insulators, bushings, etc., because of their resistance to atmospheric pollu- tants and their excellent performance under varying conditions of temperature and pressure. Synthetic polymers are the polymeric materials which possess excellent insulating prop- erties and can be easily fabricated and applied to the apparatus. These are generally divided into two groups, the thermoplastic and the thermosetting plastic types. Although they have low melting temperatures in the range 100–120◦ C, they are very flexible and can be moulded and extruded at temperatures below their melting points. They are widely used in bushings, insulators, etc. Their electrical use depends on their ability to prevent the absorption of moisture. Let us look at a few examples of the above mentioned types Organic Materials Inorganic Materials Synthetic Polymers Thermoplastic Thermosetting Amber Asbestos Polyethylene Bakelite [] Cotton Ceramic Perspex Epoxy Resins Paper Glass Polypropylene Pressboard Mica Polystyrene Rubber Polyvinyl Chloride 1 A Rushi Maadhav Now let us look at a few of them in detail 1.1 Paper and Boards The kind of paper normally employed for insulation purposes is a special variety known as Tissue paper or Kraft paper. The thickness and density of the paper vary depending on the application. Low— density paper is preferred in high frequency capacitors and cables, while Medium-density paper is used in power capacitors. High—density papers are preferable in D.C. and energy storage capacitors and for the insulation of D.C. machines. Paper is hygroscopic. Therefore, it has to be dried and impregnated with impregnants, such as mineral oil, chlorinated diphenyl and vegetable oils. The relative dielectric constant of impregnated paper depends upon the permittivity of cellulose of which the paper is made, and permittivity of the impregnant and the density of the paper. Transformer board is used mainly in oil-filled transformers where a solid insulating struc- ture is needed. This is a press board up to 8 mm in thickness. The board is built up wet on forming cylinders and cut off when at the desired thickness. This makes a sheet with the size of the width and circumference of the drum. The wet sheets are hot- or cold-press dried and finished on separate machines. 1.2 Mica Mica is the generic name of a class of crystalline mineral silicates of alumina and potash. It has got a unique combination of electrical properties, such as high dielectric strength, low dielectric losses, resistance to high temperatures and good mechanical strength. These have made it possible for it to be used in many electrical apparatus mentioned below. Muscovite and phlogopite splittings (two types of Mica) can be fabricated into various built-up mica products. Produced by mechanized or hand setting of overlapping splittings and alternate layers of binders and splittings, built-up mica is used primarily as an electrical insulation material. Flexible plate is used in electric motor and generator armatures, field coil insulation, and magnet and commutator core insulation. Molding plate is sheet mica from which V-rings are cut and stamped for use in insulat- ing the copper segments from the steel shaft ends of a commutator. Molding plate is also fabricated into tubes and rings for insulation in armatures, motor starters, and transformers. Segment plate acts as insulation between the copper commutator segments of direct- current universal motors and generators. 2 SASTRA Deemed to be University - BEEDEE613 High Voltage Engineering 1.3 Thermoplastics Thermoplastic polymers, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS) are the main products of industrial engineering, which are employed in several electrical application fields. In high-voltage equipment, polymers are commonly used in the insulation systems because of their exceptional mechanical, electrical, chemical, and thermal properties and their low production cost. In insulation materials, which are produced by mixing different polymers at different weight ratios, polymers are used to modify the physical and mechanical properties of the final products, maintaining a lower production cost. High-density polyethylene (HDPE), low-density polyethylene (LDPE), and PP are the most widely used polymers in the world market, employed for electrical equipment, auto- motive engineering material, and packaging. The most useful properties of PE and PP are oil resistance, rigidity, good stiffness, and thermal stability. Conversely, the utilization of PP and PE is restricted in certain areas owing to some weak properties such as melt viscosity. The weak properties of LDPE include a lack of mechanical and thermal resistance. Hence, numerous researchers are working on improving the properties of LDPE by mixing it with other polymers that have high temperature resistance. In addition, the poor impact strength of PP and its weak Young’s modulus restrict its applications. 3