Solid insulating materials can be classified into two main categories: organic and inorganic. There are a large number of solid inorganic insulants available, including the following:
• ALUMINA: produced by heating aluminum hydroxide or oxyhydroxide; it is widely used as a filler for ceramic insulators. Further heating yields the corundum structure, which in its sapphire form is used for dielectric substrates in microcircuit applications.

• PORCELAIN: a multiphase ceramic material that is obtained by heating aluminum silicates until a mullite phase is formed. Because mullite is porous, its surface must be glazed with a high-melting-point glass to render it smooth and impervious to contaminants for use in overhead line insulators.

• ELECTRICAL-GRADE GLASSES: which tend to be relatively lossy at high temperatures. At low temperatures, however, they are suitable for use in overhead line insulators and in transformer, capacitor, and circuit breaker bushings. At high temperatures, their main application lies with incandescent and fluorescent lamps as well as electronic tube envelopes.

• MICA: a layer-type dielectric (mica films are obtained by splitting mica blocks). The extended two-dimensionally layered strata of mica prevent the formation of conductive pathways across the substance, resulting in high dielectric strength. It has excellent thermal stability and, because of its inorganic nature, is highly resistant to partial discharges. It is used in sheet, plate, and tape forms in rotating machines and transformer coils.
Solid organic dielectrics consist of large polymer molecules, which generally have molecular weights in excess of 600. Primarily (with the notable exception of paper, which consists of cellulose that is comprised of a series of glucose units), organic dielectric materials are synthetically derived. Some of the more common insulating materials of this type include:

• POLYETHYLENE (PE): perhaps one of the most common solid dielectrics. PE has been extensively used as a solid dielectric extruded insulator in power and communication cables. Linear PE is classified as a low- (0.910 to 0.925), medium- (0.926 to 0.940), or high- (0.941 to 0.965) density polymer. Most of the PE used on extruded cables is of the cross-linked polyethylene type.

• ETHYLENE-PROPYLENE RUBBER (EPR): an amorphous elastomer that is synthesized from ethylene and propylene. It is used as an extruded-on cable where its composition has a filler content that usually exceeds 50% (comprising primarily clay, with smaller amounts of added silicate and carbon black). Dielectric losses are appreciably enhanced by the fillers, and, consequently, EPR is not suitable for extra-high-voltage applications. Its use is primarily confined to intermediate voltages (< 69 kV) and to applications where high cable flexibility (due to its inherent rubber properties) may be required.

• POLYPROPYLENE: which has a structure related to that of ethylene with one added methyl group. It is a thermoplastic material having properties similar to high-density PE, although because of its lower density, polypropylene has also a lower dielectric constant. Polypropylene has many electrical applications, both in bulk form as molded and extruded insulations, as well as in film form in taped capacitors, transformers, and cable insulations.

• EPOXY RESINS: which are characterized by low shrinkage and high mechanical strength. They can also be reinforced with glass fibers and mixed with mica flakes. Epoxy resins have many applications, including insulation of bars in the stators of rotating machines, solid-type transformers, and spacers for compressed-gas-insulated bus bars and cables.

Impregnated paper insulation is one of the earliest insulating systems employed in electrical power apparatus and cables. Although many current designs use solid- or compressed-gas insulating systems, the impregnated-paper approach still constitutes one of the most reliable insulating techniques available. Proper impregnation of the paper results in a cavity-free insulating system, thereby eliminating the occurrence of partial discharges that inevitably lead to deterioration and breakdown of the insulating system. The liquid impregnates employed are either mineral oils or synthetic fluids.

Low-density cellulose papers have slightly lower dielectric losses, but the dielectric breakdown strength is also reduced. The converse is true for impregnated systems utilizing high-density papers. If the paper is heated beyond 200°C, the chemical structure of the paper breaks down, even in the absence of external oxygen, because the latter is readily available from within the cellulose molecule. To prevent this process from occurring, cellulose papers are ordinarily not used at temperatures above 100°C.

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