Advance High Voltage Engineering

Transcription

1 Advance High Voltage Engineering Lecture # 13 Solid Dielectrics, Types and Composition Arsalan A Rahim Assistant Professor University of Engineering& Technology Lahore, Pakistan

2 Solid Dielectrics Difference from Gaseous & Liquid Case The differences observed between a Solid Dielectric and a Gaseous and Liquid Dielectric are postulated below Unlike gaseous and liquid dielectrics, any damage caused to solid dielectrics due to excessive electrical, thermal or mechanical stresses is often irreversible Their thermal and mechanical properties play a very sensitive role since these considerably affect the electrical properties Solid Dielectrics are more exposed to atmosphere, hazards of weather (rain, storm, hail, ice deposits etc.), ultra violet radiation from the sun and pollution (dust, salts etc.) so life of a dielectric has to be considered If once failure of a solid dielectric takes place, it may cause not only big damage but also a discontinuation of the power supply until alternating arrangements or tedious repairs are undertaken

3 Classification of Solid Dielectrics based on Usage Solid Dielectrics Jacketing Materials Moulding Materials Filling Materials Insulating Mechanical Support Moulding Materials : These are used for mechanically rigid forms of insulation, for example, insulators, bushings and so on e.g. are ceramics, glass (toughened glass), fiberglass reinforced plastics and epoxy - resins Jacketing Materials : Jacketing on a conductor for insulation. Polymers have been found suitable for providing extruded insulating jackets to the conductors. For example, polyethylene (PE), polyvinylchloride (PVC), natural and synthetic (ethylene propylene) rubber are extruded on the conductor in power cables. Polypropylene and paper are used in capacitors and transformers. Mica and fiberglass based polypropylene tapes are used in electrical machines.

4 Classification of Solid Dielectrics based on Usage Solid Dielectrics Jacketing Materials Moulding Materials Filling Materials Insulating Mechanical Support Filling Materials : Beside oils, wax - based draining and non - draining impregnating compounds of different types are used to impregnate paper used in power cables, transformers, capacitors, and instrument transformers. Insulating Mechanical Support : In the form of plates, pipes and ledges, insulating supports are required in transformers, circuit breakers and isolators. The products, such as pressboards, hard paper (thin paper laminates), wood (yellow teak) are used in transformers and Bakelite..

5 Classification of Solid Dielectrics based on Composition Solid Dielectrics Inorganic Materials Ceramics Glass Fiberglass Enamel Mica Asbestos Organic Materials Paper Wood Wax Leather Natural & Synthetic Resins Polymer Materials Composite Materials Impregnated Paper Insulating Board Fiber reinforced Plastics

6 Inorganic Insulating Materials They distinguish themselves in their unique ability to withstand high temperatures in addition to their being highly chemical resistant. They also don t show any sign of aging but are difficult to machine or process them These materials are inherently not homogeneous, either microscopically or macroscopically Ceramics Ceramic substances are those produced from clay, containing aluminium-oxide and other inorganic materials Their thick paste is given the desired shape and form at room temperature and then baked at a high temperature (about 1400 C) to provide a solid, inelastic final structure

7 Ceramics Ceramics, also known as porcelain in one of its forms, has a history of over a century of service as insulating materials Porcelain is widely used for insulators and bushings in the power system because of its high mechanical strength Ceramics constitute 40 to 50% of clay, 30 to 20% of aluminium oxide and 30% of feldspar, that is, (3Al 2 O 3.2SiO 2 ) and SiO 2. Ceramics with higher mechanical strength and lower dielectric losses also contain steatites (soapstone) and talc (MgO, SiO 2 ). Porcelain suitable for high frequency applications is provided with barium compounds (BaOAl 2 O 3.2SiO 2 ) The very dense nature of ceramics makes the porcelain bodies very heavy

8 Ceramics The large size and heavy weight of porcelain products make them difficult to handle, which often require cranes as well as expensive and large structural supports Ceramics are very brittle and thus break easily during handling, transit and installation Under polluted conditions, the hydrophilic surface of porcelain does not perform well. The water dissolves part of the pollution, thereby lowering the overall surface resistance due to conductive electrolytes Surface discharge or tracking process incepts at much lower voltages

9 Glass The main constituent of glass is silicon-dioxide SiO 2, available in nature in the form of quartz In electrical engineering only non-alkaline glass, or glass having alkaline content less than 0.8% are suitable The conductivity, hence the losses in such glass are low. The so - called E - Glass, having the same properties as non-alkaline glass, is used for producing glass fiber The fiberglass is used for reinforcing plastic materials to obtain high mechanical strength These fiberglass reinforced plastics are utilized as tapes in electrical machines and as moulds for insulated containers, rods and so on

10 Organic Materials (Polymer Compounds) Solid organic materials used in electrical engineering are paper, wood, wax, leather, as well as a number of natural and synthetic resins, rubbers and plastics, also known as polymers POLYMER: A very high molecular weight compound and consist of two or more polymeric compounds of several structural units normally bound together by covalent bonds. The individual structural units may consist of single atoms or may be molecular in nature, which repeat in a regular order Common applications include cable terminations, surge arresters, insulators, bus bar insulation and bushings The density of polymer materials is much lower than ceramics. Hence, the polymer products are significantly lighter and easier to handle and install.

11 Organic Materials (Polymer Compounds) The polymeric materials resist wetting, being hydrophobic. Water on the surface of such materials may form water beads. Thus the conductive contamination dissolved within the water beads becomes discontinuous Lower Surface creepage current is able to flow and the probability of dry band formation increases

12 Types of Polymer Compounds based on heat Thermoplastic Polymers Thermoplastic polymers soften and become supple on heating and solidify back on cooling The heating and cooling cycle within certain temperature limits can be applied to these materials several times without affecting their properties Examples are polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) and polyamide (PA)

13 Types of Polymer Compounds based on heat Thermoset Polymers The polymers, which soften when heated for the first time resulting into cross - linking reaction (network formation), are known as thermoset polymers This reaction, leading to the formation of network structure, is also known as curing or setting of the polymer The polymers used for electrical insulation purpose are desired to retain their rubbery (flexible) properties. Hence, these are defined as lightly cross - linked polymers Cross - linked polymer resins are, for e.g. polyester - resin, phenol - resin, silicon - resin and the most widely used in electrical engineering, the epoxy resins

14 Polymer Compounds Additives The bulk properties of a polymer can be suitably altered by the incorporation of a number of additives. The following are the most important commonly used in preparing polymer compounds for electrical insulation purposes Fillers: Applied to modify physical properties, mainly mechanical, of a polymer Plasticisers and Softeners: to lower the melt viscosity and also to change physical properties (softness, flexibility) Colorants: normally soluble colorants (dyestuffs) are used. Anti-Aging: to prevent structural degradation due to chemical reactions like oxidation, ozone attack and ultra - violet irradiation due to exposure to the sun Flame retarders: to improve the degree of fire resistance of polymers Cross - linking: to achieve intermolecular combination at the chain ends.

15 Polyvinyl Chloride (PVC) PVC is a polymerization product of the monomer vinyl-chloride derived from ethylene, where one of the H - atoms in the molecule is replaced by Chlorine - atom Vinyl Chloride PVC PVC is a polar dielectric,very hard, brittle & thermally unstable product.. Suitable PVC compounds are very widely used for low voltage power cables up to 1.1 kv for insulation, filler as well as outer sheathing A compound for these cables may contain about 68% PVC polymer; 28% plasticizer, the non - volatile solvents of PVC such as, di-iso - octyl phthalate (DIOP),di-2-ethylhexyl phthalate (DOP) etc; 3% stabilizer, lead - sulphate or lead - phthalate, and about 1% of coloring agent and other additives

16 Polyvinyl Chloride (PVC) Stabilizers are useful in improving the resistance of PVC to weathering, particularly against degradation by ultra - violet radiation Ethylene PVC Dibasic lead phthalate is used in heat resistant insulation compounds Chalk powder or china clay is used in different proportions as filler substance only for sheathing Hard-wax type smoothing agents are also added in order to achieve a smoother extrusion of these compounds Tan δ is high for PVC. So it is only used for low voltage power and house wiring cable insulation. The maximum field intensity is tried to be limited within 3 kv/mm

17 Polyethylene (PE) Polyethylene, a polymer of ethylene, is also called polythene. It is produced by the polymerization of monomer ethylene Ethylene High Density Polyethylene (HDPE) Polyethylene The low pressure process involves polymerization of ethylene in the presence of catalytic agent at around 100 C and at a few atmospheric pressure. The PE produced by this method is known as high density PE (HDPE), having a density between 0.94 and 0.98 g/cm 3. It has a comparatively higher crystalline content Low Density Polyethylene (LDPE) In high pressure the polymerization is achieved at about 200 C and a pressure between 150 to 300 atm. A low amount of oxygen serves as catalyst. The product obtained is highly branched and is known as low density PE (LDPE), having density between 0.91 and g/cm 3.It has a much lower crystalline content compared to HDPE

18 Polyethylene (PE) The PE compound is extruded on the conductor of a cable at high temperatures. It shrinks by about 15%, internal mechanical stresses may develop if cooling to room temperature is not achieved gradually PE is also susceptible to mechanical cracks The LDPE, basically a thermoset material, is generally used for obtaining cross - linked polyethylene (XLPE) insulation Chemical Process for XLPE In this process the LDPE insulation compound is added with a cross-linking agent, such as dicumyl-peroxide (DCP) to initiate a chemical reaction, accomplished under high pressure and temperature

19 XLPE A cross-link is a bond that links one polymer chain to another. They can be covalent bonds or ionic bond The basic material for XLPE cable is polyethylene (PE). PE has very good electrical properties, however, it s mechanical strength decrease s Significantly above 75 C restricting its continuous operating temperature to 70 C only The rigid structure of XLPE results in higher continuous current carrying capability and short circuit temperature of 250 o C There are three processes for converting PE to XLPE (i) Chemical Cross Linking (ii) Electron Irradiation (iii) Organic Silane Method

20 XLPE Three different processes for curing PE with peroxide in power cable industry have been developed. These are: wet curing, under high temperature and pressure of steam dry curing, under high temperature and pressure of gas (nitrogen) oil curing, under high temperature and pressure of silicon oil. 1-Core and 3-Core XLPE Insulated Cable

21 Electrical Properties of XLPE It has a very low relative permittivity ε r and a nonpolar material It also has very low loss tangent tan δ, independent of temperature and frequency PE has a very high intrinsic breakdown strength, of the order of 700 kv/mm To ensure a long service life, of the order of forty years or more, the maximum electric stress in medium voltage cables (up to 33 kv) is kept within 4 kv/mm A peculiar weakness of PE and other polymeric solid dielectrics is the development of electrical and electrochemical treeing

22 Epoxy Resins (EP-resins) The words epoxy or epoxide refer to a compound containing more than one epoxide group per molecule, that is, an oxygen atom united with two carbon atoms already joined in some other way Resin is any one of a class of solid or semisolid substances obtained from the exudation of many plants or by chemical processing of inorganic materials. Epoxy-resins are low molecular but soluble thermosetting plastics, which exhibit sufficient hardening quality in their molecules. The EP - resins are mixed with hardening and accelerating agents besides providing filler materials, coloring agents and substances to obtain a more flexible cast Epoxy resins are widely used as insulation material in electrical and electronic applications. They are employed as adhesives, sealants, coatings, impregnants, moldings and potting compounds to produce voidfree insulation around components

23 Specific Insulation, tan δ& Breakdown Strength

24 Composite Insulating System Combining different dielectric materials has produced some very high quality solid insulating systems Common composites of organic and inorganic materials are fiberglass reinforced plastics, mica based plastic tapes, quartz, fiber and mica mixed with synthetic resins, such as epoxy-resin used in electrical machines, and so on. Uni-axially oriented polyethylene (UOPE) tape has been found to have very good compatibility with oils besides having good mechanical properties Impregnated Paper Paper as such has very poor dielectric properties, but when impregnated with oil or an impregnating compound, the properties of the composite dielectric considerably improve The paper used in electrical engineering is a produce of cellulose obtained mainly from pine or spruce wood

25 Impregnated Paper Paper has an inherent micro void structure having an average capillary diameter of 10 1 to 10 2 µm. The effective inner surface of the paper is, therefore, very large and highly hygroscopic The paper used for condensers is extra thin and has a thickness of 10 to 30 µ m For transformers and cables, 80 to 130 µ m thick paper is commonly applied The relative permittivity and loss tangent of paper is much smaller than that of pure cellulose (ε r = 6.1 at 20 C)

26 Today s Text Covered from Chapter 7 of IEEE Press Book (Ravindra Book) Uptil Article 7.1 (Page# 337)