Lecture # 13

Solid Dielectrics,

Types and Composition

AdvanceHigh Voltage Engineering

Arsalan A RahimAssistant Professor

University of Engineering& Technology Lahore, Pakistan

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

Classification of Solid Dielectrics based on Usage

Solid Dielectrics

Jacketing Materials

Moulding Materials

Filling MaterialsInsulating

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.

Classification of Solid Dielectrics based on Usage

Solid Dielectrics

Jacketing Materials

Moulding Materials

Filling MaterialsInsulating

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..

Classification of Solid Dielectrics based on Composition

Solid Dielectrics

Inorganic Materials

Organic Materials

Composite Materials

• Ceramics

• Glass

• Fiberglass

• Enamel

• Mica

• Asbestos

• Paper

• Wood

• Wax

• Leather

• Natural &

Synthetic Resins

• Polymer Materials

• Impregnated

Paper

• Insulating

Board

• Fiber reinforced

Plastics

Inorganic Insulating MaterialsThey 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 orprocess them

These materials are inherently not homogeneous, either microscopically or macroscopically

CeramicsCeramic substances are those produced from clay, containing aluminium-oxideand 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) toprovide a solid, inelastic final structure

CeramicsCeramics, 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, (3Al2O3 .2SiO2 ) and SiO2 .

Ceramics with higher mechanical strength andlower dielectric losses also contain steatites (soapstone) and talc (MgO, SiO 2 ).

Porcelain suitable for high frequency applications is provided with barium compounds (BaOAl2O3 .2SiO2 )

The very dense nature of ceramics makes the porcelain bodies very heavy

CeramicsThe 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

GlassThe main constituent of glass is silicon-dioxide SiO2 , 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 ” areutilized as tapes in electrical machines and as moulds for insulated containers, rods and so on

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.

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

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)

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

Polymer Compounds AdditivesThe 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

Colorants: normally soluble colorants (dyestuffs) are used.

Plasticisers and Softeners: to lower the melt viscosity and also to changephysical properties (softness, flexibility)

Anti-Aging: to prevent structural degradation due to chemical reactionslike 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.

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

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 - octylphthalate (DIOP),di-2-ethylhexyl phthalate (DOP) etc; 3% stabilizer, lead -sulphate or lead - phthalate, and about 1% of coloring agent and other additives

Vinyl Chloride PVC

Polyvinyl Chloride (PVC)Stabilizers are useful in improving the resistance of PVC to weathering, particularly against degradation by ultra - violet radiation

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

Ethylene PVC

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

Polyethylene (PE)Polyethylene, a polymer of ethylene, is also called polythene. It is produced by the polymerization of monomer ethylene

High Density Polyethylene (HDPE)

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/cm3 . It has a comparatively higher crystalline content

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 0.923 g/cm 3 .It has a much lower crystalline content compared to HDPE

Ethylene Polyethylene

Low Density Polyethylene (LDPE)

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 ifcooling 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

XLPEA 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 sSignificantly 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 250oC

There are three processes for converting PE to XLPE(i) Chemical Cross Linking(ii) Electron Irradiation(iii) Organic Silane Method

XLPEThree 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

Electrical Properties of XLPEIt has a very low relative permittivity “ εr ” and a nonpolar materialIt also has very low loss tangent “ tan δ ”, independent of temperature and frequency

PE has a very high intrinsic breakdownstrength, 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

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 void-free insulation around components

Specific Insulation, tan δ& Breakdown Strength

Composite Insulating SystemCombining 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 havevery good compatibility with oils besides having good mechanical properties

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

Impregnated Paper

The paper used in electrical engineering is a produce of “ cellulose ” obtained mainly from pine or spruce wood

Impregnated PaperPaper 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

The relative permittivity and loss tangent of paper is much smaller than that of pure cellulose (εr = 6.1 at 20°C)

For transformers and cables, 80 to 130 µ m thick paper is commonly applied

Today’s Text Covered from Chapter 7 of IEEE

Press Book (Ravindra Book)

Uptil Article 7.1 (Page# 337)