7.1.1 Classification

Insulating materials, especially those used in generators, motors, transformers and switchgear, are often classified on the basis of their thermal stability according to the scheme described in BS 2757:1986 and IEC 60085:1984. This scheme uses nine temperature classes, allocating materials to a class with ‘temperature limits that will give acceptable life under usual industrial conditions of service’. These standards recommend that temperatures above 250°C should increase in steps of 25°C and the class is designated accordingly.

Examples of materials in each class are given below.

Class Y: Unimpregnated paper, cotton or silk, vulcanised natural rubber, various thermoplastics that have softening points that would only permit their use up to 90°C. Aniline and urea formaldehydes.

Class A: Paper, cotton or silk impregnated with oil or varnish, or laminated with natural drying oils and resins or phenol formaldehyde. Polyamides. A variety of organic varnishes and enamels used for wire coating and bonding.

Class E: Polyvinyl formal, polyurethane, epoxy resins and varnishes, cellulose triacetate, polyethylene terephthalate, phenol formaldehyde and melamine formaldehyde mouldings and laminates with cellulosic materials.

Class B: Mica, glass and asbestos fibres and fabrics bonded and impregnated with suitable organic resins such as shellac bitumen, alkyd, epoxy, phenol formaldehyde or melamine formaldehyde.

Class F: As class B but with resins that are approved for class F operation such as alkyd, epoxy alkyd and silicone alkyd.

Class H: As class B but with silicone resins or other resins suitable for class H operation. Silicone rubber.

Classes 200, 220, 250: Mica, asbestos, ceramics and glass alone or with inorganic binders or certain silicone resins. Polytetrafluoro-ethylene.

The allocation of materials to classes such that their life will be adequate under usual industrial conditions means that the materials may not give adequate life if the service is unusually severe, e.g. equipment normally operated very near to full load. Conversely, it may be economical to use materials of a lower temperature class for equipment operated infrequently or normally operated at very low load.

7.1.2 Temperature index

Various suggestions have been made for alternative temperature classification systems, especially as techniques such as cross-linking enable the thermal stability of materials to be significantly improved and new high-temperature materials are constantly being developed. IEEE 98:1984 has withdrawn the term ‘Temperature Classification’ but the institution has suggested that a ‘temperature index’ related to the temperature capability of the material should be assigned to a material on the basis of experience or comparison with materials that have established indices. The index would be based on the life of the material in particular environmental conditions and would preferably be a number chosen from the series 90, 105, 130, 155, 180, 200, 220. For temperatures above 250°C, no index has been established yet.

7.1.3 Effect of frequency

Insulating materials can have a very large variation in the dielectric loss and permittivity of the material with the frequency of the applied signal. Whilst this is important for insulation used in high-frequency electronics, it is not normally important in conventional power equipment, except for some condition of very fast transients. Table 7.1 shows the loss tangents (tan δ) at 50 Hz, 1 kHz and 1 MHz. Fortunately, the losses generally decrease with frequency for the materials that are used in electrical power insulation. Occasionally there can be significant changes at low frequencies and polymethylmethacrylate has a peak in dielectric loss at around 2 Hz, depending on temperature, where it increases by a factor of 3.

7.1.4 Fire behaviour

Despite the extensive precautions that are normally taken to prevent fire in electrical installations, fires do occasionally occur. There may be hundreds of wires bunched together in ducting and this presents serious problems in the event of a fire. One problem is that if fire propagates along the insulation, electrical systems will fail completely and these may be essential to the orderly shut-down of the equipment, or they may be responsible for important telecommunication systems. Another problem is that the fumes that are produced by combustion of the insulating material may be toxic and may also cause corrosive damage to neighbouring materials. It is therefore important that electrical insulation is assessed for behaviour in the event of fire. Some materials that have been used in the past are liable to produce toxic fumes. The former use of polychlorinated biphenyls as transformer insulation is a particular cause for concern.

Ease of ignition, or flammability, has traditionally been considered the most important property of a material when assessing fire hazards, but it has now been realised that this is only one of several factors that must be considered. The amount of heat, smoke and toxic gases released by the material and the way that the flame spreads in the material are all important. The traditional tests use very small quantities of the material which means that the information obtained is of limited value. More realistic tests require more specialised facilities and tend to be expensive to perform.

Gaseous insulation does not normally present any fire problem, although sulphur hexafluoride can decompose into toxic fractions under certain conditions. Commonly used insulating liquids, such as transformer oil, are flammable and so present a fire hazard. This is often unacceptable for transformers or switchgear situated in substations in blocks of offices or flats and alternative designs using air insulation or non-flammable liquids must be used. The same problems occur for equipment to be used in hazardous environments such as mines. Polychlorinated biphenyls were once used as non-flammable insulation in these applications, but the discovery of their toxic nature has required a search for alternative non-flammable liquids. Silicone oils are used but, although these are less flammable than transformer oil, they are still flammable.

Fire tests that are used to try to give more relevant fire hazard information for liquids use pools of the liquid from about 15 cm diameter to 9 m² of burning liquid area.

Much of the work on fire behaviour is concerned with insulation for cables. Here the standard tests such as BS 6387:1994 only require a flame of between 650 and 950°C, depending on the fire resistance category, to be applied to about 600 mm of the cable for 3 h (only 20 min for the highest temperature) without the insulation failing. However, more stringent tests involving longer lengths of cable on cable trays in vertical ducts are often required so that the conditions met in situations such as power stations can be reproduced more accurately. A typical test involves 8 m of cable on a cable rack ignited by an 88 kW methane flame with a forced air draft to propagate the flame. Measurements are made of the distance that the flame spreads and the optical density of the smoke produced.

Tests are now being made of the heat evolution of materials in a fire. There is still a lack of agreement about exactly what tests are most appropriate and how reliably small-scale tests can be scaled to possible full-scale fire scenarios. There is particular lack of agreement about toxicity testing.

(1) physical,

(2) mechanical,

(3) electrical, and

(4) chemical.

7.2.1 Physical properties

7.2.2 Mechanical properties

The usual mechanical properties of solid materials are of varying significance in the case of those required for insulating purposes, tensile strength, cross-breaking strength, shearing strength and compressive strength often being specified. Owing, however, to the relative degree of inelasticity of most solid insulations and the fact that many are quite brittle, it is frequently necessary to pay attention to compressibility, deformation under bending stresses, impact strength and extensibility; tearing strength and ability to fold without damage are important properties of thin-sheet insulations such as papers, pressboards and varnished cloths.

Methods of test for the above properties are given in British Standards.

Many other mechanical features of insulating materials have to be considered, for example: machinability (especially as regards drilling and punching) and resistance to splitting, the latter being of particular importance in the case of laminated materials, wood and pressboards.

7.2.3 Electrical properties

The essential property of a dielectric is, of course, that it shall insulate. But there are properties other than resistivity that determine the insulation value: these are the electric strength, permittivity and loss tangent.

7.2.4 Chemical properties

The chemical and related properties of insulating materials of importance may be grouped as follows:

Under (1) there are such properties as resistance to:

In group (2), typical effects of the insulating materials on other substances with which they may be used are:

These effects are generally referred to under the heading ‘compatibility’. If meaningful test results are to be obtained, all components of an insulation system must be present and they must have been treated in the same way as will be used in manufacture.

Group (3) includes such features as:

Most of these chemical properties are determined by well-known methods of chemical analysis and test. The principal tests are for acidity and alkalinity, pH value, chloride content in vulcanised fibre, and conductivity of aqueous extract (for presence of electrolytes). Some of these are dealt with in BS 5591:1978, BS 2782:1991, BS 5626−2:1979 and BS EN 1413:1998.

Increasing attention is being paid to chemical features of the raw materials and processes used in the manufacture of insulating materials—particularly varnishes, synthetic resins and all manner of plastics—and much research work is being carried out on these features and on the correlation of the chemical structure of dielectrics with their physical, electrical and mechanical properties.

7.3.1 Breakdown mechanisms in gases

As a gas is a highly compressible medium, breakdown processes in gases depend on the density of the gas and values quoted for air in Section 7.3.2 must be corrected for the air density (d) relative to normal temperature and pressure (20°C and 1013 mbar, respectively):

where p is the pressure in millibars and t is the temperature in degrees Celsius.

A discharge in a gas and subsequent development into a visible spark or flashover starts with the production of electrons in the gas by emission from one of the electrodes, or even from cosmic rays. The initial eletrons are then multiplied by various processes of ionisation that give a growth in the current and lead, ultimately, to breakdown. In most gases, when an electron collides with a neutral molecule an extra electron and a positive ion will be produced, provided the energy of the original electron is higher than the ionisation energy of the molecule (Figure 7.2(a)) but in electronegative gases such as sulphur hexafluoride an electron can be captured by the molecule to give a negative ion (Figure 7.2(b)). This process is the opposite of electron multiplication so these gases have a high breakdown strength.

The breakdown strength of a gas is affected by the uniformity of the electric field, the waveform of the applied voltage, the support insulators and solid particle contaminants, in addition to the gas density. The effect of non-uniform field is particularly significant for d.c. insulation as the breakdown voltage is quite different depending on whether the point in a point-plane system is positive or negative. For large gaps the breakdown strength for the negative point is more than twice the voltage obtained when the point is positive.

In practical apparatus the breakdown strength of compressed gas can be reduced severely by the presence of dust and solid particles. Particles near an electrode can induce a spark at a substantially lower voltage than for a particle-free situation. In one study on sulphur hexafluoride, breakdown voltages were reduced to between 20% and 90% of the particle-free values, depending on the number and size of the particles present.

7.3.2 Air

Air is the most important gas used for insulating purposes, having the unique feature of being universally and immediately available at no cost. The resistivity of air can be considered as infinite under normal conditions when there is no ionisation. There is, therefore, no measurable dielectric loss, negligible tan δ, and a relative permittivity (for all practical purposes) of unity. The electric strength under normal atmospheric conditions is 30kV/cm (peak) for a uniform field. In a practical airgap the voltage gradient is a maximum at the electrode surfaces. The sparkover (breakdown) voltage of an air gap is therefore a non-linear function of its length. The gap geometry and configuration affect the breakdown voltage, and empirical expressions have been derived to account for the ‘gap factor’.

Partial breakdown of air, locally, often occurs when the voltage gradient in a particular region exceeds the critical value for air. This happens readily at points of electric flux concentration, e.g. sharp edges of metal parts. If this local breakdown becomes unstable—as it will when the voltage between conductors is increased sufficiently—sparkover will occur. This may be an isolated spark from one conductor to the other, and the intervening air then re-heals itself. If the voltage is maintained (or increased), the spark may be followed by a continuous stream of sparks.

Typical values of breakdown voltages for gaps of different forms and sizes of electrodes (under normal atmospheric conditions) are given in Table 7.3.

Partial or complete breakdown of air in gaps can be influenced by suspending sheets of material at particular places in the electric field. In some cases, the sheets may be of metal, and in others of insulating materials; even woven fabrics can have an effect. The effect of this barrier is generally greater for more divergent fields. This solution can be useful where clearances are limited inside equipment, or particularly in high-voltage test areas.

For plane gaps, all gases exhibit a minimum breakdown voltage known as the Paschen minimum; this occurs at a given value of the product Pd of absolute gas pressure and gap length. For air this minimum occurs at Pd 6 (in torr-millimetre). Thus as the value of Pd is reduced from a higher region the breakdown voltage falls to the minimum value quoted, and further decrease in either P or d results in an increase in the voltage required to break down the gap. This explains why quite small gaps under conditions of high vacuum can sustain very high voltages. Table 7.4 gives the values of Pd and the minimum voltages for several gases.

7.3.3 Nitrogen

Instead of air, which is a mixture of approximately 21% oxygen and 79% nitrogen, nitrogen alone is sometimes used when there is a risk of oxidation of another material such as insulating oil. Nitrogen is often used in gas-filled high-voltage cables, as an inert medium to replace air in the space above the oil in some transformers, in low-loss capacitors for high-voltage testing, etc. There is no appreciable difference between the electric strength of nitrogen and that of air. Some results relating to the electric strength of nitrogen for uniform fields at pressures above atmospheric up to 20 atm are given in Table 7.6. Included are some similar results for carbon dioxide.

7.3.4 Sulphur hexafluoride

Sulphur hexafluoride is an electronegative gas which has come into wide use as a dielectric (in X-ray equipment, in waveguides, coaxial cables, transformers, etc.) and as an arc-quenching medium in circuit-breakers. Its electric strength is of the order of 2.3 times that of air or nitrogen, and at a pressure of 3−4 atm it has an electric strength similar to that of transformer oil at atmospheric pressure. The gas sublimes at about − 64°C and it may be used at temperatures up to about 150°C. Although the gas is considered to be non-toxic, non-flammable and chemically inert, under the influence of arcs or high-voltage discharges, there may be some decomposition with consequent attack on certain insulating materials and metals, and more importantly some recent environmental concerns. In circuit-breakers this problem is overcome by careful selection of materials (e.g. polytetrafluoroethylene for interrupter nozzles) and by the use of filters and absorbents to remove the products of decomposition after circuit interruption. Some figures relating to the electric strength of sulphur hexafluoride and mixtures of this gas with nitrogen are given in Table 7.7.

Numerous other electronegative gases such as perfluoropropane (C₃F₈), octafluorocyclobutane (C₄F₈) and per-fluorobutane (C₄F₁₀) have been developed, but few have found such widespread use as sulphur hexafluoride. The main interest for these gases is as dielectrics in transformers, waveguides, capacitors, etc., but one difficulty is that the temperature at which condensation occurs may not be sufficiently low for safety in outdoor equipment likely to remain un-energised for long periods. This problem can be overcome partly by fitting heaters or by using admixtures with a more volatile gas (such as nitrogen). Addition of nitrogen often improves some of the characteristics, while at the same time reducing the overall cost. Some of these gases can be used at temperatures well above 200°C.

Most of the fluorinated gases have an electric strength between two and five times that of air or nitrogen under the same conditions but, as with sulphur hexafluoride, care must be taken to prevent high-voltage discharges or arcs in the gas because of the dangers of producing decomposition products. Recent research efforts in the electrical power industry focussed on improving the use of gas mixtures (essentially 90% nitrogen and 10% SF₆), which are more environmentally acceptable. Gas insulated high voltage lines are now in operation using these gas mixtures.

Some electric machines and special devices have to operate in a gas other than air—for example, most refrigerator compressor motors operate in gaseous refrigerants mostly based on chlorofluorohydrocarbons (such as Arcton, Freon, etc.). These materials can act as solvents for some of the components used in insulating materials with consequent failure of the equipment due to blocked tubes and valves in the refrigerator circuit. Careful selection of materials for resistance to these fluids is essential.

7.3.5 Hydrogen

This gas is used as a cooling medium in some large turbo generators and synchronous motors; the main advantages are the efficient removal of heat and reductions in windage loss. Although there is a fire and explosion risk, troubles of this kind have been few during the many years that the gas has been used commercially for electrical machines. The electric strength of hydrogen at atmospheric pressure is about 65% that of air but most machines operate at pressures of 2–5 atm, and over this range the electric strength is higher than for air at atmospheric pressure. High-voltage discharges are thus not likely to be any more severe, and as discharges in hydrogen do not produce ozone or oxides of nitrogen, injurious effects are considered to be negligibly small.

7.3.6 Vacuum

Considerable investigation has been made into the utilisation of high vacua both for the insulation of equipment and as the interrupting medium in vacuum circuit breakers and contactors. The major advantage is due to the fact that very high electric field strengths can be achieved with a maximum operating pressure of 1 atm (negative), whereas with gases, very high operating pressures are generally essential and this complicates the mechanical design of the tank or other containing structure. Vast improvements in high-vacuum technology together with the need to replace oil-insulated switchgear has led to compact vacuum bottles used to retrofit old oil-filled circuit breakers.

(1) as a filling and cooling medium for transformers and some electronic equipment, and as a filling medium for capacitors, bushings, etc.;

(2) as an insulating and arc-quenching medium in switchgear;

(3) as an impregnant of absorbent insulation, e.g. paper, porous polymers and pressboard—these are used in transformers, switchgear, capacitors and cables; and

(4) as a heat transfer medium in addition to its insulation rôle in power cables, especially in force-cooled high-pressure oil-filled cables.

7.4.1 Breakdown mechanisms in liquids

Breakdown strengths of liquids are very dependent on liquid purity and all the breakdown mechanisms that are met in the practical use of liquids as electrical insulation are contamination mechanisms. Breakdown is caused by one of three contaminants; particles, water, or gas bubbles.

Particle-induced breakdown requires that there are dust particles, cellulose fibres from adjacent solid insulation or similar particles present in the liquid. If the particle has a higher relative permittivity than the liquid, electrostatic theory tells us that there will be a force acting on the particle moving it towards the region of greatest stress between the electrodes. If the particle contains moisture the force will be larger because of the high ∈ _r value for water. Other particles will be attracted to the same region and they align end-to-end, eventually forming a bridge between the electrodes. Current flows along this bridge giving localised heating and breakdown.

Water itself is inevitably likely to be present in practical liquids. Careful procedures for filling equipment and maintaining desiccants at breathing points in the apparatus can normally keep moisture levels to less than 20 ppm. Any globule of water present in the liquid will become elongated in the field direction by the action of an applied field. Breakdown channels will propagate from the ends of the globule and produce total breakdown. The electric strength of an oil can be halved by the presence of 50 ppm of water. The presence of water will significantly increase the dielectric loss in the oil and reduce the breakdown strength.

Bubbles can be formed by gas pockets in pits or cracks on surfaces containing the liquid, or they can arise from dissociation of liquid molecules, or local liquid vaporization through electron emission from sharp points on an electrode. Such bubbles will become elongated by the field in a similar way to water globules. As the breakdown strength of the gas in the bubble will be much lower than that of the liquid, the field inside the bubble may exceed the breakdown strength of the vapour. This will give a spark in the bubble which may cause dissociation of some of the surrounding liquid to generate more gas. Eventually the bubble will become so large that a complete breakdown between the electrodes will ensue.

7.4.2 Insulating oils

The insulating oils used extensively are highly refined hydrocarbon mineral oils obtained from selected crude petroleum, and have densities in the range 860–890 kg/m³ at 15°C. Oil for transformers and switchgear is dealt with in BS 148:1998. A number of special mineral oils are employed for impregnated paper capacitors and cables and others—usually of higher viscosity and flash point—for rheostats and for filling bus-bar chambers in switchgear. Typical properties are given in Tables 7.3 and 7.9.

7.4.3 Inhibited transformer oil

Oils operating at comparatively high temperatures, in the presence of oxygen and various catalytic materials, develop sludges and high acidity. These effects can be alleviated by adding various inhibiting substances to the oil, the most widely known being di-tertiary-butylparacresol used in quantities generally less than about 0.5% of the oil by weight. Materials of this type delay the point at which sludge and acid formation begin; but once the inhibitor has been used up, deterioration will proceed at the same rate as if no inhibitor had been used. For large power transformers, it has not been found necessary to use these inhibiting substances because of improvements in the construction which have reduced access of oxygen by conservators, hermetic sealing or the use of a nitrogen blanket above the oil surface. Another improvement has been the covering of copper surfaces so reducing the catalytic effect. For transformers operating under more adverse conditions of temperature such as distribution or pole-mounted units, a better case can be made for using inhibited oils.

7.4.4 Synthetic insulating liquids

Synthetic hydrocarbons are fairly widely used for power-cable insulation and as capacitor dielectrics. These are more expensive than petroleum oils but they generally have better electrical properties because of their lower contamination levels and they can have better gas-absorbing properties. A commonly used synthetic oil is poly-iso-butylene, commonly known as polybutene. Different polymer chain lengths can be produced giving a wide range of viscosities from low viscosity liquids to sticky semi-solids. The high molecular weight tacky and rubbery materials can be used mixed with oil, resin, bitumens, polyethylene and inorganic fillers to produce non-draining and potting compounds. Another synthetic oil that is used for cable insulation is dodecylbenzene, an aromatic compound. The physical properties are similar to mineral oils, but the viscosity-temperature characteristics show much higher low-temperature viscosity than comparable polybutenes. However, the gas-absorbing characteristics are good. Electrical properties are generally similar to mineral oils but the permittivity is somewhat higher for dodecylbenzene.

Polychlorinated biphenyls (also called askarels) have been used as high permittivity (3–6) fire-resistant insulating liquids since the 1930s but their effect as an ecological poison has limited their use to sealed equipment in recent years and all use of these liquids is being discouraged.

Silicone fluids (poly-dimethylsiloxanes) have been used as alternative fire-resistant insulating liquids, but their fire resistance is inferior to the askarels. They are generally gas evolving and their arc products can cause problems. However, they are very stable and have good electrical properties and are used in transformers and capacitors.

Several liquids have been developed as alternative high permittivity insulating liquids to replace the askarels for capacitor dielectrics. One possible group of materials is the organic esters. They have good viscosity temperature characteristics and are less flammable than mineral oil and can be either gas producing or gas absorbing, depending on their composition. When carefully purified, their electric strengths are about 20 kV/mm and dissipation factors average 0.001 at 20°C. Diesters have relatively high permittivities (4.3 for di-2 ethylhexylphthalate). Other liquids that may be suitable for electrical insulation are phosphate esters, halogenated hydrocarbons, fuoroesters and silicate esters. Castor oil is a good insulation material for d.c. stress with a permittivity of 4.7, but it has a high dissipation factor of 0.002 that makes it unsuitable for most a.c. applications.

BS EN 60867:1994 and BS 61099:1992 give specifications for unused liquids based on synthetic aromatic hydrocarbons and for unused synthetic organic esters respectively.

(1) for filling small cavities and large spaces, e.g. in metal-clad switchgear, transformers, cable-boxes and capacitors;

(2) for impregnating absorbent materials and windings;

(3) as the bond in laminated materials;

(4) as the basic material in moulding compounds; and

(5) for external coverings of parts and apparatus (i.e. envelopment and encapsulation).

7.5.1 Bitumens

Highly refined bitumens, which are usually steam distilled, and of numerous grades, varying from semi-liquids to hard bitumens of melting point over 120°C, are used extensively for filling cable boxes, transformers and switchgear. These have high electric strength and are very inert and stable. As the coefficient of expansion is high, care has to be taken in filling large spaces to prevent voids and cracks on cooling. Some of the bitumens, especially those of high melting point, are rather brittle; all are soluble in oil, but they have excellent resistance to moisture. BS 1858:1973 deals with bitumen-base filling compounds for electrical purposes. Properties of typical bituminous compounds are given in Table 7.10. Some bitumens are used as ingredients in varnishes and paints, rendering these very resistant to moisture and chemical attack. A few impregnating compounds contain bitumens, especially those used for treating high-voltage machine bars and coils.

7.5.2 Mineral waxes and blends

Various mineral waxes such as paraffin, ceresine, montan and ozokerite—including microcrystalline waxes—also blends and gels of these, having melting points in the range 35–130°C, are used for impregnating capacitors, radio coils and transformers, also for other purposes such as cable manufacture. Properties of mineral waxes are given in Table 7.11.

7.5.3 Synthetic waxes

A few synthetic waxes—principally chlorinated naphthalene—with melting points up to 130°C have certain advantages over natural waxes, particularly higher permittivity which enables smaller paper-insulated capacitors to be made. Properties of a typical synthetic wax of this type are given in Table 7.11.

7.5.4 Natural resins or gums

These materials, which may be classified broadly as shellac, rosin (colophony), copals and gum arabic, are used principally as ingredients in varnishes or liquid adhesives. In some cases they are used direct, e.g. as powders, for a bonding medium between layers of mica which are hot pressed, but they are usually dissolved in spirit solvents, e.g. methylated spirits (or water in the case of gum arabic), and applied as a solution to mica, paper, etc., for subsequent laminating and hot rolling, pressing or moulding (see Table 7.11).

7.5.5 Miscellaneous fusible compounds

Numerous compounds of bituminous and other types are used for all manner of cavity-filling purposes, some (mainly rosin) are oil resisting and are employed where bituminous compounds cannot be used owing to the presence of oil, e.g. for bushings of oil circuit-breakers and oil-immersed transformers. Others are used for sealing over the tops of primary batteries and accumulators. Compounds containing beeswax are useful impregnants for small coils not exposed to heat, e.g. on telephone apparatus; and sulphur, ‘sealing wax’ and ‘Chatterton’s compound’ are examples of materials finding uses for miscellaneous applications where, say, the heads of screws in insulating panels and mouldings require to be sealed over.

7.5.6 Treatments using fusible materials

Treatments with bitumen, waxes, etc., usually consist of thorough vacuum drying of the coils, capacitors or other parts to be treated, followed by complete immersion in the compound while in a molten condition and at a temperature such that the viscosity is low enough to facilitate penetration; the molten compound generally being admitted to the impregnating vessel under vacuum. Pressure (up to 10 atm) is often applied during the immersion period to assist penetration. Such treatments enable spaces in windings to be filled thoroughly, and absorbent materials such as papers and fabrics are often well saturated with the impregnants, especially in the case of waxes. These treatments provide good resistance to moisture absorption and improve transference of heat from the interior of coils, also eliminating discharges in high-voltage windings and capacitors by the filling of air spaces.

7.5.7 Synthetic resins

An increasing number of the well-known synthetic resins, which form the basis of the principal ‘plastics’, are of great use to electrical engineers on account of their fusibility or softening characteristics at elevated temperatures, which enables them to be converted to desired shapes. The synthetic resins can be divided into two groups: thermoplastic and thermosetting (see Table 7.12).

7.5.8 Thermoplastic synthetic resins

Polyethylene is waxy, translucent, tough and flexible, with a sharp melting point at about 110°C, and is used for high-voltage and high-frequency applications. It is readily injection moulded, extruded as wire coverings, and in sheet, rod and film form.

Polytetrafluoroethylene a white powder that can be moulded or extruded. It is highly resistant to moisture and chemicals, and withstands temperatures up to 250°C. It is used in high-frequency application.

Polystyrene softens at 70°C. It can be compression or injection moulded and may be used with a mineral filler to improve heat resistance.

7.5.9 Thermosetting synthetic resins

7.5.10 Encapsulation

When electronic or electrical components and circuits must resist the effects of climate, industrial atmospheres, shock or vibration they may be encapsulated. They are then generally known as ‘potted circuits’.

Certain thermosetting synthetic resins are of the greatest use as they can be easily poured from low-viscosity liquids and made to set without the use of pressure and, in some instances, with very little heat. A suitable material must: (1) be a good insulator over a wide range of temperatures (volume resistivity say 10⁶Ω-m); (2) polymerise or set without spitting off water or other products; (3) have low viscosity at pouring temperature, low vapour pressure, and freedom from deleterious side-effects on personnel who are using it; (4) show small shrinkage, especially when changing from the liquid to the solid state; and (5) must adhere to all materials commonly found in electrical equipments, e.g. to brass, solder, steel and insulating boards. Only the epoxy resins possess all the essential requirements for successful encapsulation and even these need an inorganic filler to obtain the best heat resistance, low shrinkage, good electrical properties and high thermal conductivity.

The epoxide resins of use in potted circuits are derived from a condensation reaction between epichlorhydrin and bisphenol A. They are cross-linked with aliphatic or aromatic amines, acid anhydrides and a few other chemical compounds to give thermally, electrically and mechanically stable resins. The addition of inorganic fillers improves them and reduces their shrinkage, tendency to crack at low temperatures and cost. The mixture of resin and cross-linking agent (known as a hardener) is called a system. An accelerator may be added, as well as various diluents, both reactive and non-reactive. Accelerators and promotors alter the speed of reaction and the pot life.

Typical potting formulations, in parts by weight, are:

Formulation A is satisfactory for small units of about 0.1 kg of mixture. It sets at room temperature, but is post-cured at an elevated temperature dependent on the heat resistance of the included components; 18 h at 65°C is usually satisfactory. Such a unit is suitable for small electronic packages containing small components, including semiconductors. Larger masses, depending on their geometry, may exhibit a strong exotherm (i.e. generate heat) which may damage the included components.

Formulation B is used hot (usually at about 65°C) and is very fluid at this temperature besides possessing a long pot life. This makes it suitable for the potting of transformers. Vacuum impregnation is essential to eliminate voids, which would result in ionisation and corona discharges. The large amount of filler greatly improves the thermal and mechanical properties and allows a larger casting to be produced without a high exotherm. Again, a post-curing cycle is essential to bring out the best properties; in this case about 18 h at 120°C is satisfactory and will produce a material with high volume resistivity and heat resistance. The pot life of this mixture is about two hours at 65°C, which may be compared with formulation A whose pot life is less than half an hour at room temperature.

In use the resin and dried filler of A are mixed together and stored under vacuum or in a desiccator until used. The hardener is added and thoroughly mixed just before pouring into the mould or other article to be potted. It stands at room temperature to set or ‘gel’ and is subsequently post-cured; this latter process may be carried out after removal from the mould if required.

For formulation B the resin, hardener and dried filler are all mixed and stored as before, and the accelerator is added to the mixture just before use and heated to 65°C. It is poured into the mould or other article. For transformers this is usually done under a vacuum of about 1 mmHg to 1 cmHg. Although a large amount of inorganic filler is used, this is filtered to some extent by the windings of the transformer so that the insulation between turns is mostly of unfilled resin, giving high breakdown strength and freedom from corona discharges. Good adhesion at the terminals is ensured by the nature of the resin system, but this also means that it sticks to the mould unless a release agent is used. Various preparations are used, including mixtures of high polymers, silicones, greases and waxes. The agent must be confined to the mould and not allowed to contaminate the components inside the casting.

The requirements of low shrinkage and low exotherm have been stated. The effect of the former is to damage components by compressional forces and is particularly severe on some nickel–iron alloys used in making inductors; thin-film resistors and capacitors are also vulnerable to this form of damage. Isolating the components from the resin by means of low modulus materials can appreciably reduce this defect. The heat of reaction (exotherm) is also damaging to organic materials and to semiconductors; this often results in a loss of volatile matter causing shrinkage and the effects already noted above.

All adhesives contain polar bonds in their molecular structure. These give rise to changes in dielectric properties as the frequency and temperature varies, causing the electrical behaviour of the components inside the casting to change as the frequency and temperature changes.

Another disadvantage of potted circuits is that they are irreparable: they must be designed as ‘throw away’ subunits and made at an economical price. For this reason they are rarely used in domestic applications such as radio or television, but are of particular use in military electronics, for machine control and similar purposes where the utmost reliability is essential and first costs are relatively unimportant.

7.6.1 Air-drying varnishes, paints, etc

One class of air-drying varnishes and lacquers consists of plain solutions of shellac, gums, cellulose derivatives or resins which dry (e.g. in 5–30 min) and deposit films by evaporation of the solvent. Other air-drying varnishes and paints form films which harden by evaporation of solvent accompanied by oxidation, polymerisation, or other chemical changes which harden and toughen the film. These processes take several hours.

7.6.2 Baking varnishes and enamels

Where the toughest and most resistant coatings are required, baking varnishes and enamels are used, the evaporation of solvents and hardening of the material being effected by the application of heat. Typical varnishes of this class require baking at, say 90–110°C in a ventilated oven for 1–8 h. During the baking (or ‘stoving’) the hardening is usually caused by oxidation, but in some cases polymerisation takes place. The latter process does not require oxygen and, provided that the solvents are first removed, drying can take place within the interior of coils. In consequence these varnishes are preferred to the oxidising types which skin over and leave liquid varnish underneath. Such thermosetting impregnating varnishes are used extensively for treating coils, and the windings of small machines and transformers.

7.6.3 Solventless varnishes

Thermosetting synthetic resins are used for impregnating windings and, owing to the manner of hardening during which little or no volatile matter is evolved, spaces in the interior of windings can be filled completely with non-porous resin. One type consists mainly of oil-modified phenolic resins, similar to the oleo-synthetic resinous varnishes but without solvents; they are consequently termed solventless varnishes. When used for impregnating they are in a hot liquid condition, the material solidifying within the winding by baking after impregnation.

Specially formulated low-viscosity resins of the polyester or epoxy type are also used for impregnation. It is possible to use solventless resins in conveyorised plants where the parts are dipped in the resin, allowed to drain and then passed through a heated tunnel to cure the resin. In other cases, particularly for tightly wound apparatus, the parts are treated in the resin using a vacuum-pressure process to ensure that a high level of impregnation is attained.

The most recent development in treating industrial machine windings is the ‘trickle’ process. The wound part is heated and mounted at a slight angle so that it can be rotated slowly about its axis. A metered quantity of the resin is allowed to trickle on to the winding and under the action of gravity and the rotational forces, the resin penetrates to all parts of the winding and completes the impregnation. It is possible to completely fill the interstices of the winding without loss of resin by draining. Radiant heat may be applied to complete the cure while the parts are rotating, or heating currents may be circulated through the winding. Trickle impregnation can be performed automatically and the process can be completed in a few minutes without removing the wound parts from the production line.

7.6.4 Silicone varnishes

Varnishes based on silicone resins are in general use. Some include other resins, etc., such as alkyd resins. Silicone varnishes are used for impregnating and coating cloths, tapes, cords, sleevings, papers, etc., made from glass fibres and polyethylene-terephthalate fibre; for bonding mica, glass cloths and papers, e.g. for slot insulation; for coating and bonding glass; for ‘enamelling’ wires for windings; and for all manner of impregnating, bonding, coating and finishing purposes such as the treatment of windings for classes B, F and H.

7.6.5 Properties of varnishes, etc

The properties of the solid films formed after drying and hardening varnishes, paints, etc., naturally depend mainly on the principal basic materials, e.g. gums, resins and oxidising oils. Other materials added are: ‘driers’ to accelerate drying; ‘plasticisers’ to improve the flexibility; and pigments to provide the required colour and improve the hardness and filling capabilities.

Treatments of windings and insulation parts, by varnishes, enamels, lacquers or paints, take the form of (a) application of external coatings—chiefly for providing protection against moisture and oils, or (b) impregnation of windings and absorbent materials, for rendering them less susceptible to moisture and improving their electrical and heat-resisting properties. Both air drying and baking materials are used for (a), but only baking varnishes and enamels are suitable for (b). In practically all cases, thorough drying prior to application of the varnish is essential as for compound treatment, but with varnishes, extra care is required to remove excess varnish by proper draining and to extract solvents thoroughly before hardening the material by baking. This is usually done in well-ventilated ovens at temperatures of 80–150°C and sometimes by the application of vacuum for a period to assist the removal of solvent. These processes are not required when solventless varnishes are used. In the case of silicone varnishes, the solvents must be removed and baking must then be carried out at temperatures in the range 150–260°C.

Insulating varnishes are the subjects of BS 5629:1979, BS 7831:1995 and BS EN 60464:1999. Typical properties are listed in Table 7.15.

7.7.1 Breakdown in solids

The breakdown strength measured for a solid is very dependent on the time of application of the voltage. Strengths of 100 kV/mm to 1.5 MV/mm can be obtained for short pulses under carefully controlled laboratory conditions, but in practical insulation systems breakdown strengths of only about 20 kV/mm are obtained. The high laboratory value is known as the intrinsic electric strength of the material, with breakdown due to electronic processes. In practical insulation, however, breakdown is due to: erosion, thermal effects, electromechanical effects, or treeing.

Discharge/Erosion breakdown originates at internal voids in the insulation. These are left at the fabrication stage and all solids contain them, although in a high-quality material they will be very small (microvoids). These voids will be filled with air or another gas with a permittivity and dielectric strength much lower than that of the solid. Electric field enhancement in the micro-void takes place due to the relative permittivity difference in the two media. Consequently, the gas filling the void will break down at a voltage lower than the normal breakdown strength of the material and this partial discharge will erode the solid at the ends of the spark on the surface of the void. These discharges will occur twice for every cycle of an alternating supply, so over a period thousands of millions of these discharges will occur in a cavity. The erosion effects of these discharges can eventually be sufficient to cause complete failure of the insulation. This is the primary reason why the insulation of a component may fail after a number of years in service.

Thermal breakdown is breakdown caused by internal heating in the insulation due to dielectric loss. Insulating materials are not perfect capacitors and they do have internal energy losses. The quality of the insulation is often measured in terms of how small these losses are, with the loss factor tan δ, where (90-δ) is the phase angle between the voltage and current for a sample of the material, commonly used as the parameter to assess the losses. This energy dissipated within the insulation will cause heating and electrical insulation is usually a poor thermal conductor, so a significant temperature rise in the insulation will be produced. In some circumstances the internal losses may increase as the temperature increases and a thermal runaway situation results. The temperature of the material will rise, losses will increase and the temperature will rise further until the material fails. Thermal breakdown is most likely to occur in power cables operated beyond their power rating, polymers operating near their softening points or in high-frequency applications.

Electromechanical breakdown results from the mechanical forces that an applied electric field produces. This force will reverse for every cycle of the applied voltage and can produce cracks or other damage in solid insulation.

There are two types of treeing breakdown. The first type originates at sharp points on electrodes which act as the root of fine branching channels that propagate through the insulation. Over a period of time these fine channels gradually extend towards the other electrode until complete failure of the insulation occurs. This is another mechanism which can produce failure after equipment has been in service for months or years. The second type of treeing is known as water treeing and only occurs when there is water in contact with the surface of the insulation. Microscopic damage to the insulation gradually spreads out from a high-field region until failure of the material occurs. A major difference between this type of treeing and the previous one is that the damage cannot be seen once the material has dried out.

7.7.2 Rigid boards and sheets

7.7.3 Tubes and cylinders

Tubes and cylinders can be produced by several methods. Materials capable of being moulded can be treated by compression or casting techniques. Many plastics can be extruded. Vulcanised fibre tubes are wound from paper treated with zinc chloride solutions. Laminated materials are generally wound on special machines with heated rollers while tension and pressure are applied to consolidate the layers. In this way SRB paper and fabric tubes and cylinders can be produced by rolling the treated material convolutely on heated mandrels. Some of the smaller tubes are moulded in a split mould while still on the mandrel. Cylinders (considered as tubes with internal diameters above 76 mm) are made by similar techniques. These tubes which are generally for use in large transformers may have diameters as high as 2 m and lengths of 4 m. SRB cotton fabric tubes and cylinders are produced by similar methods; SRB glass-fibre tubes and cylinders can be rolled from most of the rigid-board materials.

Pressboard tubes and cylinders (mainly for oil-immersed transformers) are produced by winding presspaper on mandrels under tension, applying adhesives resistant to transformer oil (gum arabic, casein, phenolic resins, etc.). Tubes previously rolled from shellac bonded micanite for high voltage use are now widely superseded by tubes rolled from mica paper bonded with epoxy or silicone resins.

In addition to the convolutely wound tubes, a wide range of products can be produced by helical winding of strips (papers, fabrics, film, etc.), adhesives being applied meanwhile. The edges of the strip are generally butted together and this technique makes it possible to produce tubes the length of which is limited only by the requirements of transport.

Yet another method of winding tubes, chiefly with glass fibres, is known as filament winding. Strands of resin treated glass fibre are applied to mandrels in special winding machines. Restrictions on shape are less stringent, and very good mechanical properties are obtained.

7.7.4 Flexible sheets, strips and tapes

In very many applications a certain amount of flexibility is necessary, mainly to enable the materials to be readily applied to conductors, coils and various shapes, often irregular. The following are the principal flexible insulating materials, used for such purposes as wrappings on conductors and connections of machines and transformers, bus-bars and other parts; interlayer and connection insulation of coils; and slot linings of armatures.

7.7.5 Sleevings, flexible tubings and cords

Tubular sleevings made of cotton, polyester, asbestos, glass, ceramic, quartz, silica, polyamide and other fibres supplied untreated in flat tubular form are suitable for low-voltage insulation of connections of coils, etc. Glass sleeving given a high-temperature treatment before use has the fibres set in place, thus preventing unravelling during assembly. Varnishing of all these sleevings during treatment of the coils is the usual practice.

Similar sleevings may be varnished or treated with resins and polymers before use; these types are known as coated sleevings. Most colours can be produced, thus helping in identification of circuits. The coating materials in general use for cotton and rayon sleevings are natural or synthetic varnishes. For glass fibres, high-temperature polyvinyl chlorides, polyurethanes and silicone elastomers are widely used as well as high-temperature varnishes such as those based on acrylics, polyesters, silicones or polyimide resins. Coatings of fluorinated resins on glass fibres are available.

Flexible tubings can be extruded from most of the materials mentioned in Section 7.7.4.6, but as many of these materials are thermoplastic and also liable to oxidation problems, care is required at temperatures above 90°C.

Other methods of making tubing are by helical winding with adhesives as a bond, when very thin walls (e.g. 0.025 mm) are required or only small quantities are needed. Many of the extruded tubings and some of the helix types can be caused to shrink by applying heat after the tubes have been assembled. Most shrinkage takes place in diameter but there is sometimes a small amount in length. These sleeves are useful for insulating connections and joints, for covering capacitors, resistors, diodes, etc., for colour coding, sealing and moisture proofing. The two main shrinkage processes are: (1) thermoplastics stretched during manufacture, and allowed to cool so that the mechanical strains are frozen in, shrink next time the material is heated; and when certain materials are exposed to high-energy radiation, molecular cross-linking occurs, and they shrink when the temperature is subsequently raised.

Cords for lashing bundles of switchboard wiring, holding leads in position and for tying down coils in machines and transformers, may be made from yarns laid together and twisted or braided, or from narrow woven or slit tape. The trend is away from cellulose materials (linen and hemp) to synthetics, especially glass and polyester fibres. Because of the poor abrasion resistance of glass fibres, glass cords are often pretreated with epoxy or silicone resins or with various polymers. Normally the type of treatment depends on the application.

For high-voltage machines, a useful material is a round cord 3–5 mm diameter comprising a central core of straight polyester fibres surrounded by a braided polyester sheath. This material shrinks slightly on heating, and tightens. For smaller windings, polyester mat can be slit into tapes and several tapes twisted together. Polyester fibre cords of all kinds have the advantage over glass cords that knots can be made without appreciable loss in strength. Glass and polyester bands can be treated after application with varnishes and both have good resistance to abrasion and to mould growth. For rotors straight unidirectional glass-fibre bands pretreated with epoxy, polyester, or acrylic resins are used. These bands must be applied under controlled tension to ensure that the windings are held against centrifugal force. Such bands can be placed safely near voltage carrying parts and (unlike wire bands) are not affected by magnetic fields.

For switchboard wiring, cable harnesses, etc., neat lashings can be made with small-diameter extruded poly-vinylchloride threads. These materials do not burn readily and are obviously well suited for use with PVC insulated switchboard wiring.

7.7.6 Wire coverings

Wires for coils and armature conductors are insulated with lappings of cotton, asbestos, glass fibre and polyamide fibre, all of which are hygroscopic and require treatment with oils, compounds or varnishes, usually after winding. Relevant specifications are BS 1497, 1933, 2479, 2480 and 2776. A recent addition to the range of wire and strip coverings is lapped polyimide film, bonded to itself and to the conductor with FEP polymer: the films are thin and flexible, electrically good and workable at 220°C.

Orthodox oleo-resin (BS 156) and polyvinyl formal and acetal enamels (BS 1844) are tough, resistant to abrasion and to softening by varnishes. Enamelled wires have a better space factor than those with fibre covering.

Some wires are covered in separate operations with synthetic enamels of widely differing characteristics, giving a range of finely balanced properties. The inner coat can be considered as the insulant; the outer coat can give improved resistance to solvents and abrasion, better cut-through resistance, heat-bonding of turns into a solid coil, etc. Wires coated with polyurethane polymers are useful because they can be soldered without first removing the enamel (BS 3188).

For high-temperature working, the performance of PTFE coverings is limited by cold flow of the insulant under mechanical pressure. Silicone resin gives a high-temperature covering but the solvent resistance and mechanical properties are not very good. Better mechanical characteristics are given by polyimide, polyamide imide, polyester imide and polyhydantoin, but these materials are still uncommon.

7.7.7 Moulded and formed compositions, plastics, ceramics, etc

Articles of various shapes, often quite complicated, which cannot readily or economically be matched or built up from sheet, rod or tube materials may be obtained by forming, moulding, coating or casting one of the ‘plastics’ (which are all essentially organic), or a ceramic such as porcelain, or glass or other inorganic materials. The materials can be grouped approximately as follows.

7.7.8 Methods of moulding and forming materials

7.8.1 Breakdown mechanisms in composite dielectrics

The construction of composite dielectrics usually starts with the solid material in tape form that is wound around one of the conductors. Several layers of tape are used and there is a small gap between adjacent tapes, with the gaps staggered between layers (Figure 7.5).

The most widely used composite insulation system has the spaces between the tapes filled with a mobile insulating liquid. The system that is used in many power engineering applications such as transformers and oil-insulated switchgear is paper and mineral oil. The dielectric constant of paper and mineral oil is very similar, approximately 2.5, but very different from that of air. It follows that if there are air bubbles in the system there is a much higher electric field in the air bubbles than in the rest of the insulation. Bubbles may become trapped in the gaps between the tapes. The air in such bubbles will break down at a much lower applied voltage than the rest of the insulation, producing additional gas and eroding the solid insulation in contact with the bubble perimeter. This can eventually lead to breakdown of the complete insulation structure.

7.8.2 Oil/paper systems

This insulating system is excellent for high electric strength over long periods of time, provided the insulation is free from significant partial discharges. It is therefore essential that gas bubbles are prevented from forming. This is done by applying a vacuum to the insulation to remove dissolved gas where this is possible, and in insulation systems such as high-voltage power cables by applying several atmospheres pressure to inhibit bubble formation.

In laboratory-scale tests it was found that increasing the pressure of the oil from one atmosphere to 14 atm increased the electric stress that could be applied before partial discharges could be detected from 47 kV/mm to 69 kV/mm, or from 60 kV/mm to 90 kV/mm when a thicker oil was used. There is always a problem in relating the results of laboratory investigations to the design of practical apparatus because of the reduction in strength obtained as the volume of the insulation increases. In extreme cases the breakdown strength may be halved by two orders of magnitude increase in volume under stress.

In a series of tests on samples of oil/paper insulation of reasonable size the 50 Hz strength always exceeded 40 kV/mm and the 1/50 μs impulse strength was always greater than 90 kV/mm. The presence of moisture in the paper will give a 40% reduction in breakdown strength when the moisture content is increased from zero to 8%. The impulse strength of oil-impregnated paper increases as the thickness of the paper tapes decreases. In one set of tests the impulse strength was 130 kV/mm for 0.13 mm thick tapes and 160 kV/mm for 0.03 mm thick tapes.

The proven long-life-times of oil/paper systems mean that this insulation is the normal choice for insulating power transformers, cables and power-factor-correction capacitors.

7.9.1 Type of radiation

The more common forms of radiation encountered are: neutron and γ from nuclear reactors, neutron and γ from isotopes and electrons and X-rays from particle accelerators. The unit of absorbed radiation is the rad (= 100 erg/g) or the megarad (= 10⁸ erg/g of material). The megarad is equivalent to 10 kJ/kg.

Although the effects of radiation on materials are cumulative and, therefore, dependent on the total dose, the dose rate (generally expressed as megarads per hour) may have some further effect. This point must be considered when experimental work is being carried out. Fortunately, it has been found that changes in most materials due to irradiation are practically independent of the type of radiation encountered. Hence, various sources of irradiation may be used for experimental work and those normally employed are:

The most convenient is the linear accelerator, in which a small-diameter beam can be made to scan the parts to be irradiated with uniformity of dosage. The physical conditions—temperature, ambient atmosphere (air, carbon dioxide, nitrogen, etc.), and humidity—must be carefully selected and controlled.

7.9.2 Irradiation effects

The effects of radiation are usually assessed in the first place by visual examination, as many materials discolour, crack, disintegrate or melt, while flexible or soft materials may become hard and brittle.

More advanced work relies on the determination of changes in measured properties after certain periods of irradiation. Results of tests on unirradiated samples are compared with those on similar samples after subjection to known energies for different times. Non-destructive tests (loss tangent, permittivity, resistivity, changes in weight and changes in dimensions) are particularly useful because they can be repeated on the same specimen as a function of the total dose. Other tests used to determine the effects of irradiation are electric strength; tensile, shear and impact strength; elongation; hardness; flexibility; and water absorption.

In general, organic insulating materials deteriorate mechanically and electrically as the result of irradiation, the mechanical properties usually being impaired at a greater rate than electrical ones. These results are mainly due to chemical changes which occur in the materials consequent upon certain rearrangements of their molecular structure, usually with the evolution of one or more gases, such as methane, carbon monoxide, carbon dioxide or hydrogen. The probable life-dose for typical insulating materials is given in Table 7.24. The dose figure is that at which marked deterioration is observable, there is a 50% reduction in mechanical and electric strength, or some similar factor. The life-dose is approximate: it depends on the actual formulation of the material and irradiation conditions such as dose-rate and ambients.

7.10.1 Basic definitions

An ideal dielectric is a material or medium which has no free electrons so that no conduction can take place. It is an ideal insulator. Electrically it can be represented as a pure capacitor

Real dielectrics, however, have some free electrons but much less compared with conductors. Their equivalent circuit can be represented by an RC parallel circuit.

Dielectrics have two main electrical applications:

7.10.2 Types of dielectrics

Dielectrics can have three forms: gas, liquid and solid.

In gases, the atoms/molecules do not interact but can contain free electrons which are responsible for the conduction process. Conduction mechanisms due to positive and negative ions as well as charged molecules are also found in some gases.

In liquids dielectrics, there is some interaction between molecules, and they are also known to contain impurities in real applications. This makes the conduction process essentially ionic in nature with the addition of contributions due to charged particles.

Solid dielectrics have more complex conduction mechanisms (such as thermal, tunnelling, hoping) governed by free electrons, holes and ions.

Solid dielectrics can be either:

In general, no solid or liquid is completely structureless. Ionic solids do not form glasses but covalent solids form glasses (e.g. polymers) e.g. MgO is an ionic solid (non glass) and SiO₂ is a covalent solid (glass).

7.10.3 Polarisation in dielectrics

When an insulating material is subjected to an electric field, a limited displacement of charge takes place at the atomic, molecular and bulk material levels. This charge displacement is known as polarisation.

7.10.4 Quantification of dielectric polarisation

For a vacuum-filled parallel-plate capacitor, the surface charge density Q₀ is defined as Q₀ = ε₀ V/d where V is the applied voltage, d the separation distance between plates, and ε₀ relative permittivity of free space (ε₀ = 8.854 pF/m).

If the vacuum insulating medium is replaced by a dielectric material of relative permittivity ε_r, the new surface charge density will be Q_ds = ε₀ε_r V/d.

Thus, resulting in an increase of surface density ΔQ_s = Q_ds − Q₀ = (ε_r-1)ε₀ V/d.

This quantity is known as the dielectric polarisation P. In this case, the expression can be written as P = (ε_r − 1)ε₀V/d = (ε_r-1)ε₀E = D_d−D₀.

With D_d is the electric flux density in the dielectric case. D₀ is the electric flux density in the vacuum case.

7.10.5 Properties of dielectric materials

Dielectrics can be grouped according to their structure and the way they react to the action of an electric field.

7.10.6 Example of ferroelectric material: Barium titanate and its applications

Barium titanate is one of the most important ferroelectrics. It is formed from the reaction of a mixture of BaCO₃ and TiO₂ heated at 1250°C. The product is powdered and then worked by means of common ceramic techniques. Admixtures of other oxides are employed to modify the dependence of permittivity on temperature. Its high permittivity is exploited in ceramic capacitors for the range 500–10000 pF for electronic equipment. Its piezoelectric property is used in transducer effect since it can be fabricated in a variety of complex shapes. The hysteresis in polarisation is used in timing control or carrier-frequency modulation of a voltage control.

7.10.7 Frequency response of dielectrics

Most materials are polarisable in several ways, producing complex frequency dependence. At the highest frequencies, only the electronic polarisation will ‘keep up’ with the applied field. At frequencies where permittivity varies rapidly, there is a peak in the dielectric loss. Figures 7.10 and 7.11 show typical variations of permittivity and loss angle with frequency.

7.11.1 Materials

Traditional insulators are made of ceramic, either porcelain or glass. The design of these insulators for high voltage applications evolved from telegraph wire experience. These insulators are known to perform reliably in service for several decades. However, their bulky nature, poor pollution performance due to hydrophilic surfaces and susceptibility to vandalism raised a need for better materials.

Non-ceramic insulators (NCI), also commonly known as polymeric or composite insulators, are made of a compound of materials having one of several polymers as a base. Two main materials have been used extensively on outdoor high voltages power systems: silicone rubber and Ethylene propylene diene monomer (EPDM). Their combination (EPS) has been proposed to solve particular problems of surface properties and material strength. The main advantages of these materials are their light weight (since the insulator is made from a fibre rod with a polymeric outer sheath) and more importantly their surface hydrophobicity which inhibits the formation of continuous wet paths along the insulator surface. This is particularly advantageous under pollution condition when outdoor insulators suffer from dry-bands followed by localised discharges across the dry bands.

Room temperature vulcanised (RTV) coating is used to enhance porcelain insulators pollution performance by spraying/applying a thin layer of silicone rubber on the porcelain surface giving it a hydrophobic surface similar to that of silicone rubber insulators. RTV coatings contain typically 5% (by mass) cyclic LMW polydimethyl siloxanes.

IEC 1109:1992 Standard describes acceptance test procedures for composite insulators and IEC 507:1991 specifies artificial pollution tests procedures for high voltage insulators to be used on a.c. systems. However, these latter procedures are particularly suited for porcelain and glass insulators. At present, no standard procedure exists specifically for polymeric insulators. One difficulty is related to surface pollution because the hydrophobicity of polymeric insulators makes the conventional technique not suitable. Various methods have been suggested to obtain a uniform and repeatable pollution layer on polymeric surfaces. These include surface abrasion, kaolin spraying, multiple layer spraying and the use of small quantities of wetting agents, such as Triton X-100 as used in BS 5604:1986 (same as IEC 587:1984). International Standard IEC815:1986 gives guidance on the selection of insulators for high voltage applications taking into account the insulator shape and pollution severity.

7.11.2 Hydrophobicity loss and recovery

Extensive laboratory tests and field experience have shown that these materials can lose their hydrophobicity following a number of individual stresses or their combination. In particular, discharge activity on the insulator surface is found to reduce the hydrophobicity significantly. However, silicone rubber is found to recover its hydrophobicity after a certain time (which is still not well quantified) without the damaging stress. This process of recovery is thought to be due to the diffusion to the surface of low molecular weight (LMW) molecules and/or the changes in orientation of the methyl groups of the polymer.

7.11.3 Degradation ageing factors of polymeric insulator surfaces

Despite their excellent pollution performance, polymeric insulators are found to experience faster degradation on their surface compared with conventional insulators. Although such degradation depends on insulator material and design, a number of factors have been identified as affecting the surface properties of polymeric insulators. Some of these are described in the following processes.