Properties of Insulating Materials

The main requirements of the insulating materials used for power apparatus are:

High insulation resistance.
High dielectric strength.
Good mechanical properties i.e., tenacity and elasticity.
It should not be affected by chemicals around it.
It should be non-hygroscopic because the dielectric strength of any material goes very much down with moisture content.

Vulcanized Rubber

Rubber in its natural form is highly insulating but it absorbs moisture readily and gets oxidized into a resinous material; thereby it loses insulating properties. When it is mixed with sulfur along with other carefully chosen ingredients and is subjected to a particular temperature it changes into vulcanized rubber which does not absorb moisture and has better insulating properties than even pure rubber. It is elastic and resilient.

The electrical properties expected of rubber insulation are high breakdown strength and high insulation resistance. In fact, the insulation strength of the vulcanized rubber is so good that for lower voltages the radial thickness is limited due to mechanical consideration.

The physical properties expected of rubber insulation are that the cable should withstand normal hazards of installation and it should give trouble-free service. Vulcanized rubber-insulated cables are used for wiring houses, buildings, and factories for low-power work. There are two main groups of synthetic rubber material :

general-purpose synthetics which have rubber-like properties and
special-purpose synthetics which have better properties than rubber e.g., fire-resisting and oil-resisting properties.

The four main types are:

Butyl rubber,
Silicon rubber,
Neoprene, and
Styrene rubber.

1. Butyl rubber: The processing of butyl rubber is similar to that of natural rubber but it is more difficult and its properties are comparable to those of natural rubber. The continuous temperature to which butyl rubber can be subjected is 85°C whereas for natural rubber it is 60°C. The current rating of butyl-insulated cables is approximately the same as that of paper or PVC-insulated cables.

The butyl rubber compound can be so manufactured that it has low water absorption and offers interesting possibilities for a non-metallic sheathed cable suitable for direct burial in the ground.

Silicone rubber: It is a mechanically weak material and needs external protection but it has high heat-resistant properties. It can be operated at temperatures of the order of 150°C. The raw materials used for the silicon rubber are sand, marsh gas, salt, coke, and magnesium.

Neoprene: Neoprene is a polymerized chlorobutadiene. Chlorobutadiene is a colorless liquid that is polymerized into a solid varying from a pale yellow to a darkish brown color. Neoprene does not have good insulating properties and is used up to 660 V a.c. but it has very good fire-resisting properties and therefore it is more useful as a sheathing material.

Styrene rubber: Styrene is used both for insulating and sheathing cables. It has properties almost equal to the natural rubber.

Polyvinyl Chloride (PVC)

It is a polymer derived generally from acetylene and it can be produced in different grades depending upon the polymerization process. For use in the cable industry, the polymer must be compounded with a plasticizer which makes it plastic over a wide range of temperature. The grade of PVC depends upon the plasticizer. PVC is inferior to vulcanized rubber concerning elasticity and insulation resistance. PVC material has many grades.

General purpose type: It is used both for sheathing and as an insulating material. In this compound, monomeric plasticizers are used. It is to be noted that a V.R. insulated PVC sheathed cable is not good for use.

Hard grade PVC: These are manufactured with less amount of plasticizer as compared with general purpose type. Hard-grade PVC is used for higher temperatures for a short duration of time like in soldering and is better than the general purpose type. Hard grade can not be used for low continuous temperatures.

Heat-resisting PVC: Because of the use of monomeric plasticizer which volatilizes at temperatures 80°C – 100°C, general-purpose type compounds become stiff. By using polymeric plasticizers it is possible to operate the material continuously around 100°C.

PVC compounds are normally costlier than the rubber compounds and the polymeric plasticized compounds are more expensive than the monomeric plasticized ones. PVC is inert to oxygen, oils, alkalis, and acids and, therefore, if the environmental conditions are such that these things are present in the atmosphere, PVC is more useful than rubber.

Polythene

This material can be used for high-frequency cables. This has been used to a limited extent for power cables also. The thermal dissipation properties are better than those of impregnated paper and the impulse strength compares favourably with an impregnated paper-insulated device. The maximum operating temperature of this material under short circuits is 100°C.

Cross-linked polythene: The use of polythene for cables has been limited by its low melting point. By cross-linking the molecules, in roughly the same way as vulcanizing rubber, a new material is produced that does not melt but carbonizes at 250 to 300°C. By using a chemical process it has been made technically possible to cross-link polythene in conventional equipment for the manufacture of rubber. This is why the product is said to be “vulcanized” or “cross-linked” polythene.

The polythene is inert to chemical reactions as it does not have double bonds and polar groups. Therefore, it was thought that polythene could be cross-linked only through special conditions, e.g., by irradiating polythene with electrons, thereby it could be given properties of cross-linking such as change of tensile strength and better temperature stability.

Many irradiation processes have been developed in the cable-making industry even though large amounts of high-energy radiation are required and the procedure is expensive. Polythene can also be irradiated with ultraviolet light, after adding to it a small quantity of ultraviolet-sensitive material such as benzophenone.

Under the influence of ultraviolet light on benzophenone, a radical is formed of the same type as in the decomposition of peroxide by the radical mechanism. Organic peroxides have also been used successfully to crosslink the polythene.

Impregnated Paper

A suitable layer of the paper is lapped on the conductor depending upon the operating voltage. It is then dried by the combined application of heat and vacuum. This is carried out in a hermetically sealed steam-heated chamber. The temperature is 120°–130°C before vacuum is created. After the device is dried, an insulating compound having the same temperature as that of the chamber is forced into the chamber. All the pores of the paper are completely filled with this compound.

After impregnation, the device is allowed to cool under the compound so that the void formation due to compound shrinkage is minimized. In the case of the pre-impregnated type, the papers are dried and impregnated before they are applied on the conductor. The compound used in the case of impregnated paper is a semifluid and when the cables are laid on gradients the fluid tends to move from higher to lower gradient. This reduces the compound content at higher gradients and may result in void formation at higher gradients. This is very serious for cables operating at voltages higher than 3.3 kV.

In many cases, the failures of the cables have been due to the void formation at the higher levels or due to the bursting of the sheath at the lower levels because of the excessive internal pressure of the head of the compound.

Insulating press boards: If the thickness of the paper is 0.8 mm or more, it is called a paper board. When many layers of paper are laminated with an adhesive to get the desired thickness, these are known as press boards and are used in bushings, and transformers as insulating barriers or supporting materials. The electrical properties of press boards vary depending on the resin content.

The application of these press boards depends upon the thickness and density of the paper used. For high-frequency capacitors and cables usually low-density paper (0.8 gm/cm³ ) is used whereas medium-density paper is used for power capacitors and high-density paper is used in d.c. machines and energy storage capacitors. The electric strength of the press board is higher than that of resins or porcelain. However, it is adversely affected by temperatures above 20°C. The loss angle tan δ also decreases with an increase in temperature. The main advantage of this material is that it provides good mechanical support even at higher temperatures up to 120°C.

Mica

Mica consists of crystalline mineral silicates of alumina and potash. It has high dielectric strength, low dielectric losses, and good mechanical strength. All these properties make it useful for many electrical devices e.g., commutator segment separators, aremature windings, electrical heating, and cooling equipment, and switchgear. Thin layers of mica are laminated with a suitable resin or varnish to make thick sheets of mica.

Mica can be mixed with the required type of resin to obtain its application at different operating temperatures. Mica is used as a filler in insulating materials to improve their dielectric strength, reduce dielectric loss, and improve heat resistance properties.

Ceramics

Ceramics materials are produced from clay containing aluminum oxide and other inorganic materials. The thick parts of these substances are given the desired shape and form at room temperature and then baked at a high temperature about (1450°C) to provide a solid inelastic final structure. Ceramics also known as porcelain in one of its forms have high mechanical strength and low permittivity (ε_r < 12) and are widely used for insulators and bushings. These have 40% to 50% of clay, 30- 20% of aluminum oxide, and 30% of feldspar.

The ceramics with higher permittivity (ε_r > 12) are used in capacitors and transducers. The specific insulation resistance of ceramics is comparatively low. The tan δ of these materials is high and increases with an increase in temperature resulting in higher dielectric loss. The breakdown strength of porecelain compared to other insulating materials is low but it remains unaffected over a wide range of temperature variations.

Porcelain is chemically insert to alkalies and acids and, therefore, is corrosion resistant and does not get contaminated. Alumina (Al₂O₃ ) has replaced quartz because of its better thermal conductivity, insulating properties, and mechanical strength. It is used for the fabrication of high-current vacuum circuit breakers.

Glass

Glass is a thermoplastic inorganic material consisting of silicon dioxide (SiO₂ ), which is available in nature in the form of quartz. Different types of metal oxides could be used for producing different types of glasses but for use in electrical engineering, only non-alkaline glasses are suitable having alkaline content less than 0.8%.

The dielectric constant of glass varies between 3.6 and 10.0 and the density varies between 2000 kg/m3 and 6000 kg/m³. The loss angle tan δ is less than 10^-3 and losses are higher for lower frequencies. Its dielectric strength varies between 300 and 500 kV/mm and it decreases with an increase in temperature. Glass is used for X-ray equipment, electronic valves, electric bulbs, etc.

Epoxy Resins

Epoxy resins are low molecular but soluble thermosetting plastics that exhibit sufficient hardening quality in their molecules. The chemical cross-linking of epoxy resins is normally carried out at room temperatures either by a catalytic mechanism or by bridging epoxy molecules through the epoxy or hydroxyl group. Epoxy resins have high dielectric and mechanical strength.

They can be cast into desired shapes even at room temperature. They are highly elastic and it is found that when it is subjected to a pressure of 175000 psi, it returns to its original shape after the load is removed. The dielectric constant varies between 2.5 and 4.0.

Epoxy resins being non-polar substances have high DC-specific insulation resistance and low loss tan δ compared to polar materials like PVC. However, when the temperature exceeds 100°C the specific insulation resistance begins to decrease considerably, and tan δ increases.

Compared to porcelain the breakdown strength of epoxy resin is almost double at temperatures up to 100°C but decreases rapidly at higher temperatures. As filler materials, inorganic substances like quartz powder (SiO₂) are used for casting applications.

In SF6 gas-insulated systems having epoxy resin spacers, aluminum oxide and also dolomite are used as filler materials. These are found to be more compatible with the decomposed products of SF6 by partial discharge and arcing discharges. It is to be noted that the cast or encapsulation should not contain voids or humidity, especially in high-voltage applications and the material is desired to be homogeneous. It is, therefore, desirable to dry and degas the individual components of the mixture, and casting is preferably carried out in the vacuum.

The epoxy resin casts are inert to ether, alcohol, and benzol. However, most of them are soluble in mineral oils at about 70°C. It is for this reason that they are not found suitable for applications in filled transformers. There are certain applications that require insulating materials to operate between a high range of temperatures e.g., –270°C to 400°C.

Some of the applications are space shuttle solar arrays, capacitors, transformers high-speed locomotives, microprocessor chip carriers, cryogenic cables, and other applications at cryogenic temperatures. For this, some thermoplastic polymer films are used which have a unique combination of electrical, mechanical, and physical quantities and these materials can retain these properties over a wide range of temperatures where other insulating materials may fail.

Perfluoro carbon films have high dielectric strength very low dielectric constant of 2 and low dielectric loss of 2 × 10^–4 at 100 Hz and 7.5 × 10^–4 at 100 MHz. These films are used under extreme conditions of temperature and environment. These films are used for insulation on high-temperature wires, cables, motor coils phase, and ground insulation, and for capacitors. This is also used as a substrate for flexible printed circuits and flexible cables.

Another insulating film that has the best thermal properties in this category of insulating materials is polyimide film under the trade name of Kapton manufactured by DuPont of America. These films can be used between a very wide range of temperature variations varying between –270°C and 350°C. Its continuous temperature rating is 240°C. It has high dielectric and tensile strength. The disadvantages of the film are

high moisture absorption rate and
it is affected by alkalies and strong inorganic acids.

Kepton films can be used in capacitors, transformers formed coil insulation, motor state insulation, and flexible printed circuits. The film is selectively costlier and is mainly used where its unique characteristics make it the only suitable insulation.

The use of this insulation for motors reduces the overall dimensions of the motors for the same ratings. It is, therefore, used in almost all situations whose space is a serious problem, and the other insulation results in a bigger dimension.

Another recently developed resin is polycarbonate (PC) which is good heat resistant; it is flexible and has good dielectric characteristics. It is not affected by oils, fats, and dilute acids but is adversely affected by alkalies, esters, and aromatic hydrocarbons. The film being cost-effective and fire-resistant, it is used for coil insulation, slot insulation for motors, and capacitor insulation. This is known as the lexon polymer.

General Electric Co. of USA has developed a film under the trade name Ultem which is a poly etherimine (PEI) film that has dielectric strength comparable to that of polyimide film and has higher thermal conductivity and lower moisture absorption and is relatively less costlier. It is used as insulation for transformers and motors.