Insulating Materials Used in Electrical Devices | Electricity

Almost all electrical devices depend on insulating materials in one way or the other. Most of the failures in such devices may be due to the failure of an insulating part. In general, an insulating material should have good mechanical and electrical strength, and good thermal and chemical stability.

In this article the insulating materials used in power and distribution transformer, rotating machines, capacitors, cables, transmission equipment, switchgear and electronic equipment have been discussed.

Insulating Materials Used in Power and Distribution Transformers:

The insulating materials used in power and distribution transformers must/possess the following properties:

(i) Good electrical properties to withstand the power frequency voltages and impulse over voltages.

(ii) Good mechanical properties to withstand fabrication and handling during manufacture and electromagnetic forces during over load, short circuit and normal operating conditions.

(iii) Good thermal stability and low ageing effect.

Insulating Materials Used in Rotating Machines:

For choosing an insulating material for a rotating machine proper thought is given to the temperature classification. The materials must have good mechanical characteristics and high dielectric strength for reliable operation of the machine. They should withstand thermal stresses and mechanical vibrations.

The insulating materials used are:

(a) Conductor:

Baked on enamel cotton covering or combination of both (class A)- fibre glass coated with an organic varnish (class B)- fibre glass coated with silicon varnish (class H).

(b) Slot Liner:

Paper backed mica, fibre glass, cotton tapes, synthetic resin fabric tapes are used as binders for general finishing.

After windings, the insulations are impregnated with thermosetting synthetic organic varnishes or silicon varnishes.

For high voltages generators, the conductor or turn insulation is usually mica tape. The ground insulation of the coil consists of mica flakes bonded by resins.

Insulating Materials Used in Capacitors:

A capacitor is made up of conducting plates and a dielectric. The dielectric may be air, solid insulating material, or a liquid, depending upon the duty of the capacitor. Capacitors are used in electrical utilities and in different electronic circuits.

Power capacitors use tissue power and/or polypropylene films as dielectric. This insulation system is impregnated with either mineral oil or synthetic liquids.

Electrolytic capacitors are produced in two forms, wet and dry. The wet type consists of an aluminum can holding an electrolytic solution of ammonium borate or sodium phosphate. This can form the negative electrode, while the positive electrode is made from corrugated aluminum foil, held in electrolyte by a stem which passes through a rubber gland. They are used for D.C. supply. In wet type capacitors the disadvantage are that they must be placed vertically and the electrolyte evaporates.

The dry type capacitor is made form from two very thin aluminum foil strips separated from one another by two layers of circulating paper saturated with an electrolytic paste of glycol and ammonium tetra borate.

Many type of capacitors are now used specially for D.C. applications. These are in addition to paper capacitors (paper with impregnates like castor oil, synthetic oils, mineral oil, polyesters etc.) electrolytic capacitors with aluminum or tantalum, ceramic capacitors with variety of ceramic bodies (tubular, disc, blocking etc.) and film dielectric (polystyrene, Teflon, polyethylene etc.) capacitors.

Insulating Materials Used in Cables:

The insulating materials (dielectrics) used for cables must have low discharge current, low dissipation factor (low loss tangent), and should be chemically stable. The materials generally used are impregnated paper, rubber, compressed air, mineral oil etc.

In the present days, cross-linked polyethylene cables (XLPE) are manufactured for 11 kV, 33 kV and 66 kV rating. Polyethylene which is a thermoplastic material is converted into a thermosetting material by a process similar to vulcanization of rubber, so that its linear chain structure is changed into three-dimensional network structure.

This changed polyethylene has outstanding dielectric properties, becomes resistant to extremes of temperature, more mechanically strong and resistant to environmental stress cracking. In comparison to paper insulated cable they have the advantages of inherent moisture resistance, high flexibility, simple accessories, smaller size (w.r.t. paper or pvc cable).

Liquids are used as impregnates for the insulation of solid type cables and as a filing for hollow core, liquid-cooled and gas pressurized cables. The liquid acts as a dielectric and also as a coolant (when circulated through ducts in cable-hollow cables). Liquids of low permittivity, low dielectric dissipation factor, high dielectric, strength and with gas absorbing characteristics are desirable, and all types of hydrocarbons insulating liquids are used. The gas filed cables use sulfur hexafluoride as the gas.

Insulating Materials Used in Line Insulators:

Porcelain has been in use as the most acceptable medium of insulation for transmission lines. For high voltage systems, weight has proved to be major constraint. Glass insulators which are lighter and comparable with—that of porcelain of alternate material for insulators. But glass insulators have their own inherent problems, like self-explosion, erosion and poor resistance to power arc.

Composite insulators which are lighter and can equally match the porcelain in terms of electrical characteristics have been developed. Basically, these insulators have been made out of synthetic material over a core of resin bonded glass fibre rod. These insulators degrade when exposed to ultraviolet rays.

A typical electrical grade of porcelain consists of approximately 50 per cent clays, 25 per cent crushed and ground flint and 25 per cent crushed and ground feldspar. The glaze is applied to these insulators by spraying or dipping. Glaze is virtually clay slip to which has been added metallic oxide to given a color.

Insulating Materials Used in Switchgear:

A circuit breaker is a control element in an electric circuit and possesses the following basic characteristics:

(a) When closed, it should be an ideal electrical conductor. When closed on command, it should be able to interrupt an assigned current, without causing dangerous over voltages.

(b) When open it should be an ideal electrical insulator. When open on command, it should be able to close, even on to a short-circuit, without damaging the contacts.

The miniature circuit breakers (MCB) which operate for very high currents and extra high-voltages use both liquids and gases arc interrupting medium and as coolant.

In oil-filled breakers, the petroleum oil of the same viscosity as is used in transformers is used. In bulk oil switches, motion of the liquid (to extinguish the arc) is produced by the action of the arc, which in ‘low-oil’ content switches, motion is usually produced by mechanical means. Liquids are also used in the busbar chambers associated with the switchgear. In this, very viscous liquids or compounds are normally used.

The gas-filled circuit breakers are of different types – air blast circuit breakers (use compressed air), sulphur hexafluoride breaks, and vacuum breakers. Sulphur hexafluoride breakers have high interrupting ability and are now becoming very popular at extra high voltage. Vacuum breakers are used in the voltage range of 11 kV-33 kV. They have excellent performance for capacitor switching duty.

Insulating Materials Used in Electronic Equipment:

The insulating materials for most of the electronic equipment should have high electric strength, mechanical strength, arc resistance and thermal stability. In some of the cases, it should have high moisture resistance (high power transmitters) and light weight (space electronics). For most of electronic equipments, synthetic resins are used as insulants.