In this article we will discuss about:- 1. Introduction to Insulating Materials 2. Characteristics of a Good Insulating Material 3. Classification of Insulating Materials 4. Air Spaces in Insulation 5. Effect of Moisture on Insulation 6. Protection of Insulation against Moisture.
- Introduction to Insulating Materials
- Characteristics of a Good Insulating Material
- Classification of Insulating Materials
- Air Spaces in Insulation
- Effect of Moisture on Insulation
- Protection of Insulation against Moisture
1. Introduction to Insulating Materials:
Electrical insulating materials are defined as materials which offer a very large resistance to flow of current, and for that reason they are used to keep the current in its proper path along the conductor.
A large number of substances and materials may be classified as insulators, many of which have to be employed in practice, as no single substance or material can satisfy all the requirements involved in the numerous and varied applications of insulators in electrical engineering. Such requirements involve consideration of physical properties, reliability, cost, availability, adaptability to machining operations etc.
Thus in some applications the insulating material in addition to its function as an insulator may have to act as a rigid mechanical support to the conductor and may be installed out of doors, in which case the insulating qualities must be retained under all atmospheric conditions, in other cases extreme flexibility is required.
Again, in electric heaters the insulating materials must maintain their insulating qualities over a wide range of temperatures extending in some cases to 1100°C, and for radio purposes the insulating qualities must be maintained upto very high frequencies.
In electrical machines and transformers the insulating materials applied to the conductors are required to be flexible, to have high specific electric strength (to reduce thickness to minimum) and ability to withstand unlimited cycles of heating and cooling.
A good insulating material should possess the following characteristics:
1. Large insulating resistance.
2. High dialectic strength.
3. Uniform viscosity—it gives uniform electrical and thermal properties.
4. Should be uniform throughout—it keeps the electric losses as low as possible and electric stresses uniform under high voltage difference.
5. Least thermal expansion.
6. When exposed to arcing should be non-ignitable.
7. Should be resistance to oils or liquids, gas fumes, acids and alkalies.
8. Should have no deteriorating effect on the material, in contact with it.
9. Low dissipation factor (loss tangent).
10. High mechanical strength.
11. High thermal conductivity.
12. Low permittivity.
13. High thermal strength.
14. Free from gaseous insulation to avoid discharges (for solids and gases).
15. Should be homogeneous to avoid local stress concentration.
16. Should be resistant to thermal and chemical deterioration.
The insulating materials can be classified in the following two ways:
1. Classification according to substances and materials.
2. Classification according to temperature.
(i) Solids (Inorganic and Organic):
Mica, wood, slate, glass, porcelain, rubber, cotton, silk, rayon, terylene, paper and cellulose materials etc.
(ii) Liquids (Oils and Varnishes):
Linseed oil, refined hydrocarbon mineral oils, spirit and synthetic varnishes etc.
Dry air, carbon dioxide, argon, nitrogen etc.
2. Classification According to Temperature:
4. Air Spaces in Insulation:
When an insulation is designed every attempt is made to avoid the existence of air spaces in it. Though, it is difficult to avoid air spaces in materials such as fabricated and impregnated insulation yet these may be prevented by vacuum impregnation or by gas or oil filling under pressure.
The air spaces exercise harmful effects in the following way:
When a solid insulation containing air spaces is subjected to voltage, ionization occurs (the phenomenon being known as corona).
Consequences of ionisation include-
(i) A great power loss in the insulation;
(ii) Thermal instability;
(iii) Lowering of the breakdown voltage of the insulation;
(iv) There is carbonisation, decomposition and mechanical damage to the insulating material.
Thus when there are air spaces in the insulation, it should not be overstressed and the material should have corona resistance properties.
5. Effect of Moisture on Insulation:
When an insulating material is placed in a humid atmosphere it absorbs a certain amount of moisture. The water vapours, at first, are absorbed on the surface, then they diffuse tending to reduce moisture concentration gradient and finally they are desorbed into lower vapour concentration region.
In an insulating material moisture diffusion, as a rule, takes place when the electrical equipment is inoperative. When the current is carried by the electrical equipment, the moisture diffuses from the insulating material (i.e., material dries up).
All solid dielectrics, on the basis of absorption of moisture in highly humid atmosphere, can be classified as under:
1. Hygroscopic and wettable materials.
2. Non-hygroscopic and wettable materials.
3. Non-hygroscopic and nonwettable materials.
The air having high humidity is a source of troubles in electrical insulation and can even cause failure of electrical equipment.
The effect of moisture on insulating materials brings about the following changes:
1. Changes in electrical properties.
2. Chemical changes.
3. Physical and mechanical changes.
1. Changes in Electrical Properties:
(i) The moisture absorbed by an insulating material causes- (a) a decrease in the volume resistivity, and especially surface resistivity (b) an increase in the dissipation factor and a certain increase in dielectric constant (c) decrease in dielectric strength due to a change in field distribution within the insulating material.
(ii) Conducting bridges may appear across the surface of the insulating material under high humidity and electric tension current.
In some cases the thin films formed of the moisture on the insulating material dries up when the equipment is working. Such places get a carbonized spot and such spots may join together with time and build up a conducting bridge, thus a short circuit may result.
2. Chemical Changes:
(i) High humidity often cause hydrolysis.
(ii) High humidity favours the growth of fungi in some insulating materials, which in turn degrade organic insulating materials.
3. Physical and Mechanical Changes:
(i) Some materials like plastics, polymers and materials filled with cellulose filters swell in the presence of high humidity.
(ii) Mechanical strength of the insulating material is reduced in the presence of moisture.
6. Protection of Insulation against Moisture:
The insulation can be protected against moisture by following methods:
(i) Impregnation of Winding:
The windings of all low voltage equipments are impregnated with baking varnishes (sometimes compounds are also used). Impregnating varnishes and compounds raise the moisture resistance of windings.
Impregnation treatment solidifies the windings, increases their thermal conductivity, improves their electrical and mechanical strength and heat resistance.
(ii) Making Insulation Hydrophobic (Waterproof):
The insulating materials assemblies sometimes are rendered hydrophobic to protect them against moisture. This type of treatment is specifically effective for polymers containing hydroxyls and for cellulose base insulating materials.
In comparison to old widely used techniques employing asphalts, bitumens, waxes, water-proofing by means of some hydrophobic silicon compositions free of hydroxyls and carboxyls is finding ever increasing favour. Paper, cotton fabric are rendered hydrophobic by dipping them in the solution of methyl butoxidiamine silane in carbon tetra-chloride or methyl triethoxisilane in absolute alcohol.
(iii) Hermetic Sealing:
Hermetic sealing (sealing by means of compounds) is widely used to protect insulation against moisture and help in maintaining adequate insulation properties of parts and protect them against mechanical damage. This treatment is normally affected by coating, impregnation and potting in compounds.
The sealing methods in use are- Dipping, Moulding, Injection, Encapsulation etc. The compounds most widely used in sealing the insulation of low voltage equipment are polyester-styrene, butyl methacrylate, styrene, polyurethane, silicon base compounds.
The insulating material or part to be sealed must be thoroughly dried out. The wax, asphalt or bitumens were used in old sealing methods.