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Tuesday, 1 October 2019

Types of Paper Insulation Materials

Paper and Card Board:


Paper is manufactured from coniferous wood, cellulose glass or asbestos. The organic contamination like lignin and Pentosans must be properly removed. Some alkaline reagents are added in the crushed wood which is then boiled. The material is then formed into paper. This process is called sulphate process. Paper contains 95 to 97% of cellulose, 4% of Pentosans, 3% lignin, 0.01% iron, 0.003% copper. The important physical properties to be considered for electrical papers are its thickness, density, finish, porosity, tensile strength and tearing resistance. Density has greater influence on dielectric strength and dielectric loss. Bleached paper is inferior to unbleached paper. Some of the characteristics of different papers used are given in Table.

Different Types of Paper

Sl No.
Name
Characteristics
1
Transformer paper
Good mechanical strength, high tensile strength, tearing resistance, Low co-efficient of friction, high resistance to wear, enables fast rate of winding.
2
Capacitor paper
High dielectric strength, High cellulose content (95%) to give loss factors.
3
Cable paper
Same as above but long fibres are required.



 Characteristics of Card Board

Sl No.
Name
Characteristics
1
Press Plan
Good mechanical and dielectric properties. Hygroscopic and hence it is to be impregnated. Withstands a maximum voltage of 600 V.
2
Leatheroid
More thicker than press plan. Strong, tough and flexible, Hygroscopic so it is to be impregnated, used for low voltages only.

Press board or card board is similar to paper except for its thickness. It is denser, and less flexible. It is made by passing wood pulp through heavy machines called calenders. Its insulation resistance is 10t ohm – m and dielectric strength is 50 kV/mm. It is used for making slot wedges, end liners for stator and rotor core, stacks separators in transformers, slot lining. Characteristics of two important boards are given in table.

Impregnated Paper :

Impregnated paper insulation is most reliable in electrical applications. It has good mechanical strength and chemical stability, It will be able to withstand high temperatures and is non inflammable. Comparatively the dielectric loss is less and its dielectric constant varies between 2.25 to 6.35. It is cheap and is widely available. The process normally involves heating the paper indirectly in vacuum t(; remove moisture completely and is impregnated with the pre-dehydrated oil. After this, cooling is carried out before exposing the insulation to atmosphere so that Oil does not flow out of the paper and makes paper to  absorb moisture. If the insulation is to be used for high voltages, a proper sealing of the apparatus after impregnation is very essential.

Impregnated paper insulation is more reliable. Impregnated paper is used in the underground power cables, mining and submarine cables upto a voltage range of 220 kV to 400 kV. It is also used in transformers, capacitors and in apparatus for generation, transmission and utilization of electrical power.

Capacitor Paper:

Impregnated paper free from salts and alkalies of thickness 7 to 25 microns is used as dielectric medium. Special kraft paper is generally impregnated in either petroleum jelly, mineral wax or oil polyisobutylene or chlorinated diphenyl. The manufacturing process involves rolling metal foils between two thin sheets of impregnated paper. The two methods employed are tab type and extended foil type. In the tab type a metal foil is inserted after rolling and the leads are soldered to the tabs. In the extended foil type sheets of foils are allowed to extend over the margin of the paper each foil on either side. Though the extended foil type can work on higher temperature, it occupies more space.

Mica Paper:

Since mica is crystalline in nature, therefore, it lacks flexibility. To make it flexible, mica paper is powdered. Mica is broken into small particle!, in acquous solution and then formed into thin sheets of thickness of 0.05 mm to 0.1 mm by a process similar to that used for the manufacture of cellulose paper. Mica paper is used as insulation for armature and field coils of rotating electrical machines. It has good wear resistance and high coefficient of friction. It has high arc resistance, greater dielectric strength (25 kV / mm).

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