Solid Dielectric Used in Practice
These dielectric materials are discussed in brief.
1. Organic materials
Paper
The dielectric constant is between 3.3 to 3.5. It is generally used for insulation purpose. The paper absorbs water, moisture rapidly i.e. it is hygroscopic, hence it is impregnated with mineral oil. The dielectric breakdown strength of paper is between 14 to 16 MV/ m.
Press board
Thick papers (thickness > 0.8 mm) are arranged in layers to form press boards. These pressboards are used in transformer insulation.
Rubber
|
Types of |
neoprene |
Silicon |
Vulcanized |
Butyl |
|
Dielectric |
6-6 |
3-2 |
2.9 |
2.4 |
Silicon rubber can be safely used for a temperature range of – 50°C to 200°C. It is generally used in cable insulation. The dielectric strength of silicon rubber is 20.01 kV/ mm.
Wood
Generally used as spaces, support blocks in transformer.
2. Inorganic materials
Mica
Mica can be manufactured in the form of very thin sheets.
Mica has following properties :
- Can withstand high temperatures without overheating.
- Sufficient mechanical strength.
- Low dielectric loss.
- High dielectric strength.
The dielectric constant is 6 to 7.5. The dielectric strength is 1000 kV/ mm for muscovitemica and 700 kV/ mm for phlogopite mica.
It is used in electrical machines as a separator.
Glass
The dielectric constant of glass is between 4 to 7. The dielectric strength is between 3000 to 5000 kV. Generally used as cover for electric luminaries, in x – ray equipments.
Ceramics
Ceramics are manufactured through heat treatment to minerals. Porcelain, steatite are the commonly used ceramics. The dielectric strength is 25 kV/ mm for high voltage porcelain.
These are used as insulators for transmission, distribution lines, bushings for transformers, terminals for cables.
3. Synthetic Polymers
- Polyvinyl chloride (PVC) : It can be shaped easily into different forms. Its dielectric strength is high. It is used for cable sheathing, conduits for electric wires.
- Polyester : These are used in preference to paper insulation. The dielectric strength is 2000 kV/ am. The dielectric properties are excellent and these are chemical resistant.
- Epoxy resins : These are thermosetting types of insulating materials. These have excellent mechanical and dielectric properties. The dielectric constant is between 2.5 to 3.8. The dielectric strength is 75 kV/ mm. These can be shaped into desired form. Insulators, bushings can be made with epoxy resins.