INSULATING MATERIALS:
These are encountered in every electric and electronic device and piece of equipment. They are vital for isolating conductors and for their performance. These have extremely high resistivities and high dielectric strengths below certain temperatures. The electrical properties are dielectric constant, electrical resistivity and dielectric strength of material. Traditional materials include cellulosics such as paper and organic fabrics. They also include mica, glass, and ceramics. Cellulosic materials suffer from hygroscopicity and are bound to contain gas voids.
Mica has been used where high temperatures and/or surface discharges are experienced (e.g., in vacuum tubes, heaters and dc machine commutators). Mica powder mixed with finely ground glass in a hot-pressing process yields a dense machinable material with excellent electrical qualities, particularly suitable for insulators in high-voltage high-frequency equipment.
Porcelain and glass are used in insulating overhead power lines and bus-bars. Several newly developed insulating materials fall within the classification as polymers. Their molecular arrangements are either linear or cross-linked, the former including elastomers and thermoplastics and are generally very flexible and yield with time under a given mechanical load. The higher the temperature, the greater the yield. They include polyethylene (PE), poly (vinyl chloride) (PVC), and polyamides (nylons). Polytetrafluoroethylene (PTFE), which is a linear polymer, is not flexible nor does it yield and melt at high temperatures. Because of the high binding energies of fluorine atoms, the polymer is about 95% crystalline. At temperatures as high as 400°C, it decomposes.
Electrical Properties of Some
Insulating Materials:
|
Material
|
εr
|
tan δ at 25° C,
1 MHz (average)
|
ρv at 25° C ( Ω cm)
|
Breakdown stress (kV/mm)
|
|
Quartz glass
|
3.8
|
5 x 10 -4
|
10 17
|
|
|
Lead glass
|
9
|
|
|
|
|
Soda-lime glass
|
7
|
0.01
|
|
|
|
Glass ceramic
|
5-7
|
0.001 - 0.1
|
10 12-10 14
|
10-40
|
|
Porcelain
|
5
|
0.04
|
10 12-10 15
|
20-80
|
|
Zirconium ceramic
|
7-12
|
0.003
|
|
20-30
|
|
Alumina ceramic
|
10
|
0.0005
|
|
20-30
|
|
Barium titanate ceramic
|
2000-8000
|
0.1
|
|
|
|
Micanite
|
5-11
|
0.003
|
10 14-10 16
|
50 (for 1-mm specimen), 100-200 (for 0.1-mm specimen)
|
|
PVC
|
6
|
0.1
|
|
|
|
PE
|
2.3
|
0.0001
|
10 15-10 18
|
40
|
|
Polystyrene
|
2.6
|
0.0001
|
10 16-10 18
|
30-40
|
|
PTFE
|
2
|
0.0002
|
10 16-10 18
|
25
|
|
Bakelite
|
4.5
|
0.1
|
10 13
|
|
|
Vulcanized natural rubber
|
3.5
|
0.05
|
|
|
|
Methyl methacrylate
|
3.6
|
0.01
|
|
|
Insulation for High Voltage Machines :
(A) Insulation of Rotating Machines :
Insulation is needed in the stator windings of rotating machines in the following parts:
(i) Insulation between parallel conductors of a turn,
(ii) Insulation between turns of a coil,
(iii) Insulation relative to the frame,
(iv) Insulation between coil sides.
At present most large capacity synchronous generators and motors are manufactured for 6.3 to 10.5 kV, which may go up to 18 kV. Furthermore the insulation is continuously subjected to vibrations and impact mechanical loads in operation. Micanite asbestos tape and impregnated card board find application in generators. Different synthetic insulating materials have been created under different code numbers.
(B) Insulation of Power Transformers:
Real test of insulating material arises in transformers where winding has to withstand the high voltage as well as surges.
(C) Insulation of Cables:
High voltage cables are invariably either oil filled or gas filled. Such cables are put in steel tubes and the oil or gas is under high pressure.
INSULATORS:
Insulators must have adequate electrical and mechanical strength. Failure of electrical strength of an insulator can occur by way of the puncture of the solid dielectric or due to flash over in air along the insulator surface. Electro-technical porcelain is widely used in high voltage insulators. There are 3 types of insulators :
1. Pin type insulators:
These are commonly used for voltages up to about 25 kV.
2. Suspension type:
These are preferred on systems operating above 50 kV for the following reasons:
(a) In the event of failure of an insulator, only one insulator in the string has to be replaced;
(b) For a higher voltage, additional insulator can be very easily added to the string;
(c) The flexibility of the line is greatly employed with suspension insulator.
3. Strain type insulators:
These are used when the conductors are at intermediate anchor towers. They are used on locations when there is a change in direction of the line.
String Efficiency of an Insulator:
(Pin Type Insulator)
(Suspension Type Insulators)
η = Flash-over voltage of string of n-units / n lines flash-over voltage of one unit
It can be increased by
1. Making ratio of 'capacity to earth' and capacity per insulator small.
2. Using correct grading of various capacitances by employing.
Static shielding as below:
| Line voltage in kV | no. of insulator |
| 66 | -5 |
| 110 | -8 |
| 132 | -10 |
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