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Monday, 12 August 2019

Factors affecting Dielectric - Constant, Strength and Loss

Factors affecting Dielectric Strength :


Every electrical apparatus is designed so as to work within a definite range of voltage. If the operating voltage is increased gradually, then at some value of voltage a breakdown will occur puncturing the insulation permanently. The dielectric strength or electric strength or breakdown voltage is, therefore, the minimum voltage which when applied to an insulating material will result in the destruction of its insulating properties.

Dielectric strength is expressed in kV per unit thickness of the insulating material. Breakdown potential is that value of voltage which when applied across one cm or one mm thickness of dielectric medium or insulating medium will breakdown its insulation e.g., the breakdown potential of air is 30 kV/cm which means that the maximum potential difference which 1 cm thickness of air can withstand is 30 kV. If the voltage increases this value then air insulation will breakdown. When an electrical apparatus is designed, the value of dielectric strength is of utmost importance in deciding the thickness of the insulating material. Use of the material of correct dielectric strength results in the reduction of the cost of the apparatus.



The following factors affect the dielectric strength of the insulating material :

(a) Temperature: In case of air dielectric, the dielectric strength decreases with the rise of temperature. In case of liquid dielectrics, the effect varies with the type of oil and its viscosity.

(b) Humidity: It generally decreases the value of dielectric strength. The dielectric strength of some of the insulating materials is shown in Table.

Dielectric Strength of Insulating Materials

Material
Dielectric strength (kilo volts / mm)
Low Voltage Porcelain
1.5 - 4 kV
High Voltage Porcelain
10- 16 kV
Mica
80 kV
Asbestos (Paper type)
3.5 kV
Natural Rubber
24 kV
Synthetic Rubber
4.44 kV

DIELECTRIC CONSTANT:

Every insulating material has got the general property of storing charge Q when a voltage V is applied across it. The charge Q is proportional to the voltage applied.

i.e. Q α V,
Q = CV

Where C is a constant and is known as the capacitance of the material across which the voltage is applied. Every insulating material behaves as a capacitor. The property of insulating materials that causes the difference in the value of capacitance, physical dimension remaining same, is called the 'dielectric constant or permittivity. It is denoted by E and is expressed as E = Cd / A where C is the capacitance of the material, d is the distance between the two faces, and A is the face area of insulation. Greater the permittivity, greater will be the capacitance of the insulating material. The permittivity of vacuum and air at standard temperature and pressure is about 1.00058 and for all practical purposes it is regarded as unity. The permittivity of solid and liquid insulating materials is given in Table.

Dielectric Constant for Different Insulating Materials

Material
Dielectric Constant
Wood
2.5 - 7.7
Paper
2.0 - 2.6
Mica
2.5 - 6.6
Glass
5.4 - 9.9
Marble
8.3
Water
81.0
Diamond
16.5
Oil
2.2 – 4.7
Paraffin
2.1 - 2.5
Porcelain
5.7 - 6.8
Rubber
2.0 - 3.5

Temperature and humidity affect the dielectric constant but to a very small extent. The effect of frequency and applied voltage on dielectric constant is appreciable. While selecting the insulating material for the particular job, permittivity is a prime factor of consideration.

Dielectric Loss:

When an A.C. Voltage is impressed across an ideal insulating material, no power loss takes place and the charging current leads the applied voltage by 90°. But in commercial insulating materials the leakage current does not lead the applied voltage by exactly 90° and hence there is a definite amount of dissipation of energy in the form of heat. The power dissipated is given by P = 2rc fC tan 8 where tan 6 is called the power factor of the dielectric. The power loss is dependent of tan 5 so long as the other factors like voltage, frequency etc., are constant.

Factors Affecting Dielectric Loss :

The following are the factors which affect the dielectric loss in the insulating materials.

(a) Temperature rise increases the dielectric loss.
(b) Moisture increases the dielectric loss.
(c) Voltage increase causes increased dielectric loss.
(d) The loss increases in direct proportion with the frequency of applied voltage.

Dielectric loss plays an important role in high voltage applications. Dielectric loss involves heat generation and heat dissipation. Every application of insulation requires proper understanding of the balance between the heat generation and heat dissipation.

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