Characteristics of Liquid Dielectrics:
Essential Characteristics of Liquid Dielectrics should possess good dielectric properties, excellent heat transfer characteristics and must be chemically stable under the range of conditions under which the equipment operates. These are briefly discussed below.
(a) Electrical Properties
The electrical properties that are essential in determining the dielectric performance of a Characteristics of Liquid Dielectrics are
- its capacitance per unit volume or its relative permittivity
- its resistivity
- its loss tangent (tan δ) or its power factor which is an indication of the power loss under a.c. voltage application
- its ability to withstand high electric stresses.
Permittivities of most of the petroleum oils vary from 2.0 to 2.6 while those of silicone oils from 2.0 to 73. In case of the non-polar liquids, the permittivity is independent of frequency but in the case of polar liquids, such as water, it changes with frequency. For example, the permittivity of water is 78 at 50 Hz and reduces to about 5.0 at 1 MHz.
Resistivities of insulating liquids used for high voltage applications should be more than 1016 ohm-metre and most of the liquids in their pure state exhibit this property.
Power Factor of a liquid dielectric under a.c. voltage will determine its performance under load conditions. Power factor is a measure of the power loss and is an important parameter in cable and capacitor systems. However, in the case of transformers, the dielectric loss in the oil is negligible when compared to copper and iron losses. Pure and dry transformer oil will have a very low power factor varying between 10-4 at 20°C and 10-3 at 90°C at a frequency of 50 Hz.
Dielectric Strength is the most important parameter in the choice of a given liquid dielectric for a given application. The dielectric strength depends on the atomic and molecular properties of the liquid itself. However, under practical conditions the dielectric strength depends on the material of the electrodes, temperature, type of applied voltage, gas content in the liquid etc., which change the dielectric strength by changing the molecular properties of the Characteristics of Liquid Dielectrics. The above factors which control the breakdown strength and leads to electrical breakdown of the liquid dielectrics.
Heat Transfer Characteristics
In equipments filled with a liquid dielectric (transformer, cable, circuit breaker, etc.), heat is transferred mainly by convection. Under natural atmospheric cooling conditions convection (N) is given by
where
K= thermal conductivity,
A = coefficient of expansion,
C= specific heat per unit volume,
ν = kinematic viscosity, and
n = 0.25 ~ 0.33.
The main factors that control the heat transfer are thermal conductivity (K) and viscosity (ν). From Eq. (3.1) it can be keen that a higher value for K is preferable for apparatus likely to operate continuously at a high temperature. On the other hand, a low value of K and high viscosity can lead to localized overheating or even electrical “burn out”.
Silicone oils do not exhibit these properties and therefore can pose severe overheating problems in equipment that use these dielectric oils.
Chemical Stability
In service, insulating liquids are subjected to thermal and electrical stresses in the presence of materials like O2, water, fibres and decomposition products of solid insulation. These, either singly or in combination, cause degradation of the liquid with the result that soluble solid and gaseous products are found, which can result in corrosion, impairment of heat transfer, deterioration of electrical properties, increased dielectric losses, discharges and arcing. In the absence of any remedial action, this cycle continues and produces an ever-worsening liquid purity and equipment condition.