Liquid Dielectrics

Liquid Dielectrics find wide application as insulating materials in electric equipment such as transformers, cables and switchgear.  In Transformers, insulating oils are used to provide insulation to the live parts and to transfer heat away from the hot regions.  In Circuit breakers, oil is used to extinguish the arc which occurs when the breaker opens.

Any change in the insulating properties of these dielectrics can cause damage to the equipment and result in breakdowns.  Hence, it is necessary to have a good understanding of the insulating properties of dielectrics.

From an Electrical perspective, the main properties of the insulating oil are the dielectric constant, the dielectric strength and the electrical conductivity.  

The purity of the insulating liquid is vital.  Even a small amount of water, say 0.01% can reduce the dielectric strength of oil by 20%.  The presence of other impurities can also reduce the dielectric strength sharply.  

From the perspective of insulation, liquids are classified into pure liquids and commercial liquids.  Pure liquids are those in which the amount of impurities are less than 1 in 10^9.  

Commercial liquids, on the other hand, are impure liquids which contain impurities such water, other chemical molecules and foreign particles.  These liquids are not homogeneous.  It is common to find that two samples taken from the same transformer having different properties.