Breakdown Characteristics of Liquid Insulation in the Presence of Barrier under Varying Field Conditions

Abstract

Transformer oil is one of the insulating liquid used more frequently for electrical insulation purpose. The
dielectric strength of oil gap in divergent electrode systems in presence of insulating barriers has been the subject of
only few studies contrary to the air gap where numerous works have been reported. In a gaseous or liquid insulation
system of high voltage machines, the insulating capability in the available space can be improved by inserting an
insulation barrier in the gap, which alters the space charge distribution. When the discharge develops, the
accumulation of charges on the surface of barrier makes the barrier to behave as a floating plane electrode, and the
electric field between the barrier and the plane electrode becomes uniform leading to increase in the breakdown
voltage. This improvement in the insulation strength called the Barrier effect has been investigated.
The result obtained by the experimental investigation on breakdown characteristics of transformer oil under nonhomogeneous field conditions (Cone-Sphere electrode system) as well as moderately homogenous field conditions
(Sphere-Sphere electrode system) under alternating (50 Hz) voltage stresses taking into account the effect of gap
length and the influence of electrode covering by solid insulators as barriers has been reported. The effect of electrode
covering is reported for different types of solid insulating materials. It has been concluded that the effectiveness of the
barrier depends mainly on the barrier position, its thickness and dimension as well as its constituting material. The
experimental results of the barrier effect will help the manufacturers while dimensioning and designing some
insulation structure like power transformer.