The most common dielectrics are gases.
Many electrical apparatus use air as the
insulating medium, while in a few cases other gases such as N2, CO2, CCl2F2 (freon) and
SF6 (hexafluoride) are used.
Gases consist of neutral molecules, and are, therefore, good insulators. Yet under certain
conditions, a breakdown of the insulating property occurs, and current can pass through
the gas. Several phenomena are associated with the electric discharge in gases; among
them are spark, dark (Townsend) discharge, glow, corona, and arc.1
In order to conduct electricity, two conditions are required. First, the normally neutral gas
must create charges or accept them from external sources, or both. Second, an electric
field should exist to produce the directional motion of the charges.
Various phenomena occur in gaseous dielectrics when a voltage is applied.
-When low voltage is applied, small current flow between the electrodes and the
insulation retains its electrical properties.
-If the applied voltage is large, the current flowing through the insulation increases very
sharply and an electrical breakdown occur. A strongly conducting spark formed during
breakdown, practically produces a short circuit between the electrodes. The maximum
voltage applied to the insulation at the moment of breakdown is called the breakdown
voltage.
In order to understand the breakdown phenomenon in gases, the electrical properties of
gases should be studied. The processes by which high currents are produced in gases is
essential. The electrical discharges in gases are of two types;
i) non-sustaining discharges
ii) self-sustaining types2
The breakdown in a gas (spark breakdown) is the transition of a non-sustaining
discharges into a self-sustaining discharge. The build up of high currents in a breakdown
is due to the ionization in which electrons and ions are created from neutral atoms or
molecules, and their migration to the anode and cathode respectively leads to high
currents. Townsend theory and Streamer theory are the present two types of theories
which explain the mechanism of breakdown under different conditions as pressure,
temperature, electrode field configuration, nature of electrode surfaces and availability of
initial conducting particles.
3Liquid breakdown involves a unique level of complexity compared to gas or
solid dielectric breakdown. Physical characteristics, such as fluid viscosity, electroconvection,
temperature, density and pressure dependencies complicate the analysis and
modeling of the conduction and breakdown mechanisms. Most liquids have
characteristics more similar to solids than gases, such as high permittivities, high
densities, and weak molecular order, while maintaining the “flowing quality” of a gas.
Research on conduction in liquid dielectrics, especially transformer oil, began
many years ago because of its importance for applications as an insulator. Others [1-6]
have conducted studies of the effects of age, contamination, and gas content on the
overall quality of the oil, affecting the conduction and breakdown mechanisms.
Characterizing breakdown of liquids is important for optimizing the use of materials.
Identifying the failure modes or critical conditions required for a liquid to fail is
necessary to understand how breakdown will develop and successfully model the
mechanisms involved in liquid breakdown.
The work summarized here has focused on the quantification of the conduction
mechanisms of liquid dielectrics. Three geometries with differing field enhancement
factors and different electrode materials provide a variety of test conditions. Also,
changes to the hydrostatic pressure, liquid temperature, and the particle and gas content
of the liquids provide further insight into the conduction mechanisms. After these tests, 34
Electric breakdown of dielectric liquids is a very complex process that involves a succession of
intercorrelated phenomena (electronic, thermal, mechanical, etc.). Moreover, experiments have shown
that characteristic features of breakdown phenomena greatly depend on experimental conditions
(electrode geometry, shape and duration of applied voltage, etc.) [8-12]. The processes leading to
breakdown can be separated into 3 stages: first a stage of initiation of a prebreakdown phenomena
(localized discharge, cavity formation, etc.) requiring a very high electric field ( ∼100MV/m), second a
stage of propagation of a channel (called “streamer”) into the liquid under a much lower electric field
(∼1MV/m) and, finally, a stage of transition
