Dielectric Breakdown

“The dielectric breakdown voltage of insulating liquid is a measure of its ability to withstand voltage stress without failure. It is the voltage at which breakdown occurs between two electrodes under prescribed test conditions. The test serves primarily to indicate the presence of electrically conductive contaminants in the liquid, such as water, dirt, moist cellulosic fibers, or particulate matter. However, a high dielectric breakdown voltage does not indicate the absence of all contaminants.

Two [ASTM] methods are recognized for measuring the dielectric breakdown voltage of insulating liquids as follows:

a) ASTM D877 uses thin flat-faced cylindrical electrodes with a 2.5 mm gap. The sensitivity of this method, to the general population of contaminants present in a liquid sample, decreases as applied test voltages used in this method become greater than 25 kV rms.

b) ASTM D1816 uses spherically-shaped electrodes. The liquid sample is circulated continuously in the test cell throughout the test. The gap distance standard settings are 1 mm and 2 mm.

The rounded electrodes (VDE) used in ASTM D1816 generate a more representative electrical field, closer to those present in a transformer. ASTM D1816 is more responsive to particles and dissolved water, both of which are detrimental to the transformer’s oil electrical strength. Therefore, test results from ASTM D1816 furnish a better evaluation of changes that may occur in transformer oil. The flat disk electrodes used in ASTM D877 generate an electrical field that is not representative of those normally present in a transformer.”

(Quote from IEEE C57.106-2015)

What method is used to analyze the sample?

ASTM D1816: 1mm or 2mm
IEC 60156

How do you interpret the results?

IEEE C57.106-2015

What do the results indicate?

“The dielectric breakdown voltage of an insulating liquid is of importance as a measure of the liquid’s ability to withstand electric stress without failure. The dielectric breakdown voltage serves to indicate the presence of contaminating agents such as water, dirt, cellulosic fibers, or conducting particles in the liquid, one or more of which may be present in significant concentrations when low breakdown voltages are obtained. However, a high dielectric breakdown voltage does not necessarily indicate the absence of all contaminants; it may merely indicate that the concentrations of contaminants that are present in the liquid between the electrodes are not large enough to deleteriously affect the average breakdown voltage of the liquid when tested by this test method.” (Quote from ASTM D1816)

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