Dielectric Breakdown Voltage Testing of Insulating Liquids

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The dielectric breakdown voltage test is performed to measure an insulating liquid’s ability to withstand electrical stress.  This ability is reduced by the presence of contaminants such as cellulosic fibers, conducting particles, dirt and/or water.  The test instrument that is used to measure the dielectric withstand voltage of an insulating liquid is typically referred to as an oil test set.  The set will consist of a vessel where a sample of the insulating liquid is placed, electrodes and an AC voltage source. 

 
HV_Diagnostic_BA75-1Oil Test Set by HV Diagnostics

The American Society for Testing and Materials (ASTM) define two test methods for measuring breakdown voltage in insulating liquids.  These test methods are ASTM D877 and ASTM D1816.  ASTM D877 may be used for evaluation of insulating liquids in equipment that is designed to be filled with unprocessed liquids.  A high breakdown voltage measured by this test method does not indicate that the amount of contaminants present in the sample is sufficiently low for the sampled liquid to be acceptable in all electrical equipment.  Test Method D877 is not sensitive to low levels of contaminants. 

If the insulating liquid is used in equipment that requires the application of vacuum and filtering of the oil before being placed into service, Test Method D877 is not recommended. Test Method D1816 should be used to determine the breakdown voltage of filtered and degassed liquids.

Test Method D877 describes two procedures, A and B, for determining electrical breakdown of specimens.  Procedure A is used to determine the breakdown voltage of liquids in which any insoluble breakdown products easily settle during the interval between the required repeated breakdown tests.  These liquids will include petroleum oils, hydrocarbons, and askarels (PCB) used as insulating and cooling liquids in transformers, cables, and similar apparatus.

Procedure B of Test Method D877 should be applied for liquid samples containing insoluble materials that settle from the specimen during testing.  These may include samples taken from circuit breakers, load tap changers, and other liquids heavily contaminated with insoluble particulate material.

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