Insulating liquids breakdown voltage determination: Test method efficiency

The 50-60 Hz breakdown voltage of insulating liquids is not an intrinsic property of the dielectric but strongly depends on the presence of contaminants, such as particles and suspended water. Both IEC and ASTM give tests procedures finalized to determine the 50-60 Hz breakdown voltages. This test is adopted as acceptance on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Following the IEC Standards, this test has to be performed following the IEC 60156, Ed. 2.0 published in 1995 which is presently under revision. The main reason for revising the standard for the 50-60 Hz breakdown voltage determination of the insulating liquids is the attempt to reduce the scatter in the results, the quantities of the sampling oils and the testing times. The future new edition of IEC 60156 is on the way to reconfirm the present standardized procedure but will recommend the use of a stirring system. Also in an informative annex a new advanced procedure will be included, claiming to be able to reduce the scatter in the determination of the 50-60 Hz breakdown voltages. With the aim to evaluate the mean value and related scatter of power frequency breakdown voltages of insulating liquids, testing time and quantities of samples, this paper presents a comparison of adopted test methods recommended by IEC 60156 and other methods, like the Up-and-Down- and Multiple-Level methods.