Dielectric Breakdown Voltage
Written by: Andy Shkolnik, Jason C. Dennison and Ryan Tate
Technical Authors from SDMyers
When it comes to the ability of your transformer’s insulating liquid to withstand electrical stress, there are some things you should know about dielectric breakdown voltage. Here are six things every transformer owner should know about dielectric breakdown voltage.
1. What is dielectric breakdown voltage testing?
A dielectric breakdown voltage test measures the electrical stress an insulating liquid can withstand without “breakdown”—when electrical potential is high enough to cause discharge through the dielectric liquid. This test is used to evaluate a liquid’s ability to withstand electrical stress and can be performed for accepting new insulating liquids or evaluating in-service insulating liquids.
The test is performed using a test cell with two electrodes spaced with a predetermined gap between them. A sample of the liquid being tested fills the test cell, and the two electrodes are subjected to a steadily increasing electrical potential until there is a discharge through the liquid from one electrode to the other. The voltage level at which the breakdown occurs is recorded as the test result and compared to acceptable levels.
There are different methods based on which test standard is used (D877, D1816, or IEC 60156, more on these later). The standard defines the test method and parameters required to conduct each test. These parameters are the rate at which the test voltage is increased, the size and shape of the electrodes, the gap size between the electrodes, how many times the test is repeated, and whether or not the liquid is stirred during the test.
2. Why should you perform dielectric breakdown voltage testing?
For in-service transformers, the dielectric breakdown voltage test is one way to detect contamination in the liquid before it leads to a catastrophic failure. Contamination of the liquid due to such things as fibers from the solid insulation, conductive particles, contamination by foreign matter, dirt, and water can affect the dielectric breakdown voltage. When done in conjunction with other tests, the test results can help you assess a transformer's overall reliability and lifespan.
The test can also be performed on new, reprocessed, or reconditioned insulating liquids before being used to fill equipment. Acceptable test results are necessary (but not sufficient by themselves) to ensure that the liquid was stored and transported properly.
3. Which types of insulating liquids can be tested?
For a long time, our industry has used the generic terms “oil” or “fluid” when referring to a transformer’s insulating liquid. Regardless of which type your transformer contains, each is subject to contamination, the presence of conductive particles, aging, and moisture. There are five common insulating liquid types found in transformers. * They include:
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Mineral Oil
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Silicone Fluids
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Natural Ester (vegetable oil) Liquids
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Synthetic Ester Fluids
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High Molecular Weight Hydrocarbon (HMWH) Liquids
*Note: This list represents broad categories of the most common liquid types. Additional types can be tested but are relatively rare. Consult a transformer specialist if you have questions.
4. How often should I test for dielectric breakdown voltage?
Dielectric breakdown voltage testing should be essential to your regular transformer maintenance and reliability program. It is recommended that your insulating liquid is tested at least once a year, with the test results being recorded to track trending results. Understanding the trend of your data will help you identify sudden or unexpected changes and plan accordingly with preventative maintenance actions.