Insulating oil is widely used in liquid filled high-voltage electrical equipment, such as power transformers, distribution transformers, mutual inductors, bushings, oil filled circuit breakers, oil filled cables, and oil filled capacitors. Insulating oil also acts as a coolant to dissipate heat from electrical equipment. Therefore, dielectric oils should have good thermal conductivity and chemical stability at high temperatures, with a longer duration. In order to determine the dielectric strength of oil, it is mainly defined as the maximum voltage required to produce dielectric breakdown through the material, measured in kilovolts per standard insulating medium thickness. In order to measure the breakdown voltage of the material, a higher voltage is applied to the material until the leakage current causes a spark between the electrodes, indicating that the material is broken down. The standard procedure for measuring the breakdown voltage of insulating oil is to observe the voltage at which sparks may occur between two electrodes immersed in insulating oil, and to observe a specific gap, typically 2.5 mm, at a specified standard interval.
Significance of Transformer Oil Testing
Regular insulation oil testing is an important preventive measure to help maintain the normal operation of high-voltage equipment. It is important to regularly monitor the dielectric strength of insulating oil before it causes internal arcing or complete equipment failure. Unexpected failures of critical electrical equipment during operation can lead to commercial losses and catastrophic loss of human life, and reduce the reliability of power systems.
Insulating oil test
ASTM D1816, ASTM D877, and IEC 60156 are popular standards that specify the procedures and structural aspects of the OIL TEST CELL used to test the dielectric strength or breakdown value of oil samples. Take a sample of insulating oil from the oil drain valve of the transformer and measure its breakdown voltage. Apply a test voltage to the electrode immersed in insulating oil at a constant, standard conversion rate (e.g., 2 kV/sec. The test can be conducted five to six times, and the average value of these readings can be considered the breakdown voltage of the insulating oil under test.
The dielectric strength or breakdown voltage of the insulating oil should not be lower than the specified value of the recommended value. If the breakdown voltage approaches the specified limits, further diagnostic tests must be performed on the sample, such as oil resistivity and dielectric loss factor (tan δ) Testing. According to standard practice, it is recommended to conduct an oil pressure test twice a year.
