Insulating oil is widely used in liquid filled high-voltage electrical equipment, such as power transformers, distribution transformers, transformers, bushings, oil filled circuit breakers, oil filled cables, oil filled capacitors, etc. Insulating oil also acts as a coolant to dissipate heat from electrical equipment. Therefore, dielectric oil should have good thermal conductivity and chemical stability at high temperatures, with a longer duration. To determine the dielectric strength of oil, it is mainly defined as the maximum voltage required to generate dielectric breakdown through the material, measured in kilovolts per standard insulation 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 sparks between the electrodes, indicating material breakdown. The standard procedure for measuring the breakdown voltage of insulating oil is to observe at what voltage sparks will be generated between two electrodes immersed in the insulating oil, with a specific gap typically 2.5mm according to the specified standard interval.
The insulation oil in high-voltage equipment is affected by electrical, mechanical, and thermal stresses during operation.
Types of insulation oil
Mineral oil (petroleum products)
Cycloalkane oil
Paraffin oil
Synthetic oil (chemical products)
silicone oil
Biobased oil (vegetable oil)
Usually, cyclohexane and paraffin based oils are used more than other types of oils.
What causes oil to deteriorate?
The insulation oil in high-voltage equipment will withstand electrical, mechanical, and thermal stresses during operation. This can lead to oil pollution by forming carbon deposits, sulfur, acid, various types of gases, and sludge, which are mainly oxidation products and their formation increases with temperature and contact with air. If the moisture and conductive impurities are high, the breakdown voltage in transformer oil is low.
The significance of transformer oil testing
Regular insulation oil testing is an important preventive measure that helps maintain the normal operation of high-voltage equipment. Regular monitoring of the dielectric strength of insulation oil is crucial 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, as well as reduce the reliability of the power system.
Insulation 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 the insulation oil sample from the oil drain valve of the transformer and measure its breakdown voltage. Apply the test voltage to the electrode immersed in insulating oil and at a constant, standard conversion rate (e.g. 2kV/second). The test can be conducted five to six times, and the average of these readings can be considered as the breakdown voltage of the tested insulating oil.
The dielectric strength or breakdown voltage of insulating oil should not be lower than the specified recommended value. If the breakdown voltage approaches the specified limit, further diagnostic tests must be conducted on the sample, such as oil resistivity and dielectric loss factor (tan δ) Testing. According to standard practice, it is recommended to conduct two oil pressure tests per year.
