Test equipment
Insulating oil breakdown voltage tester
Work procedure
Sample container
The volume of the sample should be approximately three times that of the sample volume. It is better to use a brown bottle for sampling. Transparent bottles should be stored in the dark before the test. Plastic containers that do not react with insulating oil can also be used, but they should not be reused. For sealing, a threaded plug with a polyethylene or polytetrafluoroe thylene gasket should be used. The container and the plug should be cleaned first with an appropriate cleaning agent to remove any residual liquid from the previous use. Then, they should be cleaned with acetone and finally dried with air.
Sampling
When sampling, 3% of container space should be reserved. The breakdown voltage test is quite sensitive to trace amounts of water or other impurities in the sample, and a dedicated sampler should be used for sampling to prevent sample contamination. The area where impurities are most likely to be present in the insulating oil is generally at the bottom of the container.
Clean the oil cup
Before the test, the electrodes and the oil cup should be cleaned and dried with absolute ethanol and ether. Adjust the electrode distance to maintain (2.5 ± 0.1) mm, and calibrate it with a gauge block. The test should be conducted under room temperature ranging from 15 to 35℃ and humidity not exceeding 75%.
Sample processing
After the test oil sample is sent to the laboratory, it should be left undisturbed for a sufficient period of time without damaging the original storage seal until the oil sample approaches room temperature. Before pouring out the oil, the oil storage container should be inverted several times to mix the oil sample and minimize the formation of bubbles. Then, the tested oil should be used to rinse the oil cup and electrodes 2 to 3 times. Next, the oil cup wall should be slowly injected into the oil cup. The glass cover or glass hood should be closed, and the container should be left to stand for 20 minutes.
Pressure test
On the measuring device, a 50Hz alternating voltage is applied to both ends of the electrode containing the oil sample. The voltage rise rate is (2 ± 0.2) kV/s until the oil gap breaks down, and the breakdown voltage value is recorded.
After reaching the breakdown voltage, pause for at least 2 minutes before applying the next voltage. Repeat this process 6 times and take the average of the breakdown voltages.
Make sure there are no air bubbles between the electrodes. If using a stirrer, it should be kept in operation throughout the entire experiment.
Waste oil treatment
After the test is completed, properly dispose of the waste oil. Special containers should be used to store the waste oil, and it should be centrally processed regularly to avoid environmental pollution.
Test result judgment basis (kV)
Rated voltage of equipment: < 15; Oil before startup ≥ 30; Operating oil ≥ 25;
Rated voltage of the equipment: 15 to 35. The oil before startup should be ≥ 35, and the operating oil should be ≥ 30.
Rated voltage of the equipment: 110 – 220 volts. The oil before startup should be ≥ 40, and the operating oil should be ≥ 35.
Rated voltage of the equipment: 500. The oil before startup should be ≥ 60, and the operating oil should be ≥ 50.
Notes for Attention
The electrodes can be spherical electrodes, mushroom-shaped electrodes or flat electrodes.
The distance between the electrodes is (2.5 ± 0.1) mm and is calibrated using a gauge block. If the distance between the electrodes is too small, it is prone to breakdown and the measurement result will be too low. On the contrary, if the distance is too large, the measurement result will be too high.
The dispersion of the test data is large, and the reason for this is that the influence of the breakdown process is quite significant. Therefore, in the test method, it is stipulated to take the average value of 6 measurements as the test result.
When there is moisture and other impurities in the insulating oil, it will have a significant impact on the breakdown voltage measurement. Therefore, the sample must be thoroughly shaken and then injected into the oil cup.
During the experiment, it was found that the breakdown voltage value increased with the increase in the number of tests. This was caused by the presence of different types of impurities in the oil. If the impurities in the oil mainly consist of fibrous substances and water, during the breakdown process, the water evaporates, so the test data keeps increasing. However, there are also times when it decreases. One should consider whether it exceeds the specified limit, etc.
The insulation breakdown voltage tester should be calibrated regularly.
Post time: Mar-09-2026