The principle and standard procedures of insulating oil breakdown voltage test

1. Definition
The breakdown voltage test of insulating oil refers to applying a continuously increasing alternating voltage to the insulating oil sample under specified experimental conditions until the insulation performance between the two electrodes in the oil fails (a breakdown occurs, forming an electric arc channel), and the measured lower voltage value is obtained, with the unit being kilovolts (kV). This test is used to measure the higher voltage threshold that the insulating oil can withstand.
2. Test Purpose and Importance
Insulating oil mainly plays the roles of insulation and cooling in power equipment. The breakdown voltage is a core indicator for measuring insulation performance, purity, and effectiveness.
Main purpose: To detect whether there are impurities such as water, fibers, dust, metal particles, carbon compounds, etc. in the oil.
Importance: Impurities, under the action of the electric field, will form “conductive bridges”, reducing insulation strength. If the breakdown voltage is too low, the risk of internal discharge in the equipment under overvoltage or normal working voltage increases, which may lead to equipment damage, explosion, fire, and major power outages.
3. Test Principle and Standards
In a standardized test oil cup, two specific-shaped electrodes are immersed in the oil sample and kept at a fixed gap (usually 2.5 mm). A constant rate (such as 2kV/s or 3kV/s) of 50/60Hz alternating voltage is applied until the current increases sharply and a breakdown occurs. The voltage value at that instant is recorded, which is the breakdown voltage.
Main international / national standards:
IEC 60156: “Determination of breakdown voltage of insulating liquids” (International standard)
ASTM D1816: Using spherical cap electrodes, sensitive to water and impurities, widely used
GB/T 507: “Breakdown voltage determination method for insulating oil” (Chinese national standard)
ASTM D877: Using circular disc electrodes, an older method, used for rapid screening, with relatively lower accuracy
4. Detailed Test Steps (Taking IEC 60156/GB/T 507 as an example)
4.1 Sampling
Take samples from the bottom sampling valve using specialized tools to avoid contamination by air and moisture.
Use clean, dry brown glass bottles or dedicated syringes.
4.2 Sample Preparation
Slowly pour the oil sample into a clean test oil cup and avoid generating bubbles.
Allow the oil to stand for 10-15 minutes to allow the air bubbles to escape from the oil.
4.3 Equipment Setup
Ensure that the oil cup and electrodes (usually brass or stainless steel spherical cap electrodes) have been thoroughly cleaned and dried according to standards.
Set the electrode gap to 2.5 mm.
Set the voltage rise rate to 2kV/s (or the standard value).
4.4 Conducting the Test
Start the automatic tester, with the voltage starting from zero and rising uniformly.
When the oil gap breaks down, the equipment automatically cuts off the voltage and records the corresponding value.
After the secondary breakdown, gently stir the oil sample with a stirring device (to eliminate carbon particles and bubbles generated by the breakdown), and allow it to stand for 1-2 minutes.
Repeat this process 6 times on the same oil sample.
4.5 Calculation and Reporting
Discard the secondary breakdown value (as “conditioning”), and take the arithmetic mean of the following 5 breakdown voltages as the final reported value of the oil sample. (Note: Some standards require calculating the average of all 6 values)