1. Scope
This work specification is applicable to the determination of dielectric strength of insulating oil (including new oil and in-service oil), and stipulates the referenced standards for testing, requirements for instruments and equipment, operating procedures, methods for judging test results, and precautions for testing, etc. The purpose of formulating this guide is to standardize operations, ensure the accuracy of test results, and provide a basis for equipment operation, supervision, and maintenance.
Other related tests of insulating oil other than dielectric strength determination are not within the scope of this work guide; please refer to the corresponding work guides.
2. Normative References
The provisions in the following documents, through reference in this work guide, become provisions of this standard. For dated references, subsequent amendments or revisions do not apply to this work guide; however, parties reaching agreements based on this work guide are encouraged to study whether the latest versions of these documents can be used. For undated references, the latest versions apply to this work guide.
GB/T507 Method for Determination of Breakdown Voltage of Insulating Oil
DL429.9 Test Method for Oil Quality in Power Systems - Determination of Dielectric Strength of Insulating Oil
3. Test Equipment: Insulating Oil Breakdown Voltage Tester
4. Operating Procedures:
4.1 Sample Container
The volume of the sample should be approximately 3 times the capacity of the test cell. Brown bottles are preferred for sampling; transparent bottles should be stored away from light before the test. Plastic containers that do not react with insulating oil may also be used, but they cannot be reused. For sealing, screw caps with polyethylene or polytetrafluoroethylene (PTFE) gaskets should be used. The container and cap should first be cleaned with an appropriate cleaning agent to remove residual liquid from the previous use, then cleaned with acetone, and finally dried with hot air.
4.2 Sampling
When sampling, leave 3% of the container space. The breakdown voltage test is quite sensitive to trace amounts of water or other impurities in the sample; a dedicated sampler must be used to prevent contamination of the sample. Take insulating oil from places where impurities are likely to be present, generally the bottom of the container.
4.3 Cleaning the Oil Cup
Before the test, the electrodes and oil cup should be cleaned with anhydrous ethanol and then dried. Adjust the electrode distance to maintain 2.5±0.1mm, and calibrate with a feeler gauge. The test should be conducted at room temperature of 15–35°C and humidity not higher than 75%.
4.4 Sample Preparation
After the test oil sample is delivered to the laboratory, it should be left for a certain period without breaking the original storage seal until the oil sample approaches room temperature. Before pouring out the oil, invert the oil storage container several times to mix the oil sample, avoiding the generation of air bubbles as much as possible. Then rinse the oil cup and electrodes two to three times with the test oil. Then slowly pour the test oil along the wall of the cup into the oil cup. Cover with a glass lid or glass cover and let it stand for 20 minutes.
4.5 Voltage Application Test
On the measuring device, apply a 50Hz AC voltage across the two electrodes containing the oil sample, with a voltage rise rate of 2kV/s±0.2kV/s, until breakdown of the oil gap occurs, and record the breakdown voltage value.
After reaching the breakdown voltage, pause for at least 2 minutes before the next voltage application; repeat the test 6 times and take the average breakdown voltage.
Note that there should be no air bubbles between the electrodes; if stirring is used, it should be maintained throughout the entire test.
4.6 Waste Oil Disposal
After the test, properly dispose of the waste oil. There should be dedicated containers for storing waste oil, and centralized disposal should be carried out regularly to avoid environmental pollution.
5. Basis for Judging Test Results:
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