How should an oil sample be taken to dielectric strength testing?
Two things are particularly important when taking oil samples. The first is to ensure that the proper sampling procedure is followed, and the second is to ensure that all of the essential information is properly recorded.
If the sample is to be sent to a test house for testing, the test house should be able to advise on the information needed, but it is important to bear in mind that the condition diagnosis will only be as good as the information supplied. The test house should also advise on the volume of the sample, and the type of container to use.
For oil samples from transformers, this information that oil test laboratories generally require is:
(1) Description of the sample
(2) List of tests to be performed
(3) Transformer name plate information
(4) Type of transformer
(5) Type of insulating fluid
(6) Any leaks noted
(7) Insulating fluid service history (has it been dried, etc)
(8) Transformer service history (has it been rewound, etc)
(9) Type of breather
(10) Type of insulation, including temperature rise rating
(11) Details of cooling equipment (fans, radiators, etc)
(12) Temperature of top of fluid, read from gauge
(13) Actual fluid temperature measured
(14) Fluid level
(15) Vacuum and pressure gauge readings
For load tap changers, it is also advisable to record the counter reading, the selector range and the sweep range.
Sampling should be performed in accordance with the appropriate standard, and is not, therefore, discussed in detail in this technical guide.
In the USA, there are two standards for sampling:
(1) D923 - Standard Practices for Sampling Electrical Insulating Liquids
(2) D3613 - Standard Practice for Sampling Electrical Insulating Oils for Gas Analysis and Determination of Water Content
Internationally, there are two further sampling standards:
(1) IEC 60475 Ed. 2.0 - Method of Sampling Insulating Liquids
(2) IEC 60567 Ed. 3.0 - Oil-filled electrical equipment - Sampling of gases and of oil for analysis of free and dissolved gases - Guidance
The IEC standards should be consulted together, especially as part of IEC 60567 has been transferred to IEC 60475.
