Sampling of transformer oil is quite an important stage in the process of diagnosing of oil-filled equipment current state. Today there are techniques that allow to make a conclusion about the need for repairs on the basis of sample analysis.
It is necessary to choose dry weather when drawing samples from electric devices of outdoor substations. This operation is performed by a team of two people. One of them is a site supervisor with an Electrical Level not less than 4, and the other one is a member of the team with an Electrical Level not less than 2.
Vessels made of transparent glass with a capacity of 0.5 to 1 liter are used for drawing transformer oil samples. They must be preliminary prepared and dried in a special oven. The delivery of such vessels to the sampling site has its own specifics, as well as transporting of samples to the place of testing. It is carried out in special wooden boxes with holes of appropriate sizes. Using the glassware for any other sampling is not allowed.
Drawing of oil samples is carried out during normal operation of electrical devices or immediately after their trip. Fulfillment of these conditions is necessary in determining the amount of moisture and other characteristics that depend on its presence in oil. The ambient temperature and sampling time is recorded during the process.
Typically, oil sampling must be carried out from the bottom of oil-filled equipment. To do this, proceed as follows:
- Clean the oil pickup tube;
- Drain oil to wash the tube hole;
- Drain 2 to 10 liters of dirty oil and condensate, excluding its contact with the ground;
- Rinse the vessels for sampling twice with the drawn oil;
- Fill the vessels at 95-98% by volume without the formation of air bubbles and undissolved precipitated water;
- Tightly plug the vessels with stoppers pre-washed with the same oil. It is necessary to avoid any contact between the samples and the air.
After sampling, all vessels are marked with special labels with the date, the substation name, parameters of the transformer or other electrical device from which this specific sample was drawn, its voltage, causes of sampling, surname of the person who executed the procedure of sampling.
Delivery of samples to the laboratory must be carried out no later than 7 days from the time of sampling. If stored for a longer time, samples may change their properties, which can lead to inconsistency of the results obtained from the state of oil in an electric device.