Oil testing can diagnose unseen problems inside a transformer. Mineral oil comes in contact with the internal workings of the transformer and contains valuable information regarding the condition of the transformer. Information about the equipment and a properly drawn sample are critical for making a correct diagnosis of the transformer's health.

Information about your electrical equipment is critical but is of limited use by itself. Laboratory testing is also required to get the most accurate diagnosis, but accurate and representative test results are dependent on getting a good representative sample.

Regarding containers for Karl Fischer Moisture Analysis, SDMyers has found that a glass bottle functions as an excellent container for moisture analysis, if all of the following conditions are met: Before sampling, the capped bottle is kept dry by using a desiccant tablet; the glass bottle has a metal cap with a Teflon liner; when sampling, the bottle is filled to the very top in order to minimize the gas space in the bottle; and the KF Moisture test is the first test performed out of the bottle.

Using proper sample containers will help ensure that your samples are representative of the actual conditions inside your electrical equipment, enabling proper diagnosis to extend the life of your transformer.

There are several methods for dehydrating oil in a transformer while simultaneously attempting to dry the paper insulation. However, drying the insulation (the most important step) is much more difficult than just dehydrating the oil. 

Moisture may exist in a transformer in several forms.

1. Water vapor in the gas space of the transformer.

2. Water dissolved in the oil.

3. Free water suspended as droplets in the oil.

4. Emulsified water contained in the decay products of oil oxidation.

5. Water absorbed into and adsorbed onto the solid insulation.

6. Free water that has settled to the bottom of the transformer.

Paper has more affinity for moisture than oil does. Once wet, moisture removal is the corrective action that should follow. However, before moisture removal, it is important to first evaluate how the transformer became wet. 

The three most common ways to reduce the moisture levels in a wet transformer are: a Field Vacuum Dry-Out, where heat and direct vacuum are applied to the transformer tank after the unit is completely drained of fluid; a Factory Dry-Out, where a transformer is removed from service, drained normally, and then transported to a repair facility; and by using an Online Dryer, such as a DryMax.



New transformers may have defects that can lead to failure. Frequently, such defects will leave signature dissolved gases in the oil. A timely Dissolved Gas Analysis may catch the fault as it begins, and before it advances far enough to do permanent damage.

Some defects may provide initial symptoms during the first ten months of transformer installations without causing or revealing more obvious indications until after the warranty period has expired. The timing of the first interval at ten months, and running the complete recommended package of tests, will serve to establish a diagnostic baseline.

A combination of dissolved gas levels and comments on the Rainbow Report provides DGA recommendations for new units. For newly installed transformers for of a small to medium size and class, SDMyers recommends retesting the dissolved gasses in three months, to obtain baseline data. For the larger, more expensive, and higher maintenance units, such as furnace transformers and generator step-ups, we recommend starting the DGA retest at one month.

Technical Briefs

Over the years, customers have asked us a variety of questions about dissolved gas analysis (DGA) results that indicate elevated ethane levels in natural ester fluid. Natural ester-filled transformers, specifically those filled with FR3, have a tendency to generate ethane—and sometimes hydrogen—as stray gasses at temperatures normally found in a properly operated transformer. While not typical in every case, it does happen frequently enough that we do not consider it to be abnormal. 

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