BY Alan Ross
Articles & Papers

A significant and growing risk for unplanned outages and lost production has been increasing in the past decade due to failures of critical power transformers. With plant capacity running at all-time highs in the US, and little room for unplanned outages, it is more critical than ever that transformer reliability become front and center of any effective reliability program. 





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.

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