DGA Testing Of Transformers | Part 2

February 27, 2014

In Part 1 of this two-part series on dissolved gas analysis (DGA) and testing of oil-filled transformers, it was stated that DGA evaluation of combustible gasses is used to determine possible causes of a transformer’s internal problem or fault. It is a highly complicated process, and not every DGA will be able to define a problem or cause of a fault within a transformer.

Each gas level has an “action limit” for which the lab will recommend a resampling. The follow-up samples tell the lab what your required course of action may be and how quick a response is needed. However, if it appears that the transformer suffers from a fault or internal problem, further investigation is usually required to determine the root cause of the fault and what course of action is needed to correct the problem.

All substation transformers, large distribution transformers, and transformers in critical applications should be DGA tested at least once a year. Transformers with a history of generating combustible gasses or that are known to have internal problems should be tested more often. At Solomon Corporation, we can help to determine if you have a transformer with a problem. We can provide an independent lab report, so that our recommendations are backed by other experts in the field. If there is a problem with a transformer, we’ll recommend corrective action and/or send out a field crew to do further inspection, determine possible causes, and perform repairs.

For oil-filled transformers that require repairs, it is recommended that the transformer be removed from service and completely tested and evaluated. The electrical testing should consist of Transformer Turn Ratio (TTR), Doble Power Factor, Megohm Resistance, Winding Resistance and Core Ground Measurement. In addition, visual inspection of internal switches, connections and core and coil assembly should be performed. And due to the fact that causes for combustible gasses can be difficult to determine, the transformer may need to be taken to a repair facility where the core and coil can be removed from the transformer’s tank to locate the problem.

A Standard Oil Quality and Karl Fischer Moisture Content should be done on an annual schedule along with the DGA Analysis. Poor quality oil affects the transformer’s circulation and can cause dielectric breakdown within the transformer. High levels of moisture can damage the cellulose insulation resulting in electrical stresses. If left uncorrected, poor quality oil and moisture can lead to the development of combustible gasses within the transformer. Solomon Corporation can provide corrective action for poor quality oil and moisture contamination. This may include an on-site hot oil treatment that reprocesses the oil from the transformer to remove oxidation and contamination, or on-site draining and filling the transformer with new oil. The transformer can be taken to one our Solomon Corporation’s qualified repair facilities for a complete reconditioning of the transformer.