DGA Recommendations for New UnitsTechnical Briefs
DGA Recommendations for New Units
On a transformer that is brand new and has very low dissolved gases, why would there be a recommendation to retest in 1 or 3 months?
The answer is that, when a unit receives a 3-month or 1-month retest recommendation on DGA, it may also receive the following comment on the Rainbow report: "The analysis of this sample shows only minor amounts of combustible gas. This baseline indicates normal operation." In some cases, this can mean total dissolved gases are only at 1! Here is an example:
On this unit, there is not a concern with the gasses, because they show no signs of a fault. This is reflected in the comment that appears underneath the dissolved gas results. However, this is a new unit and is likely a new install.
Manufacturers often have recommendations on the testing interval of newly installed units. The logic is this: if it lasts the first minute, it will last an hour; if it lasts the first hour without failure, it will last a day, etc. This logic is also mirrored in standards and guidelines. IEEE C57.104-2008 says that: "Generally, daily or weekly sampling is recommended after startup, followed by monthly or longer intervals." Similarly, according to IEEE C57.93-2007 transformers should have DGA "[…] carried out after the first day, three months, six months, and subsequently at one […] year intervals."
SD Myers takes a similar view on the testing intervals of new units. In general, for newly installed transformers of a small to medium size and class (those that would normally receive a 1 year recommendation), we recommend 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.
The purpose of this is to closely watch the unit while it is new and in use to see if any issues develop while still under the manufacturer’s warranty. The idea behind this strategy is that it is much less expensive to address potential failures early on that are brought on by manufacturing defects. Once a transformer has failed in service, it is often not possible to correct the defect without replacing the unit. Also, failure in service will often create consequential damages: damage to other equipment or facilities, loss of production, loss of goodwill, failure to meet contractual obligations, etc...
Additionally, warranties by the manufacturer may be invalidated if recommended intervals for testing of newly installed units are not maintained. Generally, a good rule of thumb is that the "newly installed" period is typically considered to last until the first routine samples are drawn. As in all of the preceding discussion, warranty requirements and manufacturer requirements should be the minimum acceptable requirements that are incorporated into a testing program.