As use of dissolved gas analysis (DGA) monitors increases as a growing component of transformer maintenance and reliability, it is imperative to understand the capabilities of monitors in their ability to align with conventional laboratory results and detect gas-related changes from a baseline. SDMyers studied DGA monitors from several manufacturers through experiments over 18 months. Technologies included in the study were gas chromatography, photo-acoustic spectroscopy, solid-state palladium, thermal conductivity detection, and selective membrane methods. This paper summarizes conclusions from that study based on technology employed.
Liquid power factor is an outstanding tool for evaluating in-service transformer oil. The test is valuable for acceptance testing of new oil from a supplier, and for evaluating conditions in newly installed equipment. For in-service oil, there are several adverse conditions that can be discovered from the liquid power factor results.
The values we obtain from liquid power tests indicate certain conditions in a transformer, allowing us to determine which condition, moisture, oil oxidation, or contamination, is causing the result. The values we use to classify liquid power factor results for in-service oil are the same for all primary voltage classes of equipment.
For in-service insulating liquids, we recommend running liquid power factor at both 25°C and 100°C on all mineral oil-filled transformers except, perhaps, for very small distribution class transformers such as small pad mounted or pole mounted units. Liquid power factor is particularly important when D1816 dielectric breakdown voltage is also performed, since the results from the power factor may indicate the cause(s) of poor Dd1816 results.
Furans form when the paper that makes up the solid insulation breaks down or depolymerizes. When this happens, the cellulose molecules break into shorter polymer chains and kick out a glucose monomer molecule. As this happens, the average polymer chain length in the paper (which can be measured and reported as the degree of polymerization. or DP) decreases. Shorter polymer chains result in weaker paper.
Interpreting the results of furanic compounds can tell us whether the paper has broken down and, maybe even more importantly, whether it still is breaking down. SDMyers tests for five different furanic compounds. Increases in the concentrations of these compounds indicate that the paper is breaking down. Further, the particular furanic compounds present provide some useful information on what conditions caused the paper to break down, and may even indicate whether those conditions are still present.
While knowing the state of the solid insulation is critical information for any transformer, it may not be the best use of resources to always run furans as a routine test. There are three very important times for performing analysis of insulating liquids for furans.
There are a couple of situations where performing the analysis on insulating liquid during every routine sampling and testing interval is appropriate.
The significance of the furan test results depends, in part, on the reason for testing the unit in the first place. If you have baseline data, or if you have other past history, any increase in total furans, and especially the presence of specific furans other than 2-furaldehyde, may be significant. Even a small increase may indicate a significant, suggesting ongoing breakdown in the solid insulation. When we do not have a history of furans analysis to follow up on other abnormal results or for a baseline determination, we have to make our judgement and recommendations based on complete information and some general guidelines.
Very high levels for furan test results may indicate substantial damage to the insulating system. The calculations that we perform to estimate the condition of the solid insulation by using the furans analysis results is performed in a two-step approach to estimate a DP and calculate insulation life remaining.