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.
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.
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.
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.
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.
There are a couple of situations where performing the analysis on insulating liquid during every routine sampling and testing interval is appropriate.
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.
The D1816 dielectric breakdown voltage is a more sensitive method of dielectric breakdown voltage testing and is generally more useful than the D877 testing method under many circumstances. This method is sensitive to dissolved gases in the oil. As a result, high dissolved gas content in the insulating oil sample may depress the D1816 value to the point where it is outside the acceptable range. In short, oil that is acceptable in every respect that affects its performance in electrical equipment may still "fail" the D1816 determination because of dissolved gas content.