The Most Powerful Tool in the Industry: Dissolved Gas-in-oil Analysis (DGA)
It may seem like a bold statement to say that dissolved gas-in-oil analysis (DGA) is the most powerful tool in the industry, but I stand by it.
DGA is the most cost-effective tool to detect a wide range of problems in the early stages and then throughout deterioration until an outage is required. It is a great asset management tool for expensive apparatus such as transformers and has also been applied to load (energized) tap changers, oil circuit breakers and other oil-filled devices. It has been applied to a variety of dielectric liquids beyond mineral oils, including natural and synthetic esters, silicone and high temperature hydrocarbons.
One of the useful characteristics of the data is that it allows differentiation of developing problems into categories, overheating of paper (cellulosic materials), oil (and other dielectric liquids), partial discharge activity and arcing. The severity of the problem can be evaluated based on the composition of gases and generation rates. The more probable type of faults can also be determined and then evaluated using other methods.
There can be some complexity to the analysis. For example, the presence of mostly high hydrogen can be from stray gassing, electrolysis of water in the valve or other locations or partial discharge activity. There are patterns of gassing that are more typical of stray gassing and an ASTM test method (D7150) would verify if this is a likely source. Proper sampling — adequately wiping out the valve and flushing with oil along with analysis of water content data — can help determine if electrolysis of free water is likely the source of the gassing. There is test equipment to determine if a transformer has active partial discharge activity. It all starts with the DGA test indicating that there is a serious enough problem to then search for the root cause.
The presence of acetylene is always a warning that there is electrical discharge activity or very high temperature localized overheating at temperatures typically >700°C. It is important to examine the relative composition of gases to determine if the most likely source is discharge activity or overheating. The gassing rate is examined to help identify the severity.
Most often DGA tests are performed in laboratories. Fortunately, samples for most equipment can be taken while on-line. Some small volume transformers should only be sampled off-line. It is important to select a reliable laboratory that can provide accurate and repeatable results in order to provide a good analysis. Doble has 5 laboratories located throughout the United States and Canada, including our Morgan Schaffer laboratory in Montréal, for this purpose. To be most effective, test data needs to be stored in a database for historic trends to be evaluated. There are also on-line devices specifically designed to monitor DGA, such as the Calisto, that provide information close to real-time every day as early warning when more extensive testing might be needed.
DGA is a great test for detecting and managing problems but does not always provide enough information to determine the root cause. Thankfully, there are many diagnostics tools for this. This includes other oil tests that will be discussed in future blogs such as those for oil quality, furans, metals in oil, water content and more.
Additionally, electrical field tests can measure power factor and capacitance, excitation current, leakage reactance, turns ratio, winding resistance, sweep frequency response analysis (SFRA) and instruments to detect and locate partial discharge activity.
Interested in Learning More?
Doble provides training seminars on diagnostics from laboratory tests, including DGA. See our Training listings for more information on when these seminars are held.
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