Advancements in Dissolved Gas Analysis: CO/CO2 Ratio

For DGA interpretation, faults identified using hydrocarbon gases are considered more serious if they appear to affect paper insulation. That is made explicit in CIGRE technical brochure 771 [1]. Production of hydrocarbon gases from the oil by electrical or thermal stress does not significantly affect the oil’s function as a coolant or electrical insulator. On the other hand, production of carbon oxide gases from paper insulation raises a concern of paper deterioration. In particular, charring of the paper by a localized hot spot, especially in the windings, can lead to transformer failure.
Recently R. Cox and C. Rutledge have developed a method for judging the location of a fault in paper insulation from the percent change of the CO2/CO carbon oxide gas ratio [2, 3]. In a controlled experiment with a sacrificial transformer and a heating element, they found that direct heating of paper tends to generate more CO than CO2. Case studies of faulty transformers have revealed that a large percent decrease in CO2/CO is associated with charring of winding paper. A moderate percent decrease in the ratio is often associated with paper charring outside of the windings, such as on bushing or tap changer leads. A minor percent decrease or an increase of the ratio (with production of carbon oxide gas) is usually associated with mild bulk overheating of paper insulation rather than a localized hot spot.

Read full article in the Special SUBSTATIONS Issue

SOURCEElectricity Today Magazine, Vol.33, No.3
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Dr. Zachary Draper is a scientist with Delta-X Research Inc.  His current research focuses on reliability statistics, multi-dimensional gas diagnostics for transformers and load tap changers, and seasonal variability and signal processing for online DGA monitors. He also provides training on how to interpret advanced analytics and reliability-based dissolved gas analysis. A member of both IEEE and CIGRE, Dr. Draper received a B.Sc. in Physics and Astronomy from the University of Washington, and a Ph.D. in Astrophysics from the University of Victoria.