May,2020, by Josué Rodríguez Díaz/ José Eulalio Contreras de Leon/ Elisa Zambrano
Just as blood tests are used to detect health problems, Dissolved Gas Analysis can be very useful for the early detection of possible transformer failures.
The degradation of the insulation materials used in transformers causes combustible gases to be released inside the unit. Therefore, before a device shows signs of a problem with its operation, the levels of dissolved gases in the oil can help the user detect potential failures promptly. To perform the Dissolved Gas Analysis (DGA), it is necessary to extract an oil sample during routine maintenance, following standard procedures to prevent altering results.
Once this is done, gases are extracted from the oil sample, and a gas chromatography is performed. With this, the volume of each gas is detected in parts per million (ppm).
By doing this, we will be able to understand what is happening inside the equipment without the need of de-energizing and opening the transformer, since the distribution of gases found in the oil is an indicative of the type of electrical failures that may occur.
The 9 main gases that were found are: • Atmospheric Gases: Nitrogen and Oxygen • Carbon Oxides: Monoxide and Carbon Dioxide • Hydrocarbons: Acetylene, Ethylene, Methane, Ethane • Other: Hydrogen
The concentration of each gas is related to the temperature reached, as well as the volume of the oil at said temperature; therefore, we can say that, the level of gas concentration is closely related to the severity of the failure.
The following failures can be detected in transformers using mineral oil: • Cellulose insulation degraded by overheating generates high levels of carbon dioxide and carbon monoxide. • Oil overheating results in the production of hydrocarbon gases. • Partial discharges can be detected when high levels of hydrogen are present. • Arcing between energized parts can be detected with the presence of acetylene. • Issues with the airtight sealing of the tank can be evidenced by the presence of nitrogen and oxygen.
Dielectric fluids for transformers are compounds formed by chemical groups with carboncarbon and/or carbon-hydrogen bonds where the rupture of some of these bonds is the first available indicator of probable equipment malfunction that could lead to a failure if they are not detected and corrected promptly. Mineral oils are, unquestionably, the most used fluids in the electrical industry, because of their cost, availability, and properties. However, unlike these oils, natural and synthetic esters have been used as an environmentally friendly solution alternative, considering that although their chemical structure contains carbon, hydrogen, and oxygen, their diverse molecular arrangement allows the presence of different characteristics while delivering a similar performance, therefore, the generation of gases is also affected.