- Dissolved Gas Analysis (DGA) Definition: Dissolved gas analysis of transformer oil is a technique to study gases produced under thermal and electrical stress in transformers.
- Gas Extraction Methods: Using specialized equipment, gases are extracted and analyzed to diagnose the internal condition of a transformer.
- Indicative Gases: Certain gases like hydrogen, methane, and ethylene signify specific types of thermal stresses based on their amounts and presence.
- CO and CO2 Levels: The levels of carbon monoxide and carbon dioxide can reveal the degradation of transformer insulation.
- Furan Analysis Importance: This method is critical for assessing the condition of paper insulation and estimating the remaining life of a transformer.
Dissolved gas analysis (DGA), also called a DGA test, studies dissolved gases in transformer oil. When a transformer sees abnormal thermal or electrical stress, the oil decomposes and certain gases form. On a major fault those gases collect in a Buchholz relay. When the stress is milder, the gases have time to dissolve in the oil.
Watching the Buchholz relay alone cannot show the full internal condition of a power transformer. The dissolved gases in the in-service oil must also be analysed. DGA of transformer oil is how those dissolved amounts are used to judge internal condition.
A routine DGA test builds a history of internal condition over the transformer’s life. In the test, gases are extracted from the oil and measured in a known volume of oil. The mix of those gases is then used to judge what is happening inside.
The gases usually found in oil in service are hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and oxygen(O2).
The usual way to measure these gases in oil is a vacuum gas-extraction apparatus and a gas chromatograph. The apparatus extracts gases by stirring the oil under vacuum. Those extracted gases are then fed to the chromatograph so each component can be measured.
Hydrogen and methane production increases when a transformer’s internal temperature reaches 150oC to 300oC. Above 300°C, ethylene (C2H4) levels rise. Temperatures over 700°C also produce acetylene, with high hydrogen (H2) and ethylene (C2H4).
Ethylene (C2H4) points to a very high-temperature hot spot inside an electrical transformer. If a DGA test finds large amounts of CO and CO2, paper insulation is likely decomposing.
Furan Analysis of Transformer Oil
Transformer core and winding mainly use paper insulation. The base of that paper is cellulose, a long chain of molecules. As the paper ages, those long chains break into shorter pieces. The same thing happens in very old books: the pages become brittle.
In a transformer, oil oxidation speeds that aging of the paper. When the paper becomes mechanically weak, it cannot take the mechanical stress of an electrical short circuit, and electrical breakdown can follow. The paper insulation inside a power transformer therefore needs to be watched.
Removing paper insulation from an in-service transformer for testing is impractical. Furan analysis lets us judge that insulation without taking the paper out.
Dissolved gas analysis can give a first view of paper condition, but it is not a sensitive method. IEC 60599 (older texts call it IEC-599) treats a CO2/CO ratio of about 3 to 11 as typical of cellulose in service. A ratio below 3 can point to hotter paper involvement. A ratio above 11 with high gas amounts often points to lower-temperature paper aging.
Healthy cellulose often sits in that same 4 to 11 band. The ratio is still a blunt check of paper. CO2 and CO also form when oil breaks down, so the ratio can mislead.
When oil soaks into paper and the paper is damaged by heat, unique oil-soluble compounds dissolve in the oil along with CO2 and CO. Those compounds belong to the Furfuraldehyde group. They are sometimes called Furfural in short. Among the furfural compounds, 2- Furfural is the most common. Furfural-family compounds form only from destructive heating of cellulose or paper.
Furan analysis is sensitive: damage to a few grams of paper shows up in the oil, even in a large transformer. Operators often treat it as the best check of remaining paper life.
The rise of furfurals in transformer oil over time helps judge the condition and remaining life of the transformer’s paper insulation.
