Dissolved Gas Analysis (DGA)

A key indicator of partial discharge and low-energy electrical disturbances. Hydrogen is generated by the decomposition of oil molecules under electron bombardment in gas-filled voids within the insulation. It is also produced by corona in oil and the reaction of water with hot metal surfaces. Rapidly rising H₂ levels with minimal other gases strongly suggest partial discharge activity. However, hydrogen alone is not fully diagnostic, as it can also appear in early stages of thermal faults. Stainless steel surfaces can catalytically generate hydrogen, complicating interpretation.

The definitive marker for arcing and high-energy electrical breakdown. Acetylene forms only at temperatures exceeding 700°C to 800°C, conditions exclusively created by electrical arcing in oil. Its presence, even in trace amounts, is a critical alarm requiring immediate investigation. Key diagnostic ratios involving acetylene include the Dörnenburg Ratio and the Duval Triangle method. A sudden spike in acetylene accompanied by hydrogen and ethylene confirms an active arcing fault, such as a winding short circuit or tap changer flashover.

The primary indicator of thermal faults in oil exceeding 500°C. Ethylene is generated by the thermal cracking of hydrocarbon oil molecules under high heat. It is the dominant gas in cases of severe overheating, such as hot spots on core laminations, circulating current paths, or overloaded windings. The ratio of ethylene to ethane helps differentiate between low-temperature and high-temperature thermal faults. A high ethylene-to-ethane ratio points toward a hot metal surface in direct contact with oil.

Associated with low-to-medium temperature thermal faults and partial discharge in oil. Methane begins forming at lower temperatures than ethylene, typically in the range of 150°C to 300°C, making it an early warning sign of developing hot spots. It is also produced by partial discharge in oil, though in lower concentrations than hydrogen. The methane-to-hydrogen ratio is a key diagnostic tool: a high ratio suggests a thermal process, while a low ratio points toward electrical discharge.

The primary indicators of solid insulation degradation. Unlike hydrocarbon gases that originate from oil decomposition, CO and CO₂ are generated by the thermal decomposition of cellulose paper insulation. Elevated CO levels, especially when the CO/CO₂ ratio exceeds 0.1, signal paper overheating or pyrolysis. This is critical because paper insulation is non-renewable; once degraded, the transformer's mechanical integrity is permanently compromised. The Duval Triangle 2 method specifically uses these gases to diagnose cellulose degradation.

Indicative of low-temperature thermal faults in oil, typically below 500°C. Ethane forms alongside methane during the initial stages of oil overheating. It is a key component in the Rogers Ratio method, where the ethylene-to-ethane ratio distinguishes between thermal faults of increasing severity. A low ethylene-to-ethane ratio with rising ethane suggests a developing hot spot that has not yet reached the temperature threshold for significant ethylene production. Trending ethane provides early warning before more severe gases appear.