Arcing
Large amounts of hydrogen and acetylene are produced, with minor quantities of methane and ethylene. Key Gas: Acetylene

Corona
Low-energy electrical discharges produce hydrogen and methane, with small quantities of ethane and ethylene. Key Gas: Hydrogen

Overheated Oil
Decomposition products include ethylene and methane, together with smaller quantities of hydrogen and ethane. Key Gas: Ethylene

Overheated Cellulose
Large quantities of carbon dioxide and carbon monoxide are evolved. Key Gas: Carbon monoxide

Interpretation and Historical Data
NDL has one of the most comprehensive insulating oil data management systems and interpretation guides. This system does graphical trend analysis for gas-in-oil data. The reports contain recommended actions based on the latest accepted guidelines and NDL’s extensive experience. NDL will maintain historical records for each customer. These data are used to update and improve the diagnostic process.

Temperature
Gas production rates increase exponentially with temperature, and directly with volume of oil and paper insulation at high enough temperature to produce gases. Temperature decreases as distance from the fault increases. Temperature at the fault center is highest, and oil and paper here will produce the most gas. As distance increases from the fault (hot spot), temperature goes down and the rate of gas generation also goes down. Because of the volume effect, a large heated volume of oil and paper will produce the same amount of gas as a smaller volume at a higher temperature.

Gas Mixing
Concentration of gases in close proximity to an active fault will be higher than in the DGA oil sample. As distance increases from a fault, gas concentrations decrease.

Gas Solubility
Solubilities of gases in oil vary with temperature and pressure. Solubility of all transformer gases vary proportionally up and down with pressure. Variation of solubilities with temperature is much more complex. Solubilities of hydrogen, nitrogen, carbon monoxide, and oxygen go up and down proportionally with temperature. Solubilities of carbon dioxide, acetylene, ethylene, and ethane are reversed and vary inversely with temperature changes. As temperature rises, solubilities of these gases go down; and as temperature falls, their solubilities increase. Methane solubility remains almost constant with temperature changes.

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