This is the single most important test performed on transformer oil and is used to determine the concentration of certain gases such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene.
The concentration and relative ratios of these gases can be used to diagnose certain operational problems and incipient faults in transformers, which may or may not be associated with a change in physical or chemical property of the insulating oil.
For example, high level of carbon monoxide relative to other gases may indicate thermal breakdown of cellulose paper while hydrogen in conjunction with methane may indicate a corona discharge. Acetylene is considered a significant gas generated as it is formed in breakdown of oil at temperatures in excess of 700oC and can indicate a serious high temperature overheating fault.