Dissolved gas analysis differences between natural esters and mineral oils used in power transformers: a review

Typically and historically mineral oil has been used as liquid insulation in power transformers. Its functions are to ensure the isolation between active parts, absorb heat transmitting it to the outer surfaces and protect other insulators (such as paper) from moisture. Moreover, the analysis of insulating oil provides a diagnosis of the state of the transformer. In the last 10 years, there has been resurgence in the use of natural ester (vegetable oil) because of their ‘green’ credentials. Their biodegradability and high fire point compared to mineral oils are some of its advantages. However, its high cost and low evaluation in service (performance) still limit its application. For mineral oils there is a large database and knowledge obtained from numerous transformers studied over many years. For this reason, certain gas patterns can be related to a specific fault. Conversely, the limited field data related to natural ester makes the analysis of dissolved gases unreliable. Some recent studies attempt to determine the differences between each type of oil when a fault takes place. This study presents a review of the difference between the generated gases in mineral oil and natural ester as a result of the most common faults in transformers.