Dissolved gas analysis

Dissolved gas analysis (DGA) is the study of dissolved gases in transformer oil.

Insulating materials within transformers and electrical equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical fault, and the rate of gas generation can indicate the severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any preventative maintenance program.

The collection and analysis of gases in an oil-insulated transformer was discussed as early as 1928. Many years of empirical and theoretical study have gone into the analysis of transformer fault gases.

DGA usually consists of sampling the oil and sending the sample to a laboratory for analysis. Mobile DGA units can be transported and used on site as well; some units can be directly connected to a transformer. Online monitoring of electrical equipment is an integral part of the smart grid.

Large power transformers are filled with oil that cools and insulates the transformer windings. Mineral oil is the most common type in outdoor transformers; fire-resistant fluids also used include polychlorinated biphenyls (PCB)s and silicone. (PCBs are amongst a broader group of harmful persistent organic pollutants (POPs) that are toxic, persist in the environment and animals, bioaccumulate through the food chain and pose a risk of causing adverse effects to human health and the environment. Hence PCB are banned in various countries.)

The insulating liquid is in contact with the internal components. Gases formed by normal and abnormal events within the transformer are dissolved in the oil. By analyzing the volume, types, proportions, and rate of production of dissolved gases, much diagnostic information can be gathered. Since these gases can reveal the faults of a transformer, they are known as "fault gases". Gases are produced by oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.

An oil sample tube is used to draw, retain and transport the sample of transformer oil in the same condition as it is inside a transformer with all fault gases dissolved in it.

It is a gas tight borosilicate glass tube of capacity 150 ml or 250 ml, having two airtight Teflon valves on both the ends. The outlets of these valves have been provided with a screw thread which helps in convenient connection of synthetic tubes while drawing sample from transformer. Also this provision is useful in transferring the oil into Sample oil burette of the Multiple Gas Extractor without any exposure to atmosphere, thereby retaining all its dissolved and evolved fault gases contents.

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Wikipedia