DGA is the study of dissolved gases. Transformer oil is used to stop the direct contact of oxygen, with cellulose made paper insulation of windings, which is susceptible to oxidation. Some gases are generated in the transformer due to the breakdown of electrical insulating materials inside the transformer.
Dissolved Gas Analysis can detect the concentration of the key gas in the unit of (“parts per million”)PPM, and can record the value of the volume of gas. It can also detect the faults like partial discharge, discharges of low energy and high energy, and a thermal fault of transformer oil.
Effect of dissolved gases:
When gases are occurring in the transformer then there are several gases created automatically. Hydrogen (H2), methane (CH4), ethylene (C2H4 ), ethane (C2H6), acetylene (C2H2), propene (C3H6), propane (C3H8), carbon monoxide (CO), oxygen (O2), and nitrogen (N2) are the gases which are generated automatically in the transformer. Some of these gases are expected for the normal aging of the transformer insulation.
The procedure of Dissolved Gas Analysis (DGA):
In this procedure we have to sample many things in the transformer like the sampling of oil from the transformer, extracting of gases from the oil, and analysis of extracted mixture gas in a chromatography (GC). After the extraction of the extracted gas mixture, the extracted gas mixture will be put into a chromatography. Inside the chromatography, the different gases are absorbed and separated in various degree and reached at the detector after different times of period. In this procedure, all mixture gases will be separated into different chemical compounds and can be identified in their particular name. And lastly, the concentration in volume gas will be calculated and expressed in pm.
|Gas concentration in ppm|
|Gas||Normal limits(<)||Action Limits(>)||Types of Fault|
|Carbon Monoxide(CO)||500||1,000||Severe overheating|
|Carbon dioxide(CO2)||10,000||15,000||Severe overheating|
Types of fault:
There are different types of fault like arching, corona, overheated oil, overheated cellulose, and etc.
Arcing: Arcing gas has occurred at the acetylene gas. In the process of arcing fault, a large amount of acetylene and hydrogen of methane and ethylene can be produced.
Corona: Corona is occurred due to the hydrogen gas. In the corona fault, low energy discharges generate the hydrogen and methane as well as the minor quantities of ethane and ethylene.
Overheated Oil: Ethylene and Methane are the cause of overheating of oil. Overheating of oil will produce ethylene and methane in the temperature of 300°F and 1,112°F respectively.
Overheated Cellulose: Overheated of cellulose is due to the Carbon monoxide. If we overheat the cellulose then it will form Carbon monoxide.