Mineral oil is the most common type in outdoor transformers; fire-resistant fluids also used include polychlorinated biphenyls (PCB)s and silicone.
By analyzing the volume, types, proportions, and rate of production of dissolved gases, much diagnostic information can be gathered.
Gases are produced by oxidation, vaporization, insulation decomposition, oil breakdown and electrolytic action.
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.
It has a septum arrangement on one side of the tube for drawing sample oil to test its moisture content.
The DGA technique involves extracting or stripping the gases from the oil and injecting them into a gas chromatograph (GC).
The gas was removed from the graduated column through a septum with a gas-tight syringe and immediately injected into a GC.
During 2004, Central Power Research Institute, Bangalore, India introduced a novel method in which a same sample of transformer oil could be exposed to vacuum many times, at ambient temperature, until there is no increase in the volume of extracted gases.
This method was further developed by Dakshin Lab Agencies, Bangalore to provide a Transformer Oil Multi Stage Gas Extractor.
These gases are isolated using a mercury piston to measure its volume at atmospheric pressure and subsequent transfer to a gas chromatograph using a gas-tight syringe.
An apparatus, in very similar design and in principle providing a multiple gas extraction, using vacuum and Toepler pump has been in service in Sydney (Australia) for more than 30 years.
Interpretation of the results obtained for a particular transformer requires knowledge of the age of the unit, the loading cycle, and the date of major maintenance such as filtering of the oil.
[7] After samples have been taken and analyzed, the first step in evaluating DGA results is to consider the concentration levels (in ppm) of each key gas.