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Did you know there is a diagnostic technique like your medical check-up that is applied to electrical transformers and that can prevent thermal and electrical failures? This technique is known as DGA or Dissolved Gas Analysis.
DGA takes advantage of the ability of mineral oil to generate specific gases under electrical or thermal stress. Quantifying these gases and identifying the specific gas pattern allows a specialist to detect the kind of failure and the severity assessment as well, still at the incipient stage, when the risk of damage to the equipment is still low.
The main gases relating to the faults are hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H6) and, if insulating paper is involved, carbon monoxide (CO) and carbon dioxide (CO2) as well. Through these gases, you can identify electrical faults such as partial discharges or electrical arcs, and thermal faults such as hot spots, but how can we relate specific gases to specific faults? This is done by considering the energy involved in the fault and the bond formation energy of the oil molecules, for example: partial discharge is a low-energy fault and the gas-key related to that is the hydrogen, because the partial discharge only has the energy to break the C-H bond and generate H2. In the same way, acetylene (C2H2) is related to electric arc because to form a CΞC bond a lot more energy is needed, and only a power electric arc can do this.
For the hot spots, like a loose connector for example, it is possible estimate the temperature of the faults and therefore the severity. For low temperatures, it is mainly methane and ethane that are formed. As the temperature increases and therefore more energy is involved in the fault, less ethane and more ethylene are formed, indicating that the severity and therefore the risk is increasing, and the time is approaching to reduce the sampling periodicity or even to schedule an internal inspection.
A good analogy for the DGA is your own blood sample test. You can compare your results to the limit values and conclude that all the results conform with to the limits and eventually some of them are out. From the same results, a medical doctor can extract a lot more information, not only about the blood itself but with regard to your general health.
DGA is currently the best monitoring technique for power transformers and is being widely used by utilities and industrial users around the world to check the health of their equipment. A DGA expert can extract a lot of useful information from an oil sample.
If you want to find out more about this, you can consult IEC 60599 - Mineral oil-filled electrical equipment in service - Guidance on the interpretation of dissolved and free gases analysis or IEEE C57.104 - IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers.
Jayme Nunes is Senior Technical Advisor at Nynas.