Oil Sampling & DGA Analysis of Power Transformers

August 14, 2022

Power transformers are one of the most important pieces of equipment when it comes to electrical distribution. They are responsible for the transformation of higher voltages from distribution to lower voltages as seen in everyday electrical equipment. The most common type of transformer is the oil filled transformer. The internal windings are insulated with paper and the tank filled with an insulating oil. This allows for transformers to be smaller in physical size while maintaining high outputs of power. Many people underestimate the importance of a healthy transformer in relation to the reliability of their power systems. They believe it is a piece of equipment that just sits there and does not do anything as it has no moving parts. While it may look that way from the outside, what is happening on the inside requires special attention.

Dissolved Gas Analysis (DGA) is the study of dissolved gasses in transformer oil. Whenever a transformer undergoes abnormal thermal and electrical stresses, certain gasses are produced due to the decomposition of the transformer oil. By monitoring the content of specific gasses present in the oil you can get a picture as to the condition of the transformer and any history behind abnormal events that may have occurred during its lifespan. This is a non-intrusive method to obtaining a condition assessment of the transformer internals. Generally, the gasses found in transformer oil are Hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H3), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), and oxygen (O2). 

The oil sample is taken from the sampling point by extraction into 50ml syringes and 500ml bottles. It is important for the sampling containers to be sterilized to ensure the sample does not become contaminated. Generally, it is found that hydrogen and methane are produced in large quantities if the internal temperature of the transformer rises above 150-3000C due to abnormal thermal stresses. If the temperature goes above 3000C ethylene is produced in large quantities. In temperatures above 7000C a large amount of hydrogen and ethylene are produced. Ethylene is a good indicator of hot spots within transformers. If the DGA analysis detects carbon monoxide and carbon dioxide it is presumed there is a decomposition of proper insulation within the transformer.  

The transformer core and windings use paper for their insulation. The paper is made from a cellulose base and its structure is a long chain of molecules. Over time the paper becomes brittle and these long chain molecules begin to shorten. When fully submersed in transformer oil, the lifespan of the paper is severely diminished due to the oxidization that occurs in the oil. When the paper weakens, it cannot contain the mechanical stresses involved with an electrical short circuit. This leads to a breakdown of the paper and therefore it is necessary to monitor the condition of paper insulation inside the transformer. 

Although the DGA test can predict the condition of the paper insulation primarily, it is not a very sensitive method. When the transformer oil soaks into the paper it becomes damaged by heat and some unique oil soluble compounds are released and dissolved into the oil. These compounds belong to the furfuraldehyde group. These furfural family of compounds can only be released from damaged cellulose paper. The furan analysis of the oil is extremely sensitive and damage to even a few grams of paper is noticeable. The percentage rate rise of furfurals in transformer oil over time is used to assess the condition and remaining lifespan of the paper insulation. 

Just by analyzing the dissolved gas and furan content within transformer oil you can get an accurate description and timeline of the internals of the transformer without turning the power off. If your site uses power transformers and does not already perform Oil testing give us a ring to see how we can help. 

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