Assessing Transformer Condition – Part 2: Dissolved Gas Analysis (DGA) Carries the Most Weight
DGA is arguably the most powerful tool in the industry when it comes to assessing transformer condition. Commonly performed according to ASTM D3612C, and known as the headspace method (also detailed in IEC60567), this diagnostic test measures the concentration of certain key gases dissolved in the oil. Additionally, provided oil samples are taken at regular intervals, the rate of gas generation can also be determined. This information enables specialists to understand which faults are emerging and their severity.
While acetylene is the most important gas to measure for detecting severe faults, all gasses are important from an incipient fault perspective. The types and quantities of gases that form within the insulating oil will unveil the nature of the fault and determine whether it involves the solid insulation, is a thermal or electrical issue, and whether there is a leak within the sealed system or premature degradation in an open system.
There are many recognized methods for interpreting DGA data, which there is not time to review here, as well as suggested gas limits in guides such as IEEE C57.104-2019 and IEC 60599. Nevertheless, it’s important to remember that allowance must be made for factors such as the type of the dielectric oil involved (silicone, mineral or ester fluids) but a high-level summary of DGA interpretation would be:
- Acetylene usually indicates arcing or a high temperature thermal condition
- To check for partial discharge, look at hydrogen levels
- For low temperature faults, pay close attention to ethane and methane
- Ethylene is an indicator of a high-temperature thermal issue
- In temperate climates, high levels of carbon monoxide are a sign of paper degradation, whereas in hotter climates, high levels of CO can persist without other indicators of paper degradation being present
- High levels of carbon dioxide can indicate general overheating of the paper insulation
A single set of DGA data fails to inform us whether the gas concentrations are stable, increasing or even subsiding, or indeed how long they have been there, if they are associated with a known incident like a transient condition, or when the transformer is stressed in a particular manner. All that is known are the gases present and the concentration of each; this may indicate an issue, but it cannot indicate whether there is an active problem. Therefore, a trend of several data points needs to be established which will inform the asset manager if the gassing is stable, becoming more intense, or if it is progressing from one fault type to another.
Even after having established the DGA trend, as with all diagnostic tests, context is paramount. Know the normal behavior for your asset, its age, and local conditions, such as ambient temperature, loading, transients, harmonics, or other circumstances that would explain the gases in the oil. Comparing gassing of an asset to sister units (if available) can provide additional information. Changes in the gassing levels may have been caused by a change in loading pattern or a through fault. Also consider any maintenance activities that have been performed. Have any repairs been made? What electrical tests have been conducted? If results from several transformers have changed, has there been a change in sampling procedure or the laboratory used?
Under some circumstances degassing of the transformer oil is undertaken; typically, when filling a new transformer or after maintenance which has exposed the core and windings. This inevitably changes DGA values and requires new benchmark tests to reestablish the trend in gas behavior over a period of time (at least 3 months). It’s important to remember that degassing the oil will not fix the underlying cause of the problem, erases the DGA trend and as a procedure is not risk free even when using competent contractors.
In the third and final part of this blog series we’ll look at Oil Quality data and the valuable information that gives us about transformer condition.
Authors:
Simon Sutton has over 25 years’ experience in the electricity transmission and distribution industry predominantly in the cables sector. He has worked in the cable materials supply industry, as the cables policy manager for a transmission utility and in the research sector. His interests also include condition monitoring, diagnostic testing, forensics and asset management. Simon now works as Director of Services for Altanova, a Doble company, and is based in the UK. His responsibilities include business strategy, external relationships and coordination of technical activities around the world. Simon holds a degree and PhD in Physics both from the University of Reading. He is active in International professional bodies representing UK on the CIGRE Study Committee for Materials and Emerging Test Techniques, Convenor of the CIGRE Strategic Advisory Group on Solids and is a member of the editorial board of the IEEE Electrical Insulation Magazine. He is a Visiting Senior Research Fellow at the University of Southampton.
Lance R. Lewand is the Technical Director for the Doble Insulating Materials Laboratory. The Insulating Materials Laboratory is responsible for routine and investigative analyses of liquid and solid dielectrics for electric apparatus. Since joining Doble in 1992, Mr. Lewand has published over 75 technical papers pertaining to testing and sampling of electrical insulating materials and laboratory diagnostics. Mr. Lewand received his Bachelor of Science degree from St. Mary’s College of Maryland. He is actively involved in professional organizations including the American Chemical Society, a representative of the U.S. National Committee for TC10 of the International Electrotechnical Commission (IEC) and ISO TC28, ASTM D-27 since 1989, Chair of ASTM Committee D-27, sub-committee chair 06 on Chemical Tests, secretary of the Doble Committee on Insulating Materials, and a recipient of the ASTM Award of Merit for Committee D-27.
Andy Davies has been working for Doble for 6+ years. His work commenced with 2.5 years in the Middle East; providing asset health indexing and maintenance guidance for over 2400 transformers for a middle eastern transmission company. Since then, he has been involved with support and training for online asset management tools like dobleARMS and INSIDEVIEW, hardware support for portable and field oil testing equipment like Calisto, Myrkos and Domino and provides transformer consultation for customers located across EMEA. Prior to Doble, he worked with an oil services company; that provided oil reclamation and mobile oil solutions that included technical consultation for all generators, HV contractors, transmission and distribution utilities across the UK and Ireland. He has led research into DBDS and Acidity in transformers and their mitigation strategies and has a sound understanding of oil chemistry.
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