Assessing Transformer Condition: Part 3
In Part 2 of this series, we looked at the most frequently applied test to transformer oil samples, namely, Dissolved Gas-in-oil Analysis (DGA): DGA is ideal for detecting incipient faults in the transformer. However, the diagnostic value in monitoring changes in the chemical, physical and dielectric properties of the oil cannot be understated, as these also can degrade over time and affect the performance of the transformer. Here, in the final installment, we consider some of the other tests which provide further valuable information about oil quality.
Checking for Oil Quality: Water Content, Relative Saturation and Breakdown Voltage
Water is the most damaging molecule in the transformer. When dissolved in the oil it catalyzes reactions, weakens bonds, attracts other polar contamination to the paper and allows acids to be aggressive. Conversely, free water in oil will generally sink to the bottom of the transformer where it contributes to tank corrosion, whereas, if it precipitates onto winding due to oversaturation, it can cause flashovers.
Water concentrations are generally much lower in the oil compared to the paper; typically, water exists at ppm levels in the oil compared to single figure percentage levels in the paper. This is because paper itself has polar components (e.g., hydrogen bonding) which although giving the paper additional mechanical strength, also attract water molecules. The presence of water in the paper is important as it disrupts the hydrogen bonds reducing the physical strength of the paper.
Knowing how water partitions between the oil and paper means that by measuring the water content in the oil, the content in the paper can be calculated. Nevertheless, different oils have different levels of affinity for water thus it is important to know what oil is being tested: the difference is particularly marked between mineral oils and ester liquids. To further complicate the situation, the polarity of the oil can be affected by ageing byproducts. It is therefore better to examine the relative saturation of water in oil rather than ppm. It should remain below 50% to retain adequate dielectric breakdown voltage.
Under particular circumstances the water in the paper can generate gas bubbles, for example, during transformer overloading events, or during startup before adequate oil circulation is established. Under these conditions, the conductors can heat the paper above 100˚C causing water to vaporize, thereby increasing the likelihood of bubble formation, which in turn can lead to partial discharge (PD) and risks localized physical PD damage. The probability of bubble formation is dependent on both the concentration of the water in the paper and temperature; for example, with 2% by dry weight of water in the paper, the risk of bubble formation is very low below 140 °C.
Transformer oils are designed to provide electrical insulation under high electrical fields. Any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function. Breakdown voltage (BDV) is a measure of the electrical stress the oil can withstand without breaking down. The test is conducted by increasing the voltage between two electrodes within a test vessel containing the test oil until the oil breaks down. Sampling technique plays a significant role in obtaining meaningful breakdown voltage results; particles and fibers accidentally introduced during cleaning the test cell or sampling bottles (chamois leather, cotton rags and paper towels) can all drastically reduce the measured result.
Accelerated Ageing: Power Factor, Color Testing, and Interfacial Tension Identify Key Characteristics
Transformers typically last at least 40 years even though the design life is usually around 25 years – but that is not by accident. Keeping the asset sealed and operating at or below nameplate will preserve this life expectancy. High temperatures, elevated levels of oxygen, water content, acidity and sludging – all in the presence of other catalytic factors, like copper in the windings, silver contacts and iron – can speed up the ageing of paper and insulating oil, as well as corrode the metal in the transformer.
There are three recommended tests to identify oil ageing or contamination, enabling early intervention:
- Power factor testing measures the dielectric losses of the insulating oil. As the oil oxidizes with increasing time in service, the polar content increases which can be detected through increased power factor. This test can also detect the presence of other contamination in the oil, and whilst it cannot identify the actual molecules, highlights the need for further investigation.
- Color tests are a simple rapid indicator of ageing in the insulation system; the darker the oil sample the more aged the oil.
- Interfacial tension (IFT) is an indirect measure of the polar nature of the oil and provides powerful insight into early oil oxidation and polar contaminants, such as water or acids. The test measures the strength of the separation between water and the oil sample. Oil and water should form distinct layers when there is little contamination in the oil, but as the oil becomes aged or wet, the tension between the liquids becomes less distinct and therefore weaker, such that a lower IFT result is worse than a higher one. It should be noted that IFT is also affected by the presence of detergents such that residual deposits from cleaning sampling equipment, sampling containers, or the test vessel with such surfactants, can have a dramatic effect on this test parameter.
There are other oil quality tests performed in tandem with the above such as acidity and relative density, which can be performed for a more in-depth examination of the characteristics of the oil. It takes gross contamination, ageing or over-processing for these properties to change significantly, so if either of these values fluctuate between tests, it could be cause for concern. If an issue with the oil does present itself there are other investigative tests that can be employed.
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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Articles:
- Assessing Transformer Condition – Part 1: Common Tests & Best Practices to Implement Now
- Assessing Transformer Condition – Part 2: Dissolved Gas Analysis (DGA) Carries the Most Weight
- Purchasing a New DGA Monitor? Read the Fine Print
- The Power of Dissolved Gas-in-Oil Analysis
