Assessing Transformer Condition – Part 1: Common Tests & Best Practices to Implement Now
Transformers are subjected to electrical, thermal, and chemical stresses during their operational life that degrade the insulating oil and solid insulation, cause corrosion and oxidation, and create the conditions for incipient faults to develop which may ultimately shorten the life of the asset. These ageing processes are necessarily considered during the transformer design phase. However, when degradation occurs faster than anticipated, this is considered to be accelerated ageing. For instance, a high resistance joint causing localized overheating or partial discharge degrading the solid insulation is an example of premature ageing.
Although simple visual inspection of the transformer tank (to look for corrosion or leaks) or IR surveys (to identify overheating pumps) yields important condition information, not all issues will be visible from the outside. Fortunately, incipient faults occurring within the transformer can be identified and diagnosed by examining the chemical, physical and electrical properties of the liquid dielectric within it. This is usually done in an external laboratory; however, some larger utilities or industrial entities may undertake in-house testing.
In this three-part series of blogs, we will:
- Look at the importance of sending high-quality oil samples to the laboratory to be tested.
- Consider the types of problems that can be identified by Dissolved Gas-in-oil Analysis (DGA) and what next steps may be taken.
- Consider other oil tests that are conducted, and the important information that can be revealed.
Importance of a High-Quality Sample
An oil sample can reveal a wide variety of information about the condition of your asset; this includes evidence of overheating, partial discharge and arcing, paper degradation, water ingress, oxidation, presence of chemical and physical contaminants, and more. Consequently, oil testing is a key method for assessing a transformer’s condition and identifying incipient faults before they become critical. A single measurement is valuable, but trending changes in the data over time enhances the diagnosis revealing the severity of the situation and enables asset managers to plan appropriate actions. This could involve offline electrical tests to determine the underlying cause, fitting online monitoring devices to monitor the condition of the asset more effectively, or scheduling a repair or replacement.
Ensuring good results for your assessment starts with delivering a good oil sample to the lab. Even perfectly performed lab tests are rendered meaningless if they are based on a poor sample. Failing to take the sample correctly will inevitably lead to bad results, and the additional cost of having to retake the sample and perform the analysis yet again. A good sample needs to be truly representative of the bulk liquid circulating within your electrical equipment. Getting to this representative oil requires several liters of oil to be flushed through the sampling pipework and into an appropriate waste oil container prior to collecting the sample proper. In the process of waiting for the flushing to complete, this oil can be used to rinse the sample container and caps, to ensure they are free from physical contamination.
When taking a sample, it is beneficial that your container is large enough to hold the amount oil needed with some extra just in case the lab needs to repeat a test to verify unusual results; this typically means about 1 liter. There are many suitable containers for taking an oil sample and each has its own benefits and pitfalls. Generally, glass/aluminum bottles or tin cans are the preferred options. The container should properly seal the sample, preventing ingress and egress of any liquids and gasses. As oil degrades in sunlight leading to the synthesis of hydrogen, the containers, sleeves and/or packaging should be light proof to protect the sample from sunlight.
Plastic bottles should be avoided, since water molecules are able to diffuse through the container walls, thus increasing the water content of the sample; studies have revealed that 10’s of ppm water can enter the sample during transportation and storage before testing. Conversely, small molecules like hydrogen can diffuse out of the oil through the plastic container walls decreasing the concentration ultimately measured in the sample.
Lastly, it’s important to pack the samples well to avoid damage during transportation to the oil testing laboratory. Make sure the bottoms of the bottles are protected as well.
Insulating oil is the lifeblood of transformers. Periodic oil testing based on condition and criticality of the asset, enables users of oil-filled high voltage equipment to detect incipient faults, monitor their progression and plan appropriate action before minor issues become bigger issues, or at worst failures.
In the next blog will we look at the information provided from Dissolved Gas-in-oil Analysis testing.
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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