Electricity and Control June 2022

TRANSFORMERS, SUBSTATIONS + CABLES : PRODUCTS + SERVICES

Assessing transformer condition

Fortunately, incipient faults occurring within the trans former 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 exter nal laboratory although some larger utilities or industrial entities undertake in-house testing.” The importance of high quality samples “An oil sample can reveal information about the condi tion of the 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. The authors emphasise that ensuring good results for an assessment starts with the key first step of delivering a good oil sample to the lab. The value of Dissolved Gas Analysis (DGA) In Part 2 the authors suggest that DGA is possibly the most powerful tool for assessing transformer condition. “Commonly performed according to ASTM D3612C (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 deter mined. This information enables specialists to under stand which faults are emerging and their severity. “While acetylene is the most important gas to meas ure for detecting severe faults, all gases are important with a view to identifying incipient faults.” Checking oil quality In Part 3 the authors consider some of the other tests which provide further information on oil quality. These include testing for water content, relative saturation and breakdown voltage, as well as power factor testing, col our testing, and interfacial tension measurement, all key indicators of accelerated aging.

In a recent three-part series of blogs under the title Assessing Transformer Condition , Doble Engineering Company looked at various test and analysis methods used in this critical service. The series is authored by Simon Sutton, who has over 25 years’ experience in the electricity transmission and distribution industry, predominantly in the cables sector, and currently works as Director of Services for Altanova, a Doble company; Lance R Lewand, Technical Director for the Doble Insulating Materials Laboratory; and Andy Davies, who, as well as his work with support and training for Doble’s online asset management tools, provides transformer consultation for customers across the EMEA region. The three-part series looks at: - The importance of sending high quality oil samples to the laboratory to be tested - The types of problems that can be identified by Dissolved Gas-in-oil Analysis (DGA) and what next steps may be taken - Other oil tests that are conducted, and the impor tant information they can reveal. The blogs are informative, offering valuable insights from Doble’s extensive experience in this field. Electricity + Control presents only a summary outline here, with brief extracts. Readers can learn more from the Doble website. Part 1 deals with Common tests & Best practices. The authors note at the start: “Transformers are subjected to electrical, thermal, and chemical stresses during their operational life that degrade the insulating oil and sol id insulation, cause corrosion and oxidation, and create the conditions for incipient faults to develop, which may 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 accelerated ageing. A high resistance joint causing localised overheating, or partial discharge degrading the solid insulation are ex amples of premature ageing. “Although simple visual inspection of the transform er 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.

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28 Electricity + Control JUNE 2022