Structural integrity of power transformers
Creators: name: Kalkan, Gokhan
Publisher: name: Imperial College London
Year: 2012

description: The subject of this thesis is to study the mechanical integrity and performance of insulation materials used in power transformers under short circuit conditions. There are a number of methods to calculate short circuit forces in the literature. These methods were developed to determine the magnetic properties related to the short circuit condition and cannot be used in the open circuit condition due to the assumption of infinitely permeable core. To this end, a new solution strategy is introduced which is able to calculate magnetic properties of power transformers both in open circuit and in short circuit conditions. A solution was derived utilizing transform techniques and multiple connected permeable regions can now be solved. Mechanical failure modes of transformer winding are presented and new solution methods are introduced for some failure modes. Dynamic representation of the transformer winding is achieved by treating the winding turn by turn. Particular attention is given to stress calculations of Continuously Transposed Cable (CTC) and resin bonded CTC. Digital Image Correlation (DIC) technique maps the strain distribution on the test specimen and the strain distribution can be extracted at any cross section of interest. 3‐D DIC technique is used to determine the response of transformerboard material under tensional and bending loads and material properties are determined. It is also shown that the DIC method provides much more accurate results compared to strain gauges due to its manufacturing technique. DIC results are used to determine material properties related to both tensional and bending type loading conditions. Resin laminate wood is also compared to transformerboard. Viscoelastic properties of transformerboard are examined. Because a short circuit event is dynamic in nature, storage and loss modulus of transformerboard are determined as a function of temperature and frequency under cyclic loading conditions. Conventional creep test setups cannot be used for transformerboard. A test setup is designed to measure creep curves of transformerboard. The test setup measures the creep curves both in oil and in air and the displacements are recorded automatically. Tests are performed at a wide range of temperatures encountered in real service conditions of transformerboard material. The developed method can be introduced in PC codes to determine the magnetic properties related to the magnetic field. Also the method can be used to determine turn to turn or disc to disc mutual inductances of a transformer. Electromagnetic forces calculated with the introduced method are also more accurate than the methods developed earlier. Methods to calculate mechanical stresses acting on transformer components are also reviewed and new solution techniques for some failure modes are developed. Mechanical performance of insulation materials under tension and bending type loadings are measured and monitored using DIC technique. Finally, time dependent material properties of the transformerboard under constant and cyclic loading conditions are determined. Both analytical and experimental techniques are utilized to determine the material properties of the transformer components and their behaviour under different loading conditions. New solution techniques are developed and the material behaviour of the insulation materials under different loading conditions is determined. With the data obtained and solutions developed, mechanical stress calculations of the power transformer components can be made more accurately.