Structural integrity of power transformers

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

Evaluation of the current disease severity scores in paediatric FMF: Is it necessary to develop a new one?

Objectives: Modified adult disease severity scoring systems are being used for childhood FMF. We aim to test the clinical consistency of two common severity scoring systems and to evaluate the correlation of scores with the type of FMF mutations in paediatric FMF patients since certain mutations are prone to severe disease.Methods. Two hundred and fifty-eight children with FMF were cross-sectionally studied. Assessment of the disease severity was performed by using the modified scoring systems of Mor et al. and Pras et al. Genetic analysis was performed using PCR and restriction endonuclease digestion methods for the presence of 15 FMF gene mutations. FMF mutations were grouped into three based on well-known genotypic-phenotypic associations. Correlation between the mutation groups and the severity scoring systems was assessed. The consistency of the severity scoring systems was evaluated.Results. The results of two scoring systems were not statistically consistent with each other (κ = 0.171). This inconsistency persisted even in a more homogeneous subgroup of patients with only homozygote mutations of M694V, M680I and M694I (κ = 0.125). There was no correlation between the mutation groups and either of the scoring systems (P = 0.002, r = 0,196 for scoring systems of Mor et al.; P = 0.009, r = 0.162 for Pras et al.).Conclusions. The inconsistency of the two scoring systems and lack of correlation between the scoring systems and mutation groups raises concerns about the reliability of these scoring systems in children. There is a need to develop a scoring system in children based on a prospective registry. © The Author 2011. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved

A simplified acute kidney injury predictor following transcatheter aortic valve implantation: ACEF score

Background: Transcatheter aortic valve implantation (TAVI) is an effective, less invasive treatment alternative for symptomatic severe aortic stenosis (AS). Acute kidney injury (AKI) following TAVI is a common complication and is associated with worse outcomes. The age, creatinine, ejection fraction (ACEF) score is a simple scoring method, including only three parameters: age, creatinine, and ejection fraction (EF). The score was well established in predicting AKI after coronary interventions.Aims: We aimed to evaluate whether this simple scoring method, ACEF, may predict a development of AKI in patients who underwent TAVI.Methods: A total of 173 consecutive patients with symptomatic severe AS who underwent TAVI were included retrospectively. The primary endpoint of the study was the development of AKI. Study population was divided into two groups according to the presence of AKI. The ACEF score was calculated with the formula: age/EF + 1 (if baseline creatinine >2 mg/dl).Results: Twenty-nine patients developed AKI. The median (interquartile range) ACEF score was 1.36 (1.20–1.58). The ACEF score was found to be an independent predictor of AKI (P <0.001). The ACEF score ≥1.36 predicted AKI development with a sensitivity of 96.6% and specificity of 58.8%. Moreover, hypertension, hemoglobin levels, contrast volume, and aortic valve area (AVA) were found to be independent predictors of AKI.Conclusions: Our study revealed that the ACEF score was an independent predictor of AKI. A simple and objective score might be very useful in predicting AKI development in patients undergoing TAVI

Structural integrity of power transformers

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Utility of near infrared spectrophotometry in mesenteric ischemia: An experimental study

Kalkan, Asim/0000-0002-5800-0201WOS: 000444424600004This study investigated the effectiveness of near infrared (NIR) spectrophotometry for measuring abdominal saturation in a rat model of induced mesenteric ischemia. This was a randomized, controlled, non-blinded interventional animal study. Twenty-eight male Sprague Dawley rats were randomly allocated into four groups. Following induction of anesthesia, the abdominal region was shaved and intra-abdominal access was achieved through a 4-cm midline incision and basal measurements were performed using an INVOS-5100c device. the rats were grouped on the basis of rSO2 measurement times. Mesenteric artery ligation was not performed on the control group rats. From the rest of the mesenteric artery ligated rats, measurements were taken at 30 min, at 2 h and at 6 h. Measurements were repeated using NIR spectrophotometry probes after the abdomen was surgically closed. Following the measurements, the abdomen was re-opened and small intestine tissue specimens were taken for histopathological examination. Initial and final NIR spectrophotometry measurements were compared. No statistically significant difference was found in the group 1 between basal rSO2 values and post-incision values (p = 0.748). There was a significant variation between the initial and final measurements in groups 2, 3, and 4 (p = 0.018). Although various radiological or biochemical markers are available for detecting mesenteric ischemia, the use of NIR spectrophotometry can be considered a good non-invasive alternative method for early diagnosis of acute mesenteric ischemia