Estudio experimental, analítico y numérico del fallo mecánico del aislamiento celulósico de conductores continuamente transpuestos en transformadores de potencia bajo la influencia de cortocircuitos y envejecimiento térmico

RESUMEN: El papel dieléctrico de un transformador de potencia está sometido a un proceso de envejecimiento, producido por altas temperaturas y reacciones químicas en el aceite dieléctrico. Las fuerzas electromagnéticas que afectan a los conductores pueden producir la rotura del papel, llegando a causar el fin de la vida útil del transformador. Para mejorar la comprensión de estos fenómenos, un conductor continuamente transpuesto (CTC) con cuatro capas de dieléctrico (papel Kraft ordinario y tipo crepe) fue impregnado en aceite nafténico y envejecido durante diferentes periodos, representando toda su vida útil. Muestras del CTC fueron sometidas a deformaciones de flexión compatibles con un cortocircuito, analizándose las grietas generadas en el papel. El papel dieléctrico envejecido fue caracterizado mecánicamente (a tracción, compresión y tangencialmente), y el ensayo a flexión se simuló numéricamente en ANSYS Static Structural, para estimar las deformaciones en el papel. Dos criterios analíticos de fallo (máxima deformación y criterio de Norris) fueron adaptados para predecir las regiones del papel dieléctrico susceptibles de fallar para un estado de envejecimiento y deformación impuesta determinados, obteniendo predicciones coherentes con los resultados experimentales y validando así el enfoque de Norris.ABSTRACT: The dielectric paper of power transformers is subjected to ageing caused by high temperatures and chemical reactions in the dielectric oil. The electromagnetic forces the conductor is subjected to can cause the breakage of the paper and even the end of the useful life of the transformer. To improve the understanding of those phenomena, a continuously transposed conductor (CTC) with four layers of dielectric paper (plain Kraft and crepe papers) was impregnated in naphthenic oil and aged during different periods, representative of its whole lifespan. CTC samples were subjected to bending deformations compatible with a short circuit, and the cracks generated in the paper were analysed. The aged paper was mechanically characterised (in tensile, compressive and shear modes) and the bending tests were simulated in ANSYS Static Structural, to estimate the strains in the paper. Two analytical failure models (maximum strain and Norris criterion) were modified to predict the regions in the dielectric paper susceptible to fail for a particular ageing state and deformation level, obtaining a good agreement with the experimental results and thus validating the Norris approach

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Technological development of cylindrical and flat shaped high energy density capacitors

Cylindrical wound metallized film capacitors rated 2 micron F 500 VDC that had an energy density greater than 0.3 J/g, and flat flexible metallized film capacitors rated at 2 micron F 500 VDC that had an energy density greater than 0.1 J/g were developed. Polysulfone, polycarbonate, and polyvinylidene fluoride (PVF2) were investigated as dielectrics for the cylindrical units. PVF2 in 6.0 micron m thickness was employed in the final components of both types. Capacitance and dissipation factor measurements were made over the range 25 C to 100 C, and 10 Hz to 10 kHz. No pre-life-test burning was performed, and six of ten cylindrical units survived a 2500 hour AC plus DC lift test. Three of the four failures were infant mortality. All but two of the flat components survived 400 hours. Finished energy densities were 0.104 J/g at 500 V and 0.200 J/g at 700 V, the energy density being limited by the availability of thin PVF2 films

A study of life time management of Power Transformers at E. ON’s Öresundsverket, Malmö

The objective of this master thesis is to review the present and future condition of generator step up power transformers at the combined heat and power plant Öresundsverket, in Malmö. The objective of this work was to prolong the lifetime of power transformers at Öresundsverket. The thermal properties of power transformer are been taking into consideration for their life time assessment. The most suitable thermal model was chosen which can prolong life to these transformers in the future. Electrical, mechanical and chemical properties of power transformers were taking in account for evaluation. The comparisons were made between different calculated models to find the most suitable thermal model for transformers

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

Conception of an online condition monitoring and assessment for transformer oil and design of a human-machine interface for real time analysis

06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır.Güç trafoları elektrik güç sistemlerinin en önemli bileşenlerindendir. Güç trafolarının sağlığı şebeke güvenilirliği ve verimliliği için oldukça hayati bir durumdur. Program dışı meydana gelen elektrik kesintilerinin veya aniden gelişen bir arızanın yol açacağı maddi zararlar genellikle büyük olur. Arıza sadece trafoya hasar vermekle kalmaz, aynı zamanda sanayi ve hizmet kuruluşları, konutlar, hastane ve okul gibi binlerce kurumu etkiler. Enerji sisteminin ve elektrikle çalışan cihazların zarar görme olasılığı artar. İş ve zaman kaybı ile birlikte maddi zararlar çok büyük rakamlara ulaşabilir. Örneğin 3 fazlı 500MVA lık yükseltici bir trafonun servis harici kalma durumu günlük sadece güç kayıpları açısından 12 milyon liralık bir bedel oluşturur. Bu nedenle, trafo hakkında erken uyarı bilgisi alındıktan sonra yeni programlar hazırlanıp gerekli önlemlerin acilen alınması çok sağlıklı ve ekonomik olacaktır. Trafoların sağlıklı çalışma göstergelerinden en önemli birisi de hiç süphesiz yağ ve gaz analizleridir. Yağlar elektrik mühendisliğinde başta izolasyon ve soğutma temel özelliklerini sağlamak için güç trafolarında yaygın olarak kullanılır. Yağın kalitesinin dielektrik mukavemet, yüzeysel gerilme, asitlik durumu, nem oranı, renk, güç faktörü gibi testlerle periyodik olarak izlenmesi trafonun izlenmesi ile eşdeğer bir öneme sahiptir. Trafo yağının içinde nem değerinin artarak yağın delinme gerilimini azaltması ve buna bağlı olarak izolasyon seviyesini aşağı düşürmesi, yağın içerisinde bulunduğu kabinde serbest halde bulunan oksijenin trafo sargılarında oksitlenme oluşumunu arttırarak trafo yağının kimyasal olarak yaşlanmasını hızlandırması ve yüksek sıcaklıklara bazen de arklara maruz kalan trafo yağının yüksek gerilim altında elektroliz ile sargılardan kopan küçük metal tozlarla birleşip trafo yağının soğutma ve izolasyon özelliklerini zayıflatması bu izlemelerin hayati rolüne örnek olarak gösterilebilir. Elektrik güç trafolarının rutin kontrolleri yapılarak ileride oluşabilecek arıza ve hasarların erken teşhisi için ve oluşmuş arızaların sebebini belirlemede gaz analizleri de çok önemli bulgular verir. Çözünmüş gaz analizi (DGA olarak anılır), azot, oksijen, karbon monoksit, karbon dioksit, hidrojen, metan, etan, etilen ve asetilen gibi yağ içinde belli gazların konsantrasyonlarını belirlemek için kullanılır. Bu gazların konsantrasyonu ve nispi oranları, yağın bir fiziksel ya da kimyasal özelliğindeki değişikliği ile ilişkili olabilir ve transformatör ile belirli işletme sorunlarını teşhis etmek için kullanılabilir. Trafoyu servis dışına çıkarmadan yağ örneğinin alınabilmesi, DGA yöntemini üstün kılar. DGA yöntemi sayesinde arıza ve olayların gelişim süreci kontrollü şekilde izlenebilir.Power transformers are amongst the most critical component in electrical power systems. The health of power transformer plays a vital role in the security and efficiency of the network. Material damages caused by unscheduled electrical outages or sudden failures are often important. Failure will not only cause damage to the transformer, but also affects industries and service providers, houses and thousands of institutions such as hospitals and schools, and on top of the afore mentioned points , the possibility for the power system and electrical devices to breakdown is increased. Business, time and financial losses can reach high proportions. For example, a 3-phases 500MV step up transformer, which is out of service, can cause a daily loss of 12 million Turkish lira. Therefore, collecting early warning information on transformers, scheduling new programs and taking necessary measures urgently on transformer will be very healthy and economical. Among the most important indicators of healthy operation of transformers are undoubtedly oil and gas analyses. In electrical engineering, the component in which are widely used oils, mainly to ensure the insulation and cooling functionalities, are power transformers. Monitoring the quality of the oil by periodically testing certain oil features such as its dielectric strength, interfacial tension, neutralization number, moisture content, colour, power factor etc. is essential in monitoring the condition of transformers. While performing transformers maintenance routine test, gas analysis also provides important information for early detection of faults or damages that may occur in the future and for the determination the causes of these faults. In a dissolved gas analysis process, the concentrations or relative concentrations of some specific gases dissolved in oil are used to diagnose certain well known transformers operational issues. These gases generally comprise azote, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene, and acetylene. Changes that appear in certain physical and chemical properties of the oil might also be utilized to determine the nature of those operational issues. Extracting oil samples from transformers while in service reinforce the DGA process. It is possible then with DGA to monitor the development process of faults in transformers

Issues of seismic response and retrofit for critical substation equipment

This study focuses on means to reduce the seismic hazard for transformer-bushing systems and different issues of the response and rehabilitation of transformers. The primary means of seismic mitigation investigated is the use of the Friction Pendulum System (FPS) bearings to seismically isolate transformers. This is done by developing a finite element model representing the behavior of FPS bearings and implementing this model on to ADINA finite element package for further use in analytical studies. This model is used to study the behavior of isolated primary-secondary systems and the effects of parameters like different FPS radii or vertical excitations. Also studied are the effects of isolation on forces applied to the foundations and the corresponding design of foundations compared to the commonly used fixed-base forces. Further, the interaction of transformer-bushings with interconnecting equipment in the substation is studied and corresponding graphs indicating the amount of required slack in connecting cables are presented. Finally, the behavior of internal components of transformers under seismic excitation has been studied. Possible failure and damage modes are identified and a model is developed and analyzed to assess damage risk