Insulation system is one of the most important parts of power transformer. Theoretical background for insulation design is given, which includes explanation of dielectric properties of main insulation materials, their withstand to breakdown voltage, their verification by dielectric tests, and statistical data based theory for breakdown probability. Two known numerical methods (BEM and FEM) for calculation of electric field are used and for each numerical method an appropriate model of transformer is made. Safety factors for main insulation between HV and LV winding are calculated after which the oil gap width is reduced. Additional analysis of safety factors and field nonuniformity along windings is performed.Index Terms-numerical calculation, boundary element method, finite element method, electric field, power transformer
Purpose -The purpose of this paper is to determine the external magnetic field density of the traction transformer for EMU and to find the model for its computation. Design/methodology/approach -The magnetic flux density in the surrounding region of the traction transformer was modeled and calculated using FEM. The transformer was modeled in a way that tank, clamps and current sources were taken into account. Most frequent operating modes for the basic 50 Hz current harmonic, and most represented higher harmonic of 1,950 Hz loading current, were analyzed. Findings -Matching calculated and measured values were obtained on the finished transformer. The developed calculation has proved to be a useful tool for the stray field calculation outside this type of transformer. Calculated values of the flux density are lower then the maximum permitted values defined by the DIN VDE 0848 standard. Originality/value -This paper presents a study of calculation compared to measurement of magnetic field density outside an oil immersed transformer.
In the distribution transformers design oval windings are used due to economic advantages. On the other hand, such windings are more susceptible to radial forces in a short circuit. A diamond dotted paper with an epoxy coating is used in order to increase the stiffness of the winding. Despite that, winding failure may occur during the short circuit, e.g. buckling of inner winding. Because of a very thin foil conductor (typically 0.5-2 mm), the most critical is inner low voltage foil winding which can collapse due to radial forces at stresses far below the elastic limit of conductor material. This paper shows an analytical approach to the calculation of critical stress in inner oval foil winding with epoxy coated insulation. Critical stress was calculated using the equation for free buckling of round winding. Equivalent Young's modulus of elasticity was obtained experimentally from the testing of the sample model loaded with bending force on a tensile test machine. A total of 12 test samples were formed from aluminium foil conductor and diamond dotted paper and cured at the temperature of 105°C. The results were successfully verified on distribution transformers subjected to short circuit withstand tests.
A simple numerical method based on integral approach was described to compute the stray magnetic field in the surrounding region of a loaded distribution transformer. The model for magnetic field computation was made of transformer windings and low voltage conductors. Windings were modeled by rectangular and round blocks with uniform amper-turn distribution throughout the winding cross section. Low voltage conductors were modeled as series of straight current-carrying wire segments. The computed and measured results of magnetic induction are in good agreement.
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