Chapter 11. Thermal Model of a Core Form Power Transformer and Related Thermal Calculations

Chapter 11. Thermal Model of a Core Form Power Transformer and Related Thermal Calculations

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429 11. THERMAL MODEL OF A CORE FORM POWER TRANSFORMER AND RELATED THERMAL CALCULATIONS Summary A model of a core form power transformer is presented which utilizes a detailed network of oil flow paths through the coils and radiators. Along each path segment, oil velocities, temperatures, and temperature rises are computed. The oil flows may be either thermally or pump driven. Throughout the disk coils, the flow is assumed directed by means of oil flow washers. Temperatures are computed for each disk along the disk coils. Coils with non-directed oil flow are also treated but in less detail. The model includes temperature dependent oil viscosity, resistivity, and oil density as well as both temperature and velocity dependent heat transfer and friction coefficients. The resulting non-linear system of equations is solved iteratively. The radiator oil flow is also treated by means of a similar network model. Radiator cooling can be by natural convection or fans. Tank cooling by convection and radiation is also included. Iterations are performed in a back and forth manner between the coils and radiators until steady state is reached where the electric power losses equal the losses to the surroundings. Some assumptions regarding the temperature distribution of the tank oil and tank oil flows are made in order to tie the coil and radiator flows together. Although detailed output information is available such as path temperatures and velocities, average coil, coil hot spot, average oil, and top oil temperatures are also computed and compared with test data. In the transient version, time constants and times at overload until a particular hot spot or top oil temperature is reached can be obtained. Other thermal issues such as the loss of life determination, cable, tieplate, and tank wall temperatures are also addressed. 11.1 INTRODUCTION A thermal model of an oil cooled power transformer is presented here, along with details of the computer implementation and experimental verification. Any such model, particularly of such a complex system, is necessarily approximate. Thus, the model assumes that the oil flows in definite paths and ignores local circulation or eddy patterns which may © 2002 by CRC Press
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THERMAL MODEL 430 arise. Since we assume that the oil flow through the disk coils is guided by means of oil flow washers, this assumption should be fairly accurate for these types of coils. Although recent studies have shown that irregular eddy flow patterns may exist in non-directed oil flow cooling in vertical ducts [Pie92], these types of coils occur to a very limited extent in our designs. Such patterns may also occur in the bulk tank oil. We assume these are small compared with the major or average convective cooling flow in the tank. We further assume that the convective flow in the tank results in a linear temperature profile from the bottom of the radiators to the top of the coils in the tank oil external to the coils. The model likewise ignores localized heating which may occur, for example, due to high current carrying leads near the tank wall. It accounts for these
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 11. Thermal Model of a Core Form Power Transformer and Related Thermal Calculations

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