2012 • 77 Pages • 3.32 MB • English

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CFD Study On The Thermal Performance of Transformer Disc Windings Without Oil Guides Yuhe Jiao Master of Science Thesis KTH School of Industrial Engineering and Management Energy Technology EGI-2012-089MSC EKV915 Division of ETT SE-100 44 STOCKHOLM

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Master of Science Thesis EGI 2012: 089MSC EKV915 CFD study on the thermal performance of transformer disc windings without oil guides Name: Yuhe Jiao Approved Examiner Supervisor Reza Fakhraie Jurjen Kranenborg Commissioner Contact person Abstract The hotspot temperature of disc windings has a close relation with the transformer age. In oil immersed transformers, oil guides are applied generally to enhance the cooling effects for disc windings. In some cases disc windings without oil guides are used. However, the lack of oil guides is expected to result in a more complicated thermal behavior of the windings, making it more difficult to predict the location and strength of the hotspot temperature (i.e. the hottest temperature in the winding). To get an improved understanding of the thermal behavior, a CFD study has been performed. This article describes the implementation of CFD simulation for 2D axisymmetry models without oil guides, and then analyzes the results of a series of parametric studies to see the sensitive factors influencing the cooling effects. These parameters include radial disc width, inlet mass flow rate, horizontal duct height, vertical duct width and the inlet/outlet configurations. Three main characteristics, the hotspot temperature, the location of the hotspot and the number of oil flow patterns are detected to describe the thermal performance. Key words: Non-oil guided, disc windings, CFD, parametric study -2-

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Table of Contents Abstract ........................................................................................................................................................................... 2 1 INTRODUCTION ............................................................................................................................................. 5 1.1 Purpose .......................................................................................................................................................... 5 1.2 Scope and structures .................................................................................................................................... 6 1.3 Review of previous experience .................................................................................................................. 7 1.4 Nomenclature ............................................................................................................................................... 9 2 POWER TRANSFORMERS ..........................................................................................................................10 2.1 Introduction of power transformers .......................................................................................................10 2.2 Operating principle of transformer .........................................................................................................10 2.3 Energy losses and thermal cooling methods .........................................................................................12 2.4 Oil immersed transformers ......................................................................................................................13 2.5 Oil guided and non-oil guided disc windings ........................................................................................16 2.6 Transformer oil ..........................................................................................................................................17 3 ANALYSIS OF THEORETICAL MODEL ...............................................................................................18 3.1 Physical models of disc windings ............................................................................................................18 3.2 Governing equations .................................................................................................................................21 3.2.1 Navier-Stokes equations ..........................................................................................................21 3.2.2 The boussinesq approximation ..............................................................................................27 3.3 Simplification and assumption .................................................................................................................28 4 WINDING MODEL IMPLEMENTATION .............................................................................................30 4.1 Winding geometry model .........................................................................................................................30 4.2 Grid model ..................................................................................................................................................31 4.2.1 Mesh solution ............................................................................................................................31 4.2.2 Grid quality analysis .................................................................................................................32 4.3 FLUENT Setting .......................................................................................................................................35 5 BASE CASE STUDY .......................................................................................................................................36 5.1 Base case setup ...........................................................................................................................................36 5.1.1 Estimation of initial condition................................................................................................36 5.1.2 Base case setting .......................................................................................................................37 5.2 Base case analysis .......................................................................................................................................38 5.2.1 Steady state solution .................................................................................................................38 5.2.2 Time dependent solution ........................................................................................................42 6 PARAMETRIC STUDIES ..............................................................................................................................44 6.1 Radial disc width sensitivity investigation ..............................................................................................44 6.2 Mass flow rate sensitivity investigation ..................................................................................................50 6.3 Horizontal space height sensitivity investigation ..................................................................................57 -3-

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6.4 Vertical duct width sensitivity investigation ..........................................................................................62 6.5 Inlet/Outlet configuration sensitivity investigation .............................................................................64 6.6 Oil guided case investigation ....................................................................................................................66 7 CONCLUSION .................................................................................................................................................71 8 FUTURE WORK ..............................................................................................................................................73 ACKNOWLEDGMENTS .......................................................................................................................................74 BIBLIOGRAPHY ......................................................................................................................................................75 APPENDICES ............................................................................................................................................................77 -4-

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1 INTRODUCTION 1.1 Purpose The main purpose of this report is to study the oil flow and temperature distributions in transformer disc windings without oil guides by using advanced CFD tools. By performing the parametric study on different design parameters, a good understanding of the sensitive factors that influence the thermal performance is obtained. Different from study of disc windings with oil guides, this project mainly focuses on disc winding transformer without oil guides. The global cooling effect, hotspot temperature as well as its location and the oil distribution are unpredictable in such a large length scale transformer ducts model. This makes it necessary to do this investigation using advanced CFD tools. As a result, we want to see the cooling effects of the disc windings without oil guides comparing to the case with oil guides, the solution that is suitable for the study of non- oil guide model, the parameters that will affect the oil distribution and the temperature distribution, etc. Three indicators proposed for the cooling effects are the hotspot temperature which has a close relation to the age of transformer, the location of the hotspot which is important if it can be predicted in design and the oil flow distribution which will give a clear view about the heat transfer capability inside the disc winding. For most of large oil immersed transformer, oil guides are applied to enhance the cooling effects by guiding the oil flow in order to increase the heat transfer coefficient and make oil distribution stable. However, for some winding with a small radial dimension, it is difficult or not necessary to place guides, then, what will happen in the disc windings such as the oil flow distribution and temperature distribution, whether the solution for oil guided case is suitable for non-oil guided cases would be a new interest. At least, a good understanding about the oil distribution and temperature distribution could also help know more about the cooling effects of oil immersed disc winding and hopefully this result could contribute to other new cooling methods. -5-

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1.2 Scope and structures This investigation gets experience from previous CFD studies by ABB, for example, (Kranenborg, 2008) on oil guided disc windings. Firstly, this report gives an introduction of transformers as well as their cooling methods and describes the problem without oil guides inside the disc winding generally. Then, the related theoretical analysis describes the physical models of the non-oil guided disc windings. The implemented process by using Ansys Fluent is given in the following part. The case study part consists of the base case study and parametric study, during which the comparison between oil guide and non-oil guide is also conducted. In the end, the final conclusion is given and the suggestions s future work is also proposed. This investigation was performed mainly by doing CFD simulation using Ansys Fluent which is a program based on FVM (Finite Volume Method). In order to get an accurate result, a large number of nodes were generated for the full winding geometry model. The calculation time for each case was quite long, especially for time dependent simulation. Thus, the conclusion drawn in this investigation mainly gives a general description and estimation of the thermal performance. For a more detailed and exact result, more work should be done in the future. This report has the following structure. Section 1 is the introduction of the purpose and scope for this report as well as the nomenclature description; Section 2 gives a basic introduction of the oil-filled power transformer including the definition, classification, application and operating principles. The thermal cooling methods applied in power transformers are also described. Section 3 analyzes the physical winding model using heat and mass transfer theory. Describe the fluid model and the heat transfer model. Preparing for the numerical methods, the related governing equations and the bousinessq approximation are introduced. Section 4 mainly describes the implementation process by applying Ansys Fluent including creating geometry model, generating grid models as well as its optimizing analysis and the Fluent settings. Section 5 performs the base case study which is close to real case. The solution and results are set as base case and reference for the following parametric studies. Section 6 is parametric study on sensitive parameters such as inlet mass flow rate, vertical duct width, horizontal duct height and the disc length. The comparison among different Inlet/Outlet configurations and the comparison between oil guided and non-oil guided models have also -6-

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been performed. Section 7 draws the general conclusion from all the studies above and gives a proposal about future work. 1.3 Review of previous experience In this article, the investigated object is the model of disc windings in transformer without oil guides. By now, few studies, especially by using CFD tools, have been done on such kind of model. However, a lot of experience can be obtained from previous work on oil guided studies. (Mufuta & Bulck, 1999) has performed a theoretical study of modeling the mixed convection to describe the flow behavior of transformer oil in the winding of a disc type transformer. In their following work (M.B. & Bulck, 2000), they focused on the phenomenon of mass flow distribution around rectangular cross-section of winding discs; (Xiu-chun, et al., 2001) analyzed the effects of the surface heat flow density of sections in the radial oil ducts on the heat exchange based on the test and researches. After then, (Xiu-chun & Jun-pu, 2008) did a research on temperature field on large forced directed oil cooling transformer; (Yan, et al., 2001) gave the analysis of the calculation and test on hotspot temperature rise in transformer windings; (Ying, et al., 2004) analyzed the heat transformer in forced-directed oil cooling transformers; (Yi-zhi, et al., 2004) proposed a calculation of transformer oil flow distribution by Newton-Raphson method; (Zhang & Li, 2006) established a coupled thermal model to investigate the thermal performance of a two-dimensional oil-filled transformer disc windings. Then, (Zhang & Li, 2006) proposed and investigated some certain designed parameters such as number of discs, horizontal and vertical ducts height, entrance oil temperature as well as the total mass flow rate which may influence the thermal performance; (Zhang, et al., 2007) conducted an experimental study to investigate oil and disc temperature in ON transformer windings for a variety of flow conditions, such as heat generation rates, and transformer winding geometries. Finally they proposed a two-dimensional axisymmetric model which is sufficiently accurate for the thermal simulation of the winding discs; (E.Rahimpour, et al., 2007) also created a model of disc windings based on experimental results. They investigated some factors, such as eddy losses, number of discs, model height, horizontal duct height and space number and width, which may affect the temperature distribution; (Kranenborg, et al., 2008) performed a numerical study on mixed convection and thermal streaking in transformer disc windings. It is found that the internal buoyancy and hot streak -7-

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formation play an important role in defining the oil flow and temperature distributions in a transformer disc winding. The hot streaks can impact the flow and temperature distributions in a disc winding; (R.Hosseini, et al., 2008) also applied the program FLUENT to describe the thermal performance of disc windings. They proposed some points for design and manufacturing of the transformers; (Taghikhani & A.Gholami, 2009) suggested a method to improve the accuracy of prediction of the hotspot temperature by solving the heat transfer partial differential equation (PDE) numerically; (Bo, et al., 2009) did a two dimensional analysis of thermal performance of disc winding by using FLUENT. (F.Torriano, et al., 2010) performed a CHT calculations by using a commercial CFD code to study the numerical model of disc windings, they also found that the hot streak plays an important role in the cooling of the windings; (W.Wu, et al., 2011) did some CFD calibration for network modeling of transformer cooling oil flows; (Tsili, et al., 2012) applied the finite element method and other numerical method to do a general thermal analysis of the power transformer; (Skillen, et al., 2011) gave a numerical prediction of local hotspot phenomenon in transformer disc windings. They descript the implementation process, explained the phenomenon and performed the sensitivity studies on mass flow rate. Even though most of the analysis of thermal performance of transformer disc windings are based on oil guided disc windings, the methods and experience for conducting such studies of disc windings can also be gained and digested for non-oil guided cases. -8-

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1.4 Nomenclature Cross-sectional area ( ) Specific heat at constant pressure ( ) d Width of vertical duct ( ) Gravity ( ) Total height of disc windings ( ) Thermal conductivity ( ) Characteristic Length ( ) Mass flow rate ( ) Pressure ( ) Heat Source (losses) (kW) Inner diameter of cylindrical disc winding )( Heat source term ( ) Temperatured ifference ( ) X component of the velocity ( ) Y component of the velocity ( ) Axis coordinate ( ) Step function -() Thermal expansion ( ) Kinematic viscosity ( ) Stress ( ) Dynamic viscosity ( ) Reynolds numbe r Nusselt numbe r Prandtl numbe r Grasholf numbe r Ra Rayleigh number Low voltage High voltage Alternating Currents -9- WGCQuvkmRPmkrSPHLPKHLJxNgApk,graETVacCemumrsp2V2/yg/s1s,2ssz131K1

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2 POWER TRANSFORMERS 2.1 Introduction of power transformers A power transformer is the electrical device which is used to change the voltage of AC in power transmission system. The first transformer in the world was invented in 1840s. Modern large and medium power transformers consist of oil tank with oil filling in it, the cooling equipment on the tank wall and the active part inside the tank. As the key part of a transformer, the active part consists of 2 main components: the set of coils or windings (at least comprising a low voltage, high voltage and a regulating winding) and the iron core, as Figure 2.1 shows. For a step-up transformer, the primary coil is low voltage (LV) input and the secondary coil is high voltage (LV) output. The situation is opposite for a step-down transformer. The iron core is the part inducing the varying magnitude flux. Nowadays, transformers play key roles in long distance high-voltage power transmission. 6 Figure 2.1 Structures of disc winding transformer 2.2 Operating principle of transformer Figure 2.2 Operating principle of transformer -10-

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