带Z型冷却通道的油浸式变压器绕组传热实验和模拟毕业论文.doc

带Z型冷却通道的油浸式变压器绕组传热实验和模拟毕业论文外文正文1. IntroductionElectric transformer is a key component in the transmission and distribution of electrical energy. The reliability, lifetime, performance and design of transformers are intimately related to and affected by the formation and temperature of the hottest spot in the transformer winding disks. Due to the requirement in security and reliability, the application of mineral oil as a dielectric and coolant is becoming increasingly important to transformer performance. Consequently, investigation for the hottest spot in oil transformer windings has attracted particular attention from both manufacturers and user sectors. Accordingly, IEEE Standard C57.12.00-2000 requires that the hottest spot temperature and its location be determined by either measurement or calculation method. The determination of the hottest spot location and temperature by measurement is a daunting task for working transformers becauseof the expense, effort and difficulty involved.From the geometrical constructions, transformer windings can be classified into two basic forms, layer and disk type. The disk-type winding with zigzag cooling duct arrangement is more popular in practice, but a difficulty arises in hydraulic and thermal predictive calculations due to complex oil fluid flow paths in it. The present study is focused on the prediction of the hottest spot temperature in this type of transformer windings. In oil transformer, the cooling oil is in practice heated in winding disks and cooled in radiators, and average winding temperature is mainly determined with oil circulation condition and temperature level. In forced oil-cooled transformer, the oil circulation condition and hence cooling capacity is mainly affected (such as OF, the oil is pumped through the transformer, with natural oil circulation in the windings), and determined (such as OD, the oil is pumped and forced through the windings), by oil pump in the circulation loop, but inappropriate flow distribution across the horizontal ducts may cause a local overheating in the windings. As a result, the coupling between fluid flow and heat transfer in the windings must be taken into consideration for the accurate prediction of the hottest spot temperature in magnitude and location. With no application of oil pump, natural oil cooling (ON) technique has the highest reliability and is hence frequently adopted in transformer design. In ON transformer cooling loop, the circulation oil is driven with thermal driving force, referred to as thermosyphon. At steady state, the thermal driving force should be equal to total flow resistances along the thermosyphon loop, and therefore higher oil temperature rise is necessary to maintain the oil circulation while the hottest spot temperature in the windings must be under control. Because of such globally coupled phenomena between the fluid flow and heat transfer along the thermosyphon circulation loop, the prediction of the hottest spot in the ON transformer windings is involved with hydraulic pressure losses in winding ducts, as well as the interaction between fluid flow and heat transfer.Mufuta 1-3 employed commercial software to simulate flow phenomena in the winding ducts, and conjugated heat transfer analysis was conducted with the thermal boundary condition of constant heat dissipation rate through the disk surface. But temperature field in the disk was neglected in the analysis, and a difficulty in model formulation may result from the complex duct geometry in the windings. At present, hydraulic piping network analysis is more popular for the prediction of fluid flow distribution, because of its simple formulation and easy coupling to thermal simulation for the disk temperature field 4. Its accuracy has been verified with experimental and numerical results5. Oliver 6,7, Burton et al. 8, and Del Vecchio and Feghali 9 have utilized the hydraulic network approach to develop coupled thermal models. The boundary condition required by the hydraulic network models was the heat flux through unit surface area of the duct. Temperature dependent oil properties and temperature and velocity dependent heat transfer and friction coefficients were taken into consideration. But heat conduction within each disk was neglected, and hence the disk temperature in the analyses was assumed uniform. It is not in agreement with Allen and Childs experimental results10, as the temperature variation along the radial direction within the disk was found to exceed 10 C. In addition, all of above models neglected cylindrical geometry involved, and approximately treated it as Cartesian. Declercq and Van der Veken 11 formulated a computer model with cylindrical analysis and accounted for the heat conduction within the disk. But the temperature variation in the disk was simply described as wall temperature and internal temperature, and the wall temperature at each surface was assumed uniform, as well as the internal temperature. Allen and Childs 10, Imre et al. 12-14 and Sarunac 15,16 applied the concept of heat flow network to simulate the temperature distribution in the winding disk, and to incorporate it into hydraulic model for fluid flow in the winding ducts. In the hybrid thermal model, each conductor within the disk was assumed as one node having uniform temperature and heat generation rate, and the heat conduction in insulation paper was reduced to 1-D. Consequently, this analysis cannot handle the heat conduction in both axial and radial direction, and hence some complex disk geometries, such as those made of Twins and CTC conductors, cannot be simulated accurately. In general, a two-dimensional (2-D), i.e., axi-symmetric, thermal analysis is essential for temperature simulation in the winding disks.At present, multi-dimensional thermal models have been substantially applied for layer-type transformer windings 17,18. For the disk-type transformer windings, Preiningerova et al. 19,20, Venkateswarlu and Prasad 21, Sarunac and Pavlicevic 22, and Carstea 23,24 employed various numerical techniques to simulate a 2-D temperature field in the winding disk, but in all of these heat conduction models, the heat convection boundary condition executed at disk surface, i.e., heat transfer coefficient and oil bulk temperature, was given as an assumed value. In reality, the heat transfer boundary condition for heat conduction model should be obtained from hydraulic flow model for oil flow in the winding ducts. Therefore, the cooling oil flow and temperature field in the winding disks interact with each other, and are coupled together, leading to the necessity of iterative solution for both.Among the various analytical methods employed in solving coupled conductionconvection problems, one frequently used is the lumped-parameter method25-28, and it has been applied to the simulation of the cooling of transformer windings 12-14. With the lumped-parameter method, the problem domain is all of disks and the cooling ducts surrounding them, and the huge mesh grid may result in a difficulty in solution convergence. If the mass flow rates depend on the temperatures, it is necessary to alternate between temperature calculations and a hydraulic network model for flow distribution. In the present study, a new iteration method is employed to solve the coupled thermal problem with multi-dimensional simulation for winding disks, in which, hydraulic piping network analysis is used for the calculation of oil bulk temperature as well as flow distribution, and while each of the winding disks is considered as a domain for multi-dimensional thermal simulation using a heat conduction model. The coupling between the hydraulic model and the heat conduction models is carried out with heat convection at solidfluid interface, and as a result the determination of heat transfer coefficient has a distinct influence on the accuracy of the disk temperature simulation. A summary of the relevant heat transfer studies is given below.Del Vecchio and Feghali 9 and Declercq and Van der Veken 11 employed average heat transfer coefficient to describe the heat transfer in oil transformer windings, and its value was determined from correlations available in literature. With regression analysis on experimental results, Imre et al. 29 also developed a series of empirical correlations for average heat transfer coefficient corresponding to various types of disk-type windings. In reality, complex flow ducts and heat loss distribution in the windings may cause oil temperature and flow significantly location dependent, and hence convection heat transfer coefficient. Therefore an employment of local heat transfer coefficient is more feasible for the accurate prediction of the hottest spot temperature in location and magnitude. And the effect of thermal entry length must also be taken into account for determining the local heat transfer coefficient, because of low flow Reynolds numbers, high oil Prandtl numbers and relatively short flow channel length encountered in oil transformers. In the analysis for winding heat transfer, the winding ducts are usually considered as parallel plates with heating from the both-side walls, and Nusselt number correlation is employed to describe local heat transfer coefficient and is defined as （1a）and it can be expressed with a function of dimensionless distance（1b）Childs and Allen 4,10 treated the cooling oil in each winding duct as a laminar flow between parallel plates with the boundary condition of specified heat flux, and the corresponding correlation is taken from Shah and London 30, as follows. （2a） （2b）（2c）Oliver 6,7 used the analytical results available from Rohsenow and Hartnett 31, and derived a correlation for laminar flow between parallel plates with the boundary condition of specified heat flux, as （3a） （3b）which was finally applied to a thermal simulation for oil transformer windings. In Yamazaki and Sakamotos studies 32,33, two correlations from Sellars et al. 34 were employed to calculate local heat transfer coefficient in the winding ducts, respectively, with the boundary condition of specified heat flux,and linearly varying wall temperatures, Eqs. (4) and (5) are only valid for the laminar flow within thermally developing region. （4） （5）In general, the value of is the highest at the duct inlet, and then decreased along the flow duct till the laminar flow enters into thermally developed region, where becomes distance independent and its value remains constant. For specified heat flux condition, the constant of within the thermally developed region is 8.235, as shown in (2) and (3).In reality, the heat transfer phenomena in oil transformer winding ducts are not within idealized specified heat flux or linearly varying wall temperatures, and the roughness of duct surface also has influence on heat transfer efficiency. Therefore, an experimental study is conducted in this work to obtain a suitable heat transfer correlation for flow characteristics and geometries encountered in ON transformer windings.2. Thermal model descriptionIn disk-type ON transformer winding, each winding disk is made of an insulated conductor horizontally wound around the concentric cylindrical core, and washers are placed periodically in the vertical direction to force the flow in a zigzag fashion up the winding. The winding section between two washers is known as a pass, as shown in Fig. 1. Each winding contains many such passes. Without loss of generality, one pass is taken for analysis, and a coupled thermal model is developed to simulate 2-D axisymmetric temperature field in winding disk, and oil bulk temperature and flow distribution. The coolant flows are treated as one-dimensional in the cooling ducts with a constant mass flow rate. Confluence and branch occur at the node, which is located at the center of each junction, as shown in Fig. 2. Basically model equations of the hydraulic network are formulated by using the conservation of mass for flow into and out of each junction and by specifying that the algebraic sum of the pressure drops around each closed loop is zero 35,36. An experimental study has been conducted for the validation of the present hydraulic network model 37. The improved model is in good agreement with the experimental and numerical results.The temperature distribution in the winding disks is simulated with a heat conduction model using the finite volume method, and the boundary condition for it is the convection heat transfer at the disk surfaces, related to bulk temperature of surrounding oil and local heat transfer coefficient. Due to the axi-symmetric geometry involved, the analysis is reduced to 2-D cylindrical section. The temperature and location dependent heat generation rate, and the thermal contact resistance are taken into account, and the model can be applied to various disk configurations, made of Single, Twins and CTC conductors as shown in Fig. 1.With coupling between the heat conduction model and the hydraulic network model, a comprehensive thermal model is developed to simultaneously investigate a 2-D temperature field in the winding disks and hydraulic and thermal fields in the cooling oil.In the horizontal duct , the temperature rise in the ith fluid control volume is dependent on the heat absorbed within, whose value should be equal to the heat transfer from the neighboring solid control volume in disk I and in disk , the energy balance equation therefore becomes（6）In the vertical duct, the temperature rise in the th fluid control volume depends on the heat transfer from the neighboring solid control volume in disk, only, since the inner wall on the other side is assumed as a thermal insulator due to negligible heat transfer there. Therefore, energy balance yields（7）The fluid properties in the control volume are determined at the bulk mean temperature,（8）At duct junctions, mixing among the fluids from different ducts occurs, and the fluid properties are determined by a bulk mean temperature. The specific heat at constant pressure is reasonably constant for the cooling oil, thus the thermal equation for the branch junction becomes, （9）For the confluence junction ,以及 （10）Clearly, the junction temperatures are already discretized. At the solid/fluid (disk/oil) interface, the heat flux at the disk surfaces is treated as thermal boundary condition input into the hydraulic model, as given in (6) and (7), and interactively the convection boundary condition executed to the heat conduction model can be calculated from the outputs of the hydraulic model. Therefore, an iterative method is used to tackle the coupling between the heat conduction and hydraulic models. The correlation from Shah and London 30 is chosen in the thermal model to determine heat transfer coefficient. It is validated against experimental data, and generally the predictive accuracy is within 38,39.3. Experimental facility and measurementsAn experimental study is conducted to investigate the disk temperature and the oil flow and temperature distribution in the windings at steady state, as affected by heat generation rate in the conductors, total flow rate across the winding ducts and various winding duct geometries. The experimental results are employed to reduce the uncertainty associated with the heat transfer coefficient, and to further improve the thermal simulation model in accuracy and reliability. The experimental facility and method are described below.3.1. Oil circulation systemThe oil circulation system of experimental facility is shown in Fig. 4. Mineral oil VOLTESSO 35 is adopted as experimental coolant. The tested winding model is setup in the test tank, and its position is fixed with four locating pins at the tank base. The two exits of the test tank, the lower and upper exits, are designed to ensure that the tested model is always completely submerged in the coolant during the experiment. The top tank is employed to supply a steady oil flow by elevation head into the tested model from the bottom of the test tank. The total flow rate is controlled with a globe valve and measured using a flow meter, OMEGA Model No. FL-6102A. It has a flow range of 7.5LPM with an accuracy of 2% full scale (0.15LPM). Other flow operations, such as running backward, filtering oil, emptying oil tank and adding more oil to the system, are also necessary during the experiment period, and they can be executed with an onoff arrangement of the various valves. No specific radiator is installed in the oil circulation system. During the thermal performance testing, the oil tank, top tank and pipes are working as radiator to dissipate the heat generated from the winding model to the ambient air.3.2. Tested winding modelsIn disk-type transformer winding, spacers and sticks are placed circumferentially to evenly divide the horizontal and vertical ducts and the winding disks in the tangential direction. As a result, taking the domain between two adjacent spacers and sticks as one part, the thermal and hydraulic phenomena in each part are similar and equivalent. Based on this consideration, one part of transformer winding is facsimiled in the present experimental study, and the model scale is designed to be 1:1.In the developed simulation model, a variety of disk configurations, such as Single, Twins, CTC and Semi-solid, etc., has been taken into account. Since the experimental validation in the present study is mainly focused on the oil flow and temperature distribution and the determination of local heat transfer coefficient as discussed above, some simplifications and modifications are made for easy model buildup and with no loss of generality. In disk construction, comparing to the actual single