Study On Flow Field And Temperature Field Coupling Finite Element Methods In Oil-immersed Power Transformer

Oil-immersed power transformer is one of the most important equipment in the transmission and distribution power grid system.The hot spot temperature rise is related to the service life and the safety stability of the oil-immersed power transformer.The calculation of temperature rise of oil-immersed power transformer is a coupling problem of electromagnetic field,flow field and temperature field.While solving the temperature rise calculation of the oil-immersed power transformer,this paper has mainly studied the flow field equation calculation problems,fluid-solid coupling heat transfer problems,flow field and temperature field coupling calculation problems,stochastic temperature field computation method and other issues.The main work of this paper includes:(1)The least-squares finite element discretization scheme is derived for the steady incompressible flow field problems.The calculation method has been studied for the finite element matrix equation while the global stiffness matrix is stored at the element level.And techniques dealing with the first boundary condition at the element level is proposed.The proposed method is applied to calculate the lid-driven cavity flow problem.The basic characteristics of the proposed method for the steady flow problems is analyzed.And the computational result verifies the validity of the proposed method.(2)In order to solve the problem of low efficiency for the calculation of transient flow field problem,a reduced order least-squares finite element method is proposed.Firstly,the snapshot matrix is constructed with partial solution of the flow field calculated with the least-squares finite element method.Then,the POD(Proper Orthogonal Decomposition)method is applied to obtain the characteristic orthogonal basis of the flow field.And combined the DEIM(Discrete Empirical Interpolation Method)and Galerkin projection,the POD-DEIM reduced order least-squares finite element model is constructed.Numerical results show the accuracy and efficiency of the reduced order method.(3)In order to solve temperature rise calculation problem in the oil-immersed power transformer,the flow field and temperature field coupling fin ite element method is proposed.The fluid governing equation is calculated by the least-squares finite element method for obtaining the velocity distribution in the oil duct.Then,the fluid-solid coupling heat transfer model is constructed,and the streamline upwind finite element method is applied to obtain temperature distribution of the whole field domain.Finally,the flow field and temperature field finite element equations are calculated respectively to obtain the convergent flow velocity and temperature distribution of the computational domain.The proposed method is applied to calculate the temperature distribution of a local winding structure model in an oil-immersed power transformer.And the computational results obtained from the proposed method is compared with the results obtained from Fluent,which verifies the accuracy of the proposed method.(4)Combined with basic structure of disc-type winding of an oil-immersed power transformer,the characteristics of the velocity distribution in the oil duct and the temperature distribution of the whole domain is analyzed with the proposed flow field and temperature field coupling finite element method.Meanwhile,referring to the power transformer design theory,this study analyzed the influence of the parameters such as the widith of the horizontal duct,the height of the vetical oil duct,subsection number of the disc-type winding and the inlet velocity on the oil flow pattern and the temperature distribution characteristic.The influence mechanism of oil flow velocity distribution on the winding temperature rise is discussed,which can provide guidance for the heat dissipation structure design and the the overheating problem analysis of the transformer winding.(5)In order to study the influence of uncertainty factors on the temperature field distribution,the reduced order Monte-Carlo stochastic finite element method is presented to solve the stochastic temperature field distribution.Firstly,the POD-DEIM reduced order finite element method for solving nonlinear flow field and temperature field is developed based on POD and DEIM method.Then,the reduced order Monte-Carlo stochastic finite element method is proposed combined the reduced order method with the Monte-Carlo stochastic finite element method.Considering the stochastic distribution of the inlet velocity,the stochastic temperature field of a local winding structure model in an oil-immersed power transformer is calculated by the proposed method.The statistical distribution types of the hot spot temperature is analyzed,and The accuracy and efficiency of the proposed reduced order method are verified.