Thermoelectric Coupling Calculation And Nonlinear Model Order Reduction Of Converter Transformer

Along with the high voltage direct current development,the manufacture of converter transformer is of the great significance to the safe operation of the power grid.An appropriate insulation design of converter transformer can effectively reduce the operation faults and avoid unpredicted economic losses,which is of great value to engineering.However,the converter transformer operates in high temperature and strong electric field,and the operation state is affected obviously by the coupling effect.Indeed,the coupling effect would change the insulation state,and even cause the safe region within the insulation margin to meet the hazard.Therefore,it is necessary to study the coupling parameters and coupling mechanism in insulation analysis of converter transformer.In this paper,the field-circuit co-simulation is adopted to study the thermal-fluid coupling of converter transformer,after the construction of experimental platform to measure the dependence of the insulation properties with temperature and electric field.Next,the adaptive time step finite element method based on the load error is adopted to study the electric field under polarity reversal,based on the fitting coupling relationship.Finally,the reduced order model of the combination of POD(Proper Orthogonal Decomposition)and LDEIM-Q(Local Discrete Empirical Interpolation Method Based on QR Factorization)is proposed to decrease the computation difficulties caused by nonlinear thermoelectric coupling.The main contributions are as follows:(1)Due to the lack of some coupling properties of HVDC materials,the threeelectro experimental platform is constructed to measure the conductivity of transformer oil and oil-immersed pressboard.The results show that the conductivity of transformer oil and oil-immersed pressboard increase with temperature,furthermore,the conductivity of transformer oil goes down at first and then go up with the increase of electric field,like U type.(2)The insulation properties with the nonlinear thermoelectric coupling are studied by the finite element method based on the scalar potential.Then the adaptive time step method based on load error is utilized due to the difference time-step requirements from constant DC excitation and reversal process in polarity reversal test.By analysis,the region with high temperature is near the end of winding which stands strong electric field,the coupling effect from non-uniform temperature cannot be ignored.Furthermore,the electric field goes up with the consideration of U type electric dependence.With the effect of nonlinear thermoelectric coupling,the insulation state and the time constant of pressboard are changed.(3)The field-circuit co-simulation is adopted to study the temperature distribution in terms of the complex structures of converter transformer which is difficult to model entirely.The method simplifies the radiator cooling ducts into circuit model,and the series coupling method is adopted to transfer the temperature and velocity data between two models under different dimensions,finally the temperature field based on the entire cooling circulation is solved out.By comparison,the results from field-circuit cosimulation approximate the actual condition more,and this method can avoid the computation cost of radiator field model construction.The results show the simplified the radiator ducts make a great contribution to temperature distribution,which can change the value and location of hot spot in converter transformer.(4)In terms of the computation cost from thermoelectric coupling nonlinear iteration,this paper proposed LDEIM-Q method which selects the local subset with the similar properties to approximate the nonlinear term by the local approaches.This method can avoid the computation complexity caused by the stiffness matrix reconstruction in every iteration.Compared with traditional DEIM,LDEIM-Q has a better stability and a clear boundary error.Finally,POD/LDEIM-Q is utilized to study the nonlinear thermoelectric coupling of an experimental transformer,the computation time is decreased dramatically and the accuracy is improved.(5)The measuring platform of scaling model of converter transformer is designed and constructed.In this platform,temperature data are recorded by fiber sensor in temperature-rise experiment,and electric field data are recorded by Pockels sensor in insulation experiment respectively.The results valid the correctness of field-circuit cosimulation and nonlinear reduced order model.