| The power electronic transformer has the advantages of small volume,light weight and friendly environment.It has a wide application prospect in the marine wind power medium voltage DC gathering,electric locomotive traction,flexible substation and other occasions,and has been widely concerned by academia and industry.Compared with ordinary high frequency transformer,the operating voltage,capacity and power density of power electronic transformer are higher,so its insulation mode is different from that of ordinary high frequency transformer.The insulation methods of power electronic transformer mainly include oil insulation and epoxy insulation.Epoxy insulation has the advantages of light weight,friendly environment and high safety coefficient,which is the trend of the development of the insulation of power electronic transformer in the future.Although epoxy insulation has the advantages above,its thermal performance is poor,and it is easy to cause the uneven temperature distribution of transformer insulation structure and lead to insulation cracking.Therefore,it is of great significance to carry out the simulation analysis of temperature and stress field of power electronic transformer,which is of great significance for the prediction of fault point and the design of package insulation of power electronic transformer.The main contents of this paper are as follows:(1)According to the actual size,the geometric model of 220 k W/30 k Hz power electronic transformer is established,and the detailed grid division is carried out for each part of the transformer.The basic equations of fluid mechanics,solid mechanics and thermodynamics are established according to the physical process of transformer operation.The internal heat source and unique heat dissipation mode of power electronic transformer are analyzed,The load loss and core loss in the operation of power electronic transformer are calculated,which provides the initial conditions for the calculation and analysis of temperature field and stress field.(2)The temperature distribution of the power electronic transformer is studied.The temperature distribution characteristics of the whole and high and low voltage windings of the power electronic transformer are simulated.The influence of the ambient temperature,operation state,wind speed and the thermal conductivity of epoxy insulation on the temperature field distribution of the transformer is analyzed.The results show that the hot spot temperature of power electronic transformer is located on the high voltage winding;When the thermal conductivity of epoxy insulation material is 1.1 W/(m·K),the temperature distribution of transformer meets the requirements of GB 1094.11,and has30% load margin;Finally,the temperature rise test of the actual power electronic transformer is carried out,and the internal temperature rise of the actual transformer is tested.The simulation results of finite element are compared with the actual test results,and the accuracy of the finite element calculation results is verified.(3)The thermal stress distribution of the insulation structure of power electronic transformer under different operating conditions and temperature shock is studied.The influence of the parameters of epoxy insulation material on the internal thermal stress is calculated and analyzed.The results show that the tensile stress of epoxy insulation in transformer is 70 MPa when the short circuit time is 3s;When the temperature is raised and cooled,the outer and inner sides of the transformer bear the larger thermal stress respectively;Under the impact of temperature,the tensile stress of power electronic transformer reaches 90 MPa;When the thermal conductivity of epoxy insulation material is greater than 1.1 W/(m·K),and the thermal expansion coefficient is less than 40 ppm/K,the maximum stress of transformer internal insulation is 90 MPa.The results of this paper are expected to provide reference for the analysis of insulation failure,insulation design and material selection in the process of power electronic transformer operation. |
