Research On Parameter Constraints And Design Of MMC-SST Four Winding High Frequency Transformer

In hybrid ac/dc distribution networks,the stable operation of solid state transformer(SST)is of great significance to the entire distribution system.High power high frequency transformer is the key equipment for voltage isolation and voltage level conversion in SST,and the volume,efficiency and leakage inductance parameters of high frequency transformer directly affect the power density and operating characteristics of SST,which makes it very important to accurately model the loss and leakage inductance of high frequency transformer and optimize the design of the whole machine.In this paper,the parameter constraints,loss characteristics,leakage inductance characteristics and overall structure optimization design of high power four winding high frequency transformers applied to SST are studied.First of all,the topology and control strategy of four active bridge SST based on inductive filtering are introduced,the fluctuating power suppression characteristics and voltage clamping characteristics of SST under hybrid synchronous phase shifting modulation are briefly described.Furthermore,the magnetic circuit model and circuit model of the four-winding transformer were established,and the influence of the current and distribution parameters of the high frequency transformer on the capacitor voltage of the three phase MMC submodule under the two operating characteristics was analyzed,so as to obtain the parameter constraints of the four winding transformer.which lays the foundation for the design of the four winding transformer later.Secondly,based on the Dowell one-dimensional electromagnetic field model,this paper introduces the theoretical calculation method of core loss and winding loss of high frequency transformer,and verifies the reliability of the theoretical calculation method through finite element simulation.Then,the analytical calculation method of leakage inductance of toroidal core transformer in the wide frequency range is derived in detail,and the rationality of the calculation method is verified by finite element simulation and prototype test.Based on the theoretical model of transformer loss and leakage inductance,the effects of transformer magnetic flux density,winding turns and insulation distance on transformer loss and leakage inductance are analyzed,and the constraint relationship between objective functions such as transformer efficiency,power density and leakage inductance is obtained.Further guide the multi-objective optimization design of four winding transformer.And then,based on the parameter constraints of the four winding high frequency transformer in the SST operating characteristics derived above,this paper compares and analyzes the electric field intensity distribution and magnetic field strength distribution under different winding modes of winding through finite element simulation,and selects the winding method with the minimum leakage inductance and consistency of the distribution parameters to complete the structural design of the four winding transformer.On this basis,the free parameter sweep method is used to scan a wide range of data on the transformer magnetic flux density,winding turns and core size,and a large number of transformer design schemes are obtained,and the final four winding transformer design scheme is determined by considering the transformer efficiency,power density,leakage inductance and transformer temperature rise.In the end,in order to verify the functionality and reliability of the designed four winding high frequency transformer,an SST working condition simulation test circuit is proposed,and the rationality of the proposed working condition simulation test circuit and the designed four winding transformer are verified by simulation and experiment.