Research On Control Strategy Of Auto-balancing Power Electronic Transformer Applied To Medium Voltage Distribution System

In recent years,with the global trend of new energy and power system power electronic rapid development,power electronic transformer(PET)has been widely concerned.As a kind of PET with auto-balancing capability,auto-balancing power electronic transformers(A-PET)acts as a new energy medium in the medium voltage distribution network.Compared with traditional transformers or general power electronic transformers,A-PET have many advantages,such as high power density,flexible controllable,and can effectively deal with the original secondary unbalanced state.The A-PET has important significance to the improvement of the grid quality,the construction of smart power grids.The main content and innovation are as follows:Firstly,three level topology of the high-voltage stage,isolation stage and low-voltage stage is widely applied in A-PET.The input stage adopts cascade H-bridge rectifier,which can effectively improve the voltage rating of the device.It is suitable for medium voltage and high power applications.The isolation stage adopts DC-DC converter with double active bridge structure,which realizes voltage level conversion and electrical isolation.The low-voltage stage uses three single-phase inverters to be interleaved in parallel to achieve the auto-balancing principle.The control strategy for the topology is the key part of the A-PET,and it is also the focus of this paper.Secondly,the mathematical model of the whole device and the three-stage topology of A-PET are established respectively.On this basis,the control strategy of three-stage structure is designed and analyzed.Aiming at the problem of interphase steady voltage and phase voltage sharing in high voltage level cascade H-bridge rectifier that widely exists,This paper presents a layered compound control,so rectification as well as adjustment of the input side power factor are both achieved effectively.The power transfer characteristics of the isolated stage LC resonant DC-DC converters is analyzed.The isolated stage uses a power feedforward compensation control strategy to achieve power transfer control.The process of parallel of single-phase inverter at low-voltage stsge,the circulation problem is inevitable.In order to suppress the system circulation effectively,mechanism of droop control is introduced in detail,and the droop robust control based on virtual impedance is introduced in the low-voltage stage inverter control to achieve power sharing.The dynamic response of the control is improved at the same time.Finally,the A-PET main circuit design and parameter calculation is given,including the selection of input and output filter design,DC side capacitor design,and intermediate-frequency transformer Then a 150 kVA A-PET system simulation model is built in the simulation software MATLAB / Simulink.Through the transient and steady state simulation,the correctness of the control strategy and theoretical analysis are verified.