Research On Parallel Operation Of Power Electronic Transformer And Its Grid Adaptability

Power electronic transformer(PET),which is composed of power electronic converter and high frequency transformer,not only has the functions of electrical isolation and voltage level conversion,but also has the functions of reactive power compensation,power flow control,power management,renewable energy DC access and so on.The main applications of PET are locomotive traction,new energy grid,transmission and distribution grid and other fields.In this paper,the problems of circulating current and DC bus voltage fluctuation in parallel operation of PETs and the adaptability of PET to power grid are studied.The suppression scheme of circulating current and capacitor voltage fluctuation is proposed.The influence of circuit parameters on the grid adaptability of PET is studied by using impedance modeling method.Firstly,according to the requirements of high-voltage side and low-voltage side,the three-stage circuit topology of PET is discussed.The circuit topology,working principle and control mode of each stage are introduced.Secondly,the problem of circulating current in the low side and high side of two paralleled three-stage PETs and the suppression method are studied.The isolation function of high frequency transformer makes that the circulating current on both sides of PET can be studied separately.When studying the circulating current on one side,the circuit of other stages can be simplified and expressed in the form of equivalent circuit.The circulating current path is drawn in the equivalent circuit,and the circulating current definition is given.According to KVL,the mathematical relationship is derived,and the factors affecting the circulating current are analyzed.In this paper,a circulating current suppression strategy is proposed,which is suitable for both the low voltage side and the high voltage side.Matlab /Simulink is used to establish simulation model.The simulation results show that the introduction of the circulating current suppression strategy can significantly reduce the circulating current.For NPC converter at low voltage side,the experimental platform of two paralleled PETs at low voltage side is built.The experimental results reveal the effectiveness of the proposed circulating current suppression strategy.Thirdly,the coordinated control of paralleled PETs is studied.When the actual transmission power of PET changes with the dispatching instruction,it will cause the fluctuation of DC bus voltage.In order to ensure the stability of DC bus voltage,it is necessary to suppress the fluctuation of DC bus capacitor voltage.Circulating current suppression should be considered at the same time in parallel operation of two PETs.Therefore,coordinated control should be implemented for circulating current and DC bus voltage fluctuation.Matlab / Simulink is used to build the simulation model of different working conditions,such as reference power change,power turnover,input and cut-out of PET.The simulation results verify the availability of the proposed coordinated control strategy.Finally,the grid adaptability of PET is studied by using dq impedance modeling method.For the equivalent circuit based on NPC converter for low voltage side of PET,a small signal model is established.Considering the influence of PLL and current loop control,the dq impedance model of the equivalent circuit is established.The Nyquist stability criterion is used to analyze the stability of the grid connected coverter system when PET is connected to the weak grid.Matlab / Simulink is used to study the influence of different grid impedance,PLL control parameters and current loop control parameters on the grid adaptability of PET.The frequency domain impedance and time domain waveforms under stable and unstable conditions are given to verify the analysis of the influence of grid connected system parameters on the stability of grid connected system.