Research On Topology And Power Balance Control Of Multi-port Cascaded Multilevel Converters For Distributed Energy Accessing

As China’s comprehensive national strength and population continue to grow,the level of energy consumption has greatly increased.In the future,traditional energy sources are facing the problem of depletion of resources.At the same time,the burning of fossil fuels will cause irreversible damage to the environment.Therefore,the development and utilization of new energy sources has become the source power of sustainable development of society.Distributed energy grid-connected or power supply to the load requires power electronic converters.Multi-port converters reduce the number and cost of multiple input and multiple output system devices by means of device multiplexing,and improve the transferring efficiency between sources.In addition,the cascaded multi-level structure can realize voltage and capacity improvement through cascaded sub-module as well as carrier phase shift control strategy,featuring high modularity,good harmonic characteristics and strong scalability,which can be used as a media for distributed power admission.Therefore,in order to realize the medium and high voltage and large capacity new energy access flexibly and efficiently,the reasonable energy management and power control methods based on multi-port cascade multi-level topology are researched to meet the effective utilization of new energy.Firstly,the idea of module multiplexing is adopted.Three bridge arms are alternately removed on the basis of the nine-bridge arm of the matrix converter.The remaining six bridge arms form a hexagonal cascaded multilevel converter,and the front ends of the interphase three sets of bridge arms are connected to the three-phase PWM rectifiers,which are connected to the grid through a phase shifting transformer to realize the expansion of the hexagonal twoport converter to the three ports.Then the topology modeling of the hexagonal converter is carried out,the connection between the port voltage and current and the bridge arm is established,and the expression of each frequency component of the bridge arm power is derived.By establishing the expression of the instantaneous power DC component of the bridge arm and analyzing the instantaneous power imbalance mechanism of the hexagonal converter bridge arm,the necessity principle of the circulation control is proposed.The voltage fluctuation component of the bridge arm is derived by calculating the voltage fluctuation component of the bridge arm sub-module,based on which the fluctuation component of the circulation is calculated.Finally,the whole calculation process is iterated n times,and the general formula of the circulation fluctuation expression is obtained.So far,the modeling and circulation analysis of the three-port hexagonal cascade multi-level converter is completed.Seeking reasonable and effective distributed energy consumption technology has important research significance.In this paper,the hexagonal converter is divided into four working modes under two connection methods.The double-closed-loop control strategy based on the third harmonic injection and the forward-feeding of the grid electromotive force,the control of the port currents of the inverter side,control of system power balance,balance control of power in bridge arm and carrier horizontal phase shift SPWM modulation strategy are studied.Inductance and capacitance parameters design methods of hexagonal converter are researched.Based on this,the feasibility of topology and control is verified by MATLAB/Simulink in distributed surplus and insufficient conditions of two connection methods,including the phase shift transformer connection and the front-end access of distribution.In order to realize the low-frequency grid-access of the offshore wind power,the hexagonal converter is used as the medium for the distributed low-frequency access grid.On the basis of the previous scene,some control methods are changed.the bridge arm current is controlled directly to control the bridge arm voltage.The circulation is controlled to maintain the energy balance control between the bridge arms and the bridge arms so as to maintain the stability of the capacitor voltage so that the power of the entire system is balanced.And finally the effectiveness of the control is verified through the system internal control waveforms and the electrical waveform of each external port.