Research On Control Strategy Of Multi-functional Photovoltaic Grid-connected Inverter Under Unbalanced Grid Voltage

With the global energy crisis and environmental pollution becoming more and more serious,making full use of renewable energy such as solar energy to generate electricity has become the trend of future development.As an effective way of integrating distributed generation into power grid,micro-grid has attracted more and more scholars' research in recent years.At the same time of a large number of distributed generation access,a variety of power quality problems appear in the micro-grid,among which harmonic pollution and reactive power pollution are the most common.Photovoltaic power generation is the most common form of generation in micro-grid.Because the topology of grid-connected photovoltaic inverter is similar to that of traditional active filter,in order to save costs,it is only necessary to study its control strategy to realize the control of power quality in micro-grid.This kind of grid-connected photovoltaic inverters with power quality control function is called multi-functional grid-connected photovoltaic inverters.At present,a large number of studies on multi-functional grid-connected photovoltaic inverters focus on how to control harmonic and reactive power compensation.These studies mainly focus on how to use the output current of the inverters to control the power quality of micro-grid.However,the unbalanced grid voltage often occurs in the micro-grid,which will lead to the harmonic component of the active current output by the inverters themselves and affect the stability of their operation.In this case,in order to control the power quality of micro-grid under unbalanced grid voltage,the control strategy of multi-functional grid-connected photovoltaic inverters under unbalanced grid voltage is studied in this paper.Firstly,this paper introduces the background and research status of the topic,and expounds the purpose and significance of the research.Then the topology structure of the multi-functional grid-connected photovoltaic inverters is analyzed,and the Boost circuit of the former stage is briefly analyzed.The back-stage inverters are emphatically analyzed,and the mathematical models of the three-phase full-bridge inverters are constructed when the grid voltage is balanced and unbalanced.Secondly,the directive current of multi-functional grid-connected photovoltaic inverters is analyzed,including harmonic,reactive directive current and grid-connected power tracking directive current.In order to detect harmonic and reactive current components in micro-grid under unbalanced gridvoltage,a harmonic detection method based on FBD power theory is adopted;in order to suppress the influence of unbalanced grid voltage on the quality of active current output by the inverter itself,the positive and negative sequence decoupling method is used to obtain the grid-connected power tracking instruction current.Then the overall control strategy of multi-functional grid-connected photovoltaic inverters under unbalanced grid voltage is studied,including positive-negative decoupling phase-locked control based on sliding average filter,new combined positive-negative sequence control and composite control based on multi-quasi-PR(proportional resonance)controller,which not only eliminates the influence of unbalanced grid voltage on the inverter itself,but also compensates the harmonic and reactive currents in the micro-grid.On this basis,a cooperative control strategy based on analytic hierarchy process is further analyzed,which makes the multi-functional grid-connected photovoltaic inverters optimize the power quality of micro-grid under the condition of limited compensation capacity.Finally,the validity and feasibility of the control strategy adopted in this paper are verified by the simulation analysis of MATLAB/Simulink and the construction of 10 kW multi-functional photovoltaic grid-connected inverter experimental platform.