Research On Three-phase Voltage Unbalance Compensation Strategy Of Voltage-controlled Grid-connected Inverter

The renewable energy technology is developed rapidly in the world because of increasingly serious energy crisis and environmental pollution.The micro-grid is a flexible and efficient way to utilize renewable energy.Three-phase grid-connected inverter is the key equipment in the micro-grid and its performance directly affects the safe and stable operation of the distributed generation system.When designing the control strategy of the grid-connected inverter,the abnormal grid condition should be taken into account.In this paper,three-phase grid-connected inverter is taken as the research object.In view of the failure of the traditional droop control strategy under the unbalanced grid,an in-depth study of the voltage-type unbalanced compensation strategy is carried out.The contents include the following aspects:Firstly,the existing unbalance compensation methods are reviewed,and their advantages and disadvantages are analyzed.LC three-phase grid-connected topology is selected and its mathematical model is established.The traditional droop control is further studied,while the controller design are carried out.Furthermore,the quantization method of the unbalanced system is introduced,and it is pointed out that the traditional droop control cannot be applied to the unbalanced grid.Second-order generalized integrator is adopted to extract the positive and negative sequence components of an unbalanced system accurately.In this study,two unbalance compensation control strategies are proposed.To reduce PCC imbalance,this paper proposes a PCC voltage unbalance compensation method based on negative sequence droop control,which compensates the unbalanced voltage by injecting negative sequence reactive power;To reduce the imbalance of the grid current,a negative-sequence current suppression method is proposed by changing the negative sequence equivalent impedance dynamically.Then,the small signal model of inverter is established.The stability of the control strategy is verified by analyzing the root locus diagram,and the stability interval of the control parameters is obtained.The simulation results in MATLAB/Simulink verify the feasibility of the control strategy.Finally,a three-phase grid-connected inverter platform based on DSP28335 is built to verify the control strategy.The experimental results verify the effectiveness of the proposed control strategy from steady state and transient state.