Control Strategies Of LCL Grid-connected Converter Under Unbalanced Grid Voltage Conditions

LCL filter has been widely used in grid-connected converter for its good filtering performance and small size.However capacitor branch may cause resonance,the active damping based on capacitor current feedback is an effective way to damp this resonance.And a proportional regulator was used in feedback loop,which can modify the feedback strength and simplify the tuning process.Under the unbalance grid voltage,LCL-filter based grid-connected converter was built,which a proportional-resonant(PR)regulator is adopted in the stationary ?? reference frame.According to theory of root locus,the paper analyzed the PR regulator,P regulator and the LCL filter,and selected appropriate parameters to guarantee the stability of the system.Finally,the reasonability and feasibility of the control strategy and regulator design method was verified by the simulation.The PWM rectifier system based on direct power(DPC)has the advantages of simple structure,simple algorithm and fast dynamic response without decoupling control and rotating transformation which offers in current control loop.So in last part,this paper presented virtual-flux based direct power control(VF-DPC),adopted the proposed PR regulator.According the characteristics of LCL filter,an active power damping control link in order to suppress the resonance was joined in.Finally,the reasonability and feasibility of the control strategy and regulator design method was verified by the simulation results of 3.5kVA grid-connected converter.