Research On Resonance Mechanism And Suppression Of Grid-connected Inverter

Grid-connected inverter is the vital equipment to connect distributed generation system with public grid,its performance directly determines the quality of grid-connected current.The resonance occurring in LCL-type grid-connected inverter is not only easy to cause the grid current distortion,but also threaten the stable operation of the grid-connected system.The problem is especially complicated in the multiple grid-connected inverters network with increasing capacity.Therefore,it is significant to research the resonant mechanism of grid-connected inverter and its suppression method.In this paper,the mathematical models of single-phase and three-phase grid-connected inverter are established,and suitable current control technologies of the latter are analyzed in three different coordinate systems,together with deriving respective grid-connected current expressions.The resonant mechanism of single grid-connected inverter is analyzed according to the output-side transfer function.Then,based on the parallel system composed of multiple 500 kW grid-connected inverters in the actual distribution network,the Norton equivalent circuit of single grid-connected inverter considering the PWM harmonic source is established.With expansion of topology,the Norton equivalent circuit of multiple grid-connected inverters is established as well.In order to make the model more realistic,it incorporates cable distribution capacitance,nonlinear harmonic source and linear load.The series resonance and parallel resonance of multi-inverter grid-connected system are studied by impedance analysis method.The influence of inverters,PWM harmonic source,nonlinear harmonic source and grid voltage on grid current are studied by gain matrix method,respectively.The factors influencing the interaction characteristics of the inverters are analyzed emphatically,including the inverter control circuit,the system parameters and the reference current.For the resonance suppression of single grid-connected inverter,the passive damping with an resistor in series with filter capacitor and the active damping with capacitor current proportional feedback are adopted,respectively.Then,an active damping method with capacitor voltage inertial feedback plus phase shifting element is proposed.The design process of system parameters is analyzed in detail,and it is pointed out that the method can still suppress the grid current distortion caused by the grid harmonics without grid voltage feedforward.For the resonance suppression of multiple grid-connected inverters,the installation of resonance suppression unit at PCC is adopted,including pure resistor,notch filter and active damper,and their advantages and disadvantages are analyzed.The further research on resonance suppression of multi-inverter grid-connected system is prospected.Finally,simulation results verify the correctness of above theories.