Research On Carrier Phase Configuration Applied To Grid-connected Inverters For Circulating Resonant Current Suppression

High performance of the grid connected inverters is of great significance to improve the capacity of distributed generation system.In the parallel system,the sideband harmonic will be affected by the carrier phase.Under the closed-loop control,the high-frequency circulating resonant current is easy to be aroused,which is not conducive to the stability of the parallel system.In order to improve the redundancy and flexibility of the parallel system,this paper proposes a new control strategy based on the virtual oscillator to synchronize the carrier phase.Firstly,the Norton equivalent model and its simplified model are established for LCL filter grid-connected inverters with active damping control,and the mathematical expressions of the grid current are deduced under the carrier synchronization and asynchronization respectively.The influence of carrier phase on the high-frequency harmonics characteristics is studied with different LCL filter parameters.It is pointed out that the carrier synchronization and/or high modulus of output impedance near the resonance frequency contribute to the suppression of circulating resonance current.The correctness of the theoretical analysis has been verified based on Matlab/Simulink.A control strategy based on virtual oscillator is proposed.First of all,the principle of virtual oscillator’s output voltage stabilization by internal non-linear control unit is introduced,and then the quantitative relation of its internal parameters of virtual oscillator is deduced by Lyapunov’s stability judgment.Then the machine of output voltage self-synchronization when two virtual oscillators are connected through impedance network is clarified,the switching frequency mathematical model of the multi inverter parallel system is established in the complex frequency domain,using the band-pass filter with zero phase shift function at the switching frequency to simplify the mathematical model.Based on the established mathematical model,the linear processing of control unit of the virtual oscillator is realized,and the linear relationship between the input-output of the virtual oscillator is obtained.The sufficient conditions for the output voltage synchronization of virtual oscillator is clarified by the definition of?₂-stability of linear system,which provides a theoretical basis for the parameter design of virtual oscillator.Lastly,a design method of virtual oscillator parameters is proposed.The correctness of the design parameters of virtual oscillator and the feasibility of the carrier synchronization have been verified based on Matlab/Simulink.Finally,a prototype with two parallel grid-inverters based on DSP+FPGA controller is developed.Under the experimental conditions of carrier synchronization and asynchronization,the carrier synchronization is helpful to suppress the circulating resonance current has been verified.Furthermore,the basic principle of virtual oscillator is also verified based on the FPGA controller.