Harmonic State-Space Modeling and System Characteristic Analysis of Grid-Connected Inverter Parallel-Operation System Considering Asynchronous Carriers

In LCL grid-connected inverter, low switching frequency makes the control loop, filter resonant peak, and sideband harmonics generated by modulation coupled in mid-frequency band. When multiple inverters operate in parallel, the asynchronous carrier phase (i.e., asynchronous carriers) will cause additional mid-frequency resonance in each inverter. This will coexist with the inherent mid-frequency harmonic coupling of each inverter. Then, the mid-frequency harmonic characteristics of parallel system will become more complex, which will increase the difficulty of accurately modeling, and analyzing the harmonic characteristics of the output current of each inverter and total grid-connected current. Based on this, a complete harmonic state-space (HSS) model of parallel system considering asynchronous carriers and its frequency-domain analysis method are proposed in this article, which realizes the accurate analysis of the impact of asynchronous carriers on system harmonic characteristics. Meanwhile, an iterative calculation method of parallel-system HSS model considering asynchronous carriers is proposed, which realizes the efficient and accurate calculation of the output current harmonic amplitude and phase of each inverter. Finally, the effectiveness of the proposed model and analysis method are verified through simulations and experiments.