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Harmonic resonance modal analysis of Multi-inverter Grid-connected system

In view of the current situation that large-scale inverters are connected in parallel to the weak grid at the point of common coupling (PCC) through LCL filters, determining the resonance frequency of a multi-inverter grid-connected system based on a frequency domain analysis method requires the establishment of a complex high-order transfer function, but the method cannot provide information on resonance distribution and the degree of participation of each component. In this paper, the resonance modal analysis (RMS) method is used to derive the output impedance model of a single inverter under the Norton equivalent form based on the grid-connected structure and control block diagram of the LCL three-phase inverter. The system's equivalent admittance matrix performs eigenvalue decomposition, obtains the system resonance frequency and the participation factors of each node, and evaluates the resonance characteristics of the system under different numbers of grid-connected inverters and different types of grid-connected inverter combinations. By analyzing the transfer function of the grid-connected current of the inverter and deeply studying the impedance coupling relationship between the inverter and the power grid, the correctness of the modal analysis method and the characteristic of resonance in multi-inverter grid-connected system is verified.