Abstract：Under the condition of weak grid，the dynamic interactions between multi-inverters and between multi-inverters and the grid affect the power quality and stability of the power system，which is likely to cause harmonic resonance. In order to study the harmonic resonance characteristics of the single-stage photovoltaic grid-connected multi-inverter system，the modal analysis method is used for systematical analysis and discussion on the resonance problem while the interaction between the photovoltaic generation and the system is considered. Firstly，according to the structure and control strategy of three-phase single-stage photovoltaic grid-connected system，the Thevenin equivalent model for the multi-inverter system is established. Secondly，a modal analysis method is applied，which can determine the system resonance frequency，resonance center and the participation degree of each node by constructing node admittance matrix of the multi-inverter system. The resonance characteristics and variation laws of the system are studied from three aspects：the number of inverters，external environment and transmission distance. Finally，with the use of MATLAB/Simulink simulation platform，the correctness and effectiveness of the modal analysis method are validated by a simulation model of a three-phase single-stage photovoltaic grid-connected multi-inverter system. The results show that when the number of inverters increases，the low resonance frequency tends to decrease，which is 30th，27th，and 25th harmonics respectively，while the high resonance frequency remains unchanged at 2 230 Hz. When the ambient temperature decreases，the low resonance frequency increases gradually，which is 22th，23th，and 24th harmonics respectively，and the high resonance frequency is stable at about 2 225 Hz. When the transmission distance increases，the low and high resonance frequencies gradually decrease and become close to each other.
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