Impedance Frequency Coupling and Stability Analysis of Parallel System of Grid-following and Grid-forming Converters in Weak Grid
In the heterogeneous system with grid-following photovoltaics and grid-forming energy storage,the influence of grid-forming converters on the heterogeneous system stability can be studied through the impedance analysis method.However,the existing studies do not fully consider the frequency coupling effect between different converters and the frequency coupling effect between the converters and the grid impedance.So it is difficult to accurately evaluate the stability of the heterogeneous system in the weak grid.Therefore,taking a two-generator heterogeneous grid-connected system as an example,this paper deeply analyzes the influence of system internal coupling on the output impedances of grid-following and grid-forming converters.The analysis shows that the grid impedance and the output impedance of the grid-forming/grid-following converter have an additional influence on the output impedance of the grid-following/grid-forming converter.In particular,compared with the converter in the independent grid-connected operation,the output impedance of the grid-following converter in the mid-and low-frequency band is easily affected in the heterogeneous system,while the output impedance of the grid-forming converter is basically unaffected.On this basis,the impedance model of the heterogeneous grid-connected system is established by combining the grid-forming converter and the grid as an equivalent grid,and the small-signal stability is discussed.The results of the hardware-in-loop experiment show that,compared with the traditional impedance model,the stability analysis based on the proposed impedance model realizes the accurate judgment of the small-signal stability of the system in the weak grid.
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维普
