Research On Self-Synchronization Control Strategy Of Grid-Connected Inverter Under Weak Power Grid

With the rapid growth of distributed power generation based on renewable energy sources such as photovoltaics and wind power,grid-connected inverters are increasingly used in power systems.Grid-connected inverters can be divided into active/reactive power control type(PQ-type)grid-connected inverters and DC voltage/reactive power control type(DCVQ-type)grid-connected inverters according to different application environments and control objectives.However,under a weak power grid environment with a large line impedance,the phase-locked loop(PLL),which is a traditional synchronization unit that synchronizes the grid-connected inverter with the large grid,will negatively affect the stability of the grid-connected inverters.Based on this,this paper makes in-depth research on the self-synchronization strategy of PQ-type and DCVQ-type grid-connected inverters.First of all,this paper accurately establishes the output impedance model of grid-connected inverters adopting traditional PLL synchronous unit by using the harmonic linearization method.On this basis,the improved Nyquist stability criterion is used to analyze the negative influence of PLL on the stability of grid-connected inverters under weak power grid,and the internal mechanism of the negative impact of PLL under weak power grid is explained.Due to the negative impact of PLL on the stability of grid-connected inverters under weak power grid,the self-synchronization strategy of grid-connected inverters is respectively studied for PQ-type and DCVQ-type grid-connected inverters considering the PQ-type and DCVQ-type grid-connected inverters have the same demand in practical engineering applications.Based on virtual synchronous machine strategy that is the existing self-synchronization strategy of PQ-type grid-connected inverters omitting PLL,using harmonic linearization output impedance modeling combined with improved Nyquist stability crite ria,the stability of PQ-type grid-connected inverters under weak power grid is theoretically analyzed.At the same time,referring to the synchronization mechanism of PLL and the self-synchronization strategy of virtual synchronous machine for PQ-type grid-connected inverters,a self-synchronization strategy of omitting PLL for DCVQ-type grid-connected inverters is designed.The output impedance modeling and stability analysis method consistent with the PQ-type grid-connected inverters are used to theoretically analyze the stability of the DCVQ-type grid-connected inverters controlled by the self-synchronization strategy under weak power grid.The accuracy in synchronize with grid and applicability under weak power grid of self-synchronization strategy of PQ-type and DCVQ-type grid-connected inverters are verified by the Plecs simulation platform.