A Sequential SISO Stability Analysis Model for Grid-Connected Converters by Considering the Impact of Power Flow Direction

For grid-connected converters operating undernonunity power factor, theirdqadmittances become off-diagonaldue to emerging of the coupling admittances. Consequently, thegrid-connected converter systems become multi-input multioutput(MIMO) systems whose stability analysis becomes challenging.To avoid the multivariable nature for system stability analysis,by considering the impact of power flow direction and converteradmittance characteristics, this article formulates a sequentialsingle-input single-output (SISO) stability analysis model for grid-connected converter systems. Compared with the existing SISOmodels, the proposed one offers salient features as follows. 1) Differ-ent power flow directions correspond to different stability analysissequences and system minor-loop gains. 2) It possesses a moreclear physical meaning and explicitly identifies the contributionsof different parts, including thed-axis subsystem, the coupling ofthed-axis subsystem with theq-axis subsystem, and theq-axissubsystem to the system stability. 3) The proposed system minor-loop gain never contains any RHP pole and therefore, the systemstability analysis and oscillation mechanism illustration are greatlysimplified. Furthermore, the classical control theory can be intro-duced to guide the system analysis and design. Simulation resultsare presented to validate the accuracy of the proposed sequentialSISO model.