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

For grid-connected converters operating under nonunity power factor, their
$dq$
admittances become off-diagonal due to emerging of the coupling admittances. Consequently, the grid-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 converter admittance characteristics, this article formulates a sequential single-input single-output (SISO) stability analysis model for grid-connected converter systems. Compared with the existing SISO models, the proposed one offers salient features as follows. 1) Different power flow directions correspond to different stability analysis sequences and system minor-loop gains. 2) It possesses a more clear physical meaning and explicitly identifies the contributions of different parts, including the
$d$
-axis subsystem, the coupling of the
$d$
-axis subsystem with the
$q$
-axis subsystem, and the
$q$
-axis subsystem to the system stability. 3) The proposed system minor-loop gain never contains any RHP pole and therefore, the system stability analysis and oscillation mechanism illustration are greatly simplified. Furthermore, the classical control theory can be introduced to guide the system analysis and design. Simulation results are presented to validate the accuracy of the proposed sequential SISO model.