Harmonic Current and Inrush Fault Current Coordinated Suppression Method for VSG Under Non-ideal Grid Condition

For the virtual synchronous generator, the voltage-controlled harmonic suppression method will seriously deteriorate the magnitude of inrush fault current in the moment of grid symmetrical fault, because the suppression effects of harmonic current and inrush fault current are mutually constrained from the perspective of its output impedance. For this issue, a harmonic current and inrush fault current coordinated suppression method is proposed to achieve both good results of harmonic current suppression in normal grid and inrush fault current limitation in fault grid, which mainly includes a passive branch, an active branch and an improved virtual synchronous generator (VSG) control. The passive branch is installed between the PCC and grid to introduce an additional voltage variable. Meanwhile, with the proper feed-forward of the PCC voltage and additional voltage, the active branch is used to increase the mid/high frequency impedance viewed from the grid fault point to VSG, and also decrease the harmonic impedance viewed from the PCC to VSG. Then, not only the transient inrush fault current is limited, but also the harmonic current introduced from the nonlinear load is mostly absorbed. Moreover, the improved VSG control is used to limit the steady-state fundamental frequency fault current. Therefore, the contradictory between harmonic current and inrush fault current suppression for VSG is well solved. Finally, the experiment results validate the proposed control method.