A Symmetrical Control Method for Grid-Connected Converters to Suppress the Frequency Coupling Under Weak Grid Conditions

The asymmetrical rotating (dq-) frame control leads to frequency coupling dynamics, which tends to bring in harmonic instability. In this article, based on the established complex signal impedance model, the influence of the grid impedance and the frequency coupling on system stability is analyzed. For the phase-locked loop (PLL) mainly affects the q-axis dynamics, it is proposed that the q-axis feedforward and the d-axis compensation control methods to decrease the asymmetric influence caused by the PLL. Similarly, for the dc-link voltage controller mainly affects the d-axis current reference, this article proposes a d-axis feedforward and q-axis compensation method to improve the asymmetry dynamics introduced by the dc-link voltage controller. When the proposed methods are adopted, the components introduced by the PLL and the dc-link voltage controller in the coupling terms are eliminated, thereby achieving the purpose of suppressing the frequency coupling phenomenon and improving the system stability. The theoretical analysis and the experimental results show that the proposed methods are effective.