Flexible Synchronization Control for Grid-Forming Converters with Regulated DC-Link Dynamics

Abstract

In grid-forming (GFM) converters with regulated dc-link dynamics, the dc-link voltage control can directly generate the frequency that is used for synchronizing GFM converters with the grid. However, this synchronization method suffers from low-frequency oscillations (LFO). To tackle this challenge, this paper proposes a flexible synchronization control scheme for grid-forming converters. In the approach, the dc-link voltage control, the active power feedforward control, and the q-axis voltage feedforward control loops are connected in parallel for the synchronization purpose. It is found that both the active power feedforward and the q-axis voltage feedforward loops can dampen the LFO. Yet, the active power feedforward loop with different controllers induces additional synchronous- or low-frequency instability issues, whereas the q-axis voltage feedforward loop does not have such adverse effect. Therefore, the paralleled dc-link voltage and q-axis voltage feedforward control-based synchronization scheme is recommended. Experimental tests of GFM converters with different grid impedances are performed. The results confirm the effectiveness of theoretical analysis and of the proposed control approach.