Direct Control Methods for Grid-Forming and Grid-Following Inverters

This article examines the performance of direct and indirect control schemes for grid-forming (GFM) and grid-following (GFL) inverters. Multiple control loops are cascaded in the indirect control schemes for both GFM and GFL controllers, with an inner loop dedicated to current regulation. In GFL mode, this current control loop is crucial to make a voltage source inverter (VSI) performs like a current source. However, in GFM mode, the current control loop serves to mitigate inrush current during transients and abnormal conditions. In contrast, the direct control approach simplifies the system by eliminating the inner current control loop. Instead, for direct control, power is directly managed in GFL mode, while voltage and frequency are regulated in GFM mode through the outer loops only. To evaluate the performance of direct and indirect control schemes, the paper conducts experiments using a laboratory-scaled testbed comprising a microgrid consisting of a 5-kVA SiC-based inverter, a 10-kVA Si-based inverter, and a 12-kW regenerative grid simulator, all operating at 208V. Hardware results back the direct method’s robust control in some operating regions, where the indirect control scheme exhibits high sensitivity to changes in controller parameters and sudden grid voltage variations.