Multifunctional Grid-Forming Cascade Control for Converters Equipped with an LCL Filter.

A multifunctional control framework is presented for grid-forming converters. This structure enables to operate the voltage source converter (VSC) in voltage-source mode in normal operation while activating a current control loop if overcurrent limits are reached. The framework is obtained by merging a multifunctional cascade controller recently proposed for stand-alone applications and a hybrid synchronization loop, which relies on a combination of a power synchronization loop (PSL) and a phase-locked loop (PLL). These two together make it possible to control a grid converter in either current or voltage source mode while maintaining synchronism with the grid. The effectiveness of the proposed control framework is assessed through experimental validation using a 12.5-kVA converter connected to strong and weak grids in terms of both power reference tracking and rejection of grid voltage sag. In particular, the proper performance of the system is emphasized in both tested grid configurations.