A Novel Design for Switchable Grid-Following and Grid-Forming Control

This paper presents the design of a novel grid-forming (GFM) control structure adapted from a typical grid-following (GFL) control structure with minimal edits, thereby enabling a switchable control structure for voltage sourced converters (VSCs) to operate in either GFL or GFM mode by simply switching a flag manually. The VSC is shown to be able to operate in the GFL control mode synchronizing to the main grid through a phase-locked-loop (PLL) and operate as a GFM controller with power-based synchronization for both grid-connected and islanded conditions. To guarantee smooth operation, the control schemes and the mode switching logic have been carefully designed and examined via a series of experiments. The experiment results show that the switchable control structure can fulfill the desired control and operation functions and enable smooth transition between control modes.