A Unified Droop-Free Distributed Secondary Control for Grid-Following and Grid-Forming Inverters in AC Microgrids

The inverter-based distributed generators (DGs) in microgrids usually operate in either grid-following (GFL) or grid-forming (GFM) control mode. In GFL mode the DG follows the voltage and frequency of the microgrid while in GFM mode it dictates voltage and frequency for the microgrid. In this paper, we propose a unified droop-free distributed secondary control for both GFL and GFM inverters based on distributed optimization. The proposed control regulates the frequency to nominal frequency, regulates the global average voltage in the microgrid to the rated voltage, and ensures accurate active and reactive power sharing among all GFL and GFM inverters. The DGs can communicate through a sparse communication network and share information about average voltage estimates and nominal active/reactive power. The effectiveness of the proposed control is validated through simulations on an eight-DG test microgrid with four GFL inverters and four GFM inverters.