Self-Synchronization Scheme for Network of Grid-following and Grid-forming Photovoltaic Inverters

Microgrids in power electronics dominated grid (PEDG) should be able to operate under islanded and grid-connected modes and switch between them seamlessly. During the islanding events severe fluctuations in voltage and frequency of the microgrid may arise that can cause unstable operation and ultimately blackout. Moreover, under this transition the inverter may lose synchronism or point of synchronization because of disconnection from grid. This paper proposes: (i) a predictive control scheme that enables dual-mode (grid-forming and grid-following) operation of inverters and enables seamless transition between these two modes; and (ii) a swift single-point synchronization scheme for the network of grid-following and grid-forming inverters. The proposed control scheme mitigates voltage, current, and frequency fluctuations and enables seamless transition while needing limited time for synchronization between the grid-connected and the islanded modes of operation. Moreover, the predictive controller ensures smooth toggling between grid-following and grid-forming modes of operation and extracting maximum power from photovoltaic energy sources. The effectiveness of the proposed control scheme with its self-synchronization capability is validated by various case studies under grid-tied, islanded and transition from grid-connected to islanded mode of operations.