Secondary control with grid-forming inverters for an island grid restoration approach without communication

If a blackout occurs, self-starting units, like grid-forming inverters, are needed for a black start and a restoration in a low voltage grid. Reliable power supply of the grid by grid-forming inverters can be achieved by storage systems connected to the grid-forming inverters. The use of storage systems as a power source for the grid-forming inverters requires that the storage systems must not be completely discharged or charged, otherwise the functionality of balancing the power of the grid-forming inverters is not guaranteed and a restoration failure would result. This paper presents a concept for grid restoration and maintaining a reliable island operation using a secondary control structure in combination with the implementation of a state machine in smart meters to handle such critical cases of the storage systems. The secondary control can also restore the nominal frequency after load changes without any communication between grid elements. RMS simulations are carried out in DIgSILENT PowerFactory showing the restoration process and island operation in different scenarios.