Kalman filter-based smooth switching strategy between grid-connected and off-grid modes in grid-connected inverters

Grid-connected inverters (GCI) in distributed generation systems typically provide support to the grid through grid-connected operation. If the grid requires maintenance or a grid fault occurs, the inverter must operate independently of the grid. In this article, a smooth switching control strategy is proposed. The proposed strategy uses a mixed voltage/current control. When the GCI needs to operate off-grid, the control of the GCI switches from the current control mode to the voltage control mode and then disconnects from the grid. When the GCI needs to be connected to the grid, the GCI switches from the voltage control mode to the current control mode and then restores the connection to the grid. To achieve smooth and fast switching between the different control modes, a Kalman filter is required to extract the fundamental current and voltage at the point of common coupling (PCC). Fundamental information is used for phase locking to ensure phase synchronization before and after switching. A series of simulations and experiments validate the proposed strategy.