Unified control scheme of grid-connected inverters for autonomous and smooth transfer to stand-alone mode

As one of the approaches for a grid-sustaining inverter, the inverter should cover not only grid-connected (GC) mode but also stand-alone (SA) mode for power supply to local loads; therefore, there are separate control loops for each mode. In order for an uninterruptible power supply to the local load, it should be seamless to change from GC mode to SA mode. This carries two types of issues. One is a transient state caused by switching the control loops. The other is an uncontrolled state during the time interval between a grid failure and its detection, called clearing time. These can lead to unstable voltage for the local load. Hence, a smooth and autonomous mode switching method is required even if the fault detection is late. Existing mode transfer methods considering both the issues have been accompanied by a slow dynamic response, additional sensors, restriction of adopting filter, or controller complexity due to third-order plant. To overcome them, in this article, a control scheme realized by a unified control loop is proposed for smooth and autonomous mode switching with a novel antiderailing control. Furthermore, the proposed control scheme can achieve a high bandwidth for the output power control in GC mode because it is based on controlling the current flowing the inverter-side inductor, and both filter types, LC and LCL, can be adopted. To validate the proposed concept, simulations and experiments were conducted. The results show that the proposed control scheme can be used to achieve autonomous and smooth mode transition even under reactive power reference and reactive load conditions.