Three-Loop-Based Universal Control Architecture for Decentralized Operation of Multiple Inverters in an Autonomous Grid-Interactive Microgrid

Abstract

This article presents a decentralized control architecture for inverters in a utility-interactive autonomous microgrid involving multiple inverter units. The architecture is novel as all the system requirements, viz., automatic/seamless-mode transitions between grid-tied to standalone mode of operation and vice versa, islanding/anti-islanding detection, low-voltage ride-through, and dynamic grid support by reactive-power injection, are organized into three cascaded control loops-voltage control, frequency control, and a PLL. The control loops are designed to operate in the synchronous reference frame so that the control structure becomes compact and free from any computational complexities and associated delays. An equivalent droop combining virtual resistance with the active (P) and reactive (Q) power-based droops is used to achieve proportional load-sharing among the inverters in the standalone mode. The references to these loops are self-modified by the controller according to the operational mode. The architecture is autonomous but straightforward, where each inverter operates on its terminal information eliminating the requirement of any centralized controller or intercommunication network. The performance of the proposed control strategy is validated through comprehensive experimental results on a laboratory prototype that tests all the functions of an autonomous microgrid.