A review of recent control techniques of drooped inverter-based AC microgrids

As the penetration of distributed generation (DG) systems in the grid is increasing, the challenge of combining large numbers of DGs in the power systems has to be carefully clarified and managed. The control strategy and management concept of the interconnected systems should be flexible and reliable to handle the various types of DGs. This can be suitably met by microgrids. This paper introduces the microgrid structure and elements and states the main objectives that should be achieved by the microgrid controllers and each DG controller in both operation modes (grid-connected and island mode). It also presents the challenges of having multiple DG units in a microgrid in terms of accurate power control/sharing, voltage and frequency regulation, power management between DGs, different renewable energy sources integration and deployment, seamless mode transfer, and the modeling issues. The centralized and decentralized control techniques as potential solutions have been discussed and compared by highlighting the advantages and disadvantages of each. Furthermore, the recent control techniques for drooped alternating current microgrids and the main proposed solutions and contributions in the literature have been exposed to finally overcome the droop control limitations and obtain a flexible and smart distributed power system. Inverter-based microgrids operate in island or grid-connected modes with three control classes of distributed generation (DG) units. Grid-forming units act as autonomous ideal alternating current (AC) voltage sources, regulating voltage and frequency in island mode and adapting to grid references in grid-connected mode. Grid-feeding units, synchronized with the main AC bus, control output current/power, typically used for photovoltaic and wind integration. Grid-supporting units combine features of both, controlling output voltage and frequency for power sharing in both modes. DGs are also classified as voltage-controlled sources for internal voltage/frequency regulation or power-controlled sources (PCSs) for output power/current control, with PCS using techniques like maximum power point tracking for renewable energy integration.image