A Synchronous Reference Frame based Decentralized Control Architecture for Inverters Connected to an Autonomous Microgrid

This paper presents a decentralized control architecture for decentralized operation of inverters in an autonomous microgrid. The proposed architecture is novel because it can organize all the system requirements viz. grid (utility)-connected/ parallel connected (stand-alone) mode of operation, islanding/anti-islanding detection, automatic/seamless mode transitions, low voltage ride through and reactive power injection for dynamic grid support into three cascaded control loops. The architecture is simple but autonomous as each inverter operates on its terminal information only. It eliminates the requirement of any centralized controller or inter-communication. The proposed technique requires three basic control loops - voltage control, frequency control and a PLL control which are designed to operate in the synchronous reference frame (SRF) so that the control structure becomes compact and free from any computational complexities and associated delays. The references to these loops are self-modified according to the mode of operation. A unique integrated droop control (IDC) strategy combining the active/reactive power (P/Q)-droop and the synthesis of virtual resistance into equivalent droop based on SRF currents is used. A novel phased locked loop (PLL) structure is used that dynamically adjust the rotational d-q reference frame depending on the system reactive loading. 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.