Power-Flow-Based Secondary Control for Autonomous Droop-Controlled AC Nanogrids With Peer-to-Peer Energy Trading

Regarding the control of micro- and nanogrids, LC- or LCL-filtered power inverters (acting as interfaces with distributed energy resources such as photovoltaic or wind) commonly perform as grid-forming or grid-supporting units to maintain both the frequency and voltage within pre-set standards. Nevertheless, these power inverters are assumed to be connected at the same point of common coupling directly or via radial feeders; thus, the voltage references are the same for each parallel power inverter, thus requiring a virtual impedance loop. In addition, classic power sharing techniques comply with their individual power rates, and circulating currents among distributed generators are not considered. Under these circumstances, energy trading among prosumers and peer-to-peer contracts is not feasible in autonomous AC micro- and nanogrid operations. This paper proposes a reformulated power flow problem, adapted to autonomous droop-controlled AC microgrids, to be used as a secondary control layer. The entire hierarchical control is implemented and experimentally validated in a laboratory-scale nanogrid with energy storage systems, photovoltaic generators and power converters. The obtained results demonstrate the proper performance of the proposed approach, with successful operation of primary and inner controllers.