Analysis of a grid-forming converter based on repetitive control in centralized AC microgrid

The power system is experiencing an expansion of distributed generators connected to the low voltage network. In this context appears the so-called microgrid, which consists of a group of electrical microsources, loads and storage elements controlled as a single unit whose may operate either connected to or islanded from the main grid. The microgrid can be arranged in a centralized structure, e.g., master-slave architecture, where the centralized converter, i.e., grid-forming converter plays a crucial role. It must promptly respond to load and grid variations, manage the transition from grid-connected to islanded mode, and define the voltage and frequency during islanded operation. This paper proposes a repetitive control tuned at even harmonic orders applied to a grid-forming converter instead of repetitive control tuned at odd harmonics as usual. Such grid-forming is devised in synchronous reference frame and applied to a centralized AC microgrid, particularly under islanded operation. Such microgrid is composed of linear and nonlinear load, a photovoltaic inverter and the grid-former converter itself. Through simulation results the microgrid power flow and power quality are analyzed evaluating the grid-forming converter operation.