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.