Control of flywheel energy storage systems as virtual synchronous machines for microgrids

Microgrids are an attractive option in remote areas with elevated renewable resources. However, with or without grid connection, microgrids often results in weak grids. Hence, microgrids are much affected by the power variations and require energy storage systems to smooth them out. Flywheel based energy storage systems (FESSs) are gaining momentum in microgrids as, despite the limited amount of stored energy, they allow to interchange high power and have long useful lifetime. In addition, the state-of-charge is very simple to estimate as it only consists on measuring the spinning speed. In FESSs, the flywheel is attached to an electrical machine, which is connected to the grid through a back-to-back converter. Controlling the three phase converter as a virtual synchronous machine allows to overcome instability issues of the PLL-based control in a weak grids. For his reason, his paper proposes to control the grid-side converter of the FESS as a virtual synchronous machine. Small signal analysis is applied to the equations of the virual synchronous machine. The DC-link voltage is regulated by the machine-side converter. Simulation results are provided to illustrate the proposed concepts.