The combination of two or more different energy storage technologies in a hybrid energy storage system appears to be advantageous if the combination of technologies fulfils the storage purpose better than a single storage technology. Herein, requirements regarding the performance of the storage system are determined for a hybrid compensation system for provision of system services in medium-voltage grids. In hybrid storage systems, a common control system ensures that the present storage technologies are utilized in such a way that total storage efficiency is highest and that usability of the application is maximized. The control system of a hybrid storage system consisting of a redox flow battery and high-speed flywheel storage is introduced and evaluated.