Flywheels can be used as storage elements in islanded power systems and, as a special case, in isolated wind plants, especially those running at almost constant speed and included as a component in diesel groups. Nowadays, systems with variable speed are under development, so the amount of available energy in the flywheel is highly increased. In this paper, a variable speed flywheel based on hydrostatic transmission ( HT) is presented. A comparison between an almost constant speed flywheel, a variable speed one based on a power electronic converter, and the proposed system is made by means of simulations in order to evaluate the behaviour of this configuration. (C) 2008 Elsevier Ltd. All rights reserved.