Hybrid flywheel (Hy-FLY) energy storage system (ESS) for offshore wind application

Several factors have raised interest in discovering low-carbon electricity production sources. Out of all renewable energy (RE) sources, offshore wind is being rapidly integrated into the electrical grid worldwide. Inertia in the grid refers to the energy stored in large rotating turbogenerators and some industrial motors, which gives them the tendency to continue rotating. The kinetic energy of the rotating masses in thermal power plants arrests the fall of frequency. Thus, the traditional power plants' inertia helps to maintain the grid stability by allowing the grid control systems to act after the initial few seconds. This helps in maintaining the frequency within the prescribed range. However, most of the renewable energy-based power generation systems are connected to the grid via a power electronic converter of some description (AC to DC and DC to AC power converters), providing little to no intrinsic inertial response to the varying load generation mismatch of the grid system. As RE systems continue to displace fossil-fuelled generation, significant amounts of real inertia are being lost. Presently this loss of real inertia is compensated by taking measures such as introducing synthetic inertia, inertia emulation etc. The grid operation strategy is also changed so that the largest loss of generation can be controlled. For the stable functioning of any electrical grid, the generation and demand must be in equilibrium without severe variations. Thus, the novelty of this study is to develop a system for providing real inertia, which helps to control the rate of change of frequency (RoCoF) when an overloading event happens in the grid. The system makes use of real inertia as well as a secondary energy store. The concept combines a flywheel (a source of real inertia) and secondary energy stores coupled to a synchronous generator. The flywheel and the secondary energy storage system are connected to the synchronous generator through an electromechanical differential drive unit that enables to take advantage of high-system inertia during the arresting period of RoCoF while allowing the flywheel and secondary storage to extract energy when the system is discharging.