## 1 Introduction  
As it is widely reported in technical and scientific literature [1; 2; 3; 4; 5; 6; 7; 8], the massive introduction of photovoltaic or wind farms in the electrical networks involves several crucial issues for the safe and reliable operation of transmission and distribution grids. Foremost among these are the non-programmability of renewable energy sources, which involves traditional power plant manage difficulties and congestions in the power transmission lines, and the decrease of the network regulating energy, which leads to network instability.  
The installation of energy storage systems in a high voltage network could be one of the key elements to effectively help in solving these problems, postponing or avoiding the grid reinforcement based on new insulated cables underground [9] or hosted in shared structure, [10; 11] or on new overhead lines [12]. In [13], an overview of different energy storage technologies for grid services, including supercapacitor is presented. Moreover, many contributions in literature focus on the possibility to counteract the grid regulating energy decrease due to renewable energy plants by exploiting the power supplied by electrochemical energy storage installations [1; 2; 3; 4; 5; 6; 7; 8]. This solution seems to be very effective, but recent studies demonstrated that battery aging during a grid frequency regulation operation could be higher than expected and difficult to foresee, especially for some battery technologies, including lithium-iron phosphate, lithium-nickel cobalt aluminum and lithium- nickel cobalt manganese technologies [14]. The flywheel system suitability for grid services has already been recognised for some time [15]. Flywheels inherent features, such as the long calendar life, fast response, high round trip efficiency, high charge and discharge rates, can provide fast acting regulation without suffering in terms of life time degradation [16]. However, some issues must be likewise considered in relation to the large power sizing, such as the high investment cost, the complex power management and the integration of sophisticated technologies (flywheel rotor, electric machine, power electronics and bearings). Therefore, hybrid storage system solutions consisting of batteries and flywheels [16] could represent a valid solution to extend the battery lifetime and performances in this context. In recent literature, several contributions deal with kinetic-electrochemical hybrid systems. In [17; 18], a general overview of the benefits that they could bring to the electrical system stability is presented, mainly focusing on Li-ion batteries. Paper [19] deals with the technical and economic feasibility of hybrid storage system installation in electrical networks. Paper [20] analyses a storage strategy based on lead-acid batteries and flywheels for wind power plants.  
In this paper, a hybrid energy storage system consisting of flywheels and batteries with a Lithium-manganese oxide (LMO) cathode is proposed and analysed, with the aim of tackling battery aging during the grid frequency regulation operation. This goal can be reached by means of a suitable control of the power supplied by the hybrid system itself.