## 3 Methodological Approach  
### A Simplified Simulation of the Greek Power System  
In this connection, a simplified simulation of the Greek power system is proposed to calculate the RES electricity that can be absorbed directly, the energy surplus and the ability of Hydro-pumped storage to exploit it. Methods for solving units commitment and optimization algorithms for economic dispatch in power systems with high wind penetration have been extensively analysed and proposed in previous works [30; 31; 32; 33]. In most of these cases, wind generation is obtained by the use of forecasting tools and unit scheduling is estimated in order to meet demand with a high probability over the scheduling horizon. It is out of the aims of the current approach to optimize the load dispatch and units commitment. The proposed methodology is based on the analysis of the steady-state operation of the Greek power system and takes into account the specific characteristics of demand, the technical features of conventional and hydropower plants and the technical constraints for the smooth and safe operation of the system. Hourly time series are used instead of a probabilistic approach [34], due to the fact that the sequence of events is essential for the simulation of hydro-pumped storage [35]. The large-scale Wind and PV integration is challenging for the power system operator due to their variable power output, the difficulty to predict wind power output accurately and the limited capacity of Greek power system with neighboring countries. However, flexible conventional and hydro units are able to cover any sudden or expected deficit, which may occur to the system. Additionally, the landscape of Greece constitutes an ideal topography for the construction of hydro-pumped storage units which could contribute significantly to the stability of the system and increase renewable peak demand supply. For the simulation of the Greek power system a steady state analysis is performed based on hourly timeseries data. Alternative scenarios for reference years 2019, 2030 and 2050 are comparably presented in terms of energy flows and Levelized Cost of Energy (LCOE). For the simplified simulation all the corresponding data of the power units (load factors, technical constraints, maintenance periods, stochasticity) are taken into consideration. The main target of this simulation is the accomplishment of energy balance, giving priority to renewable energy sources. All the relative assumptions are conservative in order to ensure the safe operation of the power system, with respect to all the technical constraints. The contribution of interconnections is not taken into consideration due to their limited capacity, and the Greek power system is considered as a remote power system. This assumption gives results on the safe side, while in reality, the management of the system could be easier with interconnections. The innovation of the analysis is based on the use of simultaneous mesoscale typical wind year data for the representation of the wind resource in the whole Greek territory. Ninety points are used to provide high geographical resolution and cover current and  
Figure 1: Normalized PV and Wind capacity (by the annual mean load) [26; 27; 28; 29]. future possible sites for wind farms. At each point, hourly wind power output is calculated for one year on the basis of hourly wind speed and wind power installed capacity. Then the aggregated hourly wind power in the whole Greek territory is used as an input for the simulation of the Greek power system. Typical wind turbines power curves are used to calculate wind power output, given the wind installed capacity and the time series of wind speed at each point under consideration. Then the aggregated wind power output \(\left(P_{W_{pred}}\right)\) for every hour is resulted. The main principles of the methodology, units' commitment and load dispatch are presented in this section in four steps.