# Simulation and Optimisation of Utility-Scale PV-Wind Systems with Pumped Hydro Storage
## 1 Introduction  
At the utility scale, power-generation systems such as photovoltaic (PV) generators or wind farms sell electricity to an AC grid in the long-term market via the daily or intraday market or through a power purchase agreement. At this scale, there are high-load consumers (towns, large factories, residential, commercial, or industrial areas) that can supply their load consumption via the utility grid and/or their own generating systems (e.g., PV and/or wind turbines with or without storage). We studied large consumers such as load-supply systems (LSS) with their own generating systems that included PV, wind turbines, and PHS. Further, we considered power-generating systems (PGS) comprising a PV generator and/or wind turbines with PHS, where the objective is maximizing the income generated by selling electricity to the grid. There is no auxiliary load consumption, i.e., the load for auxiliary components is considerably lower than the generated power. Atthe national utility-scale levels, grid stability and secure load supply suggest the necessity to increase energy storage in the generation mix with a greater presence of renewable energy and a lower weight in the mix of thermal power plants [1]. High PV penetration affects the shape of day-ahead hourly price curves, lowering the daytime electricity prices [2] ('duck' shape [3]). In grid-connected systems, PHS is a type of storage that can be used for energy arbitrage: electricity can be stored (water is pumped from the lower to the higher reservoir) during hours when electricity prices are low (off-peak hours, with low national demand), and the stored energy can be used (the water from the upper reservoir flows back down through a turbine and generates electricity) to inject electricity into the grid when the electricity price is high (peak hours, with high national demand). Thus, PHS can improve grid stability and load-supply security and reduce costs in LSS or increase benefits in PGS. Energy arbitrage with PHS could be profitable (i.e., the increase in benefits attributed to price arbitrage could help compensate for its investment and operational costs) based on the hourly electricity prices, PHS efficiency, capital expenditures (CAPEX), and operational expenditure (OPEX) costs [4]. Other PHS benefits for renewable power include curtailment reduction, fast and flexible ramping, frequency regulation, black start, and capacity firming [3]. In addition, PHS can be used for other ancillary services such as steady-state voltage control, fast reactive current injections, and island operation capability [5]. PHS is the dominant technology in electrical energy storage, with 97% of power and 99% of storage energy [6]. The primary advantages of PHS include its technological maturity, long useful life, high overall efficiency, rapid response, and strong power ramping. PHS is the most suitable technology for small islands and mass storage [7], and it has been used for energy storage on islands with high renewable penetration, such as the El Hierro [8] and Feroe islands [9]. PHS is the most suitable technology for the seasonal storage of electrical energy. [PERSON] et al. (2020) [10] showed that more than 79% of global electricity consumption in 2017 could be stored in PHS at a cost of less than USD 50/MWh. Existing PHS plants in electrical systems consume electricity during off-peak hours (pumping water) and inject electricity using the stored energy into the grid during peak hours, thereby contributing to grid services. Further, they operate in an open (one or more reservoirs are connected to a natural body of water) or closed loop (reservoirs are separate from natural waterways) [7]. Traditionally, PHS operated at a fixed speed [11], although variable-speed systems have gradually been incorporated [12; 13]. Mountainous countries show high potential for pumped-storage hydropower (for example, Nepal [14]).