## 1 Introduction
More specifically, the operation of electrical power systems requires frequency to remain tightly close to its rated value. Traditional frequency control strategies are based on synchronous generators and their ability to modify the mechanical power from their prime movers to compensate for frequency deviations. These are caused by real-time mismatches between the mechanical power input and the electrical power output (plus losses) of synchronous generators. The inertia of rotational masses of each turbine-generator set offers a natural temporal buffer for these mismatches that limits the rate of change of frequency (RoCoF). As stated, renewable generators are commonly connected to the power system through power converters, which lack an inherent inertial response to frequency deviations. This increases frequency deviations and RoCoF values as a growing share of renewable generators displace synchronous ones. Thus, the main objective of this work is to assess the ability of renewable generators to contribute to frequency control in power systems where synchronous generators are being displaced by power electronics interfaced ones. This work is focused on the first seconds immediately after an unbalance, in which the primary reserve activates to contain as much as possible the initial oscillation of frequency after a perturbation. In particular, the work faces the issue of providing primary frequency control with renewable generators as photovoltaic modules and wind turbines, which are traditionally not technically able to perform it, trying to understand how to implement it and whether the performances are acceptable. Before going through the developed model, it is extremely important to offer an overview of the methods and algorithms proposed in the literature for the power reserve control of photovoltaic systems. This bibliographic analysis is very useful, since each one of the methods can have some issues and some advantages, which can be compared to the ones of the model developed in this work, understanding its effectiveness. For what concerns the photovoltaic (PV) technology, two main types of solutions have been reported in the literature for complying with future requirements in terms of the modulation of active power as a response to a frequency deviation: the use of energy storage systems (ESSs) or the implementation of innovative control schemes for power curtailment [8]. The adoption of ESSs for primary frequency control can potentially solve the issues in highly renewable penetrated systems, as the small island ones or the power systems of the future. The biggest advantage with respect to power curtailment methods is that no energy is wasted. Indeed, to keep a certain reserve level always available, renewable generators have to exploit just a certain amount of the producible energy. If the remuneration and the incentives for the participation in primary frequency control are not enough, the profitability of renewable plants decreases. In that case, adopting an ESS integrated with the generators would allow a more flexible utilization without reducing the energy exploited. Another winning aspect of some ESSs is that they are able to adapt the power with rapid ramp rates, which is one of the key targets for the future primary frequency control [9]. On the other hand, the ESS solution presents also some drawbacks, mainly related to investment cost and complexity. In fact, the ESS solution requires an additional investment to purchase the storage system itself. Furthermore, in the case of batteries as ESSs, their lifetime is limited [8]. These negative aspects can lead to an increase in the cost of the PV electricity produced, which is in contrast to the expectations of cost reduction in the next decade [10]. It is also important to state that power curtailment and storage are not necessarily mutually exclusive solutions, since, in some situations, combining the two is the best option from the economic point of view [11].