## 4 Simulation Results
### Increasing Renewables Penetration  
The last scenario studied in this work is interesting in a long-term perspective. As explained in the introduction, in the future, a large spread of renewable sources is expected that will displace the conventional ones. In these simulations, the effect of such an increase on the frequency stability is addressed, both in the case of the absence of frequency response capability from renewables and in the presence of the advanced controls considered in this work. It is important to remember the expected result of the tests in this first scenario: the increase in renewable penetration, due to their unpredictability and non-dispatchability, requires an increase in the reserve level, which at the same time displaces the conventional plants that are responsible for providing primary frequency controls and contributing to systems' inertias. Due to these facts, the magnitude of the frequency nadir consequent to a perturbation is expected to increase as the penetration of renewables increases. The aim of the advanced controls, on the other hand, is to allow the substitution of the conventional fleet of generators with renewable ones without losing the ability to control the frequency. This is why, in the second case, a smaller frequency nadir and frequency oscillations are expected. When adapting the participation factors, it is important to take into account that the displacement of synchronous generators causes a reduction in the system's equivalent inertia. In this work, the reduction is proportional to the synchronous generator participation factor. This means, for example, that if the participation factor of the equivalent steam turbine passes from 0.7 to 0.35, the equivalent inertia constant will be halved. The first group of simulations regards an increase in VSWT penetration, whose results are shown in Figure 22.  
The solid lines represent the frequency variation consequent to a negative load power step of \(-10\%\) in the scenario of primary frequency control provided just with synchronous generators, while the dashed lines are resulting from the scenario with a _complete FC_. Taking a first look at the plot, the negative effect of the increasing penetration of renewables if they are not able to participate in the primary frequency control is immediately clear. Indeed, for the same step of load power, the nadir increases from 1.0174 pu in the base case (20% wind, 10% PV) to 1.0275 pu in the most extreme case (50% wind, 10% PV). Furthermore, the difference in terms of the nadir between one curve and the other is not equal: increasing the wind penetration, the increase in the nadir is larger and larger from one curve to the next one. This happens because there is not just an effect due to the reduction in the synchronous generator power variation weight, but also of the system's inertia. Figure 22: VSWT penetration increase. Once stated thus, it can be seen how large the benefit is associated with the introduction of the frequency response capability in renewables: the nadir is much lower than in the first scenario and, as the penetration of VSWT increases, the mismatch between the two scenarios is way more evident. Unlike what happens in the first scenario, the nadir difference between the various curves in the second scenario reduces as the wind turbine penetration increases, meaning that the provision of this service from renewables (in this case, it prevails over the effect of the wind turbines inertial control since their weight is dominant) is able to \"cover\" the reduction in the system's inertia, thus improving frequency stability. The same kind of observations are discernable from the other simulations, which regard an increase in PV penetration (Figure 23). In this case, the benefit is even larger, since, after a certain value of \(p_{pv}\), the nadir even reduces as the penetration of the photovoltaic increases. For example, in the most extreme case (40% PV, 20% wind), the nadir is lower than in the less PV-penetrated systems (10% PV, 20% PV, and 30% PV).