Abstract
This article introduces BetaMemo models, a set of advanced probabilistic forecasting models aimed at predicting the hourly power output of photovoltaic plants. By employing a semiparametric approach based on beta distributions and deterministic models, BetaMemo offers detailed forecasts, including point forecasts, variance, quantiles, uncertainty measures, and probabilities of power generation falling within specific intervals or exceeding predefined thresholds. BetaMemo models rely on input data derived from weather forecasts generated by a Numerical Weather Prediction model coupled with variables pertaining to solar positioning in the forthcoming hours. Eleven BetaMemo models were created, each using a unique combination of explanatory variables. These variables include data related to the location of the plant and spatiotemporal variables from weather forecasts across a broad area surrounding the plant. The models were validated using a real-life case study of a photovoltaic plant in Portugal, including comparisons of their performance with benchmark forecasting models. The results demonstrate the superior performance of the BetaMemo models, surpassing those of benchmark models in terms of forecasting accuracy. The BetaMemo model that integrates the most extensive set of spatiotemporal explanatory variables provides notably better forecasting results than simpler versions of the model that rely exclusively on the local plant information. This model improves the continuous ranked probability score by 13.89% and the reliability index by 45.66% compared to those obtained from a quantile random forest model using the same explanatory variables. The findings highlight the potential of BetaMemo models to enhance decision-making processes related to photovoltaic power bidding in electricity markets.

Abstract
This paper presents a comprehensive assessment of the large-scale deployment of hydropower on the provision of frequency regulation services, when equipped with the extended flexibility solutions being developed and/or tested within the scope of the XFLEX HYDRO project. The current analysis is performed on the Iberian Peninsula (IP) power grid considering its interconnection to the Continental Europe (CE) system, since this power system zone is expected to have the most severe frequency transient behaviour in future scenarios with increased shares of variable renewable energies. For this purpose, prospective scenarios with increased shares of time variable renewable generation were identified and analysed. To assess the impacts of the hydropower flexibility solutions on frequency dynamics after a major active power loss, extensive time domain simulations were performed of the power system, including reliable reduced order dynamic models for the hydropower flexibility solutions under evaluation. This research assesses the effects of synchronous and synthetic inertia, and of the Frequency Containment Reserve (FCR) and Fast Frequency Response (FFR) services as specified in European grid codes. The main findings highlight the potential of hydropower inertia and of adopting a variable speed technology for enhancing frequency stability, while contribute to better understand the role of hydropower plants in future power systems.

Abstract
System operators (SO) require Converted-Interfaced Renewable Energy Systems (CI-RES) power plants investors to provide demonstrative studies related to different operational performance capabilities and advanced system services provision to the grid. Typically, these studies rely on Original Equipment Manufacturer (OEM) simulation models for the power converters and CI-RES power plants control units. Such models might be unavailable to the SO due to confidentiality reasons and might present challenges in parametrization due to their complexity. Moreover, compatibility issues between simulation packages used by the SO and those utilized by the independent entity performing the studies creates additional difficulties. Hence, SO demand to power plant investors the proving of equivalent simulation models and resorting preferably to standardized open-source models. This work presents a methodology to derive an equivalent model of a CI-RES power plant using Generic Renewable Energy Models (GREM) in which the parameters identification is performed exploiting an Evolutionary Particle Swarm Optimization (EPSO) to capture the plant's dynamic behaviour at the Point of Interconnection (POI) in face of a set of reference network disturbances. Considering as Case-Study the integration of a PV-Battery Hybrid power plat into the electrical system of Terceira Island, the results demonstrate successful derivation of GREM parameters allowing the representation of the dynamic behaviour of the power plant in face of network disturbance events. © Energynautics GmbH.

Abstract
While the share of renewable energy in interconnected systems has been increasing steadily, in isolated systems it represents a bigger challenge. This paper presents a dispatch algorithm integrating thermal, wind, solar and hydro generation and storage for an isolated network, which allows maximizing renewable energy integration and reducing the share of thermal energy in the mix. The possibility of using the battery to provide spinning reserve is also considered. The algorithm was tested and validated using real data from the island of Madeira, Portugal. Results prove the robustness and flexibility of the algorithm, showing that a significant decrease in the thermal fraction is achievable, and that it is possible to accommodate an increase in renewable generation with minimal or no curtailment at all.

Abstract
The progressive replacement of thermal power plants by converter-interfaced generation, such as wind and solar power plants, reduces the synchronous component available in the system. Additionally, as converter-interfaced renewable energy sources do not directly provide inertia to the power grid, electric power systems are facing a notorious inertia reduction. When facing disturbances affecting the balance between the generation and demand, reduced inertia systems exhibit higher and faster frequency deviations and dynamics. This can result in the disconnection of generation units as well as load shedding, provoking cascading effects that can compel severe power outages. This work examines the impacts of the progressive integration of converter-interfaced renewable energy sources in the frequency stability, considering critical disturbances involving short-circuits in different locations. To simulate the dynamic behaviour of a network containing high shares of renewable energy generation, the IEEE 39-bus system is used while resorting to the PSS/E simulation package. After obtaining a scenario with reduced synchronous generation, the network's stability is assessed in face of key frequency indicators (frequency nadir and Rate of Change of Frequency, RoCoF). Regarding the critical disturbances applied in a low inertia scenario, different control solutions for the mitigation of frequency stability problems are tested and their performance is assessed comparatively. This involves the investigation of the performance of the active power-frequency control in the renewable energy sources, of synchronous condensers, or fast active power-frequency regulation services from stationary energy storage. Moreover, the influence of the location and apparent power of synchronous condensers (SCs) and Battery Energy Storage Systems (BESS) on the frequency indicators is evaluated.

Abstract
Grid-forming (GF) converters based on renewable energy sources are a fundamental piece of future power systems. In particular, the design specifications of GF converters in photovoltaic (PV) applications are difficult to meet because PV inverters lack energy storage. The operation of GF-PV inverters under normal conditions has already been addressed in the existing literature. However, the operation in case of large disturbances, such as faults, has rarely been explored. In this paper, a GF controller for a two-stage PV inverter that is robust against faults is presented. This control system includes several improvements compared to the traditional GF controller. Power feedforwards and saturations are applied to improve the transient performance. Also, a method to keep the virtual swing equation synchronised when the current saturates is presented. Remarkably, there is no need to change the controller structure during faults. Simulations of a PV inverter connected to a simple power system based on a diesel generator and loads are conducted. The results show that the proposed countermeasures improve the performance of GF-PV inverters in case of faults. In addition, it is shown that keeping the phase of the virtual swing equation and the grid voltage space vector synchronised is important to avoid the collapse of the dc-link voltage. Suggestions for further research are presented in the last part of the work.