Abstract
This paper investigates the stability of a converter-dominated islanded power system when the island’s battery energy storage converters are operated in different control modes (Grid Forming and Grid Following) and combined with different volumes of synchronous compensation. The study is conducted in a realistic simulation model of the future Madeira island, where no thermal generation is present, and the share of converter-based Renewable Energy Sources is large (75 to 80 % of instantaneous penetration). The impact of the different combinations of synchronous condensers and BESS converter control modes on the system stability is evaluated using a stability index-based approach that accounts for multiple operation scenarios. In this procedure, the system’s dynamic response to the reference disturbances (short-circuits in the Transmission and Distribution Network) is obtained via RMS dynamic simulation and is then analyzed to extract two stability indices (Nadir and Rocof). Such indices are computed for the synchronous generator speed and the grid electrical frequency (measured in different points using a PLL) and are later used as the basis for discussion and conclusion drawing. © Energynautics GmbH.

Abstract

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
While the share of renewable energy in intercon-nected 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.