Abstract
This paper evidences the ability of a VSM-based grid-forming to mitigate stability problems of different classes, raising a special concern towards the importance of its location in systems with large converter-interfaced renewable energy systems. Within this context, a multi-class stability assessment, that pillars on the simulation of different nature disturbances and in the subsequent evaluation of a 4 index set, was performed. Such analysis was carried out on a modified version of the IEEE39 Test System, using DigSILENT Power Factory as the simulation engine.

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.

Abstract
This paper discusses the assessment of integrating photovoltaic-battery hybrid power plants into the electrical grids of the Azores islands and their ability to comply with advanced network services. To ensure the hybrid power plant supports the grid operational requirements, a methodology was devised through steady-state and dynamic numerical simulations. On one hand, the steady-state analysis generated active-reactive power diagrams for different voltage levels at the plant’s interconnection point with the island’s grid, demonstrating that the internal grid of the PV-battery hybrid power plant allows a significant range of reactive power modulation in different operating conditions. On the other hand, dynamic analysis highlighted the plant’s crucial role in modulating reactive current production during grid faults. Additionally, it showed the plant’s capability to automatically reduce active power injection during over-frequency events and, as a result, lessening the frequency regulation effort for synchronous generators and fast energy storage system. © Energynautics GmbH.

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.