Europe is taking concerted efforts to prepare its grid for the challenges posed by the European Union’s (EU) ambitious energy transition goals. Under the Green Deal, EU aims to achieve net greenhouse gas (GHG) emission reductions of at least 55 per cent by 2030 compared to 1990 levels, and net-zero emissions by 2050. The primary tools for decarbonising Europe are achieving energy efficiency of the whole energy system, direct electrification and a drastic change in the generation mix with a growing share of renewable energy sources (RES). Given the scale of these future changes, the impacts on the system are beyond compare.
The RES installed capacity must be increased manifold to meet future energy requirements. Therefore, power generation will increasingly come from weather dependent and electronic interfaced devices, and the share of synchronous generators providing inertia will be reduced. As a result, the system behaviour will change substantially, creating a major challenge for the stability of the systems, which relies, at least as of now, largely on those synchronous generators, as well as on security of supply. To ensure the system stability of the power electronic (PE) dominated grid, conventional system development and system operation methods will no longer be adequate. Responding to this complex and diverse challenge and ensuring system stability during the transition period and in the long term is a condition to making the transition achievable and successful.
The European Network for Transmission System Operators for Electricity (ENTSO-E) released a position paper outlining the transmission system operator (TSO) community’s perspective on solving technical challenges related to stability management of the PE dominated system. It has emphasised that TSOs are willing to be the driving force together with the relevant stakeholders. Significant research and development (R&D) efforts are required for accelerating the uptake of new technologies for stability management.
In fact, finding solutions to stability management issues have been prioritised in the ENTSO-E Research Development and Innovation (RDI) Roadmap 2020-2030, as part of Flagship 5, dedicated to developing alternating current/direct current (AC/DC) grids, and a project concept has been defined in the ENTSO-E RDI Implementation Report 2021-2025. The potential solutions will be supported through EU financing instruments – such as Horizon Europe.
ENTSO-E’s position paper highlights that the assessment and control of grid stability requires consistent and pan-European methods, especially for system-wide stability challenges and certain new stability phenomena. As mentioned earlier, the decline of the share of generation by rotating machines providing inertia and other stabilising features to the system have already to a growth in already known stability phenomena. This includes rotor angle, voltage stability and frequency stability, which have increased in the recent years and have a timeframe of seconds. Additionally, due to the effects of fast-acting PE-devices, recent years have witnessed the appearance of new stability phenomena – converter driven and resonance stability. These have a much shorter timeframes of micro to milliseconds.
Figure 1: Power system stability phenomena
Further, to maintain the pace of the energy transition, network codes must be updated quickly through a harmonised process. The amendment of the Connection Network Codes (CNCs) will be crucial to ensure that equipment connected to the grid (on the generation or demand side) provide the stabilising capabilities, for instance control interactions and resonances, required for a power system with high penetration of RES, PEs and direct current (DC) connection. That said, given that the challenges have to be faced with urgency, the timeframe for implementing network codes have to be considered. Further, in case markets are not able to provide the required capabilities in time, TSOs may also acquire, own and operate capabilities within their grid, such as synchronous compensators.
New technical capabilities and system services make it essential to define new resilience requirements (for system splits, wide-meshed offshore grids, sector coupling and so on). Particularly, system services ensure system stability maintenance in daily operation and during incidents such as the disconnection of supply or system splits. In this content, it is vital to develop markets and regulations to ensure sufficient capabilities are available at best cost.
TSOs have a critical role to play and need to clarify how other grid participants are impacting the grid stability. In this context, they need to create comprehensive action plans covering all aspects such as technical, economical, regulatory and environmental across various timeframes, short-, medium and long-term.
In the short-term (less than one year), ENTSO-E and TSOs, in cooperation with policy makers and stakeholders, need to create a common and comprehensive pan-European roadmap for stability management based on a description of the necessary capabilities of the future power system and grid codes. Another important task is identifying areas for stability assessment and control where standardisation is necessary and beneficial. Further, TSOs must analyse and identify, for each synchronous area, the point in time when different stability challenges are expected to emerge based on the pace of the energy transition and the integration rate of PE interfaced devices. They must also identify a range of new technical capabilities, system services and methodologies to handle the controllable resources and flexibility necessary to maintain system stability, security and resilience. EU financing support will play a key role in boosting RD&I efforts.
In the medium-term (two to three years), ENTSO-E and TSOs should propose methods for the analysis and identification of thresholds for system inertia and calculation methods that can be applied to identify the optimal socio-economic mix of traditional inertia and inverter-based assets with grid forming capabilities. It should further propose methods to predict and monitor system stability and handle controllable resources and flexibility. It must recommend grid code amendments for further harmonisation across Europe. The CNCs should evolve to ensure the equipment capabilities (such as new fast frequency reserves or grid forming functionality) needed for a power system with high-RES penetration. TSOs need to analyse roles and responsibilities with regards to the stability management of the pan-European grid, considering the cross-regional aspect of certain stability phenomena and the impact of cross-sector integration.
In the long term (more than three years), ENTSO-E and TSOs will deploy prediction, monitoring and communication systems for stability management on a pan-European level. TSOs will collaborate with policy makers to ensure that stability issues are considered in the planning processes of network development. They need to support vendors and manufacturers in the development as well as deployment of new capabilities in inverter-based assets through the testing and assessment of new technologies and to ensure interoperability. TSOs will need to develop and verify power system models, asset models and simulation tools capable of analysing new and challenging European stability phenomena. Finally, they need to ensure liquidity in markets for new system services.