Under the European Climate Law [2021, European Climate Law], the realisation of the EU’s Green Deal 2050 net-zero emissions target requires doubling the share of generation from renewable energy sources (RES) from today’s levels to 55 %–60 % by 2030, and further increasing to around 84 % by 2050 [2020, European Commission]. This surge in RES goes alongside a quickly increasing presence of power electronics converters, as these are used to connect RES to the electricity grids. This rapid replacement of traditional synchronous generators by converter-connected RES poses severe grid stability and control challenges and also has the risk of coming at the expense of reduced grid energy efficiency.
Traditionally, fossil-fuel based synchronous generators have maintained grid stability by regulating voltage and frequency against a backdrop of fast changes of supply and demand, including faults, due to their physical and electrical characteristics. However, RES connected to the grid through conventional grid-following (GFL) power converters have no inherent ability to regulate the dynamics of the grid. Therefore, in the near future an electricity system dominated by power electronic converters will have too few synchronous generators left to stabilise the grid, leading to barriers to RES integration. In certain areas of the world with high penetration of RES, this is already happening, with a notable example being the Electric Reliability Council of Texas (ERCOT), which regularly curtails the use of renewables in that state because of stability issues arising from too many grid-following converters. Given the pressure of realising the EU’s Green Deal 2050 emission targets, there thus is an urgent need to add new synchronous generator functionality to stabilise the grid.
In a RES-dominated grid, modern power converters will need to mimic the dynamic power response of synchronous generators if the grid is to remain stable. Advanced power converters known as grid-forming (GFM) converters will be required to supply active and reactive power while holding a constant voltage magnitude and frequency on few-milliseconds timescales to provide voltage and frequency stability. GFM converters will replace the stabilising dynamics of traditional synchronous generators and will be capable of autonomously injecting stabilising power into the grid, rather than simply following synchronous generators. They will be able to collectively form the voltage and frequency of the grid, rather than following it.
However, it is critical to consider the increased risk when grid reliability stands on the shoulders of “controls and software” instead of the rugged “copper and iron” assets in conventional generation resources which relies on the laws of physics [2024, Bahrani]. If the reliability of GFM converters cannot be completely trusted, the grid operational risk is unacceptably high for the system operators (SOs) who have an obligation to ensure that the security of supply is not compromised. This becomes particularly relevant as a new edition of the European Commission regulation “Requirement for Generators RfG” [2016, RfG] is planned for the end of 2025, which will mandate GFM requirements for every newly connected plant, including the need of demonstrating compliance and on-site performance monitoring.
Next to the need for testing of the grid-forming characteristics is the need to accurately determine grid converter losses under actual operating conditions, to ensure that the move from fossil fuels to RES does not come at the expense of increased grid losses coming from the converters. Loss evaluation under dynamic operation is a particular need as converters typically only have optimal efficiency at their nominal operating point.
In summary, there is now a pressing need to develop processes, standards, and setups for testing, validation, and certification of GFM converter performance and efficiency under the full range of real-world conditions to establish the necessary trust in GFM converters [2020, ENTSO-E], that in turn is crucial to reach the EU’s Green Deal 2050 net-zero emissions targets.
Testbeds for grid-forming converter testing (objective 1)
The first specific need to address in the issue of reduced grid stability is to develop testbeds that can assess the performance of the GFM assets indicated by their manufacturer against the functional specifications required (and paid for) by the SO. This includes necessary generation equipment to be used in a controlled laboratory environment to recreate realistic grid conditions, as well as the measurement setups necessary to accurately verify the converters response in terms of accuracy, sampling rate, and synchronisation.
Laboratory methodologies to test grid-forming converters (objective 2)
The second need concerns metrological procedures on how to perform GFM evaluation tests, translating the SOs’ functional specifications into repeatable, technology-agnostic test procedures. This includes the need for measurement algorithms for quick evaluation of grid parameters under fast grid dynamics in the sub–transient timescale such as harmonic distortion, grid impedance, or rate-of-change-of-frequency.
On-site testing of converters (objective 4)
As GFM converters must work as an ensemble without setting up destructive interactions, there is also a need for traceable on-site testing capabilities to evaluate and verify GFM performance under the full range of real-world conditions, where grid variations often happen simultaneously. Implicit in these requirements is the need for the metrological infrastructure to underpin the testing of grid-forming characteristics in a quantifiable manner. Any gap in requirements definition and lack of conformity assessment poses the risk of inferior equipment being installed in the field, compromising the system security and reliability.
Metrology for converter efficiency (objective 3)
Equally critical is the need to accurately determine the grid converter losses under actual operating conditions. This is required to ensure that the move from fossil fuels to RES does not come at the expense of increased grid losses coming from the converters. High converter efficiencies are crucial for realising a sustainable, RES dominated electricity grid. In the context of grid formation, loss evaluation under dynamic operation is particularly needed as converters typically only have optimal efficiency at their nominal operating point.