The significance of power converters, their efficiency and GFM capabilities have been highlighted by the European network of transmission system operators (ENTSO-E) [2020, ENTSO-E] and in the last five years some pioneering system operators in Europe, USA and Australia have been producing guidelines outlining their essential system requirements [2021, GB ESO], [2022 UNIFI], [2022, German TSOs],[2023, Fingrid]. In the USA, the Department of Energy recently funded with $25 million the creation of a consortium to advance GFM technology in power grids, recognising its strategic relevance. Led by the National Renewable Energy Laboratory (NREL), the Universal Interoperability for Grid-Forming Inverters (UNIFI) consortium aims to facilitate the transition of electric power grids to accommodate grid-forming technology maintaining the operational stability, reliability, and resilience of the grid. This massive increase of activities by TSOs and DSOs underlines the failure of the present state of the art to adequately cover the performance evaluation of GFM converters.
The present state of the art fall short in several aspects. First, there is lack of a widely-agreed rigorous definition of GFM performance, which generates ambiguity at TSOs and DSOs on what their performances should be in practice. To surpass this dangerous ambiguity, it is urgent to develop practical and quantifiable specifications to which GFM units can be tested against [2022, Venkatramanan].
Secondly, once GFM unit performance requirements are defined, translating them into enforceable specifications is challenging due to the absence of guidance on test setups and procedures [2023, Shah]. At present, no clear testing procedures are available and GFM units thus cannot be seamlessly integrated in the grid. UNIFI has recently published roadmaps and guidance on the route to follow [2020, Lin], [2022, Venkatramanan], outlining the steps needed to reach commercial integration. In these SO-driven roadmaps, developing the capability for laboratory testing and evaluation of GFM units is highlighted as an important step [2024, Bahrani], [2023, Shah] as presently, traceable, standardised GFM testing facilities do not exist [2023, Wang].
This is where the main contribution of this project will be delivered to advance the state of the art: developing testbeds for testing, validation, and certification of GFM performance, together with repeatable procedures and technology-agnostic processes to establish the necessary trust in GFM converters. The time is now right to ensure that the metrological aspects underpinning GFM converter testing and conformity validation is not neglected in the rush to get the GFM technology to market: Europe needs access to state–of–the-art GFM testing facilities.
Evaluation of efficiency performances of the power converters is also extremely relevant: EN 61800-9-2 and EN 60146-2 indicate the general state of the art in loss evaluation of power converters, saying that present electrical efficiency measurement techniques are not reliable for efficiencies above 97 %, and that a maximum tolerance of 20 % in loss power measurements should be allowed. With modern power converter claiming efficiencies above 99 %, there is a need to improve the existing state of the art in evaluating conversion efficiency. To achieve an overall 3 % uncertainty in efficiency evaluation of typical 1 % converter loss power, the power measurements at input and output must have and uncertainty better than 0.02 % [2019, Bergman], which cannot be achieved by present grid instrumentation. In a grid-forming application, where converters need to be evaluated under actual dynamic grid conditions, metrology for the efficiency determination with an uncertainty of even 5 % of typical losses is a serious challenge due to the complexity of the required power measurement setups. At present, no NMI offers a metrological calibration service for this kind of application.
The EU Commission Regulation “Requirements for Generators” (RfG) [2016, RfG], clearly and quantitatively details all the requirements that newly connected generating units have to demonstrate. However, the resulting set of unambiguous specifications does not yet cover GFM units. Therefore, an important step to progress beyond the state of the art, instrumental to support EU regulation, is the development of a state-of-the-art measurement infrastructure for GFM converters. The technical challenge lies in the fact that GFM converters provide faster and more advanced performances than conventional generators, delivering active and reactive power in the span of few milliseconds in response to small changes in grid conditions. This requires accurate and fast measurements of a whole set of electrical quantities including challenging quantities such as internal voltage phasors, the grid ROCOF and the inertial response.
To develop the required GFM measurement framework, a joint effort is envisaged in this project, with university participants providing dedicated power electronics expertise and NMIs bringing metrology expertise and advanced instrumentation. This measurement framework consists of four parts, detailed in the following paragraphs.
Testbeds for grid-forming converter testing
Current state of the art
There is lack of a widely agreed rigorous definition of GFM performance, which generates ambiguity at TSOs and DSOs on what their performances should be in practice. To surpass this dangerous ambiguity, it is urgent to develop practical and quantifiable specifications to which GFM units can be tested against.
Progress beyond the state of the art
WP1 targets Objective 1 and goes beyond the state of the art by developing the metrological testbeds required for GFM performance evaluation tests. This includes covering several technological challenges to develop suitable generating and measurement laboratory equipment. Whereas at present testbeds with metrological accuracy are essentially lacking, the new testbeds will cover all parameters (frequency, voltage and power quality) that are mandatory for achieving reliable grid stabilisation in grids with more and more renewable power supplies. A good practice guide and peer reviewed papers will support TSOs and DSOs, the GFM converter industry and as well as science community in the improvement of grid stabilisation measures and initiate the development of new grid algorithms for a reliable and future energy supply.
Laboratory methodologies to test grid-forming converters
Current state of the art
Once GFM unit performance requirements are defined, translating them into enforceable specifications is challenging due to the absence of guidance on test setups and procedures [2023, Shah]. At present, no clear testing procedures are available and GFM units thus cannot be seamlessly integrated in the grid. UNIFI has recently published roadmaps and guidance on the route to follow [2020, Lin], [2022, Venkatramanan], outlining the steps needed to reach commercial integration. In these SO-driven roadmaps, developing the capability for laboratory testing and evaluation of GFM units is highlighted as an important step [2024, Bahrani], [2023, Shah] as presently, traceable, standardised GFM testing facilities do not exist [2023, Wang].
Progress beyond the state of the art
WP2 will target Objective 2 and will go beyond the state of the art in the following 3 aspects. Firstly, by developing metrology-sound test protocols and procedures, which at the momentpresently do not exist, turning system requirements into enforceable unit-level performance requirements, covering provision of voltage, frequency, and power quality support. This includes advanced functionalities such as black-start and islanded operations, with corresponding advanced measurement capabilities that will prepare for future testing needs. Secondly, by defining the measurement requirements, measurands and influencing factors including variations ranges, accuracy, bandwidth and noise immunity, that at the moment are undefined but at the same time essential to ensure repeatable and traceable testing. Finally, by developing advanced algorithms to measure in the sub-transient time scale, which is particularly critical under realistic grid conditions and noise, such as bad power quality or in case of combined voltage and frequency disturbances.
Metrology for converter efficiency
Current state of the art
Evaluation of efficiency performances of the power converters is also extremely relevant: EN 61800-9-2 and EN 60146-2 indicate the general state of the art in loss evaluation of power converters, saying that present electrical efficiency measurement techniques are not reliable for efficiencies above 97 %, and that a maximum tolerance of 20 % in loss power measurements should be allowed. With modern power converter claiming efficiencies above 99 %, there is a need to improve the existing state of the art in evaluating conversion efficiency. To achieve an overall 3 % uncertainty in efficiency evaluation of typical 1 % converter loss power, the power measurements at input and output must have better than 0.02 % uncertainty [2019, Bergman], which cannot be achieved by present grid instrumentation. In a grid-forming application, where converters need to be evaluated under actual dynamic grid conditions, metrology for the efficiency determination with an uncertainty of even 5 % of typical losses is a serious challenge due to the complexity of the required power measurement setups. At present, no NMI offers a metrological calibration service for this kind of application.
Progress beyond the state of the art
WP3 will target Objective 3 and will go beyond the state of the art by developing measurement facilities for traceable determination of the efficiency of GFM converters under dynamic conditions, i.e., for converters actively responding to grid voltage, frequency, or power quality variations. Whereas efficiency measurements have been performed in the 22NRM04 e-TRENY project under static operating conditions for passive transformers and grid-following converters, this project will make the next step by developing efficiency measurement techniques that can be applied to active GFM converters for varying frequencies, including the determining of efficiency during non-predefined timescales much shorter than presently available.
On-site testing of converters
Current state of the art
The EU Regulation “Requirements for Generators” (RfG) [2016, RfG], clearly and quantitatively details all the requirements that newly connected generating units have to demonstrate. However, the resulting set of unambiguous specifications does not yet cover GFM units. Therefore, an important step to progress beyond the state of the art, instrumental to support EU regulation, is the development of a state-of-the-art measurement infrastructure for GFM converters. The technical challenge lies in the fact that GFM converters provide faster and more advanced performances than conventional generators, delivering active and reactive power in the span of few milliseconds in response to small changes in grid conditions. This requires accurate and fast measurements of a whole set of electrical quantities including challenging quantities such as internal voltage phasors, the grid ROCOF and the inertial response.
Progress beyond the state of the art
WP4 will target Objective 4 and will go beyond the state of the art by developing traceable, measurement setups and suitable fast power algorithms for to enable traceable on-site testing of GFM converters at fundamental line frequency and harmonic frequencies up to at least 5 kHz. The project will use both conventional and novel normalised power measurements for on-site evaluation of grid GFM performance under actual grid conditions. A key innovation is the new ability to perform fast real-time current, voltage, power, and frequency measurements with at least 0.1 % uncertainty under highly dynamic grid conditions.
Summary
In summary, the present state of the art lacks capabilities for reliable testing and evaluation of GFM performance and efficiency, and this project addresses this gap by developing the metrological infrastructure required for performing these tests. New testbeds will be developed together with test protocols and traceable measurements that will provide a comprehensive testing capability for Europe’s manufacturers and grid operators. An important further advancement is the new capability for field evaluation of GFM performance, that allows testing the performance of installed converters in response to real grid events.
This project will provide an important competitive advantage to European GFM manufacturers with by providing access to state-of the-art facilities to underpin EU regulation such as RfG. To ensure effective realisation of the objectives, the project plans to interact with TSOs and DSOs, GFM converter manufacturers, as well as the US UNIFI consortium, to maximise the benefits of collaborative research and leveraging the advancements already made in the USA.