Short Circuit Level (SCL) is the standard measure of Grid Strength to indicate the electricity system's stability. However, Grid "strength" is decreasing in some regions of the GB system with a steady reduction of thermal power plants and increasing integration of Inverter-Based Resources (IBRs) in the drive to meet the UK's net-zero targets.
Consequently, various problems are starting to emerge such as: substandard voltage regulation, increased recovery times from voltage dips, potential instability of grid-following inverters, and protection faults. Short circuit level is no longer viewed as a good all-purpose indicator due to the different disturbance behaviours of inverter-based resources. Hence, this project will explore appropriate alternatives to short circuit level to measure Grid Strength in the future GB system, particularly with high penetration or dominance of IBRs.
Benefits
This project will help improve our understanding of the intricacies of grid strength, thus ensuring that grid strength levels can be tailored to operational circumstances and services as strictly needed are procured to maintain secure and reliable operation. Doing so will help reduce relevant operational and investment costs for GB's energy system transition, particularly when introducing decentralised and decarbonised IBR technologies/applications within and in connection to the GB electricity system.
This project can potentially reduce constraints on connections of decentralised and decarbonised IBR installations and boundary flows with refined definitions of limits to help the GB energy system transition towards Net-Zero.
This project will prepare the market for a trial deployment in a pathfinder project by raising awareness of services likely to be needed. If the trial is successful, then learning from this trial can be rolled out for business model evolution of the GB market, ensuring accuracy and transparency of services as procured to facilitate GB energy system transition.
Learnings
Outcomes
The project has achieved several outputs to date. WP1 and WP2 have been completed The learning from the project has been disseminated through:
1) ESIG 2023 Spring Technical Workshop, 29/03/2023 Tucson, AZ, US.
2) 2023 ESO External Engagement Webinar - System Operability Framework (SOF) Development, 27/04/2023 Faraday House, Warwick, UK.
3) Technical report for the NESO covering WP1 and detailing the Impedance Margin Ratio
4) IEEE Transactions on Power Systems publication: Impedance Margin Ratio: a New Metrics for Small-Signal System Strength, URL: https://ieeexplore.ieee.org/document/10452832
5) Knowledge exchange and continued development of the IMR metric via the SYSMet SIF project. SYSMet will incorporate the learning from the Strength-to-Connect project and develop the TRL. Details about the project can be found at: https://smarter.energynetworks.org/projects/sif_shet_024_sysmet-1/
6) Theoretical evaluation to quantify short circuit contribution from IBRs shared to NESO via on-going progress meetings
7) Technical report for NESO covering WP2 and detailing the Power Margin Ratio
Lessons Learnt
WP1 set out to find the best system strength metrics to assess each potential problem as replacements or refinements for short-circuit level.
During WP1, measures of system strength were fully reviewed and newly classified as a small-signal system strength and large-signal system strength based on the different characteristics. Such classification separates the problems under study, making explicit difference in strength evaluation towards different problems. This is recognised as a milestone of this project and influenced the way the project proceeded in the following WP2, i.e., studies on small-signal system strength and large-signal system strength will be carried out in parallel. A new metric to indicated strength in terms of avoidance oscillatory behaviours and small-signal instability is described as small-signal system strength metric and named Impedance Margin Ratio (IMR). Accordingly, a new metric to address large-signal system strength, which is the ability of a system to recover well from large disturbance such as a short-circuit fault at a given node, named as type-dependent short-circuit ratio (TDSCR) was proposed. This progress also led the direction for WP2, in which the development of a large signal metric was undertaken. This metric uses power flow to ascertain the strength of the system and has been called the Power Margin Ratio (PMR) . The key lesson and insight from this part of the work was the use of power flow to help to identify the potential strength of the system. The use of power flow is more straightforward as an approach to use and could be integrated far easier as a screening tool.
One of the three expected benefits of this project was stated as:
Avoidance of sudden disconnection of load or generation because of inadequate system strength is a direct benefit to customers and a core duty of the NESO. In the more complex world of an IBR dominated network, this needs to be based on a deep and nuanced understanding of at least four distinct aspects of system strength and the putting aside of the traditional one-size-fits-all measures. On the other hand, an over-cautious approach to system strength could put obstacles in the way of new connections of, for instance, wind farms.
The project has made a significant contribution by analysing system strength in ways specific to small and large disturbances. The new metrics for system strength offer a way of assessing the system voltage stiffness towards different dynamics, such as for small perturbations which can cause voltage oscillations, or large perturbations which can cause voltage dips. This work is enabling NESO to understand whether newly connected devices can increase the risk of system being unstable and discover the ‘weak point’ in the system.
There are also several points/lesson for future projects to take into account or that warrant further exploration, these are:
SCR variants are expected to be improvements on the standard SCR but it does not consider the interactions among adjacent IBRs during large disturbances. To include the interactions, the principles of ESCR could be adapted but the types of the interreacting IBRs will need to be considered, i.e., different combinations of voltage-type and current-type sources in close electrical proximity. Such an extension of SCR will be an item of further work within “Strength to Connect”.
SCR style metrics treats IBR as an ideal voltage source with an associated impedance but omits the internal control design of the IBR. The influence of PLL, droop controller and other control loops that are non-linear should be included to study the interactions among IBRs in large-signal conditions in a more accurate way.
The situation that the limited fault current IBR (low fault-current system strength) may lead to mal-operation of protection and failure to properly clear faults has not yet been discussed. This needs to be included in future work.
