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Reference Number NIA2_NGESO020
Title Strength to Connect
Status Started
Energy Categories OTHER POWER and STORAGE TECHNOLOGIES(Electricity transmission and distribution) 100%;
Research Types Applied Research and Development 100%
Science and Technology Fields ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 100%
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
National Grid ESO
Award Type Network Innovation Allowance
Funding Source Ofgem
Start Date 01 October 2022
End Date 31 March 2024
Duration ENA months
Total Grant Value £350,000
Industrial Sectors Power
Region London
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , National Grid ESO (100.000%)
  Industrial Collaborator Project Contact , National Grid plc (0.000%)
Web Site
Objectives Strength to Connect will examine what measures (small-signal impedance, synchronising power, over-load current) best indicate stable and secure operation for each known type of network disturbance. It will test the measures analytically and on simulation of an example network with varying levels of inverter-based resources.The opposite side is the strength services that IBR, Synchronous Compensators etc., can provide. This project will analyse how well each resource mitigates each stability problem, and a simulated example network will be used to verify the analysis.This project will consist of the following four Work Packages (WPs) as listed below:WP1: Grid Strength AssessmentAs mentioned previously, SCL is no longer a good all-purpose indicator of grid strength. Since IBRs have different large and small disturbance behaviours, and PLL stability and voltage recovery measure different aspects of grid strength. This first work package will deepen our understanding of how each potential problem is influenced by grid strength and determine which candidate metrics give a more precise indication of problem onset than SCL. The following research areas have been identified:A candidate indicator is a small-signal impedance for stability issues caused by PLL, as measured by dv/di.The equivalent synchronising torque and large-signal impedance are promising for the angle stability of grid-forming inverters and grid-following inverters.For voltage collapse and recovery issues, a V against P&Q nose plots and current-overload availability could be used.For issues caused by low fault-current and maloperation of protection, the actual short circuit current with more detailed information (e.g., magnitude and phase angle, positive- and negative-sequence components, etc.) can be used as metrics to assess the strength of this.Alongside general results from analytical methods, case study grids (using IEEE test systems modified to include IBR) will be analysed to compare and contrast the strength metrics for each problem. WP2: Capability investigationIn this work package, the project team will investigate the capabilities of IBR and other resources to add strength to the grid. Furthermore, an analysis of how well each resource mitigates each stability problem will be carried out. Hence, the second work package will:Assess the ability of various IBR control structures to enhance system strength, including types of voltage and reactive-power droop settings on both grid-forming and grid-following IBR.Assess the influence of various amplitudes and arrangement of IBR current-limit, including more sophisticated considerations of d- and q-axis currents, current angles, virtual impedances, etc., on system strength metrics.Assess the capabilities of other power electronic apparatus (e.g., STATCOM - Static Synchronous Compensator) to add strength.Map the capabilities of IBR and other resources to the four measures listed in WP1 to understand the mechanisms by which IBRs enhance grid strength and identify ways of mitigating each type of stability problem with the help of IBR.Analysis of how to balance the cost of maintaining a high grid strength against improving the ability of IBR to operate with low grid strengthThe case-study networks from WP1 will be utilised in this analysis.WP3: EMT simulationsElectro-Magnetic Transient (EMT) simulations will verify the analytical results in this work package. The EMT simulation is helpful for the transient and nonlinear analysis, which are the primary focuses of this project. The potential simulation software is MATLAB and Simulink. Besides, with the increased complexity of the system, a standard PC may not be capable of handling the simulation, so an OPAL-RT real-time simulator together with RT-LAB could be employed to carry out large-scale simulations. PSCAD, a widely used tool in industries, is also considered a candidate software. Three sets of simulations should be fulfilled, as below:The project team will build a case-study network in the simulator software. Candidates for this are the IEEE 39-bus transmission network, NETS-NYPS 68-bus network and the reduced GB 36-bus network. This ensures the simulations are consistent with a standard transmission network, offering credible results. Additionally, it guarantees the steady-state characteristics can be verified from power flow, and the dynamics can be verified from the eigenvalue plot.The standard model will then be modified to exhibit the different stability problems arising from low grid strength. Modifications will include line outages that increase impedance and substituting synchronous machines for the grid following IBR. The newly defined metrics (dv/di, large-signal impedance) will be assessed for their ability to predict the onset of instability and appropriate countermeasures. Four case studies showing four types of stability problems will be demonstrated. This series of simulations will be used to verify the analysis in WP1.Simulations will be carried out on the capabilities of IBR (including grid-forming capability) and STATCOM to add strength. This is to verify the capability analysis in WP2. The results are intended to show that by introducing a well-designed IBR, the four stability problems can be mitigated, and the strength can be improved.The accuracy of simulations is influenced by two factors mainly: a) software solver; b) physical models. The numerical calculation algorithm of software determines the former factor; the latter aspect is determined by how the physical and control features of power apparatuses, lines, etc., are modelled. Cross-verification between two or more software suites (e.g., PSCAD and MATLAB/Simulink) could be used to validate analytical results further if resources permit.WP4: Compatibility levelsChallenges arising from low grid strength can be addressed in three ways:By changing or expanding grid equipment, such as adding STATCOMS or changing protection relays, from the grid side;On the inverter side, by delivering a service such as a change to grid forming operation and;Immunising against low grid strength by re-tuning vulnerable control functions such as PLL. All three approaches have implications for local and system costs, and the objective is to find a system design that leads to minimum system cost. Thus, the following steps will be taken:Costs will be specified using a proxy for monetary costs, such as the apparent power rating of the additional equipment. For instance, modifying a grid following IBR to grid-forming to raise system strength attracts charges in extra control capability but also in physical terms, such as the need to expand the current rating of inverters. Also, in economic terms, if an operation below the optimal dispatch point is needed.Various compatibility levels will be tested for each metric. For example, a value of small-signal impedance could be declared as a maximum for the system. If so, what costs would the system bear adding equipment to keep the impedance below the compatibility level, and what costs would connecting parties pay in configuring a wind farm, for instance, to operate with grid impedances up to that compatibility level. Such considerations will be made to assess ways of declaring compatibility levels.Compatibility levels for grid strength are likely to be locational, so proposals will be made to express these, such as plotting heat maps showing the compatibility levels across the system. In line with the ENAs ENIP document, the risk rating is scored Low.TRL Steps = 2 (3 TRL steps)Cost = 1 (£350k)Suppliers = 1 (1 supplier)Data Assumptions = 2Total = 6 (Low) This project will develop:A Deeper understanding of the intricacies of grid strength: Avoiding sudden disconnection of load or generation because of inadequate system strength is a direct benefit to customers and a core duty of the NGESO. 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 a change from the traditional one-size-fits-all approach. On the other hand, an over-cautious approach to system strength could put obstacles in the way of new connections, e.g., wind farms.New measures and compatibility levels for system strength: The new measures will allow NGESO to carefully judge the type and volume of service provided and avoid over-or under-provision. Similarly, opening up new service definitions that enable IBRs to provide aspects of strength rather than only traditional generators or synchronous compensators creates downward pressure on costs. Further cost savings can be realised by adjusting compatibility levels so that connecting parties do so at lower system strength where possible and by raising transfer limits (rather than reinforcing) where system strength and voltage regulation were previously considered a limit. Further considerations to prepare a plan for the trial of new measurers: The market will need to be prepared to bring forward new service types and resources to achieve these benefits from the project. Stakeholder engagement will help gauge the industrys readiness to provide further services, and a trial plan for the pathfinder projects will be prepared to facilitate the introduction of new services. This project will implement a total of four WPs within a pre-defined timescale and budget plan to:Find the best measures to assess each potential problem listed in Section 2.1 and define metrics as replacements or refinements for short-circuit level.Investigate the capabilities of IBR and other resources to add strength and methods to improve their abilities to work in low grid strength conditions.Verify the analytical results with EMT simulations.Propose a method to declare compatibility levels for grid strength and tools for locational metrics, including plotting heat maps showing the compatibility levels of the whole system. The final outputs should include: Project Progress Reports for WPs 1-4 as listed in Section 2.2 (Total 4 Reports).Final Project Report as a documented guidance on the assessment of IBR capability to add strength and evaluation on their ability to work in low grid strength (Total 1 Report).A tool for locational metrics for compatibility level and heat maps to describe the compatibility of the whole system.2-3 Training sessions and documented training materials concluded from the guidance mentioned above to ensure NGESO and relevant network licensees can independently implement grid strength assessment for those problems as mentioned in 2.1-Problem based on methods/tools developed from this project. Knowledge dissemination event(s) for NGESO, other relevant Network Licensees and stakeholders during and after delivery of this project.Where relevant, the project will seek to publish in well-recognised international journals and at conference events.
Abstract Short Circuit Level (SCL) is the standard measure of Grid Strength to indicate the electricity systems 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 UKs 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. 
Publications (none)
Final Report (none)
Added to Database 14/10/22