CM097 Code Administrator Consultation

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Public Code Administrator Consultation CM097: Electromagnetic Transient (EMT) and Root Mean Square (RMS) Model Submission for Transmission Owners (TOs) Overview: As Great Britain’s (GB) power system moves towards a net zero carbon operation; the number of Inverter-Based Resources (IBR) is expected to increase, with the amount of synchronous generation on the GB power system to decline, which will significantly change the characteristics of the GB network. These changes give rise to the need for more accurate dynamic modelling and the need for analysing the effect of potential control interactions between the devices across the network leading to risks of oscillations and inverter stability.

Modification process & timetable

Have 5 minutes? Read our Executive summary Have 60 minutes? Read the full Code Administrator Consultation Have 120 minutes? Read the full Code Administrator Consultation and Annexes. Status summary: The Workgroup have finalised the Proposer’s solution. This modification is expected to have a: High impact for the Transmission System Operator and Transmission Owners (onshore & offshore). Governance route Standard Governance modification with assessment by a Workgroup Who can I talk to about the change?

Proposer: Frank Kasibante frank.kasibante1@neso.energy 07812774066 Code Administrator Chair: Claire Goult claire.goult@neso.energy How do I respond? Send your response proforma to stcteam@neso.energy by 5pm on 27 May 2026.

Workgroup Consultation 25 April 2025 Workgroup Report 21 April 2026 Code Administrator Consultation 05 May 2026 – 27 May 2026 Draft Final Modification Report 16 June 2026 Final Modification Report 16 July 2026 Implementation In line with GC0168 1 2 4 3 5 6 7 Proposal Form 29 May 2024

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Public Contents What is the issue? ....................................................................................................................................................................... 4 What is the defect the Proposer believes this modification will address? ....................................... 4 Why change? ................................................................................................................................................................................. 4 What is the solution? ................................................................................................................................................................ 5 Proposer’s Original solution ................................................................................................................................................. 5 Workgroup considerations .................................................................................................................................................. 5 Workgroup Discussion ahead of the Workgroup Consultation ................................................................ 5 Workgroup Consultation Summary .............................................................................................................................. 7 Workgroup Discussion post Workgroup Consultation ..................................................................................... 9 Terms of Reference Overview ........................................................................................................................................... 13 What is the impact of this change? ............................................................................................................................ 14 Original Proposer’s assessment against Code Objectives ........................................................................ 15 Workgroup Vote .......................................................................................................................................................................... 17 When will this change take place? .............................................................................................................................. 18 Interactions ................................................................................................................................................................................... 18 How to respond .......................................................................................................................................................................... 19 Acronyms, key terms and reference material ..................................................................................................... 19 Annexes ........................................................................................................................................................................................... 20

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Public Executive Summary This modification seeks to require Transmission Owners (TOs) to provide National Energy System Operator (NESO) with Root Mean Square (RMS) and Electromagnetic Transient (EMT) models to enable analysis of issues such as system oscillations, inverter instability and Transient over Voltage (ToV) on the National Electricity Transmission System (NETS). What is the issue? As Great Britain’s power system moves towards net zero carbon operation, the network is transitioning from large synchronous Generators to a large number of smaller Inverter-Based Resources (IBR) which are causing new and varying challenges to the power system, for example control interactions, low fault level, inverter instability and ToV. NESO requires RMS and EMT models from TOs so that it can analyse and understand how these interactions affect the network under different system conditions. There are currently no requirements in the System Owner Transmission Owner Code (STC) for TOs to submit EMT and RMS models of their assets to NESO and for NESO to share these models with relevant Users, as well as enabling NESO to share Users’ EMT and RMS models to TOs. This restricts the ability of NESO to perform system studies, modelling and post fault analysis.

What is the solution and when will it come into effect? Proposer’s solution: The proposed solution is to: 1. Mandate the collection of EMT and RMS models from TOs. Enable the sharing of these TO models with relevant Users and allow NESO to share Users' EMT and RMS models with TOs for conducting studies. These models will contribute to a comprehensive Great Britain (GB) Model, facilitating investigations, post-fault analyses, and planning studies. 2. Create a new STCP (12-2) to specify the model exchange process between TOs and NESO (STCP Modification PM0147). Implementation date: In line with Modification GC0168: Submission of Electro Magnetic Transient (EMT) Models

What is the impact if this change is made? There is a high impact for the Transmission System Operator and Transmission Owners (onshore & offshore), through the development of an efficient, economical, and coordinated electricity transmission system; the facilitation of new connections; the

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Public efficient discharge of licence obligations; and improved security and reliability of the GB electricity supply.

Workgroup conclusions: The Workgroup concluded unanimously that the Original Solution better facilitated the Applicable Objectives than the Baseline.

Interactions GC0168: Submission of Electro Magnetic Transient (EMT) Models

CMP456: Cost recovery for legacy plant in relation to GC0168 CMP466: CMP456 Consequential Charging Modification

A separate Modification will be established to introduce a new STCP (STCP Modification PM0147). What is the issue? What is the defect the Proposer believes this modification will address? As Great Britain’s power system moves towards net zero carbon operation, the network is transitioning from large synchronous Generators to a large number of smaller IBRs which are causing new and varying challenges to the power system, for example control interactions, low fault level, inverter instability and ToV. To address these challenges, NESO requires RMS and EMT models from TOs. These models can help NESO to analyse and understand the impact on the network under various system conditions. There are currently no requirements in the STC for TOs to submit EMT and RMS models of their assets to NESO and for NESO to share these models with relevant Users as well as enabling NESO to share Users’ EMT and RMS models to TOs. This restricts the ability of NESO to perform system studies, modelling and post fault analysis. Why change? For an evolving system with a high penetration of IBR and thus due to a high penetration of asynchronous generation, EMT and RMS models are required to perform more

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Public detailed analysis. This will provide more certainty in the studies and analyses outcomes which will benefit NESO in meeting its legal obligations. What is the solution? Proposer’s Original solution The proposed solution is to: 1. Mandate the collection of EMT and RMS models from TOs. Enable the sharing of these TO models with relevant Users and allow NESO to share Users' EMT and RMS models with TOs for conducting studies. These models will contribute to a comprehensive Great Britain (GB) Model, facilitating investigations, post-fault analyses, and planning studies. 2. Create a new STCP (12-2) to specify the model exchange process between TOs and NESO (STCP Modification PM0147). Workgroup considerations The Workgroup convened 8 times to discuss the identified issue within the scope of the defect, develop potential solutions, and evaluate the proposal in relation to the Applicable Code Objectives. The Workgroup held their Workgroup Consultation between 28 March 2025 to 25 April 2025 and received 3 non-confidential responses and 1 confidential response. The full non-confidential responses and a summary of the responses can be found in Annex 04. Workgroup Discussion ahead of the Workgroup Consultation During the first Workgroup, the Proposer advised Workgroup members Modification CM097 was closely aligned with GC0168 and was looking to achieve the same goal but in a different code. Legal Text Discussions The Legal Text for CM097 was reviewed, and suggested amendments were agreed by the Workgroup members. During discussions, Workgroup members expressed concerns regarding the definitions of EMT Models. Specifically, issues were raised about the accuracy and comprehensiveness of the current definition, emphasising the need for precise wording to avoid ambiguity. One Workgroup member remarked that the existing definition might

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Public be too detailed and may not accurately represent all EMT Models. The Proposer noted that RMS and EMT have not yet been defined in the Grid Code and confirmed that they would align the definitions to those found in STCP Modification PM0147. Cost Recovery Mechanism Workgroup members discussed cost recovery mechanisms for both TOs and Offshore Transmission Owners (OFTOs). It was identified that NESO needed to consult with their legal team and the Authority would review existing arrangements and address potential issues, particularly for OFTOs who lack a formal price control mechanism. A Workgroup member advised that incumbent Transmission Owners (NGET, SPT, and SHETL) do not have a specific mechanism in place for cost recovery associated with the provision of RMS and EMT models to NESO. A Workgroup member highlighted that the current income adjustment clauses within the Offshore transmission licences only allow cost recovery between £500k and £4 million, which is deemed insufficient for the anticipated expenses. It was suggested that this mechanism does not adequately support TOs and proposed introducing a contingent event clause within the license as a potential solution for cost recovery. This clause would enable TOs to recover costs in specific contingent events, providing a more satisfactory mechanism. During Workgroup 5, the Proposer provided an update to the Workgroup on their discussions with Ofgem and referred the Workgroup to an Ofgem letter that had been shared in the Workgroup Papers. Ofgem confirmed that they’re consulting internally to assess if onshore TOs could have cost recovery arrangements covered and would provide an update in due course. The Workgroup discussed offshore and onshore cost recovery and compensation arrangements. One Workgroup member raised concerns about a reference in the Ofgem letter to a threshold of £1m per annum for OFTO cost recovery, suggesting that it could lead to insolvency if mismanaged. The Proposer and other Workgroup members will meet with Ofgem to discuss cost recovery issues and provide an update at a later Workgroup meeting. The action will remain open pending results of these discussions. Draft STCP Workgroup members reviewed and made comments on a draft STCP 12-2 that was shared by the Proposer, it was agreed that amendments were to be made before the STCP proposal was submitted to STC Panel.

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Public Workgroup members deliberated on the necessity for User manuals, Dynamic Performance Studies (DPS) reports, and validation reports. The Proposer agreed to address these concerns separately and to conduct a detailed review of the model validation and documentation requirements. Workgroup members agreed that the modelling team from NESO would initiate discussions to provide clearer guidance in the updated document. The STCP would follow the normal governance route. A modification PM0147 was raised subsequently. Guidance Notes/Electrical Standards Further discussions were held regarding the distinction between guidance notes and electrical standards. Workgroup members emphasised the need for clarity in the proposed STCP 12-2 documentation, as lack of distinction of references could lead to confusion. The Proposer agreed to collaborate with NESO colleagues to make the necessary updates to the draft STCP 12-2 to ensure clarity and accuracy. Workgroup Consultation Summary The Workgroup held their Workgroup Consultation between 28 March 2025 to 25 April 2025 and received 3 non-confidential responses and 1 confidential response. The full responses and a summary of the responses can be found in Annexes 04 and Annex 05. Objectives which the Proposer’s solution better facilitates the Applicable STC Objectives than the baseline: One respondent chose objective (a), one respondent chose objective (b), two respondents chose objective (c), three respondents chose objective (e), two respondents chose objective (g), and one respondent chose objective (h). Support for implementation approach: Two respondents were supportive of the chosen implementation approach, with one respondent noting that detailed requirements will be included in a separate STCP (Modification PM0147), making a full assessment difficult. They also highlight significant costs and timescales for obtaining EMT models for existing plants and support a cost recovery mechanism. One respondent was not supportive of the chosen implementation approach, citing the following reasons: • Data Availability: Most OFTO sites lack necessary data, especially older assets. • Time Commitment: Significant time needed, can't be done immediately. • Resource Diversion: Diverts resources from other important work. • Necessity: Requirement should be justified based on need and cost. • Existing Data: NESO may already have much of this data. • Cost Recovery: Costs should be recoverable, but high threshold may not be met.

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Public • Technical Challenges: OFTOs need resources they were never funded for.

Draft Legal Text: All respondents were satisfied that the draft Legal Text satisfied the intent of the modification, with one respondent noting that the majority of the detail will be contained in STCP Modification PM0147. Cost-recovery mechanism: All respondents confirmed that they were supportive of a cost-recovery mechanism. The first respondent stated that they supported a cost recovery mechanism for TOs in respect of provision of EMT models for already connected equipment. For the established onshore TOs (NGET, SPT, and SHETL) the price control arrangements in their licences could be used for this purpose. However, for OFTOs it is unclear how the cost recovery mechanism would work, and they do not believe it would be necessary for a cost recovery for new plant going forward. As Competitively Appointed Transmission Owners (CATOs) are a new concept, they do not believe it would be necessary for a cost recovery mechanism for a new plant going forward. The second respondent highlighted the need for a cost recovery mechanism to ensure that TOs can recover legitimate additional costs incurred in obtaining EMT models for existing plant. They stated that for all new plant, their organisation already requires the delivery of RMS and EMT models as part of the specification. However, the costs associated with obtaining EMT models for existing plant are very high. Based on work that is ongoing, the total cost to their organisation is estimated to be in the order of £500k, not including the engineering time to test such models before submitting them to NESO. For TOs with a higher number of Flexible AC Transmission Systems (FACTS) devices and other active plant, this cost will be substantially higher. The absence of a cost recovery mechanism would, as a minimum, slow down the process of acquiring validated EMT models for GB plant. The third respondent was supportive of a cost-recovery mechanism but noted that the cumulative impact of many small changes is gradually affecting the belief that the OFTO regime is low-risk. They stated that implementing this modification without funding will result in a longer and less technically competent process, suggesting a cost-benefit analysis for providing the data, as NESO's assumption that this comes at no cost will not lead to the best decision for the whole industry. They highlighted that the decision should consider the impact on the entire industry, rather than focusing solely on individual TOs.

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Public Signposting Electrical Standard detailing how an EMT model would be submitted in Power System Computer Aided Design (PSCAD) Version 5: All respondents agreed that with appropriate signposting to the Grid Code from the STC, the proposal to prepare an Electrical Standard detailing how an EMT model would be submitted in PSCAD Version 5, would be an appropriate and cost-effective method of providing this guidance to transmission licences. The first respondent confirmed that they agreed with the proposed approach, for PSCAD Version 5 or higher. The second respondent favoured an approach based on open standards and interoperability between simulation platforms, rather than binding the industry to a single vendor and simulation platform. The noted existing standards such as Functional Mock-up Interface (FMI) and the approach developed by IEEE and CIGRE. The third respondent confirmed that this proposal is in line with several similar situations from STC to Grid Code. They noted that governance of the Grid Code is independent to the STC and there have been issues in the past with the Grid Code Review Panel making changes to Grid Code documents which impact on the STC, without input from the STC community. Alternative Requests: One respondent indicated that they wished to raise an Alternative Request. The responses received in the Workgroup Consultation, the related issues raised in Workgroup discussions, and the subsequent decisions made by Workgroup members are all addressed in the ‘Workgroup Discussion post Workgroup Consultation’ section below. Workgroup Discussion post Workgroup Consultation Other options/Alternatives The Proposer of Alternative Request following the Workgroup Consultation talked the Workgroup through their proposal. They noted that their proposal recognises the significant cost and engineering resource needed to deliver the Original Proposal and suggested a more pragmatic approach, where the highest priority items are delivered first. The Alternative Proposer noted that instead of an absolute obligation to submit models, their Alternative suggests that there should be a prioritisation based on the necessity

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Public and cost-effectiveness of each submission. This emphasises that not all networks require the same level of modelling, especially older and smaller networks that may not have the necessary equipment or models available. They also raised concerns about the potential delays in fulfilling the obligations of the Original Proposal, which could lead to breaches if not managed properly. The Workgroup discussed the importance of obtaining necessary data without incurring excessive costs and recognised the challenges posed by the offshore transmission process, which often leads to a lack of direct relationships with original developers of the equipment. The Alternative Proposer and Workgroup members provided insights into the model collection process, suggesting the establishment of a clear priority list for model submissions based on current system challenges. They discussed a structured approach for users to submit existing models within three months, while new models would have a nine-month timeline for development. The Alternative Proposer noted that regions facing significant issues, such as Sub- Synchronous Oscillations (SSO) challenges, could be prioritised with a clear rationale for prioritisation. This structured approach would seek to ensure that the most critical models are addressed first, promoting efficiency and effectiveness in the modelling process. The Workgroup discussed a draft Guidance Note on EMT, shared by the NESO SME. The Workgroup discussed governance issues relating to the incorporation of this guidance into existing frameworks such as the Grid Code and STCP. The Proposer and NESO SME took an action to consider how to incorporate the Guidance Note into STCP 12-2 and if/how this impacts CM097. The Alternative Proposer committed to consider the points raised during the Workgroup discussion and submitted a refined Alternative proposal (Annex 05). The updated Alternative proposal highlights concern regarding the limited availability of Electromagnetic Transient (EMT) data for most Offshore Transmission Owner (OFTO) sites, especially older ones, as manufacturers may not possess the data or may no longer exist. Acquiring the necessary information could require costly commercial agreements and significant internal resources, which cannot be completed swiftly or by a fixed deadline. It is suggested that obligations to provide such data should be based on actual need and cost, rather than applied universally. Furthermore, NESO might already hold relevant data via the OTSDUW process, and obtaining OEM controller models often involves NDAs and, sometimes, less detailed models due to confidentiality.

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Public Further to Workgroup’s advice, NESO proposed to include an appendix in the STCP 12-2, RMS and EMT model requirements and the approach & process for retrospective submission. NESO SMEs shared the updated STCP 12-2 and Appendix B with the Proposers of the Alternative. The Alternative Proposers indicated that the proposed modification, together with the supporting STCP and Appendix B, satisfactorily addresses the concerns identified in the Alternative Proposal, with the exception of cost recovery. A similar approach has been applied for Users in CMP456/CMP466 and can be considered for STC parties. The Alternative Proposer noted extensive reference to controller models, active power control, IBRs, synchronous generators, and users. These references seem irrelevant to the guidance note, as TO models should only address their own assets, not generators. NESO SMEs concurred with this recommendation and resolved to revise STCP 12-2 and Appendix B to explicitly clarify the intended references. The second concern raised by the Alternative Proposer was that Transmission Operators (TOs) are required to have a model maintenance and support framework with vendors or suppliers throughout the asset’s lifetime. The Alternative Proposer argued this is not something that can be mandated in the STCP, as it is a business decision about meeting obligations and suggested further rewording for clarification. NESO SMEs agreed to revise the STCP 12-2 and Appendix to address this concern. After conducting offline consultations with SMEs, the Proposer of the Alternative confirmed that if the revisions made to STCP 12-2 and its Appendix were deemed satisfactory, then the Alternative proposal would become unnecessary. The Alternative Proposer requested that it be noted that concerns regarding cost recovery mechanisms for both model development and ongoing support frameworks remain unresolved. The Alternative Proposer stated that while they supported the principle of providing models, the mechanism for recovering these costs was not addressed within CM097 and should be highlighted transparently rather than resolved within this modification. STCP 12-2 and Appendix B Review Appendix B A Workgroup member noted that there were inconsistencies in Appendix B, as it contained references to both 20 seconds and 50 seconds for the required simulation duration. The NESO SME confirmed that the document would be updated so that the time requirement is consistently stated as 50 seconds, aligning with the relevant Grid Code provision.

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Public The Workgroup member also raised concerns regarding clarity on percentage deviation requirements, noting ambiguity as to whether this related to rated megawatts or initial reference values. The NESO SME confirmed that the wording would be clarified to ensure the requirement is correctly described. The Workgroup member also noted inconsistent use of time units in the document, with the NESO SME agreeing to amend the document to ensure consistency. A question was raised about whether mentions of Grid Code requirements might be interpreted as extending beyond TOs. The NESO SME clarified that the plan is to revise the wording so it's clear these requirements apply only to TOs. Clarification was requested regarding 'User specific RMS models,' especially with respect to library, modified library, and DLL-based models. The NESO SME confirmed that DLL based RMS models are not acceptable and that the drafting would be updated to clarify that DLL‑based RMS models are excluded and confirm what is meant by ‘User specific RMS models’. The Workgroup agreed that further clarification in the document is required. A Workgroup member asked whether RMS model submission requirements had previously been captured through earlier Grid Code modifications. The Proposer stated that this would need to be checked, including whether any consequential STC modifications arose from GC0141. The Chair noted this will need to be confirmed. The Proposer later explained that Appendix B would be incorporated into STCP 12-2 to create a single consolidated document. They confirmed that the STCP content was aligned with Grid Code material, noting that Grid Code guidance currently covered EMT only, whereas the STCP addressed both EMT and RMS. STCP 12-2 Workgroup members raised concerns regarding the wording used on sharing TO models with Users, particularly around intellectual property and liability. The NESO SME clarified that any sharing would be subject to Non-Disclosure Agreements (NDAs) between OEMs, third parties, and NESO, and would be limited to defined purposes such as connection studies. The Proposer noted that Schedule 3 of the STC already contains confidentiality provisions that should be considered in relation to this point. The Workgroup agreed that the drafting should provide greater clarity on safeguards and on the reciprocal nature of model sharing. Later discussions highlighted ongoing concerns regarding IP constraints, NDAs, and the burden of providing additional models solely for sharing purposes. The Proposer stated that the issue would need further consideration, including whether additional

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Public mechanisms might be required to address IP limitations without creating disproportionate obligations. The document demonstrates inconsistent usage of the terms ‘Users’ and ‘Network Assets,’ which may lead to ambiguity. A Workgroup member clarified that ‘Users’ has a specific meaning in the STC that is distinct from TOs. The Workgroup agreed that this terminology should be applied consistently and accurately. The NESO SME will review and correct consistency of terminology throughout the document. Further discussions around TOs highlighted that the drafting seemed to imply TOs would be required to indicate acceptance or rejection of user models submitted by NESO. The Proposer clarified that this was not the intent and that TOs were not expected to undertake compliance assessments. The Workgroup agreed that the drafting should better reflect reciprocal responsibilities between sending and receiving parties. Subsequent discussions on the clarity of Legal Text drafting saw concerns raised that it did not clearly describe expectations for DPS‑style reports, validation reports, and supporting documentation. The Proposer noted that the relevant expectations were already set out in Appendix B but proposed amending the drafting of STCP 12-2 to directly reference Appendix B. A Workgroup member raised concerns about model submission timelines, particularly for projects already contracted or delivered significantly after the modification. The Proposer confirmed that the drafting was not intended to apply retrospectively and that the wording ‘unless otherwise agreed’ was intended to allow for pragmatic, case‑by‑case agreement. The Proposer of the Alternative Request confirmed that their previous concerns had been addressed in the updated STCP 12-2 draft Legal Text and they were content to formally withdraw their Alternative Request. The revised STCP 12-2 and Appendix B (Annex 07) were shared with Workgroup members prior to Workgroup Meeting 8.

Terms of Reference Overview a) Implementation; Implementation has been detailed in STCP 12-2. Workgroup members have been central to its drafting (Annex 07).

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Public b) Review and support the Legal Text drafting; Legal Text has been robustly discussed during Workgroup meetings, all attended with quoracy (Annex 03). c) Ensure the appropriate Industry experts or stakeholders are engaged in the Workgroup to ensure that all potentially affected stakeholders have the opportunity to be represented in the Workgroup; All affected stakeholders (apart from CATOs, who are not yet operational at the time of CM097) have been represented and their expert representatives have contributed actively towards discussions regarding impacts (Annex 08). d) The cross Code impacts this Modification has, in particular the CUSC; This modification has no direct impact to the CUSC. However, the leading Grid Code modification GC0168 has also driven Industry stakeholders to raise CUSC Modifications CMP456/CMP466 to address cost recovery (Pages 4, 6, 12, 15, and 21). e) Consider STCP 12-2 alongside CM097; CM097 Workgroup members have been central in drafting STCP 12-2 (Annex 07 and Pages 6-9, 12-15). f) Consider how to produce/gather models for existing assets; The STCP 12-2 has considered this (Annex 07). g) Assess the materiality of costs/resources needed for STC Parties to comply with additional requirements brought forward by the Proposer's solution; Materiality has been discussed extensively by Workgroup. Separate sessions have been held with the Regulator to seek guidance which has been provided (Pages 7-10). h) Compliance with the Electricity Regulation and any Relevant Legally Binding Decisions of the European Commission and/or the Agency. Workgroup members agreed that there were no assessed compliance issues (Page 19).

What is the impact of this change? There is a high impact for the Transmission System Operator and Transmission Owners (onshore & offshore), through the development of an efficient, economical, and

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Public coordinated electricity transmission system; the facilitation of new connections; the efficient discharge of licence obligations; and improved security and reliability of the GB electricity supply. Original Proposer’s assessment against Code Objectives Original Proposer’s assessment against STC Objectives Relevant Applicable Objective Identified impact (a) efficient discharge of the obligations imposed upon Transmission Licensees by Transmission Licences and the Electricity Act 1989; Positive NESO and TOs will have the ability to meet their licence obligations relating to operating the system securely. (b) efficient discharge of the obligations imposed upon the licensee by the Electricity System Operator licence, the Energy Act 2023 and Electricity Act 1989; Neutral (c) development, maintenance, and operation of an efficient, economical, and coordinated system of electricity transmission; Positive EMT and RMS models for TO assets, for assets with Power Electronic Devices, will facilitate system analysis and enable to operate the evolving and future system with anticipated high penetration of IBR resources. This will enable achievement of an efficient, economical and coordinated electricity transmission system. (d) facilitating effective competition in the generation and supply of electricity, and (so far as consistent therewith) facilitating such competition in the distribution of electricity; Neutral

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Public (e) protection of the security and quality of supply and safe operation of the National Electricity Transmission System insofar as it relates to interactions between Transmission Licensees and the licensee*; Positive EMT and RMS models for TO assets, for assets with Power Electronic Devices, will facilitate system analysis and enable to operate the evolving and future electricity system. (f) promotion of good industry practice and efficiency in the implementation and administration of the arrangements described in the STC; Positive The availability of EMT and RMS models from TOs will help NESO to analyse the impact of potential new connections to the system and undertake post-system incident analysis. This will identify any modifications and / or control measures required to operate the system. (g) facilitation of access to the National Electricity Transmission System for generation not yet connected to the National Electricity Transmission System or Distribution System; and Neutral (h) compliance with the Electricity Regulation and any Relevant Legally Binding Decisions of the European Commission and/or the Agency. Neutral * See Electricity System Operator Licence

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Public Stakeholder / consumer benefit categories Proposer’s view Improved safety and reliability of the system Positive NESO will be able to carry out pre-fault and post-fault analysis studies when provided with EMT and RMS models by TOs. The outputs will inform accurate operational decisions in the interest of safety and reliability of the system. Lower bills than would otherwise be the case Neutral

Benefits for society as a whole Neutral

Reduced environmental damage Neutral

Workgroup Vote The Workgroup met on 08 April 2026 to carry out their Workgroup Vote. The full Workgroup Vote can be found in Annex 06. The table below provides a summary of the Workgroup Members view on the best option to implement this change. For reference, the Applicable STC Objectives are: a) Efficient discharge of the obligations imposed upon Transmission Licensees by Transmission Licences and the Electricity Act 1989; b) Efficient discharge of the obligations imposed upon the licensee by the Electricity System Operator licence, the Energy Act 2023 and Electricity Act 1989; c) Development, maintenance, and operation of an efficient, economical, and coordinated system of electricity transmission; d) Facilitating effective competition in the generation and supply of electricity, and (so far as consistent therewith) facilitating such competition in the distribution of electricity; e) Protection of the security and quality of supply and safe operation of the National Electricity Transmission System insofar as it relates to interactions between Transmission Licensees and the licensee*; f) Promotion of good industry practice and efficiency in the implementation and administration of the arrangements described in the STC;

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Public g) Facilitation of access to the National Electricity Transmission System for generation not yet connected to the National Electricity Transmission System or Distribution System; and h) Compliance with the Electricity Regulation and any Relevant Legally Binding Decisions of the European Commission and/or the Agency. * See Electricity System Operator Licence The Workgroup concluded unanimously (out of 5 votes) that the Original better facilitated the Applicable Objectives than the Baseline. Option Number of voters that voted this option as better than the Baseline Original 5 When will this change take place? Implementation date In line with GC0168. Date decision required by TBC Implementation approach No systems will have to change because of this modification. Interactions ☒Grid Code ☐BSC ☐CUSC ☐SQSS ☐European Network Codes ☒Other modifications ☐Other GC0168 Submission of Electro Magnetic Transient (EMT) Models CMP456: Cost recovery for legacy plant in relation to GC0168 CMP466: CMP456 Consequential Charging Modification A separate modification will be established to introduce a new STCP (STCP Modification PM0147).

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Public How to respond Code Administrator Consultation questions • Please provide your assessment for the proposed solution against the Applicable Objectives versus the current baseline. • Do you support the proposed implementation approach? • Do you have any other comments? Views are invited on the proposals outlined in this consultation, which should be received by 5pm on 27 May 2026. Please send your response to stcteam@neso.energy using the response pro-forma which can be found on the modification page. If you wish to submit a confidential response, mark the relevant box on your consultation proforma. Confidential responses will be disclosed to the Authority in full but, unless agreed otherwise, will not be shared with the Panel or the industry and may therefore not influence the debate to the same extent as a non-confidential response. Acronyms, key terms and reference material Acronym / key term Meaning CATO Competitively Appointed Transmission Owner DPS Dynamic Performance Studies Report EMT Electromagnetic Transient FACTS Flexible AC Transmission Systems FMI Functional Mock-up Interface GB Great Britain GC Grid Code IBR Inverter Based Resources NESO National Energy System Operator OFTO Offshore Transmission Owner PSCAD Power System Computer Aided Design

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Public RMS Root Mean Square SQSS Security and Quality of Supply Standards SSO Sub-Synchronous Oscillations STC System Operator Transmission Owner Code STCP System Operator Transmission Owner Code Procedure TO Transmission Owner ToR Terms of Reference ToV Transient over Voltage T&Cs Terms and Conditions

Annexes Annex Information Annex 01 CM097 Proposal Form Annex 02 CM097 Terms of Reference Annex 03 CM097 Final Legal Text Annex 04 CM097 Workgroup Consultation Responses and Summary Annex 05 CM097 Alternative Request Annex 06 CM097 Workgroup Vote Annex 07 CM097 Draft STCP 12-2 Annex 08 CM097 Workgroup Attendance Record Annex 09 CM097 Workgroup Action Log

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Code Administrator Consultation CM0107: Update for Centralised Strategic Network Plan Overview: Due to the upcoming introduction of the Centralised Strategic Network Plan (CSNP), several sections of the System Operator Transmission Owner Code (STC)/ System Technical Code Procedure (STCP) need to be changed as the references are becoming outdated, including the Electricity Ten Year Statement (ETYS), the Seven Year Statement (SYS), the Network Options Assessment (NOA) and the Offshore Development Information Statement (ODIS). Modification process & timetable

Have 5 minutes? Read our Executive Summary Have 30 minutes? Read the full Code Administrator Consultation Have 45 minutes? Read the full Code Administrator Consultation and Annexes. Status summary: We are now consulting on this proposed change. This modification is expected to have a: Low Impact Any STC parties currently impacted by the ETYS, SYS, NOA or ODIS.

Governance route

Self-Governance modification to proceed to Code Administrator Consultation Who can I talk to about the change?

Proposer: Thomas Goss Thomas.goss2@neso.energy 07522 998 916 Code Administrator Chair: stcteam@neso.energy How do I respond? Send your response proforma to stcteam@neso.energy by 5pm on 27 May 2026 Code Administrator Consultation 06 May 2026 - 27 May 2026 Draft Final Modification Report 16 June 2026 Final Modification Report 13 July 2026 Implementation 31 August 2026 1 4 5 6 7 Proposal Form 10 April 2026

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Public Contents What is the issue? ............................................................................................................... 3 Why change? ...................................................................................................................... 3 What is the solution? .......................................................................................................... 4 Legal Text .............................................................................................................................. 4 What is the impact of this change?................................................................................. 5 When will this change take place? .................................................................................. 8 Implementation approach ............................................................................................... 8 Interactions ......................................................................................................................... 9 How to respond................................................................................................................... 9 Acronyms, key terms and reference material ............................................................. 10 Annexes.............................................................................................................................. 11

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Public What is the issue? The introduction of the Centralised Strategic Network Plan (CSNP) requires that any older references are updated to previous Electricity Planning Data methodologies such as the Electricity Ten Year Statement (ETYS), which provides NESO’s view of the future of transmission requirements. The final iteration of the ETYS is currently scheduled for release in summer 2026. NESO’s licence has been changed to include the Centralised Strategic Network Plan (CSNP). The Grid Code and System Operator – Transmission Owner Code (STC) will need to have any references to previous iterations of planning documents removed, including the ETYS and remaining references to the previous Seven Year Statement (SYS), Network Options Assessment (NOA) or Offshore Development Information Statement (ODIS). As functions of the ETYS and the data it produces are still valid, where the CSNP is directly taking over the function this needs to be included, and the wording of the legal text amended to ensure continuity. Why change? As above, the CSNP is taking over the functions previously provided by the ETYS, which is being phased out in 2026 as per the mandate from Ofgem, including the data which is input to NESO and the reports being produced. Therefore, there is a need to ensure that the STC, STCPs, Grid Code and Connection and Use of System Code (CUSC) are amended in line with the new situation and do not contain references to redundant processes, which would cause confusion and uncertainty among stakeholders. The Proposer is progressing the changes in these codes in parallel with this request. The SYS, NOA and ODIS are already redundant at the time of writing and references need to be removed from the codes.

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Public What is the solution? As the ETYS is being removed and many of the functions replaced by the CSNP there is a clear need for the Codes to keep pace with these changes. The references to the ETYS and SYS will need to come out as they will be redundant, and where the function is being taken over by the CSNP this will need to be added into the legal text. A list of STC references to ETYS and SYS has been compiled (see Appendix) and there will be potentially three scenarios: 1. References to ETYS/SYS where the function as described in the code section is being taken over in its entirety by the CSNP, and the reference can therefore simply be replaced (i.e. a straightforward replacement of ETYS for CSNP or similar). 2. References to ETYS/SYS where part of the function as described in the code section is being taken over by the CSNP but not all. The code section will need to be amended to ensure that the new reference to CSNP is correctly describing the functions it will fulfil, and any others are either removed or amended to describe the function and how they are being fulfilled. References to ETYS/SYS where the function is either completely redundant or is only a description or confirmation of the ETYS/SYS. For example, this would include mentions in a definitions section, or a passage which is being removed from the Licence and can safely be taken out of the Code without needing further modification. Legal Text As per the above sections there will be several different changes to the existing STC legal text depending on how the change from ETYS to CSNP affect the wording, and other than the three types of changes there is no standard pattern. Documents outlining the specific proposed changes for each reference to ETYS, SYS, NOA and ODIS are included as Annex 02.

The specific sections of the STC affected are as follows: • Section D Planning Co-ordination, Part 1 Introduction 1.1.3,

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Public • Section D Planning Co-ordination, Part 1, 4 Electricity Ten Year Statement: Entire section, • Section H - Disputes: Section 4.3.12, • Section J – Interpretation and Definitions • Schedule 3 2.1 (j) and (k), 2.1.6 (b, ii), 3.7 • Schedule 8 1.3.3, STCP 16-1 Appendix D 'Abbreviations and Definitions’, • Schedule 8, TO Construction offer 1.1.3.2 (c), • Schedule 9 A: 2.62, B: 2.4, Contents Headings, • Schedule 9 C: 1.1, 2.7.2, Appendix

For reference, a future STCP modification will be raised and is likely to make changes to the below STCPs.

• STCP 03-1 Issue 008 Post Event Analysis and Reporting • STCP 18-1 Issue 016 Page 34, Stage 3, • STCP 20-1 Issue 007 Electricity Ten Year Statement: (Entire document), • STCP 21-1 Issue 006 Network Options Assessment, • STCP 22-1 Issue 006 Production of Models for GB System Planning

There will also need to be code modifications raised for the Grid Code, STCPs and CUSC as these will also be impacted by the same changes to the ETYS. What is the impact of this change? This modification will impact any current STC parties who use or are involved with ETYS, i.e. anyone who provides data input to the ETYS process, and those who use the outputs of the ETYS. Ofgem decision states that the next iteration of the ETYS report (summer 2026) will be the final one. However, NESO’s licence has already been updated regarding the requirements for CSNP. The Proposer will be updating the codes to ensure that the information is submitted in accordance with the new processes.

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Public Proposer’s assessment against STC Objectives Relevant Objective Identified impact (a) efficient discharge of the obligations imposed upon Transmission Licensees by Transmission Licences and the Electricity Act 1989; Positive This modification ensures that we have continuity in terms of the data gained from the affected parties. The expectations from customers will not be changing except for the new name, location and timing of the data they already provide. (b) efficient discharge of the obligations imposed upon the licensee by the Electricity System Operator licence, the Energy Act 2023 and Electricity Act 1989; Positive See (a), removes any ambiguity between the requirements of all parties. (c) development, maintenance, and operation of an efficient, economical, and coordinated system of electricity transmission; Positive Ensuring that the references in the STC are up to date and redundancy is removed, ensures a correct flow of information and facilitating more effective future planning operations. (d) facilitating effective competition in the generation and supply of electricity, and (so far as consistent therewith) facilitating such competition in the distribution of electricity; Positive By ensuring the codes refer to the correct name for the published network planning data, Generators are better directed and can research the market better, facilitating more effective competition. (e) protection of the security and quality of supply and safe operation Positive The STC is, by this modification, kept up to date with the latest NESO

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Public of the National Electricity Transmission System insofar as it relates to interactions between Transmission Licensees and the licensee; Independent System Operator and Planner (ISOP) licence conditions C12/C17 regarding the production of a Centralised Strategic Network Plan in future. (f) promotion of good industry practice and efficiency in the implementation and administration of the arrangements described in the STC; Positive See (e) (g) facilitation of access to the National Electricity Transmission System for generation not yet connected to the National Electricity Transmission System or Distribution System; and Neutral

(h) compliance with the Electricity Regulation and any Relevant Legally Binding Decisions of the European Commission and/or the Agency. Neutral

* See Electricity System Operator Licence

Proposer’s assessment of the impact of the modification on the stakeholder / consumer benefit categories Stakeholder / consumer benefit categories Identified impact Improved safety and reliability of the system Neutral

Lower bills than would otherwise be the case Neutral

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Public Benefits for society as a whole Positive The investment NESO is putting into the CSNP project will not only take over the functions which the ETYS provided but expand them as well. The new system will better utilise the data in terms of whole system analysis and future planning, ensuring that the network is as efficient and cost-effective as possible. Reduced environmental damage Neutral As the data that users send to NESO will be used as part of the overall CSNP process, analysis produced will better mitigate against environmental damage thanks to the whole system view the new process is conducting. Improved quality of service Positive See ‘Benefits for Society as a whole’.

When will this change take place? Implementation date: 31 August 2026 Date decision required by: Mid-August 2026 Implementation approach The Proposer has been discussing the implementation of the necessary STC changes with the Transmission Owners (TOs) since the middle of 2025, and the draft of the changes has since been shared with them as well as approaching the STC Panel. Changes have also been discussed with Ofgem during this period. We have also been receiving comments in return over the last 6 months from the TOs, and the proposal has been raised in the Joint Planning Committee (JPC)

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Public sessions and their sub-groups, as well as the STC Panel in February 2026 in readiness for the full Proposal in the April Panel.

Interactions ☒Grid Code ☐BSC ☒CUSC ☐SQSS ☒European Network Codes Other modifications

☐Other

Alongside this STC modification we are also raising one for the Grid Code changes as well as in the CUSC, and we will be raising STCP changes as a Consequential modification. How to respond Code Administrator Consultation questions • Please provide your assessment for the proposed solution against the Applicable Objectives versus the current baseline. • Do you support the proposed implementation approach? • Do you have any other comments? Views are invited on the proposals outlined in this consultation, which should be received by 5pm on 27 May 2026. Please send your response to stcteam@neso.energy using the response pro-forma which can be found on the modification page. If you wish to submit a confidential response, mark the relevant box on your consultation proforma. Confidential responses will be disclosed to the Authority in full but, unless agreed otherwise, will not be shared with the Panel or the industry and may therefore not influence the debate to the same extent as a non-confidential response.

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Public Acronyms, key terms and reference material Acronym / key term Meaning BSC Balancing and Settlement Code CM Code Modification CSNP Centralised Strategic Network Plan CUSC Connection and Use of System Code ETYS Electricity Ten Year Statement ISOP Independent System Operator and Planner JPC Joint Planning Committee NESO National Energy System Operator NOA Network Options Assessment ODIS Offshore Development Information Statement SQSS Security and Quality of Supply Standards STC System Operator Transmission Owner Code STCP System Technical Code Procedure SYS Seven Year Statement TO Transmission Owner

Reference material • ETYS-CSNP STC Legal Text changes proposal v1.xlsx • Centralised Strategic Network Plan (CSNP) | National Energy System Operator

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Public • Electricity Ten Year Statement (ETYS) | National Energy System Operator

Annexes Annex Information Annex 01 CM0107 Proposal Form Annex 02 CM0107 Legal Text

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Public Code Administrator Consultation GC0168: Submission of Electromagnetic Transient (EMT) Models Overview: This modification seeks to require certain Users to provide National Energy System Operator (NESO) with EMT models to enable the analysis of issues such as system oscillations, inverter instability and transient overvoltage (ToV). Modification process & timetable

Have 5 minutes? Read our Executive summary Have 60 minutes? Read the full Workgroup Report Have 120 minutes? Read the full Workgroup Report and Annexes. Status summary: The Workgroup have finalised the proposer’s solution as well as one alternative solution. This modification is expected to have a: High impact on Generators (including both GB Generators and EU Generators), National Energy System Operator, Network Operators, Interconnector Owners, Transmission Owners and Non-Embedded Customers. Modification drivers: Efficiency, GB Compliance, Harmonisation, New Technologies, System Operability, System Planning, System Security, Transparency Governance route Standard Governance modification with assessment by a Workgroup Who can I talk to about the change? Proposer: Frank Kasibante (NESO) frank.kasibante1@neso.energy Code Administrator Chair: Kat Higby (NESO) Katharine.Higby@neso.energy How do I respond? Send your response proforma to grid.code@neso.energy by 5pm on 08 June 2026 Workgroup Consultation 23 January 2025 - 21 February 2025 Proposal Form 05 March 2024 Workgroup Report 15 April 2026 Code Administrator Consultation 05 May 2026 - 08 June 2026 Draft Modification Report 17 June 2026 Final Modification Report 16 July 2026 Implementation 10 Business Days after decision 1 2 4 3 5 6 7

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Public Contents Executive Summary ....................................................................................................................... 3 What is the issue? .......................................................................................................................... 5 What is the defect the Proposer believes this modification will address? ............................. 5 Why change? ................................................................................................................................. 6 What is the solution? ..................................................................................................................... 6 Proposer’s Original solution.......................................................................................................... 6 WAGCM1 Solution ........................................................................................................................... 6 Legal text ........................................................................................................................................ 7 Workgroup considerations ........................................................................................................... 8 Workgroup Discussions following Panel Send Back ................................................................ 16 Terms of Reference Overview .................................................................................................... 24 What is the impact of this change? .......................................................................................... 25 First Workgroup Vote ................................................................................................................... 29 Second Workgroup Vote............................................................................................................. 29 When will this change take place?............................................................................................ 30 Interactions .................................................................................................................................. 30 Acronyms, key terms and reference material ......................................................................... 31 Annexes ........................................................................................................................................ 33

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Public Executive Summary This modification seeks to require certain Users to provide National Energy System Operator (NESO) with Electromagnetic Transient (EMT) models to enable analysis of issues such as system oscillations, inverter instability and transient overvoltage (ToV) on the Transmission System.

What is the issue? Great Britain’s power system is moving towards net zero carbon operation. Connections to the National Electricity Transmission System (NETS) are transitioning from large synchronous generation to a large number of smaller Electronic Power Converters (EPCs). This is causing new and varying challenges to the power system, especially in view of the different operating and performance characteristics of EPCs whose power electronics interact with the NETS in a different way to the older Generators. Examples of these challenges include control interactions, low fault level, inverter instability and ToV. NESO requires EMT models from certain Users in order to analyse and understand how these interactions affect the NETS under different system conditions.

What is the solution and when will it come into effect? Proposer’s solution: The proposed solution is to require the provision of EMT models from Users with certain legacy assets with a completion date before 01 September 2022, upon request from The Company. A new Electrical Standard which will indicate model requirements and the approach for retrospective submission will also be introduced (available in Annex 08). Implementation Date: 10 Business Days following an Authority decision.

Alternative Request 1: The proposed alternative WAGCM1 Introduces a deferred effective date for GB Code Users, allowing immediate effect of GC0168 for EU Code Users but delaying applicability to GB Code Users until cost recovery is in place within the CUSC. The alternative was voted in by the Workgroup and became Workgroup Alternative Grid Code Modification 1 (WAGCM1). Implementation Date: 10 Business Days following an Authority decision.

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Public What is the impact if this change is made? The modification will positively impact system security and reliability by requiring EMT models, enabling NESO to analyse oscillations, inverter instability, and ToV risks more accurately. It also supports efficient system planning and helps avoid unnecessary investment or operational costs by improving the accuracy of pre and post‑fault studies. Workgroup conclusions: The Workgroup concluded by majority that WAGCM1 better facilitated the Applicable Objectives than the Baseline. The Workgroup did not agree by majority that the original solution better facilitated the Applicable Objectives than the Baseline.

Interactions CUSC modifications CMP456: Cost recovery for legacy plant in relation to GC0168 and CMP466: CMP456 Consequential Charging Modification have been raised relating to the introduction of a cost recovery mechanism. Corresponding STC modification CM097 – Electromagnetic Transient (EMT) and Root Mean Square (RMS) Model Submission for Transmission Owners (TOs). A separate modification PM0147 was established to introduce a new System Operator- Transmission Owner Code Procedure (STCP) 12-2. Grid Code Modification GC0139: Enhanced Planning-Data Exchange to Facilitate Whole System Planning – due to the interaction with GC0139, which could be implemented ahead of GC0168, the Workgroup proposed two sets of legal text for both the original and alternative solutions. This approach was proposed as futureproof (with options to cater for whichever decision was made for GC0139 by Ofgem), aiming to avoid a send back and save Industry time. Grid Code Modification GC0103: The introduction of harmonised Applicable Electrical Standards in GB to ensure compliance with the EU Connection Codes – the Workgroup noted ongoing GC0103 work to introduce a harmonised electrical standard and advised that GC0168 consideration be made when tables in the General Conditions are being amended. Ongoing work to align electrical standards and what is published on the NESO website will capture this interaction.

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Public What is the issue? What is the defect the Proposer believes this modification will address? As Great Britain’s power System moves towards net zero carbon operation, connections to the network are transitioning from large synchronous generation to a larger number of smaller EPCs. This is causing new and varying challenges to the power System, especially in view of the different operating and performance characteristics of EPCs, whose power electronics interact with the NETS in a different way to older generation. Examples of these challenges include control interactions, low fault level, inverter instability and ToV. NESO requires EMT models from Users so that it can analyse and understand how these interactions affect the NETS under different System conditions. For some European Code Users, the demonstration of compliance requires EMT models to be provided and assessed through the Compliance Process. For Users subject to modification GC0141 ‘Compliance Processes and Modelling amendments following 9th August Power Disruption’, the requirements for EMT model provision and the processes surrounding their provision have been clearly articulated. However, for GB Code Users (prior to the implementation of GC0141) there is currently no clarity over how these models should be made available. Equally for early EU Code User’s, the Grid Code does enable NESO to request EMT simulation where required (PC.A.6.1.3) however as with GB Code Users which have not been subject to the GC0141 process, there is no clarity over how these models should be made available. The requirements in the current Grid Code for these relevant Users require RMS models which are inadequate to accurately model the Transmission System with high proportions of EPCs. This situation arises through the complex switching sequences that EPCs introduce, therefore requiring a detailed three-phase representation through an EMT model. Therefore, to accurately represent the behavioural characteristics of the Transmission System, detailed EMT models will be required from Users. Grid Code modification GC0141 (as approved by The Authority on 12 December 2022) already requires Users which either connected to the System after 01 September 2022 or were subject to a control system change or a modification to already supply an EMT model. In addition, Grid Code modification GC0102 ‘EU Connection Codes GB Implementation Mod 3’ also enabled NESO to request an EMT Model from EU Code Users

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Public where required. However, this modification (GC0168) is now seeking NESO to require relevant Users (which do not fall under the requirements of GC0141 or GC0102) to provide an EMT model on a retrospective basis where such a model is required. Why change? Unlike a System with a previously high penetration of synchronous generation which could be adequately analysed and studied with RMS models, an evolving System with a high penetration of EPCs requires more detailed EMT models to perform investigations and analysis. This is largely as a result of the complex switching arrangements that take place in EPCs unlike their synchronous counterparts. The current requirements in the Grid Code are insufficient to cover the User data that NESO requires. As noted above, EMT models from Users are becoming essential to ensure that NESO can accurately model the Transmission System. Without these models, it restricts the ability of NESO to perform accurate System studies, modelling and post- fault analysis. What is the solution? Proposer’s Original solution The proposed solution is to mandate the provision of EMT models by Users with certain legacy assets with a completion date before 01 September 2022, upon request from The Company. A list of types of Users who will be affected by this proposed obligation can be found in Annex 04. The list of User types has been included within the proposed legal text (PC.A.9.2.2). This solution will require updates to Grid Code Planning Code (PC) and an amendment to the Annex of the General Conditions (referenced in GC11) to introduce a new Electrical Standard which will indicate model requirements and the approach for retrospective submission. These models will feed into a wider GB model enabling investigations, post fault studies and planning studies. This will help to enable safe, reliable and economic operation of the System and enhance the security of GB electricity supply. WAGCM1 Solution In relation to provision of EMT models by GB Code Users, the WAGCM1 Proposer considers it is essential that a compensation/cost recovery mechanism is directly acknowledged

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Public within the solution. This was a core part of the Original proposal through Workgroup development of the original solution, and a core element of the implementation plan in the Workgroup Consultation. The Original solution was amended to remove the above paragraph from the proposed legal text at the request of the Original Proposer, who now considers that GC0168 should be introduced as a stand-alone modification without the link to the CUSC and Bilateral Agreement. WAGCM1 Introduces a deferred effective date for GB Code Users, allowing immediate effect for EU Code Users but delaying applicability to GB Code Users until cost recovery is in place within the CUSC. Legal text The legal text for the original solution and WAGCM1 can be found in Annex 03 and Annex 10. The table below illustrates the difference between each of the solutions:

Planning Code General conditions Original Includes a requirement for Users to provide EMT models to NESO. Updated to include a new Electrical Standard. WAGCM1 Includes a link to cost recovery for legacy assets None

Both sets of legal text have also been added to the proposed legal text for modification GC0139. If modification GC0139 is implemented ahead of GC0168, these sets of legal text can be used to implement GC0168. These sets of legal text are available in Annex 11 and Annex 12. The following considerations were taken into account when creating the legal text: Original: • Due to the retrospective nature of application of the Proposer’s solution, which can be found in the proposed requirements, concerns attributed to the potential costs incurred resulting in retrospective requirements were raised by Users during several workgroup discussions, the Workgroup consultation and bilateral sessions.

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Public • Several Workgroup members highlighted the need to introduce a mechanism in the Grid Code which could provide a guarantee that costs met by Users via their efforts to meet the proposed new, retrospective requirements would be reimbursed via the CUSC route. • The original solution does not contain any guarantees that Users’ costs will be recovered, neither is there a market mechanism in current arrangements to address this challenge. Users could be unfairly treated and in more extreme cases, consider closing some assets. WAGCM1: Two CUSC modifications (CMP456 and CMP466) have been raised to propose a cost recovery mechanism. The two approaches below were considered by the Workgroup regarding drafting cost recovery for GC0168. 1. adding in a link to the (future) CUSC cost recovery mechanism, which would mean that GC0168 could not be implemented until the CUSC is amended; and 2. protection written into the legal text through deferring the obligation in relation to certain Users until a provision is available within the CUSC Workgroup opted for option 2 and legal text was drafted accordingly. This can be found in Annex 10, (Section PC.A.9.2.2.2).

Workgroup considerations The Workgroup convened 13 times in total to discuss the identified issue within the scope of the defect, develop potential solutions, and evaluate the proposal in relation to the Applicable Grid Code Objectives. Consideration of the Proposer’s solution Legal text identified to be amended (Grid Code Planning Code) During Workgroup discussions, members highlighted areas of the legal text that either needed clarification or to be changed. Workgroup members agreed that a table should be included within the legal text to provide clear and structured information regarding the requirements for model provision, with the aim to: • Distinguish between different types of connections and the specific dates and conditions under which models are required;

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Public • Make it easy for readers to understand their obligations by presenting the information in a tabular format rather than in dense legal text; and • Align the legal text with practical implementation details, such as the distinction between pre and post-September 2022 connections and the conditions for model updates after modifications. This table has been included within Grid Code Planning Code, section PC.A.9.2.2. Workgroup members discussed the use of the term mathematical models, emphasising the need to distinguish RMS and EMT models. They discussed the challenges of converting models between different software versions and the importance of validation. This has been clarified within the legal text. The Proposer amended the legal text, which included technical clarifications and changes to specific sections of the Grid Code and shared this with Workgroup members for review. Model collection timescales (Grid Code Planning Code, section PC.A.9.7.3) Workgroup members raised concerns regarding the three-month timescale for sending validation reports to NESO in relation to model simulation results against measurements. Workgroup members agreed that more time was needed. The Proposer agreed to amend the text to state the 3 months would start after compliance testing had been completed. Cost of model development and cost recovery The Proposer noted that the Grid Code Review Panel (GCRP) had advised GC0168 Workgroup members to discuss and consider if a cost recovery mechanism was needed. Workgroup members agreed that there should be a cost recovery mechanism, but it was difficult to quantify as it would depend on the size of the unit, its age, type and if the manufacturer was still supplying equipment. A NESO representative highlighted that cost recovery would apply to historical sites only and going forward this would be less of an issue, as new sites would not be eligible to claim. Workgroup members agreed that a modification to the CUSC is required to introduce a cost recovery mechanism. CUSC modifications CMP456 and CMP466 have now been raised to propose a cost recovery mechanism. Additional Term of Reference (ToR) It was suggested by a NESO representative that an additional item was added to the ToR relating to the need to reflect international best practice. They suggested that the recent consultations relating to EU Requirements for Generators 2.0 (RfG 2.0) and High Voltage Direct Current 2.0 (HVDC 2.0) would provide an appropriate benchmark which was agreed

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Public by the Workgroup. ToR (l) was approved by the Grid Code Review Panel. The ToR was amended to show the new reference. Terms of Reference discussion The Terms of Reference have been reviewed by the Workgroup to confirm that they have been met. The view of the Workgroup regarding the Terms of Reference is summarised as follows: a) Implementation and costs A Workgroup member noted that when Panel members agreed the ToR for the Workgroup, they were not aware of what the outcome would be. Discussions have taken place, and members agreed that there is a need for a cost recovery mechanism. As funding is a commercial issue that technically falls outside the Grid Code, the issue would need to be addressed through a CUSC modification. The CUSC Panel and a CUSC Workgroup will consider the most appropriate cost recovery mechanism, especially noting that some parties affected by this change, for example owners of Licence Exempt Embedded Medium Power Stations (LEEMPS) are not CUSC Parties. b) Review draft legal text The legal text has been discussed thoroughly by Workgroup members and all comments and amendments have been considered by the Proposer. The amended legal text, which includes technical clarifications and changes to specific sections of the Grid Code, has been reviewed and agreed by Workgroup members. c) Consider whether any further Industry experts or stakeholders should be invited to participate within the Workgroup to ensure that all potentially affected stakeholders have the opportunity to be represented in the Workgroup. Demonstrate what has been done to cover this clearly in the report Two sets of nomination requests were issued to industry to encourage stakeholders to register for GC0168 Workgroups. Individual emails were also issued to equipment manufacturers to encourage their participation to Workgroups, this was done early in the process at the request of current Workgroup members. The Proposer has engaged with owners of large Combined Cycle Gas Turbines (CCGTs) Units and/or Modules, who have been given the opportunity to review and contribute to this modification. d) Consider Electricity Balancing Guideline (EBR) implications It was agreed that there are no EBR changes, as there is no impact on the Balancing code.

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Public e) Consider a cost recovery mechanism to receive the model data required to share with a CUSC Workgroup Discussions have taken place in relation to the cost recovery mechanism, Workgroup members felt that the cost recovery mechanism was difficult to quantify as it would depend on the size of the unit, its age, type and if the manufacturer was still supplying equipment. Workgroup members agreed that a CUSC modification will be required to implement a cost recovery mechanism. As a result, CUSC modifications CMP456 and 466 were raised. f) Consider the use/introduction into the Legal Text of generator classification types C, D as opposed to Medium and Large The use of User types was discussed in Workgroup meetings, the Proposer felt that the use of a list would be easier for users and more transparent. As currently drafted the legal text refers to Large, Medium and Small Power Stations as the contractual requirements are based on these terms. Following the implementation of the EU Requirements for Generators (RfG) in the GB Grid Code in 2018, technical requirements were specified based on Type A, B, C, and D Power Generating Modules as defined in the Grid Code. Although a Large, Medium or Small Power Station could comprise of any combination of Type, A, B, C, or D Power Generating Modules, the actual contractual obligation on the Generator under CUSC is with respect to the Power Station not the Power Generating Module, although it is true to say that under RfG most of the technical obligations are with respect to the Power Generating Module. Noting that this GC0168 modification applies to all Generators, including GB Generators who are not caught by the requirements of RfG, the legal drafting (PC.A.9.2.2) has therefore been based on Power Station size (i.e. Large, Medium and Small) rather than Type A, B, C, and D Power Generating Modules. g) Consider approach on collecting models and where that guidance would sit This issue was discussed at the Workgroup, members agreed that additional guidance and the approach used could either be included as an appendix to the Planning Code in the Grid Code, under the relevant Electrical Standards, or as a standalone guidance note. The Proposer suggested that including the guidance as an Electrical Standard would be the best approach as this does introduce a governance process (Grid Code GC11) around the document in a simple way. It was noted that if a bespoke appendix was added to the Planning Code, a full Grid Code modification would be required each time the document needed to be updated which would be demanding in terms of resource and time. It was also noted that a pure guidance note published on the NESO’s website would be subject to no governance arrangements. Workgroup members agreed that neither of these

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Public options would be desirable from an Industry perspective, and therefore the best approach would be to consider including the appropriate guidance as an Electrical Standard. A draft has been produced and content discussed extensively by workgroup members. h) Consider codifying the list of Users who are required to submit EMT models Workgroup members agreed this was considered and has been included in the proposed legal text under PC.A.9.2.2. which lists the User types who are required to submit an EMT model. See Annex 04. i) Consider the scenario where a User is unable to provide an EMT model This issue was discussed and the proposed legal text updated in PC.A.9.2.2.1 stating that where an EMT model is requested, this is required to be provided within 9 months of a request from NESO unless otherwise agreed in the case of a GB Code User, and 3 months of a request from NESO unless otherwise agreed in the case of an EU Code User. This wording also accounts for potential problems in preparing old plant EMT models that cannot meet all the requirements in PC.A.9. It should also be noted that for GB Code Users and early EU Code Users, a cost recovery mechanism is being introduced to provide a mechanism of compensating Users for the ability to supply a model when requested by NESO. The difference in model submission timeframes between EU Code Users and GB Code Users arises through differences in treatment between EU Code Users and GB Code Users as introduced through the EU Connection Network Codes (RfG, Demand Connection Code (DCC) and HVDC Codes). Under the EU Connection Network Codes, there is a requirement for Users to provide an EMT Model when requested from the System Operator, however this is not the case for GB Code Users which accounts for the difference in approach between EU Code Users and GB Code Users. The draft legal text has addressed the challenges raised by workgroup members. j) Consider whether there is a need for any consequential changes to the DCode and / or Distribution Connection and Use of System Agreement (DCUSA) It was noted that it appears that a Distribution Code modification is unlikely to be required to ensure the technical requirements are adequately transferred to LEEMPS, and that there may be a need for a DCUSA modification to deal appropriately with LEEMPS in relation to any compensation mechanism that could arise from CUSC modifications CMP456 and CMP466. It was agreed that the DCUSA administrator should be contacted and informed about this modification and Workgroup discussions. Contact was made with the DCUSA administrator in October 2025 to that effect.

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Public k) Consider whether there is a need to obtain EMT models from Medium Power Stations embedded in distribution networks and, if so, the mechanism for engaging with the host Distribution Network Operator (DNO) and the Generator and the process to be followed in the event that the Generator is unable to provide the EMT models or would incur significant costs in doing so This was discussed at Workgroup 4, and it was noted this was a particular concern as LEEMPS are not CUSC signatories but are bound by some of the requirements of the Grid Code Planning Code through obligations in the Distribution Code. The obligation for LEEMPS to comply with these requirements falls on the DNO, which the DNO would pass on to their LEEMPS customers through the connection agreement. However, as the LEEMPS would have no funding for the provision of the model under CUSC, this would be unfair if Generators with a CUSC contract were compensated. l) Consider if we are reflecting international practice including observation of the modelling developments proposed for RfG 2.0 and HVDC 2.0 NESO advised that some additional requirements had been included in the proposed EU Connection Network Codes (e.g. RfG 2.0, DCC 2.0 and HVDC 2.0) relating to EMT models. It was suggested by NESO that these documents were reviewed with respect to their requirements on modelling to reflect international best practice. NESO has reviewed these documents, and the Workgroup has considered the impacts of these proposals. Workgroup Consultation Summary The Workgroup held their Workgroup Consultation between 23 January and 21 February 2025 and received 7 non-confidential responses and 2 confidential responses. The full non-confidential responses and a summary of the non-confidential responses can be found in Annexes 05 and 06.

Support for the requirement for users providing EMT models to NESO when requested: Six respondents supported the implementation approach, including the requirement for Users to provide EMT models to NESO when requested, noting the importance of accurate power System modelling, including planning, operation, and post-event analysis. • Most respondents noted that this support is conditional on a sufficient cost recovery mechanism being in place to allow for compensation for older Plants in particular, due to the significant cost of obtaining models. • One respondent noted that it is reasonable to require Users to provide an EMT model, but the approach must consider the difficulties in obtaining models from legacy Plants.

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Public • Two respondents noted that it is reasonable to require EMT models from Users identified in the draft legal text, but clarity is needed on which Users need to provide models by default, and which need to provide them when requested by NESO.

Workgroup feedback: Workgroup members agreed that the legal text must be clear on the process and obligations around model provision.

Non-support of the solution: One respondent did not support the solution, noting that NESO needs to appreciate the significant cost involved in doing this for legacy Plants and the impacts of this cost, with the worst-case being closure of Plant.

Workgroup feedback: Workgroup members consider that providing Users with flexibility to make use of their existing RMS data will alleviate concerns around the implementation of this modification. It was noted that there is flexibility in the legal text to address practical challenges and timelines on a case-by-case basis. This will also be clarified within the Electrical Standard.

Cost recovery: All of the respondents noted the need for a cost recovery mechanism, noting the following key points: • Not having a cost recovery mechanism could lead to premature closure of older Plants. • Having a cost-recovery mechanism in place before GC0168 is implemented is necessary to compensate parties who incur unexpected and significant costs as a result of providing models. • A cost recovery mechanism is necessary to avoid discrimination against older Plants and potential non-compliance issues, including the need to apply for derogations. • CUSC modification CMP398 ‘GC0156 Cost Recovery mechanism for CUSC Parties’ was noted as a relevant example of cost recovery. • Sites connected before September 2022 were not required to develop an EMT model by the Connection Agreement or Grid Code. A cost recovery mechanism is required because Users should not necessarily bear the cost of requirements introduced retrospectively by NESO.

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Public Workgroup feedback: Workgroup members clarified that EU Code Users connected before September 2022 may still be required to provide EMT models if requested, as per the requirements of the EU connection network codes (RfG, HVDC).

Governance Six respondents agreed it is appropriate to define the detail of the model submission in an Electrical Standard rather than in a specific part of the Grid Code, or as a separate guidance note.

• Respondents that preferred the model submission to be defined in an Electrical Standard noted its appropriate flexibility and that it would strike the correct balance between administrative burden and governance. • Some respondents noted that ideally, everything should be in the Grid Code as a one-stop-shop for Code Users, however respondents acknowledge that they would support the use of Electrical Standards over guidance notes.

Workgroup feedback: Workgroup members agreed that the Electrical Standard will detail the process for model collection and submission, ensuring clarity and consistency.

Model submission method Five respondents noted that it is appropriate for EMT models to be submitted in Power Systems Computer Aided Design (PSCAD) Version 5, however one raised concerns about future compatibility with newer software versions. One respondent noted that NESO should not define specific software, as it adds expense and complication.

Workgroup feedback: Workgroup members agreed that EMT models should be submitted in PSCAD Version 5, with considerations for future compatibility and flexibility in software choice.

Dealing with LEEMPS: There were mixed views around whether LEEMPS had been adequately dealt with within the Proposal:

• Two respondents noted that the Proposal does deal with LEEMPS adequately, requiring them to provide an EMT model where requested. However, both responses question how cost recovery will work, considering LEEMPS are not CUSC Parties. Another response also questioned this.

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Public • Two respondents note that further clarification is required, as within the draft legal text, the arrangements for obtaining EMT models from a LEEMPS owner / operator is not clear. • One respondent does not believe the proposal deals adequately with LEEMPS, stating that pre-September 2022 LEEMPS Generators should receive the same compensation as other Users.

Workgroup feedback: Workgroup members agreed that dealing with LEEMPS in terms of cost recovery is out of scope for the Workgroup and should be discussed within the CUSC modification which is to follow.

Target Implementation Date Six respondents agreed with the proposed Implementation Date. However, the following concerns were raised:

• Consideration is needed for the timeline of associated CUSC, STC and potentially DCUSA modifications. • One respondent does not believe the proposed timeline for Workgroup meetings and target date of September 2025 are reasonable, suggesting cost recovery mechanisms should be decided and implemented first.

Workgroup feedback: Workgroup members agreed that the implementation of this modification should be in line with the future CUSC modification relating to cost recovery and the implementation of modification CM097.

Workgroup Discussions following Panel Send Back Following an initial Workgroup Vote (which has now been superseded by a second Workgroup Vote), the Workgroup Report was presented to the Grid Code Review Panel on 26 June 2025. The Panel did not believe the Workgroup had met the modification Terms of Reference and did not agree for the modification to proceed to Code Administrator Consultation. The Panel referred the modification back to the Workgroup for further discussions and clarification on the following: • Cost recovery, including a possible link to CUSC modifications CMP456 and CMP466.

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Public • Legal text amendments and clarifications, including clarification on the interaction with the electrical standards and correction of grammatical errors. • Reference to DCUSA within the interactions section of the modification. • Possible derogation route – what happens if a model is not available and cannot be obtained or cannot be obtained within the timescales given. Workgroup meetings were resumed on 17 December 2025. The Workgroup met 5 times to address the Panel feedback. Cost recovery, including a possible link to CUSC modifications CMP456 and CMP466 Workgroup Debate on Cost Recovery Linkage to the CUSC and Legal Text Protection The Workgroup held a detailed and robust debate on whether the technical Grid Code modification should progress independently of the associated commercial cost recovery arrangements.

The Proposer stated that the intention behind removing the explicit link to cost recovery was not to diminish User protections, but to prevent GC0168 being delayed by parallel commercial processes. The Proposer emphasised that GC0168 is a system-critical modification, necessary to enable effective EMT analysis and to support the secure and stable operation of the Transmission System.

A significant number of Workgroup members stated that they would not support the removal of the cost recovery linkage. They expressed strong concern that, in the absence of an established and enforceable compensation mechanism, Users could be exposed to substantial and potentially unrecoverable costs associated with the development of EMT models for legacy assets.

These Workgroup members considered that the inclusion of legal text explicitly linking the technical obligation to cost recovery was essential legal protection, rather than a procedural preference.

As a result, no consensus was reached. Workgroup members were unwilling to agree to the removal of the cost recovery linkage from the legal text without firm guarantees or legally binding assurances that costs incurred in complying with GC0168 would be recoverable. This position reflected a clear and deliberate decision by those members to prioritise legal certainty and protection against retrospective cost exposure.

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Public It was communicated to the Workgroup by the Chair that under Grid Code governance the original Proposer was permitted to amend the original solution without agreement from the Workgroup. The Proposer determined that the link will be removed, and the final original solution does not contain a link to cost recovery.

Alternative Request (WAGCM1) raised, including a cost recovery link The Chair advised the Workgroup that if Workgroup members wanted to proceed with the link to cost recovery, an alternative solution could be raised that could include a link to the CUSC, and that both options could be presented to the Authority for decision on which (if any) solution would be implemented.

WAGCM1 was raised by a Workgroup member on 12 December 2025. Full details of this alternative solution can be found in Annex 09. WAGCM1 was voted in by the Workgroup on 17 December 2025 by majority vote.

The WAGCM1 solution is similar to the original solution in all ways except the cost recovery link, which has been added to the proposed legal text (PC.A.9.2.2.2).

Workgroup discussion on WAGCM1 and corresponding legal text The majority of Workgroup members explicitly welcomed the WAGCM1 solution, noting that it avoids placing GB code users at commercial risk and provides a pragmatic bridge between Grid Code obligations and the proposed CUSC implementation.

WAGCM1 was seen as consistent with pre-Panel send back Workgroup discussions and decisions.

The Workgroup discussed two potential implementation approaches for WAGCM1: • Option 1: Aligning the effective date of GC168 EMT obligations with the availability of a CUSC cost‑recovery mechanism. • Option 2: Introducing a deferred effective date for GB Grid Code users, allowing immediate effect for EU Code users but delaying applicability to GB users until cost recovery is in place

The Workgroup favoured Option 2 for the alternative legal text. This option keeps GC0168 implementable now, while deferring NESO’s obligation to request retrospective EMT models until appropriate cost‑recovery arrangements exist within the CUSC. Members

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Public felt this approach provides clarity on technical requirements without delaying the modification.

There was broad agreement that Option 1 would create unnecessary implementation delay and risk stalling progress.

The Workgroup also highlighted the need for legal drafting to reflect the new deferral mechanism clearly and requested that the scope be checked to ensure it captures all Users affected by retrospective application, not only those defined under a specific code classification.

A clause has been added to the proposed Legal Text (PC.A.9.2.2.2), stating that the process for NESO to request EMT models from Users for relevant Plant with a Completion Date (or commissioning date in the case of LEEPMS not subject to a Bilateral Agreement) on or before 01 September 2022 and which Plant has not been subject to a Modification or control system change, shall not apply unless and until a cost recovery process as described in CUSC modification proposal CMP456 is implemented.

The text ‘unless otherwise agreed (such agreement not to be unreasonably withheld)’ was added to the requirements regarding model submission for the above category of Users to provide flexibility and for further engagement with NESO where challenges to meet those requirements arose.

Legal text amendments and clarifications, including clarification on the interaction with the electrical standards and correction of grammatical errors. Treatment of LEEMPS in Planning Code Legal Text The Workgroup considered whether the existing Planning Code drafting, which places obligations on the “User”, appropriately captures the technical and ownership realities of LEEMPS.

Workgroup members highlighted that, in many LEEMPS arrangements, the entity defined as the Grid Code “User” (i.e. the Network Operator) does not own, operate, or control the plant or equipment that determines dynamic performance or for which EMT models are required.

Workgroup members noted that in such cases, control of converters, firmware, and modelling capability typically sits with the Generator or original equipment

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Public manufacturer, rather than the Grid Code user. The workgroup agreed that drafting obligations directly against the “User” risks placing legal requirements on parties who may not have the practical ability to comply.

Workgroup members also noted that LEEMPS assets frequently sit behind complex ownership and interface boundaries. Relevant equipment affecting EMT behaviour (including converters, transformers, and reactive compensation devices) may not align neatly with Grid Code interface points or User definitions. Several members expressed concern that “User”-based drafting could therefore create ambiguity over which assets are in scope and increase the risk of unintended non‑compliance.

To address these issues, the Workgroup supported revising the legal text in certain clauses to refer to the “relevant plant or apparatus” rather than the “User”. Workgroup members considered this asset‑focused approach to better reflect the physical reality of LEEMPS arrangements by anchoring obligations to the equipment that gives rise to EMT behaviour, without prejudging contractual or organisational responsibility for submitting models. This approach was also seen as reducing ambiguity where ownership and control do not sit with the Grid Code User. The Proposer agreed to make these required changed to the proposed Legal Text.

The Proposer also agreed to explicitly clarify the LEEMPS position in the legal text, so there is no ambiguity about who submits models and in what circumstances this is done by the Network Operator on behalf of the Generator. Consideration was made in the updated drafting to reflect the challenges represented by LEEMPS. It was also noted that regarding modification to LEEMPS, DDRC5.3 and EREC G99 20.3.2 had provisions for change notifications to Network Operators, , but only for EREC G99 compliant LEEMPS. The Distribution Code requires a small modification to apply the requirements to LEEMPS connected under EREC G59.

Workgroup Discussion in the Interface Point definition (PCA 6.7) The Workgroup discussed the use of the term Interface Point in the Planning Code, particularly in relation to LEEMPS.

Workgroup members noted that, for embedded assets, the Grid Code term Interface Point is inappropriate as it is defined in relation to Offshore assets. For Embedded Plant, NESO’s modelling needs do not always align with ownership, control, or modelling responsibility. In many cases, generators cannot reasonably provide EMT models

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Public beyond assets they own or control (e.g. upstream transformers or network equipment owned by a DNO, TO, or OFTO).

As a result, obligations framed solely by reference to a point remote from that party’s relevant Assets risk extending requirements beyond what parties can practically deliver.

The workgroup expressed concern that relying on a point remote from that party’s assets could create ambiguity over scope and responsibility, particularly where control of voltage or dynamic behaviour occurs at a different location from the formal interface definition. This was seen as increasing the risk of unintended non‑compliance, especially for legacy LEEMPS.

To address this, members supported clarifying the point where obligations apply so they do not assume modelling responsibility for assets outside that party’s control. This aligned with the wider agreement to use asset‑focused drafting, referring where appropriate to the relevant plant or apparatus rather than relying on interface‑point definitions to determine obligations.

The Workgroup agreed that references to interface point should not be used and supplemented or clarified so that EMT modelling obligations relate to the relevant Plant or Apparatus a party can reasonably control. The revised Planning Code text reflecting this approach was accepted by the workgroup.

Electrical Standard (EMT Modelling) – Summary of Workgroup Discussions The Workgroup undertook a detailed review of the proposed Electrical Standard, with particular focus on its scope, structure, and suitability for application to both new and legacy assets. There was broad support for the principle of consolidating EMT modelling requirements within a formally governed Electrical Standard, noting that this would provide transparency, consistency, and appropriate governance, and would replace reliance on informal guidance material. This approach was also taken in response to stakeholder feedback at several Grid Code related forums which indicated concerns that guidance documents could be used to introduce requirements on Users. Concerns were raised regarding the content and structure of the draft document. Workgroup members noted that parts of the draft, particularly sections describing collection processes and timelines, largely duplicated existing Grid Code obligations without adding clarity.

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Public The Workgroup agreed that the Electrical Standard should avoid restating Grid Code requirements verbatim, but should instead add value by providing technical detail, explanatory material, and practical implementation guidance, especially where retrospective application to legacy Plant is required.

Regarding the treatment of legacy assets. Workgroup members emphasised that standards developed for new connections could not be applied retrospectively without flexibility, given constraints such as obsolete technology, unavailable OEM support, historic design choices, and software limitations.

The Workgroup agreed that the Electrical Standard must explicitly recognise these constraints and describe pragmatic or alternative approaches where full compliance with current EMT specifications cannot reasonably be achieved. These challenges are recognised in the draft.

Extensive discussion took place regarding 32-bit and 64-bit EMT models. Industry participants highlighted that many legacy assets, and some relatively recent ones, cannot readily provide 64-bit models. Concerns were raised that rigid requirements in this area could be impractical and create unnecessary barriers to compliance. The drafting of the Electrical Standard also addresses this issue. The Workgroup agreed that clearer drafting was required, including explicit recognition of legacy limitations and flexibility in acceptable approaches.

The Workgroup also discussed prioritisation and resourcing, noting concerns about the cumulative impact of multiple EMT model requests on OEMs, developers, and asset owners. Members sought greater clarity on how NESO would prioritise and sequence requests, particularly where multiple assets in the same geographic area may be affected. It was noted that, in such situations a phased approach to notification and submission will be followed to ensure an orderly and manageable process. These scenarios are also reflected in the draft Electrical Standard.

Language and terminology within the draft Electrical Standard were also scrutinised. Members expressed concern that terms such as “required” could be interpreted as introducing new obligations beyond those set out in the Grid Code. The Workgroup stressed the importance of careful drafting to ensure that the Electrical Standard supports, but does not extend or unintentionally amend, Grid Code obligations.

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Public Interactions with DCUSA The Workgroup discussed DCUSA as a potentially impacted Industry Code. A requirement to submit EMT models by Embedded Medium Power Stations not subject to a Bilateral Agreement (Data to be provided by relevant Network Operator to NESO) arose from this Modification.

The Proposer engaged the DCUSA Administrator in October 2025 and informed them about this ongoing work and requested for audience to discuss progress of GC0168 and potential DCUSA interactions.

Possible derogation route – what happens if a model is not available and cannot be obtained or cannot be obtained within the timescales given. The drafting of PC.A.9.2.2 was agreed by the Workgroup as providing the solution to this challenge. Chapter 5 in the draft Electrical Standard also added detail on potential approaches to develop legacy Plant model recognising the challenges that Users will encounter in developing EMT models for this Plant.

Primary challenges noted include a) lack of data, b) obsolete technology, c) compatibility issues and d) original design versus current regulatory requirements.

The Electrical Standard can be found in Annex 08.

GC0139 interaction Due to the interaction with GC0139, which could be implemented ahead of GC0168, the Workgroup proposed that two sets of legal text for both the original and alternative solutions. This approach was proposed as futureproof (with options to cater for whichever decision was made for GC0139 by the Regulator), avoid a send back by Ofgem and save Industry time.

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Public Terms of Reference Overview a) Implementation and costs The majority of Workgroup members noted that they would not support this modification unless an appropriate cost-recovery mechanism is in place. b) Review draft legal text Legal text for both solutions have been reviewed and agreed by the Workgroup, including versions that can be used should modification GC0139 be implemented ahead of GC0168. c) Consider whether any further Industry experts or stakeholders should be invited to participate within the Workgroup to ensure that all potentially affected stakeholders have the opportunity to be represented in the Workgroup. Demonstrate what has been done to cover this clearly in the report. Good representation in the Workgroup from all sections of the industry. d) Consider EBR implications The Workgroup agrees there is no impact on EBR. e) Consider a cost recovery mechanism to receive the model data required to share with a CUSC Workgroup. CUSC modifications CMP456 and CMP466 have been raised by a GC0168 Workgroup member to consider cost recovery within the CUSC.

f) Consider the use/introduction into the Legal Text of generator classifications types C, D as opposed to Medium and Large. Discussed during the Workgroup and agreed that it would add unnecessary complexity to add these definitions. g) Consider approach on collecting models and reference to published guidance/phased application of approach. The Workgroup has agreed on a new Electrical Standard, which details the process of model collection. h) Consider codifying the list of Users who are required to submit EMT models. Completed and included within the draft legal text.

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Public i) Consider the scenario where a User is unable to provide an EMT model. The drafting of PC.A.9.2.2 was agreed by the Workgroup as providing the solution to this challenge. j) Consider whether there is a need for any consequential changes to the DCode and / or DCUSA. The Proposer engaged the DCUSA Administrator in October 2025 and informed them about this ongoing work and requested for audience to discuss progress of GC0168 and potential DCUSA interactions. k) Consider whether there is a need to obtain EMT models from medium power stations embedded in distribution networks and, if so, the mechanism for engaging with the host DNO and the Generator and the process to be followed in the event that the Generator is unable to provide the EMT models or would incur significant costs in doing so. CUSC modifications CMP456 and CMP466 have been raised by a GC0168 Workgroup member to consider cost recovery within the CUSC. l) Consider if we are reflecting international practice including observation of the modelling developments proposed for RfG 2.0 and HVDC 2.0 Considered by the Workgroup and included within the Workgroup Report.

What is the impact of this change?

Proposer’s assessment against Grid Code Objectives Relevant Objective Identified impact (i) To permit the development, maintenance and operation of an efficient, coordinated and economical system for the transmission of electricity Positive EMT models will be required to carry out analysis such as system oscillation, inverter stability, ToV analyses, especially noting that EPCs require a more detailed model than that available

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Public from a current RMS representation. Without being able to conduct this type of analysis using EMT models, it could lead to unnecessary investment by Users or TOs, significant increase in constraint costs, single events leading to tripping of a number of generators and could ultimately lead to loss of supply. (ii) Facilitating effective competition in the generation and supply of electricity (and without limiting the foregoing, to facilitate the national electricity transmission system being made available to persons authorised to supply or generate electricity on terms which neither prevent nor restrict competition in the supply or generation of electricity); Positive As new generation technologies connect to the network, most of which will rely on power electronic converters, more detailed models will be required not only in respect of the new generation itself but also the impact they have on existing generation. This will drive greater impact on competition. (iii) Subject to sub-paragraphs (i) and (ii), to promote the security and efficiency of the electricity generation, transmission and distribution systems in the national electricity transmission system operator area taken as a whole; Positive Due to the increase in EPCs connecting to the grid which is in line with the UK government’s Net Zero ambition, this modification will enable a greater volume of EPCs to connect whilst ensuring a more thorough

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Public *See Electricity System Operator Licence

evaluation of the source of oscillations or disturbances and to plan mitigating actions. (iv) To efficiently discharge the obligations imposed upon the licensee by this licence* and to comply with the Electricity Regulation and any relevant legally binding decisions of the European Commission and/or the Agency; and Positive EMT analysis is important for investigating the dynamics of converters and control interactions with the System, which enables the NESO to meet its licence obligations. (v) To promote efficiency in the implementation and administration of the Grid Code arrangements Positive At the moment, NESO has a need for analysis to be done, and in many cases the obligations for that analysis are on new entrants in the first instance, without the data to support the analysis. Then beyond that we have requirements to support the planning and operation of the system which are lacking these same models. This modification will give NESO access to models of already connected Plants.

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Public Proposer’s assessment of the impact of the modification on the stakeholder / consumer benefit categories Stakeholder / consumer benefit categories Identified impact Improved safety and reliability of the system Positive When generators provide EMT models to the NESO, it will be able to carry out pre-fault and post-fault analysis studies, the outputs of which will lead to accurate operational decisions in the interest of safety and reliability of the system which could ultimately lead to lower operational costs for the benefit of the end consumer. Lower bills than would otherwise be the case

Positive More accurate models will enable greater Transmission System optimisation which would have the benefit of reducing consumer bills. Benefits for society as a whole Positive More accurate models will enable greater Transmission System optimisation and reduce the need to run other Plant to compensate for inaccurate models. Reduced environmental damage Positive More accurate models will enable greater Transmission System optimisation and a reduced need to run other Plant, some of which could be carbon based which will have a positive environmental impact. Improved quality of service Positive More accurate models provide greater optimisation resulting in lower bills and therefore improving quality of service.

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Public First Workgroup Vote The Workgroup met on 09 May 2025 to carry out their Workgroup Vote. The Workgroup concluded by majority that the Original better facilitated the Applicable Objectives than the Baseline. This vote was superseded by a second Workgroup Vote following the Panel send back. Second Workgroup Vote The Workgroup met on 24 March 2026 to carry out their second Workgroup Vote. The Workgroup concluded by majority that the Original better facilitated the Applicable Objectives than the Baseline. For reference the Applicable Grid Code Objectives are: i. To permit the development, maintenance and operation of an efficient, coordinated and economical system for the transmission of electricity; ii. Facilitating effective competition in the generation and supply of electricity (and without limiting the foregoing, to facilitate the national electricity transmission system being made available to persons authorised to supply or generate electricity on terms which neither prevent nor restrict competition in the supply or generation of electricity); iii. Subject to sub-paragraphs (i) and (ii), to promote the security and efficiency of the electricity generation, transmission and distribution systems in the national electricity transmission system operator area taken as a whole; iv. To efficiently discharge the obligations imposed upon the licensee by this license* and to comply with the Electricity Regulation and any relevant legally binding decisions of the European Commission and/or the Agency; and v. To promote efficiency in the implementation and administration of the Grid Code arrangements. * See Electricity System Operator Licence

The Workgroup concluded by majority that WAGCM1 better facilitated the Applicable Objectives than the Baseline. The Workgroup did not agree that the original solution better facilitated the Applicable Objectives than the Baseline. Option Number of voters that voted this option as better than the Baseline (out of 7 votes) Original 3 WAGCM1 6

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When will this change take place? Implementation Date 10 Business Days following Authority decision. Date decision required by As soon as practicable. Implementation approach No systems will have to change as a result of this modification. Interactions ☒CUSC ☐BSC ☒STC ☐SQSS ☐European Network Codes ☐ DCODE ☐ EBR Article 18 T&Cs1 ☒DCUSA ☐Other modifications

☐Other

Two CUSC modifications have been raised relating to a cost recovery mechanism: CMP456: Cost recovery for legacy plant in relation to GC0168 CMP466: CMP456 Consequential Charging Modification DCUSA: DCUSA Change team has been contacted to consider interactions. Corresponding STC modification CM097 – Electromagnetic Transient (EMT) and Root Mean Square (RMS) Model Submission for Transmission Owners (TOs). STCP modification PM0147 has been raised to introduce a new System Operator- Transmission Owner Code Procedure (STCP) 12-2. A new Electrical Standard will go through the governance process in the General Conditions (GC11) following an Authority decision.

1 If the modification amends any of the clauses mapped out in Annex GR.B of the Governance Rules section of the Grid Code, it will change the Terms & Conditions relating to Balancing Service Providers. The modification will need to follow the process set out in Article 18 of the Electricity Balancing Regulation (EBR – EU Regulation 2017/2195). All Grid Code modi- fications must be consulted on for 1 month in the Code Administrator Consultation phase, unless they are Urgent modifi- cations which have no impact on EBR Article 18 T&Cs. N.B. This will also satisfy the requirements of the NCER process.

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Public How to respond Code Administrator Consultation questions • Please provide your assessment for the proposed solution(s) against the Applicable Objectives versus the current baseline. • Do you have a preferred proposed solution? • Do you support the proposed implementation approach? • Do you have any other comments? • Do you agree with the Workgroup’s assessment that the modification does not impact the Electricity Balancing Regulation (EBR) Article 18 terms and conditions held within the Code?

Views are invited on the proposals outlined in this consultation, which should be received by 5pm on 08 June 2026. Please send your response to grid.code@neso.energy using the response pro-forma which can be found on the modification page. If you wish to submit a confidential response, mark the relevant box on your consultation proforma. Confidential responses will be disclosed to the Authority in full but, unless agreed otherwise, will not be shared with the Panel or the in- dustry and may therefore not influence the debate to the same extent as a non- confidential response. Acronyms, key terms and reference material Acronym / key term Meaning BSC Balancing and Settlement Code CCGT Combined Cycle Gas Turbine CMP CUSC Modification Proposal CUSC Connection and Use of System Code DCC Demand Connection Code DCUSA Distribution Connection and Use of System Agreement

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Public DNO Distribution Network Operator EBR Electricity Balancing Regulation EMT Electromagnetic Transient EPC Electronic Power Converter GCRP Grid Code Review Panel HVDC High Voltage Direct Current LEEMPS Licence Exempt Embedded Medium Power Stations NESO National Energy System Operator OEM Original Equipment Manufacturer PSCAD Power Systems Computer Aided Design RfG Requirements for Generators RMS Root Mean Square SQSS Security and Quality of Supply Standards STC System Operator Transmission Owner Code STCP System Operator-Transmission Owner Code Procedure TO Transmission Owner ToR Terms of Reference ToV Transient Overvoltage T&Cs Terms and Conditions WAGCM Workgroup Alternative Grid Code Modification

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Public Annexes Annex Information Annex 01 GC0168 Proposal Form Annex 02 GC0168 Terms of Reference Annex 03 GC0168 Legal Text Annex 04 GC0168 List of types of Users required to provide EMT models Annex 05 GC0168 Workgroup Consultation Summary Annex 06 GC0168 Workgroup Consultation Responses Annex 07 GC0168 Second Workgroup Vote Annex 08 GC0168 Proposed Electrical Standard - EMT Modelling Requirements and Process for Submitting EMT Models Annex 09 GC0168 WAGCM1 Proposal Form Annex 10 GC0168 WAGCM1 Legal Text Annex 11 GC0168 Original Legal Text adjusted for GC0139 Implementation Annex 12 GC0168 WAGCM1 Legal Text adjusted for GC0139 Implementation Annex 13 GC0168 Workgroup Attendance Record Annex 14 GC0168 Workgroup Action Log