Gemini changes overview

Get an overview of all the current Gemini System changes in one place.

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    Current Gemini changes

    As part of our role as Central Data Service Provider (CDSP), we regularly improve our services and systems.


    XRN 5564 - Gemini Sustain Plus

    Target delivery date: 30 September 2024

    XRN 5564 was submitted by National Gas Transmission, to request for our managed service provider Correla to upgrade the Gemini System.

    This programme, known as Gemini Sustain Plus, will enable us to modernise the Gemini System. Gemini will continue to provide Capacity and Balancing services, while also delivering system compliance against new Uniform Network Code (UNC) modifications.

    Scope of delivery:

    • Conversion of all the current Gemini functionality and code into modern (Java microservices) code
    • New and existing functional requirements
    • Delivery will also align to non-functional requirements
    • Consolidation and redesign of the Gemini user interface screens
    • Implementation of Azure cloud infrastructure components (including replacing the current Oracle database)
    • Improvements to the support service arrangement to respond better to incidents and change (in-line with new service levels and KPIs)


    The Gemini Sustain Plus programme will be delivered over a two-year period.

    The target delivery date is currently planned for 30 September 2024.

    More information on this project is available on our dedicated Gemini Sustain Plus webpage.


    Previously delivered changes

    Below is an overview of the changes we have made to the Gemini System, with delivery dates and helpful links.

    XRN5579 was a parent change to enable the delivery of Gemini System changes for the 2022/23 financial year.

    Child change proposals were submitted to accommodate regulatory changes and other system amendments as and when they were required.

    NGT and Distribution Network Operators (DNOs) are now able to enter LTFS details for a period of time. The change also introduced ‘cancel’ and ‘accept’ functionality to reject/accept the flow swap details.

    This change only impacted NGT and DNO users of the Gemini System.

    Implemented on 19 March 2023.

    National Gas Transmission raised a change proposal for Xoserve to progress the associated Gemini System changes required to implement either UNC0779 and its alternative 0779A in a single solution, whilst Ofgem considered them for approval.

    The parameterised solution was updated following Ofgem's decision to reject these modifications. You can find details of the proposals for UNC 0779 and 0779A on the Joint Office website.

    Find out more

    This change delivered the following improvements to Gemini, as described in the roadmap of our 2021 Business Plan:

    • 5368.1 – Batch Process Review – streamlined Gemini Batch Jobs – Internal change
    • 5368.2 – Single Sign-On – enhanced user experience through single sign-on (SSO), multi-factor authentication (MFA) and self-serve password reset functionality – Implemented 29 May 2022
    • 5368.3 – API Enhancements – improved user experience with Gemini reports and implement application programming interfaces (APIs) over the internet gateway – Implemented 7 May 2022
    • 5368.4 – Workflow Automation – feasibility and analysis to identify improvements – Internal change
    • 5368.5 – SiteMinder Upgrade – keeping SiteMinder within support by upgrading to the latest software – Implemented 22 May 2022 

    These improvements were carried out as separate workstreams under a portfolio of work.

    For more information or if you have any queries please email

    You can find more information on the single sign-on solution on our Gemini Single Sign-On webpage.

    This change was to deliver the associated Gemini System changes required to implement the following three modifications in late April 2022:

    • Mod 0752S Introduction of Weekly Entry Capacity Auctions
    • Mod 0759S Enhancement to NTS Within-Day Firm Entry and Exit Capacity Allocations
    • Mod 0755S Enhancement of Exit Capacity Assignments

    Full details of XRN5393 can be found on the Xoserve change proposal webpage.

    There is more information on our XRN 5393 Change Awareness Training e-learning course.

    If you have any queries please email

    Implemented on 24 April 2022.

    This change was to deliver the associated Gemini system requirements to implement UNC Modification 0785.

    This change became effective for the gas day of 1 March 2022, the first applicable auction was the rolling day ahead auction on the 28 February 2022.

    • MOD 0785 Application of UNC Processes to an Aggregated Bacton (exit) Interconnection Point  

    Full details can be found in the approved Change Pack 2973.1and the Shipper awareness pack for this change delivery.

    For more information or if you have any queries please email

    Delivered in two parts, part A on 25 February 2022 and part B on 3 April 2022.

    GSE was a change project to adapt and improve Gemini, delivering over 65 enhancements based on the needs of a changing industry.

    The changes and enhancements included:

    • changes to existing screens
    • changes to the Meter Look-Up facility
    • introduction of two new report screens
    • introduction of a dashboard facility
    • introduction of 13 new application programming interfaces (APIs)
    • enhancement to one existing API – view re-nomination detail
    • provision of alternative access to APIs via the internet

    Except for access to APIs through the internet, all other changes were delivered on 25 July 2021.

    The internet-based APIs were launched in Spring 2022.

    With the implementation of Modification 0678A on 1 October 2020, the Optional Commodity Charge (OCC), also known as ‘short-haul discount’, ceased to exist.

    As part of Modification 0728 - Introduction of a Conditional Discount for Avoiding Inefficient Bypass of the NTS - an alternative charging method was developed which will be an NTS Optional Capacity Charge.

    It proposed a change to Capacity based Transmission Charges to provide a discount on routes where the Exit Point is less than or equal to 28 km from the entry point.

    Modification 0728B received Ofgem approval for delivery and the associated changes became effective from 1 October 2021. This Change to Gemini was required in order to implement these changes in the system.

    More information can be found in the May 2021 Extraordinary Change Pack and in the MOD0728B online awareness material.

    Delivered in October 2021.


    There have been several instances of incorrect prices and/or capacity values being entered by Users during the bid capture process for short term capacity auctions in both Gemini and Gemini Exit.

    These bid errors have impacted on other auction participants (via invoicing) due to neutrality processing. There was also increased potential for the application of higher overrun charges at sites where bid errors have occurred.

    The purpose of the Change

    The Change was to reduce the risk of Users making bid submission errors by mandating the use of User and/or Shipper Preferences allowing for Bid Validation to be put into place prior to bid capture/requests.

    The requirements as specified in approved Uniform Network Code (UNC) Modification 0745 will make the setting of the existing bid parameter function mandatory in the Gemini System.

    Users must first set up User/Shipper preferences for the Validation Parameters – Bid Price and Bid Capacity for all relevant locations and short-term auctions before placing the bids/requests.

    This change only applies to the below short-term auction Methods of Sale (MoS) and became effective 1 October 2021:

    Gemini Entry

    • DADSEC
    • WDDSEC
    • DISEC

    Gemini Exit

    • DADNEX
    • WDDNEX
    • DONEX

    More information can be found in the May 2021 Change Pack and in the MOD0745 External Screen Pack.

    Delivered in October 2021.

    The Gemini System was built on ageing equipment, with a 2017 health assessment recommending a new platform on new hardware.

    Download the CP4550 Change Proposal from the Joint Office of Gas Transporters website.

    The project relocated the Gemini System to new equipment.

    We worked with National Gas Transmission to understand the:

    • key requirements for hosting the Gemini System
    • potential impacts to customers and interfacing systems

    This project was implemented after the GB Charging changes (CP4376) in 2020.

    This was a large project, with a key element being the delivery of changes from the Great Britain Charging Review.

    This was a significant change to how National Gas Transmission received its revenue.

    The change was also likely to impact future UK Link releases.

    Download the CP4376 Change Proposal from the Joint Office of Gas Transporters website.

    The CP4376 project was split into parts A and B.

    Part A – CP4376

    Part A was implemented in November 2018.

    Part B – CP4376

    The majority of the changes are included in Part B with the Analysis and Design phase which started in May 2018.

    Full implementation was expected in September 2019.

    A number of components were effective from 1 October 2020 and details of these can be obtained from the Gemini Projects team.

    The Capacity Allocation Mechanism (CAM) code requires a capacity conversion mechanism to be offered to Users at Interconnection Points (IPs). Modification ‘616 Capacity Conversion mechanism for Interconnection Points’, introduces this requirement to the Uniform Network Code (UNC).

    The introduction of the conversion mechanism risked unfair competition. With the processing of the mechanism, Shippers who were matching unbundled capacity on either side of the IP would also need to have purchased bundled capacity because of prioritisation restrictions.

    To ensure fairness, the existing buyback functionality of Gemini can now be used. By using the facility and introducing a new manual process by National Gas Transmission, it ensures compliance with the EU obligation and a level playing field for all Users.

    Download the CP4368 Change Proposal from the Joint Office of Gas Transporters website.

    The change was made live on 18 February 2018.

    A new Entry Method of Sale has been introduced to support the change. Two new charge types have been created on the NTS Entry invoice:

    • CEC Capacity Conversion Entry Charge
    • CEA Capacity Conversion Entry Adjustment

    These charge types were effective from 1 March 2018.