Transmission Active Network Management (TANM) & Appendix G

The Appendix G process is used to inform National Grid Electricity System Operator (NGESO) of the amount of generation per GSP by National Grid Electricity Distribution (NGED). The available capacity is subsequently calculated by NGESO based on this and can lead to the initiation of reinforcement, operational requirements or Load Management Schemes to permit new connections.

Transmission Active Network Management (TANM) ensures that new generation connected on a GSP does not cause a reverse power overload of the transmission assets, usually the Super Grid Transformers (SGTs). This is achieved my monitoring the loading on the SGTs and curtailing generation according to a LIFO stack where required.

While both the Appendix G and TANM look at all generation on a GSP, the categorisation and purpose of them are different.

The Appendix G is split into Parts 1 – 4, where Part 1 contains legacy sites and Parts 2 – 4 contains those with requirements or restrictions as defined by NGESO. See Appendix G Information page for more details. As a NGESO restriction, TANM will therefore not be applicable to Part 1 sites but could be applicable to sites in Part 2, 3 or 4. Part 5 of the Appendix G will list the total of all generation on a GSP. This total is be subtracted from the Total Aggregated Developer headroom, as calculated by NGESO, to give the Materiality headroom. The value is based on static, pre-calculated values and the capacity of the GSP. It does not account for generators that have not accepted an offer from NGED.

TANM is used when NGED have identified a risk that the SGTs will experience a reverse powerflow overload. This is done using a load flow calculation that accounts for changes in background demand and running arrangements. In the production of a TANM curtailment report, generators will have relevant output profiles applied; this diversity can reduce the loading seen when compared to the calculation done using the maximum capacity of all generators. The report will also include generators that have applied but not yet accepted, leading to larger volumes of generation being considered when compared to that within the Appendix G.

Below is an example of a list of generators on a GSP, which has a reverse powerflow capacity of 120MW and background minimum summer demand of 30MW. This demonstrates that the part of Appendix G that a generator falls into does not have a direct correlation to a TANM requirement.

Depending on the previously agreed capacity between NGED and NGESO the Total Aggregated Developer headroom may be breached by the addition of generator H, for example. This could be increased through a Modification Application.