'User' Based Forecasts

ACS Peak GB Demand

This chapter focuses on the demand defined in the Glossary of Terms as "ACS Peak GB Demand" and discussed in Demand terminology. Accordingly, the "ACS Peak GB Demand" includes, amongst other things, distribution and transmission losses, station demand (i.e. station auxiliary demand supplied through the station transformers) and exports to External Systems.

Row 1 of Table 2.1 gives the 'Unrestricted' peak demand (as defined above) on the GB transmission system in ACS (average cold spell) conditions based on the projections made by the system 'Users'. This demand increases from the provisionally estimated outturn of 61.3GW in 2006/07 to 67.4 GW by 2013/14, which represents a growth of 1.4% per annum. An explanation of the ACS correction procedure is given in the Supplementary Demand Information section of this chapter. The forecasts are in respect of the time of the simultaneous peak demand on the GB transmission system and accordingly take account of any diversity between the individual peak demands on each of the systems of the three Transmission Licensees (i.e. NGET, SPT and SHETL). As a point of interest, no pumping demand at pumped storage stations is assumed to occur at peak times.

Peak demands represent the highest demands on the GB transmission system to be met by Large Power Stations (directly connected or embedded), Medium and Small Power Stations which are directly connected to the GB Transmission System and by electricity imported directly into the GB transmission system from External Systems. They are therefore net of any allowance the User makes in his forecasts for the output of Medium Power Stations, Small Power Stations or Customer Generation embedded within distribution networks, and imports across embedded External Interconnections to these systems (i.e. Isle of Man). The allowances made by the Users for such embedded generation is discussed in Embedded and Renewable Generation; Tables 4.1 and 4.2 are of particular relevance.

As mentioned above, both the distribution and transmission system losses are included in the demand forecasts, as are exports across External Interconnections to External Systems. The distribution losses are included as part of the Users' submissions and estimated transmission losses are made at the time the forecast is formulated. Pragmatic assumptions, based on historical evidence and market intelligence, are made with respect to exports to External Systems. For instance, while the Moyle inteconnector between Scotland and Northern Ireland is capable of a 500MW export, a 300MW export is assumed for the time of the GB peak demand.

Since the introduction of the British Electricity Trading and Transmission Arrangements (BETTA) in 2005, Seven year Statements have been extended to encompass the GB transmission system. In addition to widening demand forecasts to include Scotland, the ACS correction methodology was also updated. An explanation of the ACS correction procedure is given in the Supplementary Demand Information section of this chapter.

One particular change to the ACS methodology was made in order to address the significant fall-off experienced in the amounts of demand control being notified by suppliers under the Grid Code. This was making it increasingly difficult to derive realistic historical 'unrestricted' demands, i.e. actual metered ('restricted') demands plus notified demand control, on which to base the ACS correction. As a result, ACS demands are now calculated from historical 'restricted' rather than 'unrestricted' demands. (For the avoidance of doubt, 'restricted' demand is the level of demand after taking into account any demand control, i.e. it represents the actual metered outturn, whereas 'unrestricted' demand takes no account of the impact of any demand control).

Infrastructure planning for the transmission system continues to be based on ACS 'unrestricted' demands - a prudent approach to transmission planning made on the basis that demand control cannot be fully relied upon to be enacted at peak times. Historical 'unrestricted' ACS peak demands are now derived by analysing winter weekday evening peaks to estimate the total amount of customer demand control (both notified and un-notified) in force at such times. The resulting amounts, approximately 1GW, are similar to the levels of load management being notified during the 1990s. Adding the load management estimates onto the historical 'restricted' ACS peak outturns yields 'unrestricted' demands which form the basis of the ACS outturns and forecasts given in this Statement.

As a cautionary note, other related documents may refer to 'restricted' rather than 'unrestricted' demands, a case in point being National Grid's 'Winter Outlook Report'. Naturally, therefore, care should be exercised when making comparisons between demand forecasts on different bases.

As previously mentioned, station demands (i.e. power station demand supplied via the station transformers directly from the GB transmission system) are included in the main forecast ACS peak GB demand presented in row 1 of Table 2.1.

However, Table 2.1 also presents (in rows 3 and 5) the 'User' demand forecasts excluding power station demand. This recognises that Transmission Entry Capacity (TEC), which is a key term used to describe power station output, is used extensively in other analyses presented elsewhere in this Statement (e.g. power system analyses and plant margin evaluation). By definition, TEC is net of station demand and accordingly ACS Peak GB Demand excluding station demand should be used where relevant to avoid it being double-counted.

In addition, row 5 of Table 2.1 presents the 'User' demand forecasts excluding exports across the Moyle interconnector between Scotland and Northern Ireland as well as excluding power station demand. This forecast is compatible with the generation ranking order of Table 7.1 , which treats exports to Northern Ireland as negative generation. Table 7.1 is presented in GB Transmission System Performance.

Figure 2.1 shows recent ACS peak outturns and the current 'User' forecasts of ACS peak demand on the GB transmission system given in row 1 of Table 2.1.

Figure 2.1

Click to load a larger version of Figure2.1 image

It is explained under the Customer Demand Data section that, while the local peak demand is used for Grid Supply Point planning, the demand at the time of the GB system peak is used for infrastructure planning purposes. That section also explains that transmission losses are added to the Users' demand submissions, after which they are adjusted such that the aggregate of 'User' demand projections for the base year (2006/07) is scaled to the provisional or, if known, final ACS corrected outturn for the winter. The resulting adjustment factor is applied to subsequent years, thus retaining customers' forecast aggregate annual growth rates. These forecasts are amended when necessary in SYS Quarterly Updates to align with final base year ACS outturns.