The purpose of system warnings is to communicate to all parties (via BMRS / Fax) possible or actual future power shortages, system problems or imminent demand reductions. There is an expectation on all parties not to make the situation worse (do not start outages), if possible to make the situation better (make more plant available), to take preparatory action if required (applicable to DNO), and to review imbalance positions and trade accordingly. In the case of a Risk of System Disturbance, users are requested to warn their operational staff and maintain plant and apparatus in a condition best able to withstand the anticipated disturbance.

There are 5 categories of system warning:

• Inadequate System Margin (NISM) OC7.4.8.5 - there is insufficient system margin to meet demand and reserve requirements - contingency reserve has been eroded.
• High Risk of Demand Reduction (HRDR) OC7.4.8.6 - there is insufficient margin to meet operating reserve requirements.
• Demand Control Imminent (DCI) OC7.4.8.7 - there is still insufficient margin to meet operating reserve requirements close to real-time. Demand Control is expected in the following 30 minutes.
• Risk of System Disturbance (RSD) (e.g. severe storm) OC7.4.8.8 - Likelihood of system disruption, often localised warning targeted at Generators / DNOs in a specific area.
• Cancellation of GB System Warning OC7.4.8.9.

System warnings are shown on the summary page only while they remain in force. Details of cancelled warnings are available on the History page.

Please note that NISMs are only the first stage of the system warning process, and the least severe category of margin warning. They normally indicate an erosion of the contingency reserve requirement several hours ahead of real-time, and should not necessarily be interpreted as an indication that demand is not likely to be met in full.

Following the implementation of P219 in November 2008, two consistent streams of demand data are now available on the BMRS from National Grid, namely National Demand (ND) and Transmission System Demand (TSD) as defined below.

National Demand (ND) is the total demand in England, Wales and Scotland including transmission losses but excluding station transformers load, pumped storage demand and interconnector demand. This is defined in the Grid Code as the term "GB National Demand".

Transmission System Demand (TSD) is the total demand in England, Wales and Scotland including transmission losses and also including station transformers load, pumped storage demand and interconnector demand. This is defined in the Grid Code as the term "GB Transmission System Demand".

In the interests of consistency, it was agreed that all demands shown on the electricity summary page would be based on the TSD - Transmission System Demand definition throughout. This includes outturn and forecast values in all timescales shown on the summary page. The only exception to this is the daily energy volumes which are based on the daily sum of the National Demand (ND) outturns (INDO). This is because it is not possible to derive reference values for daily energy based on Transmission System Demand (TSD) because the interconnector demand component of TSD is not related to temperature conditions.

The Demand Forecasts shown here are the National demand forecasts made by National Grid based on historically metered generation output for Great Britain. As above, they are TSD-based, i.e. they include transmission losses, station transformers load, pumped storage demand and interconnector demand. The outturn demands are also TSD-based.

Please note:

The dates shown in this table are purely indicative, being based on peak demands calculated using operational metering, and so the final triad dates may well be different from these dates.
When available, the final definitive triad dates ( as calculated in accordance with the Statement of Use of System Charging Methodology (chapter 4) at http://www.nationalgrid.com/uk/Electricity/Charges/chargingstatementsapproval/ ) based on the latest available settlement metering data, are published on the National Grid website at http://www.nationalgrid.com/uk/Electricity/Charges/usefulinfo/ normally at the end of March each year for the preceding Nov-Feb winter triad period.

The Triad is used as a short hand way to describe the three settlement periods of highest transmission system demand within a Financial Year. These represent the half hour settlement period of system peak demand and the two half hour settlement periods of next highest demand. These two periods of next highest demand must be separated from the system peak demand and from each other by at least 10 Clear Days, between November and February of the Financial Year inclusive. The Indicative Triad Demand Information on the BMRS does not constitute a forecast of when the Triads are expected to occur, but is a summary of publicly available demand data that may be of assistance in parties' own triad forecasting activities.

Highest Demands so Far

Between November and February, this table shows the three settlement periods of highest demand (subject to the 10 Clear Day rule) in the triad period so far (i.e. since the start of November). Between March and October it shows the three settlement periods of highest demand in the previous triad period. In both cases the demand figures are based on operational metering, not settlement metering, and are therefore indicative only.

Highest Forecast Demands

Between November and February, this table shows the three highest demand peaks in the remainder of the triad period (i.e. up to the end of February), based on the 2-52 week ahead demand forecast. Between March and October it shows the three highest demand peaks for the next triad period, again based on the 2-52 week ahead demand forecast. Because the source data is at a weekly resolution rather than a daily resolution, the 10 Clear Day rule cannot be accurately applied. Instead, as an approximation to it, only demand peaks in non consecutive weeks are shown.

Indicative Triad Demand Information on the summary page is based on the TSD definition (see "Peak Demands" info tab).

Following the end of each Settlement Period two Imbalance Prices, known as System Buy Price (SBP) and System Sell Price (SSP) respectively, are calculated and published on the BMRS. These prices are purely indicative, as they are calculated before all the necessary information for definitive calculations, such as actual transmission losses, is available. The definitive calculations for Settlement purposes are carried out separately, several days later. These definitive calculations may also include amendments to the Balancing Mechanism data following post event analysis.

The Imbalance Prices are used for the settlement of the energy imbalances incurred by participants - that is to say, the differences between the volumes of energy which a participant contracts for (in all trading, not just in the Balancing Mechanism) and the volumes actually generated or consumed. A participant whose imbalance is a net spill on to the system will be paid for the energy which is put on to the system, at the System Sell Price. A participant whose imbalance is a net shortfall will have to pay for the volume of energy which is in deficit, at the System Buy Price. It is thus equally possible for generators or consumers alike to be out of balance in either direction in a given half hour.