The demand is continually changing, thus generation has to be scheduled and dispatched to track it, plus provide adequate response and spare, which can react in the appropriate timescales to cater for innacuracies in demand prediction or unexpected generation output. Here are some examples of different weekday demand patterns in Great Britain (GB).
The metering from all generation sources +/- interconnector flows will of course summate to the demand as the system is in balance. The system operator will maintain contiunous and integrated metering for the main plant, transmission system and interconnection flows. Total and nodal demand histories are derived from this and stored.
Demand prediction is normally carried out from analysis of total historical demand data against weather for cardinal point periods (time of day) in the relevant groups (day of the week, period of the year) with forecast base patterns and weather then applied to predict the future demand profile.
Operation of coventional main generation is of course under the plant operator's control, with output commited and and dispatched through system operation and market mechanisms. The instructed profile is compared with the demand prediction and plant ordering and dispatch adjusted to match across all lead timescales. This diagram shows the business elements of the 'unbundled' industry in Great Britain.
The predicted loading profile on transmission is also derived by application of nodal demand and generation data, derived from nodal history and and instructed generation output and applied to the grid technical data. The system is then analysed to ensure it will be secure - loading, voltage and stability in the steady state and after fault.
For demand prediction to be accurate, the metering must not be 'distorted' by omission of large amounts of embedded generation meters from the generation summation.
Passive distribution systems are designed and customer connections analysed to ensure the system will be secure at the peak and trough conditions in each year. Thus the passive system is always sized to meet the maximum demand on it.