There is generally about 1.5 GW of so called spinning reserve
NG pays to have up to 8.5 GW of additional capacity available to start immediately but not running, referred to as warming or hot standby, that is ready to be used at short notice which could take half an hour to 2 hours to bring on line
A similar amount of power stations (8–10 GW by capacity) are operable from a cold start in about 12 hours for coal burning stations, and 2 hours for gas fired stations
Short term and instantaneous load and generation response mechanisms
The national grid is organized, and power stations distributed, in such a way as to cope with sudden, unforeseen and dramatic changes in either load or generation. It is designed to cope with the simultaneous or nearly simultaneous failure of 2 × 660 MW sets
Spinning Reserve National Grid pays to keep a number of large power station generators partly loaded.
Pumped Storage Pumped storage as in Dinorwig Power Station is also used in addition to spinning reserve to keep the system in balance.
Frequency Service For large perturbations, which can exceed the capability of spinning reserve, NG (National Grid plc) who operate the national grid and control the operations of power stations (but does not own them) has a number of partners who are known as NG Frequency Service, National Grid Reserve Service or reserve service participants. These are large power users such as steel works, cold stores, etc. who are happy to enter into a contract to be paid to be automatically disconnected from power supplies whenever grid frequency starts to fall.
Standing Reserve Operating closely with NG Frequency Response is the National Grid Reserve Service now called STOR or Short Term Operating Reserve. NG Standing Reserve participants are small diesel engine owners, and Open Cycle gas turbine generator owners, who are paid to start up and connect to the grid within 20 minutes from the time Frequency Response customers are called to disconnect. These participants must be reliable and able to stay on and run for an hour or so, with a repetition rate of 20 hours.
National Grid has about 500 MW of diesel generators on contract, and 150 MW of gas turbines with about 2,000 MW of disconnect-able load.
Sources of intermittency on the UK National Grid The largest source of intermittency on the UK National Grid is the power stations; in fact, the single largest source is Sizewell B nuclear power station. Whenever Sizewell B is operating the entire 1.3 GW output is liable to stop at any time without warning. Its capacity is 2.16% of the national grid maximum demand, making it the single largest power source and therefore the largest source of intermittency. Despite this issue, NG readily copes with it using the methods outlined above including the use of diesel engines. An industry-wide rate of unplanned scrams (shutdowns) of 0.6 per 7000 hours critical means that such a shut-down without warning is expected to happen about once every year and a half. However, no matter how low the rate of unplanned scrams, this is largely irrelevant - what matters is the fact that it can and does happen, and measures have to be in place to deal with it.
In 2008 both Sizewell and Longannet power stations both stopped unexpectedly within minutes of each other, in fact causing widespread power failures, as substations were tripped off using prearranged under-frequency relays.
Reports of May 2008 outage
National grid http://www.nationalgrid.com/NR/rdonlyres/E19B4740-C056-4795-A567-91725ECF799B/32165/PublicFrequencyDeviationReport.pdf