National Grid (UK) reserve service
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[edit] National Grid Frequency Response
This is a service that large power users such as steel works, cold stores, large water pumps, can offer to the United Kingdom National Grid. These contractors have frequency sensitive relays fitted to the incoming breakers, and these disconnect the load if the system frequency falls beyond a pre-set figure – which may be 49.8HZ at one site for example, but the relays are all set at slightly different values to ensure a smooth progression of disconnection as frequency falls.
For example, during the loss of a large 660 MW generating set, the frequency immediately begins to fall, and sufficient load to equal 660 MW is shed until the frequency stops falling.
These loads are contracted to stay off for a maximum of about 20 minutes.
After twenty minutes Standing Reserve (Reserve Service) diesels of similar capacity (i.e. around 660 MW total) start up, and enable the Frequency Service loads to be re-connected and the relays re-armed.
Frequency Service is designed to cope with the loss of two 660 MW sets in quick succession.
There is about 2.5 GW of such Frequency Service loads available in the UK to cover a peak demand of about 60 GW.
If the total Reserve Service payments, about 2250 MW x £7,000 = £15.75 million are divided by the total kWh delivered by the National Grid to all customers, approximately 3 trillion kWh then this amounts to a total cost of about 0.005 p/kWh.
Similar arrangements operate in the USA and France and all other large power grids.
[edit] National Grid Standing Reserve
Standing Reserve or Reserve Service is intimately related to Frequency Service and consists of large numbers of small diesel generators, some as small as 250 kW, but with around 2 GW in total capacity, and these are started soon after a Frequency Service event by central despatch from National Grid control room. They are contracted to come on line within twenty minutes, and to stay on for up to two hours, with a recovery period of twenty hours. Typically they will start for about twelve times in any one year, and are paid around £7,000/MW per year plus fuel and operating costs.
Similar arrangement operate in the US and France and all other large power grids.
[edit] Other National Grid measures
The above measures happen routinely and without any interference with normal supplies to consumers. There are other similar arrangements which are used only as a last resort since they do involve disconnecting consumers. If Frequency Response and spinning reserve fails to control grid frequency and it falls too far, then the fans feeding air into power station boilers begin to lose power since they are synchronous, and the grid's entire power-station output then goes into irreversible decline. To prevent this, frequency sensitive relays on entire substations trip out, disconnecting entire customer areas on a pre-determined schedule.
This continues until, as a last resort, large areas can be switched out manually.
[edit] See also
Control of the National Grid (UK)
