National Grid Reserve Service

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In order to balance the supply and demand of electricity on short timescales, the UK National Grid has contracts in place with generators and large energy users to provide temporary extra power, or reduction in demand. These reserve services are needed if a power station fails for example, or if forecast demand differs from actual demand. National Grid has several classes of reserve services, which in descending order of response time are: BM Start-Up, Short-Term Operating Reserve, Demand Management and Fast Reserve.[1][2]

National Grid Frequency Response[edit]

This is a service that large power users such as steel works, cold stores, large water pumping stations, can offer to the UK 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 (49.7 Hz). These loads are shed for a contracted period of at least 30 minutes. Within this period 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 × £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 0.005p/kWh.

Similar arrangements operate in the USA and France and all other large power grids.[3]

National Grid Short Term Operating Reserve (STOR)[edit]

Formerly known as Standing Reserve, Short Term Operating Reserve (STOR) 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.[4][5]

Similar arrangement operate in the US and France and all other large power grids.

Other National Grid measures[edit]

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.

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