The electricity network in Great Britain is managed by National Grid. As the system operator, it is responsible for managing the flow, security and quality of electricity. Management is a real-time activity that involves matching demand and supply as closely as possible and to maintain the voltage and frequency within certain limits. The frequency of the mains varies with the difference between demand and supply; when demand exceeds supply the frequency drops and additional supply needs to be brought online. Conversely, when supply exceeds demand the frequency will rise and supply needs to be reduced. Different techniques are used to match demand and supply but the average frequency needs to be maintained at 50Hz so that clocks and other devices run neither fast nor slow over time.


The United Kingdom includes Northern Ireland but Great Britain and Northern Ireland run separate 50Hz networks that are not synchronised. Unfortunately, the .gb top level domain is closed for new domain name applications so this site uses .uk instead.


You can read about the site's design and how it works here.


The GB mains frequency is nominally 50Hz. National Grid is obliged by its licence commitments to control the frequency within ±1% of 50Hz so it can fluctuate between 49.5Hz to 50.5Hz. However the normal operational limits are 49.8Hz to 50.2Hz. Sufficient generation and/or demand needs to be held in automatic readiness to deal with all credible events that might result in a deviation to the normal frequency variation. Credible events include one generator set tripping, failure of an overhead line, switchgear failure, loss of a single HVDC link, etc. Multiple simultaneous losses are not usually considered.

As a result, there are two types of frequency response: Dynamic and Non-Dynamic Response. Dynamic Frequency Response is a service that is provided continuously and is used to manage the normal second by second changes on the system. In contrast, Non-Dynamic Frequency Response is usually a discrete service that is triggered at a defined frequency deviation.

There is a variety of options to increase supply and they are rated by:

Additional supply can be in the form of CCGT capacity, plant on spinning reserve, generator (peaking) farms or hydro-electric stations. An emerging supply source, known as Enhanced Frequency Response, is based on energy storage and can be made available online in under a second.

On the other hand, there are measures for reducing demand by automatically disconnecting large industrial consumers when a local monitor detects that the frequency has fallen below a predetermined amount. These consumers are typically refrigeration plants, chemical works, water pumping stations, industrial gas plants and data centres (who have online UPSs available and generators).

You can find more information at the National Grid website.


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25 countries in Europe share a common transmission network that is synchronised across them all. Insufficient power generation in Kosovo, and Serbia's unwillingness to bolster capacity even thought it is legally obliged to do so, is believed to have led to the shortage of supply and so the average frequency has remained below 50Hz for some time. This has caused certain mains powered domestic clocks to run slowly with a typical error of 6 minutes since mid-January 2018. There is a BBC article here and a statement by ENTSO-E here.

JST Lawrence, © Copyright 2017