With most UK generation capacity in the north and consumption predominately in the south, the grid transports electricity north to south on average rather than in both directions. It is not possible to store large amounts of energy as electricity, and so the balancing of supply and demand is a constant juggling act coordinated by the National Grid, who co-ordinate directly with major energy consumers to reduce demand or call on spare generation capacity to increase the supply temporarily.
Most large power stations are connected directly to the transmission network and have to meet strict Grid Code requirements to ensure the connections are of sufficient capacity and quality to meet both safety and reliability standards.
The existing grid system is a passive design with limited information flow about consumption or supply between end-users, electricity generators or the grid managers. Most countries, including the UK, USA and China, are upgrading their transmission and distribution networks to be ‘smart grids’, an umbrella term for a range of options around embedding intelligence in the power network. This can include demand management, smart meters, and net metering, however most ‘smart grids’ aim for better security, stability, resilience and safety.
Adding new distributed power sources can cause problems with local power quality and safety. For example, power cables have a temperature limit, above which they are not considered safe – too much power from local supplies could go over these limits without careful control, leading to grid failure and instability.
Today the distribution system has been built to take electricity from central generators to consumers, connecting to the transmission system and ‘stepping’ the voltage down to residential levels. As ‘distributed’ generation will feed into the distribution system directly, a house with micro-generation equipment such as solar PV panels or a CHP unit may at times be exporting surplus electricity to the grid; the distribution network will need to support and measure electricity flow in both directions.
Large-scale renewable generation, and in particular large wind farms, may connect directly into the transmission system, in which case they must meet the Grid Code requirements in areas such as voltage control and fault ride-through. Wind farms some distance from shore may be connected to the grid using high voltage direct current connections, rather than the more usual alternating current.