The National Grid

• • The National grid is a system of cables and transformers that covers the UK
  • The gird connects power stations to consumers
  • National Grid transfers electrical power from power stations on grid (supply) to where it is needed (the demand)
• • Demand for electricity changes frequently
  • Power stations often run below their maximum power output so there is spare capacity to cope with a high demand
• • The National Grid runs on a high potential difference and a low current
  • The grid does not use a high current because large amounts of energy is lost when the wires heat up and energy is transferred to the thermal store of the surroundings
  • It is cheaper to have a high potential difference
  • for a given power, increasing potential difference decreases current, which decreases energy lost by heating wires and surroundings - makes national grid efficient in transferring energy
• • 400,000V is the potential difference the National Grid is run at
  • the voltage is increased from 25,000 to 400,000
  • this causes the current to decrease
  • less current means less energy transferred through heating the wire
• • Transformers and pylons with large insulators are used to get the voltage high for efficient transmission
• • transformers have two coils, a primary and secondary coil joined by an iron core
• • potential difference is increased by using a step up transformer
  • they have more turns on the secondary coil than the primary coil
  • as the potential difference is increased, the current is decreased
• • potential difference is reduced by using a step down transformer
  • they have more turns on the primary coil than the secondary coil
  • as the potential difference decreases, the current increases
  • the transformer reduces the potential difference to 230V for home
• • the power of a primary coil - power = pd x current
  • transformers are nearly 100% efficient so:
  • pd x current in secondary coil = pd x current in primary coil
• • thick cables are used so that their resistance is low, to ensure minimum amount of power is lost
• • a low current and low resistance means the transmission wires not will heat up as much
  • so, most power is transferred to the consumer