National Grid

• The national grid is a giant web of wires that covers the whole of Britain, getting electricity from power stations to homes everywhere
• Electricity is distributed via the national grid
• The national grid is a giant system of cables and transformers that covers the UK and connects power stations to consumers
• The national grid transfers electrical power from power stations anywhere on the grid (the supply) to anywhere alse on the grid where it's needed (the demand)
• Electricity production has to meet demand
• Throughout the day, electricity usage (the demand) changes. Power stations have to produce enough electricity for everyone to have it when they need it
• Power stations can predict when the most electricity will be used. Demand increases when people get up in the morning, come home from school or work and when it starts to get dark or cold outside
• Popular events like a sporting final being shown on TV could also cause a peak in demand
• Power stations often run at well below their maximum power output, so there's spare capacity to cope with a high demand, even if there's an unexpected shut-down of another station
• Lots of smaller power stations that can start up quickly are also kept in standby just in case
• The national grid uses a high pd and a low current
• To transmit the huge amount of power needed, you need either a high potential difference or a high current (as P = IV)
• The problem with a high current is that you lose loads of energy as the wires heat up and energy is tranferred to the thermal energy store of the surroundings
• It's much cheaper to boost the pd up really high (400,000 V) and keep the current as low as possible
• For a given power, increasing the pd decreases the current, which decreases the energy lost by heating the wires and the surroundings. This makes the national grid an efficient way of transferring energy
• Potential difference is changed by a transformer
• To get the voltage up to 400,000 V for efficient transmission we use transformers - and big pylons with huge insulators
• Transformers all have 2 coils, a primary coil and a secondary coil, joined with an iron core
• Potential difference pd is increased using a step-up transformer. They have more turns on the secondary coil than the primary coil. As the pd is increased by the transformer, the current is decreased
• The pd is then reduced again at the local consumer end using a step-down transformer (the current is therefore increased by this transformer). They have more turns on the primary coil than the secondary
• The power of a promary coil is given by power = pd x current
• Transformers are nearly 100% efficient, so the power in primary coil = power in secondary coil, this means that:

  p.d accross secondary coil (V) x current in secondary coil (A)

  = p.d. accross primary coil (V) x current in primary coil (A)