The National Grid is the network of cables that covers the whole country to take electricity from      the power station to consumers.    Electricity is generated in the power stations but cannot be transmitted to consumers just yet as        the current would be too high (energy will be lost as heat from the cables).   The solution is to use a step up transformer that steps up the voltage to a very high value      (thousands of volts). This makes the current smaller, minimizing the heat loss.   For the electricity to reach consumers at the appropriate voltage, a step down transformer is used,      this lowers the voltage from its very high value (thousands of volts) to a lower value.