Question

In the national grid,what does the step up transformer do?

Answer 1

The power that's lost when current flows through anything is

       (current)² times (resistance).

You can see that in order to minimize the power lost in transporting electrical
energy from one place to another, you'd want to make both the resistance of
the conductors and the current through the lines as low as possible.

The resistance is reduced by using heavier (fatter) conductors, and using
as much silver or copper in them as possible ... both very expensive.

Power = (current) times (voltage), so the way to reduce the current while
shipping the same power from place to place is to increase the voltage. 
That's exactly what the transformer does ... it changes the voltage and
current, while keeping their product the same.  It's used to step the voltage
up for transmission, in several steps from the generating station all the way
to the residential neighborhood, and then finally back down, on the pole
near your house.

Answer 2

In the National Grid, the Step Up Transformer increases the amount of voltage that is passing through the grid at any one time. These are used at power stations, as without them, there would not be enough power to transport the electricity around the national grid.
Hope this helps :)