6.18 explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy

The voltage produced by power stations is too low to be transmitted efficiently. Power = VI (voltage x current) so for a given amount of power the current must be higher to accommodate for the lower voltage and current causes the wires to heat up. 

1. A step up transformer is used to boost the voltage, thus lowering the current, before it's transmitted.
2. Step down transformers are used at the end of the journey to lower the voltage to 230V and a much higher current.