A substation transformer transforms high voltage electricity into a lower voltage. Electricity is generated by a power station and is distributed by an electrical grid. The distribution of the electricity through the grid is made at high voltage because the electricity can travel for long distances without losing a lot of energy. If the distribution was made at the low voltage of 110 volts, there would be a big loss of energy because low voltage transmission is not very efficient.
When the electricity gets to the local area in which it will be used, it arrives at a substation and the high voltage is stepped down to a low voltage before it enters the homes, schools, factories and public power equipment. The substation uses a transformer to transform the high voltage into a low voltage and this is achieved by electro-magnetic induction. The high voltage charges a coil of wire which induces a magnetic field in a smaller coil of wire and that is how the electricity is stepped down from the high to the low voltage using substation transformers.