How are step up transformers used in the transmission of electric energy?

Step-up Transformers used in the transmission of electrical energy increase the voltage going over Transmission Lines (over 100,000 Volts). For a given amount of POWER to be Transmitted, the higher the Voltage, the LOWER the CURRENT. This reduces the amount of Power LOST to the Resistance of the Transmission Lines.

Power Lost is calculated by the Formula:

POWER Lost = I2 x R, where I=Current and R=Resistance

So, the lower the Current (I), the lower the transmission line losses.

At the end of the Transmission Path the Voltage is "Stepped Down" to a value usable for the Customer, usually (220 volts for Residential use in the USA).