Which transformer is used to bring electric power from high power transmission to house? why?

There is high electric potential in high power transmission line but we need only 220V to run our electrical device . Hence, to convert high voltage to low voltage, a step down transformer is used. 

A step-down transformer has more primary windings than the secondary side. ... The secondary coil connects to the load, or secondary voltage outflow, distributing the electrical power away from the transformer. The alternating current flowing in at the primary voltage creates an alternating magnetic flux.A Step down Transformer is a type of transformer, which converts a high voltage at the primary side to a low voltage at the secondary side. If we speak in terms of the coil windings, the primary winding of a Step down Transformer has more turns than the secondary winding.

Step down transformers are used in power adaptors and rectifiers to efficiently decrease the voltage. They are also used in electronic SMPS. Other applications include: Power transmission lines.A transformer converts alternating current (AC) from one voltage to another voltage. It has no moving parts and works on a magnetic induction principle; it can be designed to "step-up" or "step-down" voltage. So a step up transformer increases the voltage and a step down transformer decreases the voltage.

The concept of a step down transformer is actually quite simple. The transfer has more turns of wire on the primary coil as compared to the turns on the secondary coil. This reduces the induced voltage running through the secondary coil, which ultimately reduces the output voltage.

A transformer that increases the voltage from primary to secondary (more secondary winding turns than primary winding turns) is called a step-up transformer. Conversely, a transformer designed to do just the opposite is called a step-down transformer.

You will need a step-down voltage transformer if you're traveling to any country with a power standard that is higher than what your appliances use. Conversely, taking appliances that run on 220–110 volts to the U.S. or Canada requires a step-up voltage converter that can transform 110–120 volts up to 220–240 volts.