When power is generated at a power plant, it is transmitted through a very high voltage line; the voltage of such high capacity lines can be as high as several hundred thousand volts. From there, the high voltage lines are directed to power substations. A power substation involves several transformers that reduce to voltage to a few thousand volts; this number is usually around 7,000 volts. The substation outputs its voltage throughout several transmission lines, each of which transmits its own 7,000 volts.
These transmission lines end at another kind of transformer; these transformers are often mounted on utility poles outside homes and places of business. It is from these comparatively small transformers that the appropriate voltage for use in homes and places of business is distributed. By the time the voltage from this transformer reaches an individual power outlet, it usually ranges between 120 and 220 volts (in the United States and some other countries).
Power transformers are used primarily to couple electrical energy from a power supply line to a circuit system. They provide the power source for all of the electronic devices used in homes and places of business. There are many variations among power transformers. Power transformers may be dry-type or cast coil design and be pole-mounted, pad-mounted, or wall-mounted. They can have 3R or 4 enclosures. Power transformers are also available with aluminum or copper windings.
Some considerations for the choice of a power transformer include the voltage that is available, the voltage load required for equipment, current and future KVA load, Hz frequency, single or three phase needs, auto or double winding, mounting configuration and whether the transformer will be used indoors or outdoors. Transformers can also be used to manage the currents transmitted through underground power lines. They are necessary for the safe and effective function of the largest and smallest electronic devices in industry, commerce and in consumer products contexts.