A transformer substation is a location where electric power is converted from bulk power to local power or vice versa. These drab gray stations sit at the center of many electrical wires, both above and below ground. They generally contain various pieces of electrical equipment and enclosed systems. This equipment transforms the electrical current to allow it to keep moving through the bulk power system or to move into the local grid, where it goes to consumers.
In the old days, power wasn’t on a nationwide grid; a single company owned and maintained all the lines in an area. At that time, there were power stations that generated electricity and substations that converted it for use. The term "substation" referred to the fact that the power-transforming location was connected to a single main station. With the modern electricity grid, a single transformer substation may be connected to several power stations, but the name remained the same.
When a power plant generates electricity, it goes out in a form that most electrical devices are unable to use. This power format, often called bulk power, is great for the movement of the current, but not much else. Bulk power moves through high-voltage transmission lines and enters a transformer substation. These stations convert the bulk power to premium power and send it out to homes and businesses.
In addition to converting power for local use, a transformer substation will send bulk power back out into the system. Since it is nearly impossible to store electricity of this type, the transformer takes excess power out of the local system and converts it back to bulk power. In addition, whenever the local system has enough power, the transformer substation will send any bulk power it receives back out into the grid.
Not all transformer substations have the same capabilities. Some may only convert power for local use, while others may only retransmit power. This allows the power company to both reduce the amount of its machinery in any one spot and lessen the likelihood that a single accident will disrupt the entire local system.
Outside of the basic transformer substation functions described above, some stations have specific specialized functions. Collector substations connect to a power generation system that relies on sporadic or uneven factors such as wind or water power. These stations take the power generated in these systems and convert it for bulk power transmission.
Railway substations take bulk power and use it to power subways, monorails and other electric people-moving systems. The power used by these large machines is much closer to the form transmitted in bulk, so they use their own substation rather than the local grid. These transformer substations act just like a standard one.