A transmission substation is one whose main purpose is to connect together various transmission lines.The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected together or isolated for maintenance.

          Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest substations can cover a large area (several acres/hectares) with multiple voltage levels and a large amount of protection and control equipment.

          A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area. It is uneconomic to directly connect electricity consumers to the main transmission network (unless they use large amounts of energy), so the distribution station reduces voltage to a value suitable for connection to local loads.

          The input for a distribution substation is typically at least two transmission or subtransmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

          The feeders or will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers ar or near to the customer premises.

          Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

          Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

          The main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost.

          What follows is a description of a typical substation.

          In the largest stations, all the incoming lines have a disconnect switch and a circuit breaker. In some cases, the lines will not have both: either a switch or a circuit breaker may be all that is needed. These will, most typically, also have a current transformer, to measure the current coming in or going out on a given line.

          Once past the switching components, the lines of a given voltage all tie in to a common bus. This is a number of thick metal bus bars, almost always three bars, since three-phase current is almost universal.

          The most sophisticated substations have a double bus, in which the entire bus system is duplicated. Most substations will not have this, as it is mainly for ultra-high reliability in, for example, a substation whose failure could bring down the whole system. Other compromises between a single and double bus can found be found; for example, the breaker-and-a-half setup.

          Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a 'short circuit').

          Along with this, a substation always has control circuitry needed to command the various breakers to open in case of the failure of some component.

          An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".

          A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer.

          To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.

          Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The job of substations is to isolate the faulted portion of the system.

          There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.