In electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
networks, spot network substations
are used in interconnected distribution networks. They have the secondary network (also called a grid network) with all supply transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s bussed together on the secondary side at one location. Spot networks are considered the most reliable and most flexible arrangement of connecting power to all types of loads. Switching can be done without interrupting the power to the loads.
Electricity distribution networks are typically of two types, radial or interconnected. A radial network leaves the station and branches like a tree with no connection to any other supply. This is typical of long rural lines with isolated load areas. In general, the radial distribution network has more power failures than the interconnected distribution networks.
In a secondary network the transformers are distributed across an area (e.g. in streets) and have multiple supplies. The transformers are wired together on the secondary side. The system is arranged so that nearby transformers do not use the same feeder. In case of an issue with a feeder (or transformer) the load is fed by nearby transformers, so there is no interruption, perhaps some voltage drop. Usually a fault on the secondary side is not a big problem since the conductors will destroy themselves.
A spot network
is basically a secondary network condensed to a point. Several transformers have multiple supplies and their secondaries are bussed together. Besides a region-wide blackout
-Loss of lighting or communication:* Power outage, an accidental loss of electric power* Rolling blackout, an intentional loss of electric power* Blackout , the practice of minimizing outdoor lighting for protection from attack...
, they are vulnerable to a bus fault, which is extremely rare. The simplest case is where each transformer connects to one feeder and vice versa ("unit system"). Of course, high-voltage switching can be used to handle more cases, e.g., working transformer but busted feeder or the reverse.
Urban [Spot] network substations can be used to make the interconnected distribution networks to serve a single facility. Examples of such single facilities include airports, hospitals, major data processing centers (especially those using uninterruptible power supply
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...
), and sports arenas that regularly broadcast nationally televised events.
EPRI lists urban network supply systems has having Momentary and Long-duration Interruptions on 50 year intervals.
In large cities, many electric utility companies use grid feeders to make interconnected distribution networks to serve the downtown core. The interconnected network has multiple connections to the points of supply.
A network protector is a type of electric protective device used in electricity distribution systems. The function of the network protector is to automatically connect and disconnect its associated power transformer from the secondary network when the power starts flowing in reverse direction...
s, i.e. reverse current relays, are used to detect any open circuits that are letting the electrical current flow back towards its source.
A local spot network of 2 to 8 primary transformers can be connected to the same secondary bus to provide reliable power to a particular facility, like a large hospital or computer and major data processing center. St. Jude Children's Research Hospital
St. Jude Children's Research Hospital, founded in 1962, is a leading pediatric treatment and research facility focused on children's catastrophic diseases. It is located in Memphis, Tennessee. It is a nonprofit medical corporation chartered as a 501 tax-exempt organization under IRS regulations.In...
in Memphis, Tennessee
Memphis is a city in the southwestern corner of the U.S. state of Tennessee, and the county seat of Shelby County. The city is located on the 4th Chickasaw Bluff, south of the confluence of the Wolf and Mississippi rivers....
has 8 primary transformers that are connected to the same secondary bus. The FedExForum
The FedEx Forum is an arena located in downtown Memphis, Tennessee. It is the home of the Memphis Grizzlies of the NBA and the NCAA Division I men's basketball program of the University of Memphis, both of whom previously played home games at the Pyramid Arena...
(home of the NBA Grizzlies basketball team) in Memphis has a network of 4 primary transformers connected to the same secondary bus. In some arrangements with 4 transformers, any of the 4 transformers can carry all of its connected loads. The Toronto Pearson International Airport
Toronto Pearson International Airport is an international airport serving Toronto, Ontario, Canada; its metropolitan area; and the Golden Horseshoe, an urban agglomeration that is home to 8.1 million people – approximately 25% of Canada's population...
is electrically fed by 4 grid feeders, each capable of carrying the entire 20+ MW load.