Modern distribution systems
The modern distribution system begins as the primary circuit leaves th
e sub-station and ends as the secondary service enters the customer's meter socket. A variety of methods, materials, and equipment are used among the various utility companies, but the end result is similar. First, the energy leaves the sub-station in a primary circuit, usually with all thre
e phases.
Most areas provide three phase industrial service. There is no substitute for three-phase service to run motors and heavy industrial equipment. A ground is normally provided, connected to conductive cases and other safety equipment, to keep current away from equipment and people. Distribution voltages vary depending on customer need, equipment and availability. Delivered voltage is usually constructed using stock transformers, and either the voltage difference between phase and neutral or the voltage difference from phase to phase.
In many areas, "delta" three phase service is common. Delta service has no distributed neutral wire and is therefore less expensive. The three coils in the generator rotor are in series, in a loop, with the connections made at the three joints between the coils. Ground is provided as a low resistance earth ground, sometimes attached to a synthetic ground made by a transformer in a substation. High frequency noise (like that made by arc furnaces) can sometimes cause transients on a synthetic ground.
In North America and Latin America, three phase service is often a Y (wye) in which the neutral is directly connected to the center of the generator rotor. Wye service resists transients better than delta, since the distributed neutral provides a low-resistance metallic return to the generator. Wye service is recognizable when a grid has four wires, one of which is lightly insulated.
Many areas in the world use single phase 220 V or 230 V residential and light industrial service. In this system, a high voltage distribution network supplies a few substations per city, and the 230V power from each substation is directly distributed. A hot wire and neutral are connected to the building from one phase of three phase service.
Rural services normally try to minimize the number of poles and wires. Single-wire earth return (SWER) is the least expensive, with one wire. It uses high voltages, which in turn permit use of galvanized steel wire. The strong steel wire permits inexpensive wide pole spacings. Other areas use high voltage split-phase or three phase service at higher cost.
The least expensive network has the fewest transformers, poles and wires. Some experts say that this is three-phase delta for industrial, SWER for rural service, and 230 V single phase for residential and light industrial. The system of three-phase Wye feeding split phase is flexible and somewhat more resistant to geomagnetic faults, but more expensive.
Two frequencies are in wide use. 60 Hz permits slightly smaller transformers and is usually associated with 120 V wall sockets. 50 Hz is more common outside North America and is associated with 230 V wall sockets. Large electrical networks tightly control the line frequencies. The short term accuracy is normally better than 0.1 Hz. The long term accuracy is controlled by making up "lost" cycles so that electric clocks maintain correct time.
Electricity meters use different equations for each distribution system
