The primary distribution system is that part of the electric distribution system between the distribution substation and distribution transformers. It is made up of circuits called primary feeders or distribution feeders.These feeders include the primary feeder main or main feeder, usually a three-phase, four-wire circuit, and branches or laterals, which can be either three-phase or single-phase circuits. These are tapped from the primary feeder main, as shown in the simplified distribution feeder diagram of below Figure. A typical power distribution feeder provides power for both primary and secondary circuits.
In primary system circuits, three-phase, four-wire, multigrounded common-neutral systems, such as 12.47Y/7.2 kV, 24.9Y/14.4 kV, and 34.5Y/19.92 kV, are used almost exclusively. The fourth wire of these Y-connected systems is the neutral, grounded at many locations for both primary and secondary circuits. Single-phase loads are served by distribution transformers with primary windings that are connected between a phase conductor and the neutral. Three-phase loads can be supplied by three-phase distribution transformers or by single-phase transformers connected to form a threephase bank. Primary systems typically operate in the 15-kV range, but higher voltages are gaining acceptance.
The primary feeder main is usually sectionalized by reclosing devices positioned at various locations along the feeder. This arrangement minimizes the extent of primary circuitry that is taken out of service if a fault occurs. Thus the reclosing of these devices confines the outage to the smallest number of customers possible. This can be achieved by coordinating all the fuses and reclosers on the primary feeder main.
In block diagram in above Figure distribution substation voltage is 12.47 kV line-to-line and 7.2 kV line-to-neutral (this is conventionally written as 12,470Y/7200 V). However, the trend is toward higher primary four-wire distribution voltages in the 25 to 35-kV range. Single-phase feeders such as those serving residential areas are connected line-to-neutral on the four-wire systems
The use of underground primary feeders that are radial three-conductor cables is increasing. They are serving urban areas where load demand is heavy, particularly during the hot summer months, and newer suburban residential developments.
Both cost factors and the importance of reliability to the customers being served influence the design of primary systems. The simplest and least expensive (as well as least reliable) configuration is the radial distribution system shown in below Figure, because it depends on a single power source. Despite their lower reliability, radial systems remain the most economical and widely used distribution systems for serving homes because an electrical power outage there is less likely to have serious economic or public safety consequences. As a hedge against outages, most utilities plan their distribution systems so that they will have backup if those events occur. The goal of all electrical distribution systems is the economic and safe delivery of adequate electric power to serve the electrical loads
The reliability of the primary feeder can be improved with the installation of a loop distribution system,as shown in above Figure b. In loop systems the feeder, which originates at one bulk power source, “loops” through the service area and several substations before terminating at the original substation or another bulk source. The strategic placement of switches at the substations permits the electric utility to supply customers in either direction. If one power source fails, switches are opened or closed to bring an alternative power source online.
Loop systems provide better service continuity than radial systems, with only short service interruptions during switching. However, they are more expensive than radial systems because of the additional switching equipment requirements. As a result, loop systems are usually built to serve commercial and light industrial buildings and shopping malls, where power outages are more likely to endanger human lives or result in property losses.
Reliability and service quality can be significantly improved at even higher cost with a multiple parallel circuit pattern.In these systems, two or more circuits are tapped at each substation. The circuits can be radial or they can terminate in a second bulk power source. These interconnections permit each circuit to be supplied by many different substations.
The secondary distribution system is that part of the electrical power system between the primary system and the customer’s service entrance. This system includes distribution transformers, secondary circuits (secondary mains), customer services (consumer drops), and watthour meters to measure customer power consumption. Secondary voltages are provided by distribution transformers that are connected to the primary system and sized for the voltages required for specific parts of the service area.