What are the common AC power supply modes in distribution system?

The link in power system that directly connects with users and distributes electric energy to users. The distribution system consists of distribution substations (usually reducing the transmission voltage of the power grid to distribution voltage), high-voltage distribution lines (i.e. voltage above 1 kV), distribution transformers, low-voltage distribution lines (voltage below 1 kV) and corresponding control and protection equipment. The distribution voltage is usually 35 ~ 60 kV and 3 ~ 10 kV.

The commonly used AC power supply modes in distribution system are: ① Three-phase three-wire system. It is divided into triangle wiring (for high voltage distribution, three-phase 220 volt motor and lighting) and star wiring (for high voltage distribution, three-phase 380 volt motor). ② Three-phase four-wire system. Used for 380/220 volt low-voltage power and lighting hybrid distribution. ③ Three-phase, two-wire and one system. Mostly used in rural power distribution. ④ Three-phase single-wire system. It is often used for traction power supply of electrified railways. ⑤ Single-phase two-wire system. Mainly to supply electricity to residents. The DC power supply mode of distribution system is as follows: ① Two-wire system. Used for power supply of urban trolleybuses, subway locomotives, mine traction locomotives, etc. ② Three-wire system. Supply power plants, substations, distribution stations, secondary equipment, electrolysis, electroplating.

The primary distribution network is the network from the outlet of the distribution station to the entrance of the distribution station (or substation). It is also called high voltage distribution network in China. The voltage is usually 6 ~ 10 kV, and 10 kV distribution is often used in cities. With the increase of urban load density, 20 kV distribution scheme has been adopted. The main part of the primary distribution line from the distribution station is called the trunk line. The part separated by the trunk line is called the branch line. The branch line is connected with the distribution transformer. There are two connection modes for primary distribution network: radial and ring.

The secondary distribution network is a system composed of lines and components between the secondary outlet of distribution transformer and the customer's household line, also known as low-voltage distribution network. In addition to radial and ring connection, important users in cities can also use double-loop connection. In urban areas with high power load density, grid wiring is adopted. This kind of network is powered by several primary distribution trunk lines, and after being stepped down by distribution transformers, it is connected to the secondary distribution network through low-voltage fuses. Because the adjacent distribution transformers in the secondary system are connected to different primary distribution trunk lines, the downtown power failure caused by primary distribution line failure can be avoided.

Distribution lines have overhead lines and underground cables according to the structure. Overhead lines can be used in rural areas and small and medium-sized cities, and underground cables should be used in big cities (especially downtown areas), tourist areas and residential areas.