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The voltage levels of power system include 220/380V (0.4 kV), 3 kV, 6 kV, 10 kV, 20 kV, 35 kV, 66 kV, 110 kV, 220 kV, 330 kV and 500 kV. With the improvement of motor manufacturing technology, 10 kV motors have been produced in batches, so 3 kV and 6 kV motors have been less used, and 20 kV and 66 kV motors have also been rarely used. The power supply system is mainly 10 kV and 35 kV. The power transmission and distribution system is mainly above 110 kV. There are two types of generators in the power plant: 6 kV and 10 kV. Now, 10 kV is the main generator. Users are all 220/380V (0.4 kV) low-voltage systems.

According to the Design Rules for Urban Power Network Regulations, the transmission network is 500 kV, 330 kV, 220 kV and 110 kV, the high voltage distribution network is 110 kV and 66 kV, the medium voltage distribution network is 20 kV, 10 kV and 6 kV, and the low voltage distribution network is 0.4 kV (220 V/380 V).

The power plant sends out 6 kV or 10 kV power. In addition to its own use (auxiliary power), the power plant can also send 10 kV voltage to users near the power plant. The power supply range of 10 kV is 10 Km, 35 kV is 20~50 Km, 66 kV is 30~100 Km, 110 kV is 50~150 Km, 220 kV is 100~300 Km, 330 kV is 200~600 Km, 500 kV is 150~850 Km.

Various voltage levels of the power system are converted through power transformers. The voltage rises to step-up transformers (substations are step-up stations) and decreases to step-down transformers (substations are step-down stations). A two-coil transformer with two coils (windings) is selected when one voltage is changed to another voltage, and a three-coil transformer with three coils (windings) is selected when the other voltage is changed to two voltages.

In addition to boosting and reducing voltage, the substation is also divided into hub station, regional station and terminal station according to its size. The voltage level of the hub station is generally three (three coil transformers), 550kV/220kV/110kV. Regional stations also generally have three voltage levels (three-coil transformer), 220 kV/110 kV/35 kV or 110 kV/35 kV/10 kV. Terminal stations are generally directly connected to users, and most of them have two voltage levels (two coils of transformer) of 110kV/10kV or 35kV/10kV. The user's own substation generally has only two voltage levels (double coil transformer): 110 kV/10kV, 35 kV/0.4kV, and 10kV/0.4kV, of which 10kV/0.4kV is the most.

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