The basic process of transmission is to create conditions for electromagnetic energy to travel along the direction of the transmission line. Line transmission capacity is governed by various laws of electromagnetic fields and circuits. With the ground potential as the reference point (zero potential), the line conductors must be at the high voltage applied by the power supply, called the transmission voltage.
The maximum transmission power determined by the transmission line after comprehensive consideration of various factors such as technology and economy is called the transmission capacity of the line. The delivery capacity is roughly proportional to the square of the transmission voltage. Therefore, increasing the transmission voltage is the main technical means to achieve large-capacity or long-distance transmission, and is also the main indicator of the development level of transmission technology.
In the process of transmission, the level of the transmission voltage depends on the transmission capacity and the transmission distance. The general principle is: the larger the capacity, the farther the distance, the higher the transmission voltage. There are ten levels of long-distance transmission levels such as 3, 6, 10, 35, 63, 110, 220, 330, 500, and 750.
From the development process, the transmission voltage level is increased by about twice. When the power generation increases to about 4 times, a new higher voltage level appears. Generally, the transmission voltage of 220 kV and below is called high voltage transmission, and the transmission voltage of 330 to 765 kV is called ultra high voltage transmission, and the transmission voltage of 1000 kV and above is called UHV transmission. The table shows the approximate range of transmission voltage, delivery capacity, and delivery distance. Increasing the transmission voltage not only increases the transmission capacity, but also reduces the transmission cost, reduces the consumption of metal materials, and increases the utilization rate of the line corridor. By 1987, the AC transmission voltage already used in the world reached 765 kV. The 1150 kV UHV AC transmission has been industrially tested. The largest DC transmission project that has been built has a transmission voltage of ±750 kV, a transmission distance of 2,400 km, and a designed transmission capacity of 6 million kilowatts.
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