Power transmission involves creating the right conditions for electromagnetic energy to move efficiently along a transmission line. The capacity of a transmission line is governed by both electromagnetic field principles and circuit laws. When using ground potential as the reference (zero potential), the conductors in the line must be at a high voltage supplied by the power source, known as the transmission voltage.
The maximum power that a transmission line can carry, after considering technical and economic factors, is referred to as its transmission capacity. This capacity is generally proportional to the square of the transmission voltage. Therefore, raising the transmission voltage is a key method for achieving large-scale or long-distance power delivery, and it also serves as an important indicator of the advancement in transmission technology.
The level of transmission voltage depends on both the required transmission capacity and the distance over which power is delivered. The general rule is that higher capacity and longer distances require higher voltages. Common long-distance transmission levels include 3 kV, 6 kV, 10 kV, 35 kV, 63 kV, 110 kV, 220 kV, 330 kV, 500 kV, and 750 kV.
Historically, transmission voltage levels have increased by approximately double each time. When power generation increases significantly—by about four times—a new higher voltage level typically emerges. Voltages up to 220 kV are generally considered high voltage, while 330–765 kV is classified as ultra-high voltage (UHV). Transmission voltages of 1,000 kV and above are referred to as Extra High Voltage (EHV) or UHV. The table below outlines the approximate ranges of transmission voltage, capacity, and distance.
Raising the transmission voltage not only enhances the line's capacity but also lowers transmission costs, reduces the use of metal materials, and improves the efficiency of the corridor used by the line. By 1987, AC transmission voltages had reached 765 kV globally. The 1,150 kV UHV AC transmission system has already been tested industrially. The largest DC transmission project built so far operates at ±750 kV, spans 2,400 km, and has a designed capacity of 6 million kilowatts.
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