Electrical power transmission involves creating the right conditions for electromagnetic energy to flow along a transmission line. The capacity of a transmission line is influenced by electromagnetic field laws and circuit principles. With the ground serving as the reference point (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 roughly proportional to the square of the transmission voltage. As a result, raising the voltage is a key method for achieving high-capacity or long-distance power transmission, and it reflects the advancement of transmission technology.
The transmission voltage level depends on both the capacity and the distance of the transmission. 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. These levels have evolved over time to meet growing energy demands.
Historically, transmission voltage has increased approximately twice with each major development. When power generation increases by about four times, a new voltage level typically emerges. Transmission voltages up to 220 kV are considered high voltage, while 330–765 kV is classified as ultra-high voltage (UHV). Voltages of 1,000 kV and above are called extra-high voltage (EHV) or UHV transmission. These systems allow for more efficient power delivery, lower costs, and better use of existing infrastructure.
By 1987, AC transmission systems had reached 765 kV globally. Industrial testing of 1,150 kV UHV AC transmission was underway. The largest DC transmission project built so far operates at ±750 kV, spans 2,400 km, and is designed to carry 6 million kilowatts. As energy needs continue to grow, advancements in transmission technology will play a critical role in ensuring reliable and efficient power delivery across vast distances.
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