What Is AC Power Transmission?

AC power transmission, also called alternating current power transmission, is the bulk movement of electrical energy from generating stations to substations using conductors energized with alternating voltage. It is the dominant form of electric power transmission worldwide, operating at 60 Hz across North America and much of the Americas and at 50 Hz across most of Europe, Africa, and Asia. The defining advantage of alternating current is the transformer, which changes voltage levels with high efficiency and no moving parts, allowing power to be generated at tens of kilovolts, transmitted at hundreds of kilovolts, and delivered to customers at a few hundred volts.

That advantage settled the engineering argument of the 1890s in favor of alternating current, and it still governs system design. Because line losses vary with the square of current, raising transmission voltage and lowering current for the same power reduces resistive heating in the conductors. Modern grids stack several voltage classes, from 69 kV subtransmission through 138 kV, 230 kV, and 345 kV, to extra-high-voltage circuits at 500 kV and 765 kV, with transformers at each substation stepping between them. The structure of the transmission and distribution system documented by the US Energy Information Administration follows this pattern of long-distance high-voltage lines feeding progressively lower-voltage networks.

Three-Phase Circuits

Nearly all AC transmission is three-phase, using three conductors carrying voltages of equal magnitude separated by 120 degrees in phase. A balanced three-phase set delivers constant instantaneous power to a linear load, which produces smooth torque in rotating machines, and it requires less conductor material than three separate single-phase circuits of the same capacity. Under balanced conditions the return current sums to zero, so a neutral conductor is unnecessary on transmission circuits and the ground wires above the phase conductors serve mainly for lightning shielding. Unbalanced loading and asymmetric line geometry are handled by transposing conductor positions along the route and by analysis in symmetrical components, the standard method for studying faults on three-phase systems.

Reactive Power and Transfer Limits

An AC line stores energy in the magnetic field around its conductors and in the capacitance between them and ground, which produces reactive power flows that carry no net energy but occupy conductor and transformer capacity. Series inductance causes the voltage drop and the phase angle difference that limit how much real power a long line can carry, while shunt capacitance raises voltage on lightly loaded circuits, the effect first observed by Ferranti. These effects grow with line length and set the practical ceiling on AC transfer distance, which is why underground and submarine cables, with far higher capacitance than overhead lines, become impractical after a few tens of kilometers unless the frequency is changed or the link is converted to direct current. Research on multi-frequency high-voltage AC transmission explores operating parts of a network below the nominal frequency to reduce reactive charging current on long cable routes.

Flexible AC Transmission Systems

Flexible AC transmission systems, or FACTS, are power-electronic controllers that adjust the voltage, impedance, or phase angle of an AC circuit within a fraction of a cycle. Shunt devices such as static VAR compensators and static synchronous compensators inject or absorb reactive power to hold voltage during disturbances. Series devices such as thyristor-controlled series capacitors change effective line impedance to steer power between parallel paths, and combined controllers do both. The US Department of Energy survey of advanced transmission technologies describes how these devices relieve congestion and improve stability on existing corridors where building new lines is not practical.

Applications

AC power transmission has applications across the electricity sector, including:

• Bulk transfer from thermal, hydroelectric, and nuclear generating stations
• Interconnection of regional grids and cross-border power exchange
• Collection and delivery networks for wind and solar generation
• Industrial supply for electric arc furnaces, electrolysis plants, and rail traction
• Urban distribution networks fed through step-down substations