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A polyphase system (the term coined by Silvanus Thompson) is a means of distributing alternating-current (AC) electric power that utilizes more than one AC phase, which refers to the phase offset value (in degrees) between AC in multiple conducting wires; phases may also refer to the corresponding terminals and conductors, as in color codes. Polyphase systems have two or more energized electrical conductors carrying alternating currents with a defined phase between the voltage waves in each conductor. Early systems used 4 wire two-phase with a 90° phase angle, but modern systems almost universally use three-phase voltage, with a phase angle of 120° (or 2π/3 radians).
Polyphase systems are particularly useful for transmitting power to which rely on alternating current to rotate. Three-phase power is used for industrial applications and for power transmission. Compared to a single-phase, two-wire system, a three-phase three-wire system transmits three times as much power for the same conductor size and voltage, using only 1.5 times as many conductors, making it twice as efficient in conductor utilization.
Systems with more than three phases are often used for rectifier and power conversion systems, and have been studied for power transmission.
Two-phase systems can also be implemented using three wires (two "hot" plus a common neutral). However this introduces asymmetry; the voltage drop in the neutral makes the phases not exactly 90 degrees apart.
Two-phase systems have been replaced with three-phase systems. The move from two to three phases was originally motivated by making a more ideal rotating field for AC motors: Mikhail Dolivo-Dobrovolsky calculated that, for simple winding configurations of the time, the magnetic field fluctuation can be reduced from 40% to 15%. This is less important in modern machines that create a nearly ideal rotating field using sinusoidally distributed windings, but three-phase systems retain other advantages.
A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer, which can also produce three-phase power from a two-phase input.
A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase. Such systems are sometimes described as split-phase.
High-phase-order (HPO) power transmission has been frequently proposed as a way to increase transmission capacity within a limited-width right of way. Transmitted power is proportional to the square of the phase-to-ground voltage drop, but transmission lines require conductors spaced adequately distant to prevent both phase-to-ground and phase-to-phase . For three-phase power, the phase-to-phase voltage, which is times the phase-to-ground voltage, dominates. Higher-phase systems at the same phase-to-ground voltage have less voltage difference between adjacent phases, allowing a tighter conductor spacing. For six- and higher-phase power systems, the dominant effect becomes the phase-to-ground voltage instead.
Six-phase operation thus lets an existing double-circuit transmission line carry more power without requiring additional conductor cable. However, it requires the capital expense and impedance losses of new phase-converting transformers to interface with the conventional three-phase grid. They are particularly economical when the alternative is upgrading an existing extra high voltage (EHV, more than 345 kV phase-to-phase) transmission line to ultra-high voltage (UHV, more than 800 kV) standards.
Between 1992 and 1995, New York State Electric & Gas operated a 1.5 mile 93kV 6-phase transmission line converted from a double-circuit 3-phase 115kV transmission line. The primary result was that it is economically favorable to operate an existing double-circuit 115kV 3-phase line as a 6-phase line for distances greater than 23–28 miles.
Three-phase power lines rely on transposition to equalize across all phases transmission losses due to slight deviations from ideal geometry. This is not possible with higher-phase lines, because a transposition can only swap adjacent phases, and the dihedral group on elements coincides with the full symmetric group only for . Full application of even that limited transposition scheme is necessary to arc suppression against ground faults.
Multi-phase power generation designs with 5, 7, 9, 12, and 15 phases in conjunction with multi-phase induction generators (MPIGs) driven by wind turbines have been proposed. An induction generator produces electrical power when its rotor is turned faster than the synchronous speed. A multi-phase induction generator has more poles, and therefore a lower synchronous speed. Since the rotation speed of a wind turbine may be too slow for a substantial portion of its operation to generate single-phase or even three-phase AC power, higher phase orders allow the system to capture a larger portion of the rotational energy as electric power.
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