Polyphase system



In the very early days of commercial electric power, some installations used two-phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor and, by using a four-wire system, conceptually the phases were independent and easy to analyse with mathematical tools available at the time.
Two-phase systems have been replaced with three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer.
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-or-more-phase supply completes one full cycle, the magnetic field of a two-poles-per-phase motor has rotated through 360 in physical space; motors with more than two poles per phase require more power supply cycles to complete one physical revolution of the magnetic field and so these motors run slower. Induction motors using a rotating magnetic field were independently invented by Galileo Ferraris and Nikola Tesla and developed in a three-phase form by Mikhail Dolivo-Dobrovolsky in 1889. Previously all commercial motors were DC, with expensive commutators, high-maintenance brushes and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting and have little vibration compared with single-phase motors.
Between 1992 and 1995, New York State Electric & Gas operated a 1.5 mile converted from a double-circuit 3-phase 115KV transmission line to a 93KV 6-phase 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 2328 miles.
High-phase-order (HPO) power transmission has been frequently proposed as a way to increase transmission capacity within a limited-width right of way. The required conductor spacing is determined by the phase-to-phase voltages, and six-phase power has the same voltage between adjacent phases as between phase and neutral. However voltages between non-adjacent phase conductors increases as the difference increases between phase angles of the conductors. Conductors can be arranged so that non-adjacent phases are spaced farther apart than adjacent phases.
This lets an existing double-circuit transmission line carry more power with minimal change to the existing cable plant. This is 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.