Editing Polyphase system (section)

==Higher phase order==
Once polyphase power is available, it may be converted to any desired number of phases with a suitable arrangement of transformers.  Thus, the need for more than three phases is unusual, but higher phase numbers than three have been used.

High-phase-order (HPO) power transmission has been frequently proposed as a way to increase transmission capacity within a limited-width [[Right-of-way (property access)|right of way]].<ref name=TDWorld>{{Cite news |url=http://tdworld.com/overhead-transmission/high-phase-what |title=High-Phase What? |date=July 1, 2011 |first=Vito |last=Longo |journal=Transmission & Distribution World |archive-url=https://web.archive.org/web/20160728074923/http://tdworld.com/overhead-transmission/high-phase-what|archive-date=28 July 2016}}</ref>  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-[[electrical ground|ground]] and phase-to-phase [[electrical arc]]s.  For three-phase power, the phase-to-phase voltage, which is {{math|{{sqrt|3}}≈1.7}} 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.<ref>{{cite conference|doi=10.1109/STIER.1990.324628|title=High phase order transmission|first=T.&nbsp;F.|last=Dorazio|publisher=IEEE|pages=31–32|conference=Technical Conference on the Southern Tier|date=25 April 1990|location=Binghamton, NY}}</ref>  

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 [[power transformer|transformer]]s to interface with the conventional three-phase grid.<ref name=TDWorld/>  They are particularly economical when the alternative is upgrading an existing [[extra high voltage]] (EHV, more than 345&nbsp;kV phase-to-phase) transmission line to ultra-high voltage (UHV, more than 800&nbsp;kV) standards.<!--A number of promising references can be found by web searching for "high phase order power transmission", all locked up behind the IEEE paywall-->

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.<ref name=NYTrial>{{cite web|title=High Phase Order Transmission Demonstration|url=http://www.cerc-reactors.com/articles/miso16sep2004seminar/6-PhaseOTLs/eseerco.pdf|website=CERC-Reactors.com|publisher=NY State Electric & Gas}}</ref>{{rp|pp=xvii-xviii}}  

Three-phase power lines rely on [[transposition (transmission lines)|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 {{mvar|n}} elements coincides with the full [[symmetric group]] only for {{math|''n''&leq;3}}.  Full application of even that limited transposition scheme is necessary to [[arc suppression|properly protect]] against ground faults.<ref name=NYTrial/>{{rp|45-52}}  

Multi-phase power generation designs with 5, 7, 9, 12, and 15 phases in conjunction with multi-phase [[induction generator]]s (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.{{dubious|date=June 2024}}{{citation needed|date=December 2018}}