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{{Short description|Transmits electricity between two alternating current frequency domains}}
{{More citations needed|date=May 2016}}
A '''variable-frequency transformer''' ('''VFT''') is used to transmit [[electric power|electricity]] between two ([[wikt:asynchronous|asynchronous]] or synchronous) [[alternating current]] [[frequency]] domains. The VFT is a relatively recent development. Most asynchronous grid inter-ties use [[high-voltage direct current]] converters, while synchronous grid inter-ties are connected by lines and "ordinary" [[electrical transformer|transformer]]s, but without the ability to control power flow between the systems, or with [[quadrature booster|phase-shifting transformer]]
It can be thought of as a very high power [[synchro]], or a [[rotary converter]] acting as a [[frequency changer]], which is more efficient than a [[motor–generator]] of the same rating.
==Construction and operation==
A variable-frequency transformer is a [[doubly fed electric machine]] resembling a vertical shaft [[hydroelectric]] generator with a [[three-phase power|three-phase]] wound [[rotor (electric)|rotor]], connected by [[slip ring]]s to one external power circuit. The [[stator]] is connected to the other.
The physical orientation of the shaft affects the phase angle developed across the rotor windings, similar to the behavior of a [[synchro]] or [[induction regulator]]. For two networks of the same frequency, a [[direct-current]] torque motor mounted on the same shaft can hold the shaft at a fixed position, effecting a [[quadrature booster|phase-shifting transformer]]. Changing the direction of torque applied to the shaft changes the direction of power flow.
The variable-frequency transformer behaves as a continuously adjustable [[quadrature booster|phase-shifting transformer]]. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back [[high-voltage direct current|HVDC]], the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are the [[current-carrying capacity]] of the slip rings for the rotor winding.▼
Alternatively, the shaft can be freed to orient naturally. If the two connected networks have different frequencies, the shaft will rotate at the difference in line frequency. It thus acts as a [[rotary converter]], syncing two networks of different frequency through a mechanical system locked to the relative frequency.
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==Projects==
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Langlois Substation in [[Québec]], [[Canada]] ({{Coord|45|17|13.76|N|74|0|56.07|W|}}) installed a 100 MW variable-frequency transformer in 2004 to connect the asynchronous grids in Québec and the [[northeastern United States]]. This was the first large scale, commercial variable frequency transformer, and was installed at [[Hydro-Québec]] Langlois substation and is located electrically near sixteen hydro generators at [[Les Cèdres, Quebec]] and thirty-six more hydro generators at [[Beauharnois, Quebec]]. The operating experience since April 2004 has demonstrated the VFT's inherent compatibility with the nearby generators<ref>Marchen, P.E, Marczewski, J.J, D'Aquila, R, Hassink, P., Roedel, J.H. and Bodo, R.L. (2014). [https://www.academia.edu/10527124/VFT_A_Smart_Transmission_Technology_That_Is_Compatible_With_the_Existing_and_Future_Grid?auto=download&campaign=weekly_digest VFT – A Smart Transmission Technology That Is Compatible With the Existing and Future Grid]. Power Systems Conference and Exposition, 2009, 1-7. doi:10.1109/PSCE.2009.4840083</ref>
[[AEP Texas]] installed a 100 MW VFT substation in [[Laredo, Texas|Laredo]], Texas, [[United States]] ({{Coord|27|34|13.64|N|99|30|34.98|W|}}) in early 2007. It connects the power systems of [[ERCOT]] (in the United States) to [[Comisión Federal de Electricidad|CFE]] (in Mexico).
Smaller VFTs are used in large land-based [[wind turbine]]s, so that the turbine rotation speed can vary while connected to an [[electric power distribution]] [[electrical grid|grid]].{{cn|date=August 2025}}
[[General Electric]] installed a 3 × 100 MW VFT substation in [[Linden, New Jersey]], in the United States in 2009. It connects the power systems of [[PJM Interconnection|PJM]] & [[New York Independent System Operator]] (NYISO). This installation is in parallel with three existing phase-shifting transformers to regulate synchronous power flow.<ref>{{cite web |url=http://geenergyfinancialservices.com/press_releases/view/140 |title=GE to Offer Additional Electric Transmission Capacity from its Smart Grid Transformers at New Jersey Power Plant |author=<!--Staff writer(s); no by-line.--> |date=11 Dec 2012 |website=GE Energy Financial Services Press Release |access-date=7 Apr 2014}}</ref>
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==See also==
*[[HVDC]]
*[[Induction regulator]]
*[[Quadrature booster]]
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==References==
{{Reflist}}
{{Electric transformers}}
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