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A '''variable -frequency transformer''' ('''VFT''') is used to transmit [[Electricelectric power|electricity]] between two ([[asynchronous]] or synchronous) [[alternating current]] [[frequency]] domains. The VFT is a relatively recent development, (first deployed in 2004). Most asynchronous grid inter-ties use [[high-voltage direct current]] converters, while synchronous grid inter-ties are connected by lines &and "ordinary" transformers, but without the ability to control power flow between the systems.
Five small variable -frequency transformers[[electrical transformer|transformer]]s with a total power rate of 25 MVA were in use at [[Neuhof Substation]], Bad Sachsa, Germany for coupling power grids of former East and [[West Germany]] between 1985 and 1990.
For instance, the 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 north-eastern[[northeastern United States]].
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). (See [http://www.ettexas.com/projects/vft.asp The Laredo VFT Project].)
GE[[General Electric]] installed a 3 × 100 MW VFT substation in [[Linden, New Jersey]], [[in the United States]] ({{Coord|40|38|0.03|N|74|12|46.04|W|}}) in 2009. It connects the power systems of PJM & NYISO (in the United States). This installation is in parallel with three existing phase -shifting transformers to regulate synchronous power flow. (See [http://www.geenergyfinancialservices.com/LindenVFT_Inauguration.asp The Linden VFT Project].)
Smaller VFTs are used in large land-based [[wind turbine]]s, so that the turbine rotation speed can vary while connected to aan [[electrical distribution]] grid.
==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]], connected by [[slip ringsring]]s to one external power circuit. A [[direct-current]] torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and [[stator]].
The variable -frequency transformer behaves as a continuously adjustable [[Quadraturequadrature booster|phase-shifting transformer]]. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back [[Highhigh-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.
==See also==
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