Variable-frequency transformer: Difference between revisions

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A '''variable frequency transformer''' is used to transmit [[Electric power|electricity]] between two [[asynchronous]] alternating current domains. The VFT is a relatively recent development (first deployed in 2004). Most grid intertiesinter-ties use [[high-voltage direct current]] converters.

Five small variable frequency transformers 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]] has installed a 100 MW variable frequency transformer to connect the asynchronous grids in Québec and the northeasternnorth-eastern United States.

AEP Texas installed a VFT substation in [[Laredo, Texas|Laredo]], Texas, [[United States]] in early 2007. It connects the power systems of ERCOT (in the United States) to CFE (in Mexico). See [http://www.ettexas.com/projects/vft.asp The Laredo VFT Project]

Smaller VFTs are used in large land-based [[wind turbine]]s, so that the turbine rotation speed can vary while connected to a 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 wound rotor, connected by slip rings 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 [[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.

== External links ==

* [http://www.gepower.com/about/press/en/2004_press/052504j.htm GE press release for the Langlois installation]

* [http://www.spectrum.ieee.org/nov07/5714 Power Transmission Without the Power Electronics]