- Variable frequency transformer
A variable frequency transformer is used to transmit electricity between two
asynchronousalternating current domains. The VFT is a relatively recent development (first deployed in 2004). Most grid interties use high-voltage direct currentconverters.
For instance, the Langlois Substation in
Québechas installed a 100 MW variable frequency transformer to connect the asynchronous grids in Québec and the northeastern United States.
AEP Texas installed a VFT substation in
Laredo, Texasin 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 turbines, 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 machineresembling a vertical shaft hydroelectricgenerator 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 phase-shifting transformer. It allows control of the power flow between two networks. Unlike power electronics solutions such as back-to-back
HVDC, the variable frequency transformer does not demand harmonic filters and reactive power compensation. Limitations of the concept are the current-carrying capacityof the slip rings for the rotor winding.
Variable frequency transformers have been installed at Langlois, Quebec and at an interconnection point between Laredo, Texas and the Mexican power system.
* [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]
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