A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. The VFT is a relatively recent development (first deployed in 2004). Most grid interties use high-voltage direct current converters.
For instance, the Langlois Substation in Québec has 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, Texas in early 2007. It connects the power systems of ERCOT (in the United States) to CFE (in Mexico). See 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 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 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 capacity of 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.
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