Variable frequency transformer
A variable frequency transformer is used to transmit electricity between two (asynchronous or synchronous) alternating current 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 & "ordinary" transformers, but without the ability to control power flow between the systems.
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.
AEP Texas installed a 100 MW VFT substation in Laredo, Texas, United States ( ) in early 2007. It connects the power systems of ERCOT (in the United States) to CFE (in Mexico). (See The Laredo VFT Project.)
GE installed a 3 × 100 MW VFT substation in Linden, New Jersey, United States ( ) 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 The Linden 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.