A variable-frequency transformer (VFT) is used to transmit electricity between two (asynchronous or synchronous) alternating current frequency domains.
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, or with phase-shifting transformers with some flow control.
It can be thought of as a very high power synchro, or a rotary converter acting as a frequency changer, which is more efficient than a motor–generator of the same rating.
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.
With no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator.