Variable-frequency transformer

A variable-frequency transformer (VFT) is used to transmit electricity between two (asynchronous or synchronous) alternating current frequency domains. The VFT is a relatively recent development. 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 transformer 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.