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.
For instance, the Langlois Substation in Québec, Canada (45°17′13.76″N 74°0′56.07″W / 45.2871556°N 74.015575°W / 45.2871556; -74.015575) installed a 100 MW variable frequency transformer in 2004 to connect the asynchronous grids in Québec and the north-eastern United States.
AEP Texas installed a 100 MW VFT substation in Laredo, Texas, United States (27°34′13.64″N 99°30′34.98″W / 27.5704556°N 99.5097167°W / 27.5704556; -99.5097167) 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 (40°38′0.03″N 74°12′46.04″W / 40.6333417°N 74.2127889°W / 40.6333417; -74.2127889) 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.
Read more about Variable Frequency Transformer: Construction and Operation
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