VFT Operational Overview - The Laredo Project

A variable frequency transformer (VFT) is being installed at AEP's Laredo substation, where it will be used to exchange up to 100 MW of power between the asynchronous power grids of ERCOT (Texas) and CFE (Mexico). Functionally, the VFT is similar to a back-to-back HVDC converter. The core technology of the VFT is a rotary transformer with three-phase windings on both rotor and stator. A motor and drive system are used to adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flowing through the VFT. The world's first VFT was installed at Hydro-Quebec's Langlois substation. Some additional control features have been included in the Laredo VFT design, related to tie flow regulation, power runback, reactive power control, and black start capability. This paper describes the VFT application at Laredo and includes simulation tests of performance during severe grid disturbances.