Variable frequency transformer is a flexible ac transmission system equipment that can connect asynchronization power grid. Frequency transformer is a kind of integrated transformer, phase shifter, hydro-generator, doubly-fed motor and dc drive device such as a new asynchronous power grid interconnection technology and phase of the core parts in side has a single phase winding of the stator and rotor rotating transformer, and adjust the rotor magnetic field through the dc motor drive system relative to the phase displacement of the stator magnetic field, so as to control the power frequency transformer output size and direction.The inverter transformer mainly consists of three parts: rotating transformer, dc drive motor and collector ring.Among them, rotating transformer is the subject of power exchange between rotor and stator on both sides of the system through the principle of magnetic coupling, dc motor is the key equipment to control the rotor speed and the transmission power of frequency conversion transformer, and collector ring is the equipment to realize the electrical connection between rotor winding and external system.

The VFT is equipped with asynchronization networking,  frequency adjustment, power supply to weak system, black start power supply, low frequency power oscillation .It is similar to dc back-to-back in function, but more suitable for weak power system than back-to-back, which is more conducive to reducing the footprint.

Variable frequency transformer is a series of flexible ac transmission system devices that is also a kind of new smart grid equipment, which has a smooth adjustment conveying power to suppress low frequency oscillation, system frequency, and other functions what can realize the asynchronous interconnection between different frequency power grid, optimizing the allocation of adjust the transmission power of parallel transmission channels and proportion.It is conducive to enhancing the power network interconnection and transmission capacity, improving the flexibility of power network operation and control, and improving the safety and stability level of power system which can play an important role in building a strong, intelligent and efficient modern large power grid.