Smart Transformer as a Variable Frequency Transformer

A variable frequency transformer (VFT) is a power transfer device used to exchange power between two grids operating at different frequencies. Conventional VFTs are rotational machines working on the principle of induction. This paper proposes the use of a smart transformer (ST) as a VFT. Conventional VFTs have issues like uncontrolled reactive power flow, high power torque fluctuations, etc. These issues can be mitigated by using ST as a VFT. To handle power levels at which VFTs generally work, design procedures for the ST converters are explained. Such a connection of asynchronous grids with ST is also advantageous over conventional HVdc solutions. The line frequency transformers used in HVdc systems are replaced, and high frequency transformer used in ST helps reduce the size and weight of the system. Moreover, additional dc links of the ST allow for connection to MVdc grids, renewable energy systems, storage, etc. Simulation results show satisfactory operation of the proposed scheme.