Immunizing Variable Frequency Transformer From Dual-Side Asymmetrical Grid Faults via a Single-Converter-Based Novel Control Strategy

Both variable frequency transformer (VFT) and high voltage direct current converter are feasible solutions for asynchronous interconnection of different power grids. The VFT possesses inherent natural damping and high overloading capability, which is important for stability and reliability of power grids. The VFT, however, has not been widely used, since it cannot well solve some grid faults like asymmetrical grid faults. In this paper, a novel control strategy is designed on a single series compensation converter (SCC) of traditional VFT. With only replacement of this proposed strategy, the traditional VFT is immune to dual-side asymmetrical grid faults. In detail, three controllers are designed and implemented in a single SCC, i.e., two negative sequence controllers are designed to deal with asymmetrical grid faults at stator and rotor sides of the VFT, respectively; A positive sequence controller is adopted to control the reactive power flowing through the VFT, which reserves the original function of the SCC. Detailed theoretical analysis and hardware-in-loop experiments are carried out to verify the proposed control strategy.