Brushless doubly fed induction machine as a variable frequency transformer

Variable frequency transformer (VFT) has been recently used as an alternative to HVDC to control power flow between asynchronous networks. VFT consumes less reactive power than a back-to-back HVDC system, provides faster initial transient recovery, and has better natural damping capability. VFT is simply a doubly-fed induction machine (DFIM) where the machine torque controls the power flow from stator to rotor and vice versa. The main disadvantage of this VFT is the slip rings and brushes required for the rotor circuit, especially in bulk power transmission. The brushless doubly-fed machine (BDFM) with nested cage rotor machine is proved to be a comparable alternative to conventional DFIM in many applications with the advantage that all windings being in the stator frame with fixed output terminals. In this paper, the BDFM is used as a variable frequency transformer (BVFT). A prototype machine is designed and simulated to verify the system validity.