A novel bidirectional synchronized transfer method for multilevel electric drive systems based on discrete Fourier transformation

Bidirectional synchronized transfer (BST) in variable frequency drive (VFD) systems implies automatic synchronization with seamless transfer of the motor to the grid or take‐over of the motor from the grid. This feature is realized if the magnitude, frequency, and phase angle of the VFD voltage and the grid voltage are synchronized. So, an accurate synchronization algorithm is necessary. This paper proposes a new BST method based on the discrete Fourier transformation (DFT) that obtains the magnitude, frequency and phase angle of VFD and grid voltages precisely through determining the fundamental components. This new BST coordinates the output voltage of the VFD with the grid using two error amplifiers. Also, additional decoupling reactors or transformers are no longer required by using this method due to exact estimation and synchronization. Furthermore, the algorithm can be integrated into any motor control strategy. This paper presents the principle of the proposed BST comprehensively and evaluates it by experimental results. The algorithm is tested on an induction motor via a 13‐level cascaded H‐bridge (CHB) inverter and the V/Hz constant control. The effect of BST on the motor voltage and current, the inverter current and dc‐link voltage is also examined. Results confirm the proposed strategy.