Stability analysis and control for power electronic transformer with FRT capability

In this paper, a power electronic transformer (PET) topology with the fault ride-through (FRT) capability is presented. In the PET, modular multilevel converters (MMCs) are adopted to provide medium voltage DC (MVDC) ports. The positive and negative isolated modular multilevel DC converters (IMMDCs) are cascaded respectively with the positive and negative MVDC bus. Under pole-to-ground (PTG) fault condition, the healthy pole of MMC and the healthy IMMDC can operate as normal with the capability of half rated active power transmission. As a cascaded system, the stability of the PET may be affected by the impedance-mismatching of the cascaded converters. Therefore, based on impedance analysis method, the stability analysis of the proposed PET is developed. Then, the stability control method based on series virtual resistance is presented to improve the PET stability. Besides, to fast the dynamic respond speed, a coordinated control scheme based on low voltage DC port current feedforward is introduced. Finally, the presented PET topology and the control strategy is verified by simulations in MATLAB/Simulink.