An Online-Optimization-Based High-Frequency Link Control of an MMC-Driven Power Electronic Transformer for Wind-Energy Systems

Wide-scale grid integration of wind-energy renewable systems at the utility scale has sparked interest in novel power-electronic architectures for the interface at medium-voltage (MV) grid. A recent effort in this direction involves a Power Electronic Transformer (PET) comprising back-to-back connected Modular Multilevel Converters (MMCs); here, one MMC interfaces with the MV grid, and the other so-called High-Frequency MMC (HF-MMC) interacts with the wind-energy system via an HF transformer and a low-voltage (LV) 2 L voltage-source converter (VSC). In this paper, a novel control architecture for operating the PET is presented, that achieves superior power transfer characteristics across the HF transformer. On the HF-MMC end, an online optimization-based modulation scheme is developed that ensures unity displacement power factor (DPF) operation and current harmonics minimization; while at the 2 L VSC end, the voltage phase angle is controlled to maintain the LV dc-bus. To achieve these and the desired performance improvements, two actions are performed: (a) the target control variables - voltage magnitude of HF-MMC and phase angle of 2L-VSC voltage - are enforced via appropriate control loops, and (b) the switching instances of the HF MMC sub-modules are engineered by solving an optimization problem online using the coordinate-gradient-descent method. Detailed simulation results on analysis and control are presented in MATLAB-SIMULINK. Furthermore, verification of LV dc-bus regulation, unity DPF operation during power transfer, and minimization of HF-link current harmonics are carried out in OPAL-RT-based hardware-in-loop (HIL) real-time simulations and experimental results on a scaled hardware prototype.