Modeling, analysis, design and control of a power electronic transformer using back-to-back connected MMCs for grid integration of renewables and storage

Abstract

Utility-scale renewables, wind and solar, are already becoming cheaper than power from conventional fossil-fuel sources. Their further penetration for achieving the goal of 100% carbon-free electricity will be stalled unless their cost and efficiencies are further improved. Power electronic interfaces enabling their grid integration should provide ancillary services to keep the grid stable. In addition, it has become the norm to be able to integrate multiple heterogeneous forms of energy through a common interface scheme and thus achieve higher power densities. The power electronic interface in this research serves these purposes for the integration of renewables and storage to the medium voltage grid between 5 kV - 34:5 kV .A Power Electronic Transformer (PET) topology involving the direct back-to-back connection of Modular Multilevel Converters (MMCs), where one of the MMCs is connected to the grid and the other called High-Frequency MMC (HF-MMC) is connected to a step-down HF transformer, is studied in this dissertation. The low-voltage (LV) side of the HF transformer is connected to another back-to-back arrangement of two-level voltage source converters (2L-VSCs) for extracting power from renewables such as wind. The proposed interface is based on a modular topology to render it highly reliable, in conjunction with a high-frequency transformer which can be lighter in weight by a factor of 150 compared to the line-frequency transformer. It can provide ancillary services and control flexibility to offer "smart" solutions to maintain grid stability even when the penetration of renewables begins to approach conventional sources. Specifically, the contributions of the dissertation in making the interface deployable in the industry are as stated below. The modeling, design, control, and operation of the proposed interface scheme have been presented and validated through OPAL-RT-based Hardware-In-Loop simulations, MATLAB/SIMULINK simulations, and results from the experimental hardware prototype. The PWM switching ripples across the dc link have been quantitatively characterized and investigation of their propagation in the dc-link control schemes has been performed. Harmonic analysis and online optimization-based control schemes to improve the efficiency of power transfer across the high-frequency link have been presented. The DC side model of the HF-MMC which unveils the sensitivity of HF-MMC to voltages at frequencies other than dc being injected at the dc link is developed. The potential for virtual inertia with the proposed PET interface has been identified along with formulating associated control schemes for inertial support to the grid. Finally, an invention of a power converter topology employing several modular high-frequency transformers with drastically reduced voltage stresses and with multiple parallel power transfer paths has been made to improve on the initial architecture under study. This novel power conversion architecture achieves the critical objective of unifying dispatchable forms of power such as storage and the highly intermittent forms of energy such as wind and solar.