Multi-Functional Intelligent MMC-Based HVDC Converter

Abstract

Modern power systems are undergoing a transformation in which renewable generation, data centers, electric-vehicle charging, and other power-electronics-based resources increasingly shape system dynamics and stability. Under these conditions, converters can no longer be treated as simple power-processing interfaces. Instead, they must evolve into multi-functional intelligent devices capable of sensing, analyzing, and actively interacting with the grid. Modular multilevel converter based HVDC has become the dominant technology for high-voltage applications because of its efficiency, scalability, and waveform quality. However, the low switching frequency of conventional MMC submodules makes it difficult to inject controlled perturbations for broadband sensing. This thesis makes a core contribution by proposing a multi-functional intelligent MMC-based HVDC converter that embeds in-situ broadband perturbation injection capability directly into the converter. The proposed architecture replaces only a small fraction of conventional silicon half-bridge valve submodules with fast-switching silicon carbide full-bridge valve submodules, thereby adding an auxiliary high-bandwidth voltage-synthesis channel while preserving the basic structure and cost profile of a standard MMC. Building on this architecture, the thesis develops three key elements: selective perturbation injection at either the AC terminal or DC terminal by exploiting the inherent DC/AC decoupling of the MMC, a novel DC-bias valve-energy-balancing method for the auxiliary valve which preserves desired injection waveform, and least-squares-based signal-processing methods for impedance extraction in both the synchronous dq frame and the sequence domain. The proposed converter and measurement framework are validated in OPAL-RT real-time simulation using a point-to-point MMC-HVDC system. The results show that the converter can inject small perturbations while remaining close to its nominal operating condition, and that the measured AC-side and DC-side impedances agree well with analytical or benchmark references over most of the studied frequency range.