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UBC Theses and Dissertations

Numerically efficient models of modular multilevel converters for high voltage direct current transmission system and AC-DC super grid Meng, Xuekun


High Voltage Direct Current (HVDC) technology has been widely used in providing interconnection between the remote renewable power plants and the existing power grid infrastructures. The latest development of Modular Multi-level Converter (MMC) in Voltage Source Converter (VSC) family can independently control active and reactive power and provide stability enhancement in weak AC grids. Additionally, harmonic filters are unnecessary in the implementation of MMC as it provides nearly perfect sinusoidal shape waveforms with a sufficient number of levels and switches. In recent years, various projects are developed based on the MMC-HVDC, and it proves its potential in becoming the backbone of the long-distance power transmission in the near future. However, the massive number of power switches in MMC-HVDC also brings notable challenges in digital simulation. Especially for simulating an interconnected system that involves both the MMC-HVDC system and large-scaled AC networks, a large number of switching operations and massive input/output data transfer will introduce a tremendous computational burden. Developing a numerically efficient model for MMC-HVDC interfacing with large-scale AC grids is essential for experimental testing and real-time simulations. This thesis addresses several aspects of the numerically efficient modeling of MMC-HVDC and its integration with a large-scale AC grid. First, a simplified model of MMC is proposed with improved simulation efficiency while preserving accuracy for various transients. The proposed model offers the flexibility in converter operation modes to accommodate the MMC with different submodule topologies and its relevant detailed controls. Next, we extend the concept of the simplified model of MMC into a unified framework to incorporate different levels of simplification into a general circuit structure. The proposed model is validated against state-of-the-art models to demonstrate its significantly improved simulation accuracy. Finally, the simplified MMC model is incorporated into hybrid transient stability and electromagnetic transient simulation for simulating the large-scale AC grid interconnected with MMC-HVDC links. The case studies demonstrate the superior numerical efficiency of the proposed approaches and their advantage in capturing the system-level details. It is envisioned that the proposed modeling approaches can be implemented by various power system transient simulation tools to streamline the HVDC converter design process.

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