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Analysis of hybrid quasi-two-level converter based HVDC transmission system

University of British Columbia

Voltage Source Converters (VSC) are universally utilized in the areas of power generation, conversion and transmission systems in different forms and high voltage rating of VSC becomes necessarily required for the High Voltage direct current (HVDC) which is admitted as a critical factor to the anticipation of constructing a high-capacity transmission system. Modular Multilevel Converter (MMC) makes it possible to realize a VSC topology with high voltage level and to avoid drawbacks, e.g. it processes high scalability and modularity for higher voltage and increases converter efficiency. Mathematical analysis of conventional HB-MMC is provided in Chapter 2, which lays foundation for the newly proposed topology. Basic controlling methods are demonstrated in Chapter 2. However, there are also some drawbacks for HB-MMC operation. Firstly, HB-MMC is vulnerable when it comes to DC-side faults. Secondly, the submodule losses are higher than thyristor based HVDC technology, leading to higher operating cost. Additionally, the submodule capacitance needs to be reduced in order to enable a smaller footprint. Based on the concept of Quasi-Two-Level Converter proposed for DC/DC converter application, Hybrid Q2LC is proposed for AC/DC converter application in this thesis. The contributions of the proposed topology include the reduction of switching and conduction losses, decreasing d_v/d_t stress for the AC transformer and loads. Additionally, third-order harmonic voltage injection technique is realized to limit the voltage range of AC chain-link which works as active filters to attenuate high-order harmonics. When the DC pole-to-pole fault takes place, normal and protection branches of AC chain-link are available to provide sufficient reverse blocking voltage to mitigate AC surging current. Besides, submodule capacitance of the main converter part can be further reduced, which is beneficial for the converter system to operate with high compactness and low losses. Simulation results of the proposed hybrid Q2LC with AC chain-link topology are provided in Chapter 4 based on MATLAB Simulink including both dynamic performances under normal and DC pole-to-pole fault operations. Comparisons of losses among different converter types are made. Basic parameters of the mentioned topologies are analyzed. In the last chapter, contribution summary and future works are illustrated respectively.

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2020-04-28

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/74203

Electrical Engineering

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/