UBC Theses and Dissertations
Control and modeling of a hybrid quasi-three-level converter for high voltage direct current applications Pfannschmidt, Joel Andrew
The inherent advantages of high voltage direct current systems for bulk power transmission and the interconnection of power grids, distributed energy sources, energy stores, and industrial loads have given rise to their widespread implementation. High power converters are vital components of these systems, and the continued analysis of existing and novel topologies is crucial to their increased efficacy and deployment. The voltage source converter, based on self-commutating semiconductors, encompasses significant advantages over the previously dominating line-commutated converter such as black-start capability and advanced reactive power control. However, the voltage source converter is inherently susceptible to DC-side faults and often requires a significant tradeoff of efficiency or converter footprint to counteract this vulnerability. A novel Hybrid Quasi-Three-Level Converter topology proposed herein offers competitively low losses and compactness while maintaining DC-fault ride-through capability and static synchronous compensator operation simultaneous with fault clearing. The implementation of two separate full-bridge chainlinks located at the midpoint and AC-side junction of the converter facilitates the respective generation and harmonic filtering of a three-level trapezoidal voltage. This unique operation enables lossless switching of the integrated director switches, along with rapid and efficient charging of the midpoint chainlink through active control and manipulation of their currents. This current-regulating procedure mitigates the reduced average director switch conduction duration, maintaining the lowest possible conduction losses throughout. Additionally, the injection of a third-order harmonic voltage into the AC-side chainlink minimizes its semiconductor count through a reduced rated voltage while preserving a pure fundamental frequency voltage output. The use of a fast mechanical switch in series with each director switch provides isolation during DC-side faults, enabling ride-through capability and AC-grid reactive power compensation throughout fault clearing. This thesis provides a comprehensive analysis of both the practical and theoretical operation of the Hybrid Quasi-Three-Level Converter, along with simulation-based case studies offering validation of and insightful perspective into its operational principles.
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