Abstract
This thesis is focused on the modular multilevel converter (MMC) for High-Voltage DC (HVDC) systems. It is an attempt to address the issues associated with the modelling, simulation, control, efficiency, and fault-handling capability of the MMC. Thus, to address the modelling of the MMC, a new and more accurate steady-state harmonic model is proposed. The proposed harmonic model is capable of predicting the amplitude of the harmonic components of the MMC arm voltages, submodule capacitor voltages, and arm currents. Further, based on the proposed harmonic model, a capacitor sizing method is proposed to determine the capacitance of the submodule capacitor for a desired level of voltage variation, without a need for numerical algorithms or graphs used by the existing methods. In addition, the proposed capacitor sizing method can accurately determine the required capacitance even if circulating currents are injected to mitigate dc voltage fluctuations. The thesis also proposes a simple equivalent-circuit-based simulation model for MMC-based HVDC systems, which assumes ideal submodule switches to speed up the simulation, but is nonetheless capable of capturing the transients as well as harmonic components of the voltages and currents. Further, the thesis proposes a simple compensation strategy that calculates the magnitude of the second harmonic component of an arm voltage, and uses the calculated value as a feedforward signal to cancel the circulating current of the corresponding MMC leg. The proposed feedforward compensation strategy, if combined with a closed-loop circulating current suppression strategy, greatly mitigates the possibility of control saturation and, also, results in better damped closed-loop dynamics. Finally, the thesis proposes two new MMC topologies for enhanced efficiency and dc-side fault handling capability. In the first proposed topology, that is the lattice modular multilevel converter (LMMC), the entire MMC arm is modified to accommodate networks that allow shortcuts between the arm capacitors, thus, reducing conduction power losses of the converter. In the second topology proposed, however, only the submodule is modified. In the proposed submodule topology, referred to as lattice submodule (LSM), the conduction power losses are decreased, as it is the case for the LMMC, with the difference that the voltage stress in the switches are also reduced. Keywords: Control, lattice modular multilevel converter, lattice submodule, modelling, modular multilevel converter, simulation model.
Highlights
1.1 Background and MotivationA high-voltage dc (HVDC) system for bulk transmission of electric energy over large distances is more efficient and economical than an ac counterpart, especially if undersea cables are involved, HVDC transmission systems are on occasions the only choice, e.g., for interconnecting power systems of different frequencies, or for other strategic purposes
Since the SEC simulation model is focused in the converter dynamic behavior, it works when the SM topology is different than the full-bridge submodule (FBSM), but retains the same dc-side fault handling capability for disable switches
FBSM has been considered as the building block of the modular multilevel converter (MMC), due to its dc-side fault handling capability, given by the fact that, when the switches have their gate pulses blocked, the insolated-gate bipolar transistor (IGBT) anti-parallel diodes insert the submodule capacitor in the MMC arm, with its voltage polarity against the grid voltage
Summary
1.8 Illustration of the Nemo HVDC transmission system, connecting Richborough, 1.9 Future site of the Ultranet HVDC transmission system, connecting Osterath to. 2.2 Main MMC SM topologies currently known:(a) Half-bridge submodule - HBSM; 2.11 First, second, and third harmonic components in the submodule capacitor volt-
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.