Modular Multilevel Converter-Based Hvdc Transmission Systems

High-Voltage Direct Current (HVDC) transmission systems based on Voltage Source Converter (VSC) technology has attracted significant interest recently for transmitting large amounts of power over long distances using back-to-back or point-to-point configurations. VSC-HVDC has been addressed for various HV applications such as DC interconnections, Multi-Terminal HVDC Transmission (MT-HVDC), installation of offshore wind power generation such as Europe super DC grid and installation of other renewable energy sources. Several classes of VSC topologies can be employed in HVDC systems including the conventional two and three-level converters, multilevel converters, and Modular Multilevel Converters (MMCs) that has been recently introduced and investigated for HVDC applications. MMC is penetrating the modern HVDC transmission market, due to its inherent features such as scalability, modularity, and fault ride through capability. Therefore, this thesis investigates and models a point-to-point VSC-based HVDC transmission system using nine-level MMC transient model, and 25-level MMC averaged model using MATLAB/Simulink platform to meet the requirements of HVDC systems such as HV requirements and fault ride through capability. However, a point-topoint HVDC system using conventional two-level converter is modeled and simulated using MATLAB/Simulink as a starting and benchmarking model. MMC transient model employed in this study is based on Half-Bridge Sub-Modules (HB-SMs) due to its simple structure, yet, other structures are discussed. Nevertheless, balancing of the floating capacitors is one of the challenges associated with MMCs. Therefore, capacitor voltage balancing and its modeling is addressed. Then the average model of the MMC-based HVDC system is investigated. Moreover, the behavior during DC side faults is investigated, and the employment of hybrid DC circuit breakers and Hybrid Current Limiting Circuit (HCLC) are introduced for protection and limiting the DC fault current. This introduces a platform for studying large MMC-based HVDC systems in normal operation and during faults

Study and evaluation of distributed power electronic converters in photovoltaic generation applications

This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria

Hybrid HVDC transformer for multi-terminal networks

There is a trend for offshore wind farms to move further from the point of common coupling to access higher and more consistent wind speeds to reduce the levelised cost of energy. To accommodate these rising transmission distances, High Voltage Direct Current (HVDC) transmission has become increasingly popular. However, existing HVDC wind farm topologies and converter systems are ill suited to the demands of offshore operation. The HVDC and AC substations have been shown to contribute to more than 20% of the capital cost of the wind farm and provide a single point of failure. Therefore, many wind farms have experienced significant delays in construction and commissioning, or been brought off line until faults could be repaired. What is more, around 75% of the cost of the HVDC and AC substations can be attributed to structural and installation costs. Learning from earlier experiences, industry is now beginning to investigate the potential of a modular approach. In place of a single large converter, several converters are connected in series, reducing substation individual size and complexity. While such options somewhat reduce the capital costs, further reductions are possible through elimination of the offshore substations altogether. This thesis concerns the design and evaluation the Hybrid HVDC Transformer, a high power, high voltage, DC transformer. This forms part of the platform-less (i.e. without substations) offshore DC power collection and distribution concept first introduced by the Offshore Renewable Energy Catapult. By operating in the medium frequency range the proposed Hybrid HVDC Transformer can be located within each turbine’s nacelle or tower and remove the need for expensive offshore AC and DC substations. While solid state, non-isolating DC-DC transformers have been proposed in the literature, they are incapable of achieving the step up ratios required for the Hybrid HVDC transformer [1]– [3]. A magnetic transformer is therefore required, although medium frequency and non-sinusoidal operation does complicate the design somewhat. For example, inter-winding capacitances are more significant and core losses are increased due to the added harmonics injected by the primary and secondary converters [1], [2]. To mitigate the impact of these complications, an investigation into the optimal design was conducted, including all power converter topologies, core shapes and winding configurations. The modular multilevel converter in this case proved to be the most efficient and practical topology however, the number of voltage levels that could be generated on the primary converter was limited by the DC bus voltage. To avoid the use of pulse width modulation and hence large switching losses, a novel MMC control algorithm is proposed to reduce the magnitude of the converter generated harmonics while maintaining a high efficiency. The development and analysis of this High Definition Modular Multilevel Control algorithm forms the bulk of this thesis’ contribution. While the High Definition Modular Multilevel Control algorithm was developed initially for the Hybrid HVDC Transformer, analysis shows it has several other potential applications particularly in medium and low voltage ranges