Multi-terminal current source based DC transmission system for multiple wind turbine interfacing

This thesis focuses on using power electronic techniques to addresses grid integration for wind energy conversion systems. Different approaches to mitigate the low frequency generator torque ripple caused by diode bridge rectifiers are proposed. The advantages and disadvantages of the methods are discussed. A relationship for maximum power point tracking is theoretically analysed. Based on this relationship, two new maximum power point tracking techniques are proposed, which show benefits over conventional tracking methods. Then a pulsewidth modulated current source converter based parallel connected wind energy conversion system is investigated. A new inverter controller for this system is proposed, which is able to maintain a constant average DC network voltage to give satisfactory system performance whilst controlling output reactive power. Practical results support the presented simulations. Furthermore, a fault ride through scheme is proposed for the current source converter based system. The protection scheme uses a shunt resistive chopper to dissipate the active power during faults, and allows the inverter to supply reactive power to support the grid. The space vector modulation for the current source inverter is modified for this application and the design of the dumping resistor is discussed. The system shows riding through capability to both AC and DC network disturbances. This aspect is substantiated by simulation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Wind turbine power coefficient analysis of a new maximum power point tracking technique

A small-scale wind energy conversion system can track the maximum power point (MPP) based on a linear relationship between $V_{rm dc}^{2}$ and $I_{rm dc}$. Unlike conventional MPP tracking (MPPT) methods using a lookup table, an advanced technique is proposed based on this relationship as a variant of the perturb and observe (P&O) method. It not only has the advantages of the conventional P&O method but also has a faster tracking speed and better performance. This paper theoretically analyzes the possible power coefficient drop when using a linear relationship for MPPT and establishes that the turbine design can ensure that the possible power coefficient drop is small. The simulation results show that the analysis is precise. The validity and performance of the proposed MPPT method are confirmed by both simulation and experimentation

Production of Biodegradable Board using Rape Straw and Analysis of Mechanical Properties

This study investigated the glueless preparation of biomass board using rape straw on a laboratory scale. The board-making process was broken down into four steps: soaking, refining, shape recovery, and hot-pressing. To observe the effect of pressure during the hot-press stage on the strength of the bio-board, five panels were manufactured at various pressures. Moreover, density functional theory (DFT) was used to explore how varying the pressure influenced the strength properties of the board. As pressure increased, the density of these five panels changed from 0.95 to 1.12 g/cm3. The mechanical tests showed that the bending rupture strength of these panels changed from 43 to 53 MPa, while the tensile rupture strength changed from 27 to 33 MPa. The bending strength of these biomass boards performed well enough to qualify them as Type-35 board, and their density classified them as hardboard, according to the Japanese industrial standards (JIS). This study showed that board-making was feasible using rape straw. The experimental results and the density functional theory results were consistent, in that the mechanical properties of the panels improved with increasing pressure. The DFT method was shown to be useful in exploring the factors that influenced the strength properties of the biomass board on the microscopic scale

Thermal Mitigation and Optimization Via Multitier Bond Wire Layout for IGBT Modules Considering Multicellular Electro-Thermal Effect

The stitch wire configuration is widely adopted for large-area IGBT chips. However, an inhomogeneous wire current density introduces uneven self-heating and nonuniform chip heating, which exacerbates the chip thermal stress. In this article, a multicellular electro-thermal model considering the stitch-bonding wires is derived to optimize the wire layout parameters. Furthermore, the current density alleviation of wire bonds is investigated to be the most sensitive and effective method to comprehensively mitigate the thermal nonequilibrium and local overheating. Consequently, an improved multitier layout is proposed to further achieve thermal stress suppression without any additional components and advanced materials. Finally, a triple-tier-bonded IGBT module with optimal design parameters is fabricated to validate the thermal mitigation performance. The prototype experimental results demonstrate that the modeling error is less than 3.0%. The multitier layout decreases the current density by 57.6%, hence, the maximum and average chip temperature are reduced by 21.8% and 12.4%, respectively.</p