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Voltage source converter based high-voltage direct-current (VSC-HVDC) systems have shown their advantages over line commutated converter (LCC) based systems in renewable energy integrations, weak ac grid connections and passive network energisations. Worldwide applications of VSC-HVDC have created great potentials to build multi-terminal DC (MTDC) grids to further improve the efficiency and flexibility of power networks. However, technical challenges still exist in operating MTDC grids safely and reliably. Current flow controls and fault protections within MTDC grids have not been fully addressed and remain critical aspects that need to be further studied. This thesis focuses on conducting investigations on operations and protections of VSC based dc grids. Current flow controls in meshed MTDC grids are needed to avoid the overload of transmission lines. Analysis of different types of current flow controllers (CFCs) is conducted in this thesis. It is revealed that the half bridge CFCs (HB-CFCs) are of low cost and high flexibility. A level-shift modulation method, as well as a dual-loop control, is proposed to reduce the switching losses of the HB-CFC and improve its controll ability. To guide its ontroller design, small-signal models of HB-CFCs are derived. The function of the HB-CFC and the effectiveness of the proposed modulation method are verified through simulations Protection of dc faults is deemed to be of high cost since dc circuit breakers (DCCBs) are expensive. To cope with this issue, a new device, integrating of DCCBs and HB-CFCs, is proposed. The presented new device can significantly reduce the number of semiconductor devices while containing the functions of the two devices. Detailed analysis of the proposed device is conducted, and simulations are carried out in PSCAD/EMTDC to verify the analysis. Besides dc faults and grid-side ac faults, valve-side ac faults will induce severe consequences to half-bridge (HB) and full-bridge (FB) modular multilevel converters (MMCs), especially in bipolar HVDC systems. However, fault behaviour and protection methods have not been fully studied for such faults. To bridge this gap, the analysis of valve-side single-phase fault for MMC based HVDC systems are investigated in this thesis. Protection methods against such faults are proposed. For completeness, the effectiveness of the proposed methods is verified by simulations conducted in PSCAD/EMTDC. The study of this research is expected to contribute to the operation and protection of VSC-HVDC grids.

Power devices based on wide bandgap (WBG) or ultrawide bandgap (UWBG) semiconductors have attracted more and more interests in recent years. The wide energy bandgaps of these semiconductor materials will lead to a high breakdown voltage and improve the power loss of the power devices. GaN, β-Ga2O3 and AlxGa1-xN are three typical WBG materials that are widely discussed and studied in these days. The first part of this master thesis is to study the design rules of WBG or UWBG power diodes. I will start from the definition of three important parameters that determine the performance of power diodes: breakdown voltage (BV), specific on-resistance (Ron,sp) and Baliga’s figure of merit (BFOM). Through calculation of these three parameters of the power diodes on GaN, β-Ga2O3, Al0.3Ga0.7N and Al0.7Ga0.3N, we can obtain the plots of BV vs doping concentration, Ron,sp vs doping concentration and BFOM vs doping concentration at different thicknesses of drift layer. Compared with the three parameters calculated of power diodes based on different semiconductors, the conclusion is that semiconductors with wide energy bandgap will make power diodes have high breakdown voltage and low power loss. In addition, according to 2D mapping of BFOM, the optimum solution of the thickness and doping concentration of drift layer for a peak BFOM can be obtained. The physical meaning of this optimum solution is that the optimal thickness and doping concentration of drift layer can be given if the design target of breakdown voltage of the diode is known. The second part of this master thesis is to design a circuit for testing switching performance of a GaN pn power diode. The schematic of Double Pulse Test (DPT) circuit will be introduced. To realize this DPT circuit, I will first model a 1 kV GaN pn diode fabricated in our laboratory based on the I-V and C-V characteristics measured in the lab and perform simulation of the DPT circuit in LTspice. The simulation results show that the rise-time in the turn-on transient is 3.06 ns and fall-time in the turn-off transient is 4.83 ns. To verify the simulation results, a PCB layout of this DPT circuit has been designed and the experiment will be carried out in the future.

Phenolic content is an important dimension of Pinot noir wine quality. This study aimed to find generic relationships between the phenolic content of Pinot noir wines produced from well-defined locations (single vineyards) in New Zealand, as determined from various chemical analyses, and aspects of terroir relating to local climate, soils and methods of production and information collected from questionnaires. Two New Zealand Pinot noir wines were stored at room temperature (with and without headspace sparging with N₂), at 4°C (with and without sparged N₂), at - 20°C, and at - 80°C following flash freezing with liquid nitrogen, after the original, sealed bottles were opened. The changes in total phenolics, total tannins, anthocyanins and colour parameters over 134 days were quantified. Results showed that the colour-related parameters were more sensitive to storage conditions and time compared with the other parameters. Storage at - 80°C could be the optimal way to preserve colour parameters, as it generally caused the least change in values and induced the least precipitation over the whole experimental period, followed by storage at 4°C. A total of 86 single-vineyard Pinot noir wines were collected from the 2013 Bragato wine competition. Analyses, using standard methods for total phenolics, total tannins and colour measurements, and HPLC (high-performance liquid chromatography) analysis of monomeric species separated using a published SPE (solid-phase extraction) method, were carried out on these wines. Some strong correlations were found between the different ways of taking measurements on the same, or similar, compounds in wines. Generally, the regional or vintage differences in total phenolics content and total tannins, were not dramatic, but there were evident regional and vintage differences in the colour parameters. Wines awarded different medals grades also differed in their colour parameters. Specifically, wines with gold or silver medals tended to have deeper colour densities and were higher in total anthocyanins than the bronze and no-medal wines, but were not necessarily lower in colour hue. The overall phenolic content differences investigated using PCA (principal component analysis), showed separations between Otago wines and Marlborough wines, and also between vintage 2012 wines, 2011 wines and 2010 wines, although there were some overlaps in these separations. A total of 41 viable questionnaires were returned. Viticulture, winemaking and barrel ageing practices were all quite similar among wines and influences for these parameters on individual phenolics were not able to be drawn. In contrast, soils differed considerably between regions and there were consistent negative and linear correlations between vine potential vigour as affected by soil parameters (carbon content, potential rooting depth and profile readily available water) and key colour parameters (total anthocyanins, total red pigments, ionised anthocyanins, and malvidin-3-glucoside), while positive correlations with some hydroxybenzoic acids were also found.

The current decades have witnessed the explosive increase of traffic-data demand. It is predicted that indoor wireless communications will be one of the fastest growing markets, since the vast majority (over 80%) of data demand occurs in indoors. Facing such a huge data demand, the dense deployment of small cells (SCs) in indoor environments is boosted, which brings breakthroughs of throughput for in-building communications. However, the densification of indoor small-cell (SC) networks also poses new challenges, such as complex propagating environments, severe blockage effects and short link distances, which significantly influence the evaluation of network performance. This thesis mainly investigates the performance analysis of indoor dense SC networks. Firstly, the probability of Line-of-Sight (LOS) propagation is crucial to model the real signal propagation channels and to evaluate the performance of cellular networks. However, existing LOS probability models are oversimplified to provide the exact LOS probability in indoor scenarios. By considering the realistic layout of building structures, this thesis proposes a novel and analytical LOS probability model for downlink radio propagations in typical indoor scenarios, which have rectangular rooms and corridors. Through the proposed model, the LOS probability can be calculated directly without the measurement and simulation. Next, in terms of the impact of LOS and Non-Line-of-Sight (NLOS) transmissions, the traditional works do not distinguish them, which is not practical for dense cellular networks. Thus, a tractable path loss model considering both LOS and NLOS propagations is proposed for the performance analysis of indoor dense SC networks. Based on the theory of stochastic geometry, the performance metrics, such as coverage probability, spectral efficiency (SE) and area spectral efficiency (ASE), are analytically derived. The analytical results provide insights into the design of indoor dense SC networks in the future. Thirdly, regarding the severe effects of blockages in indoor environments, the traditional approach that simply considers it as a log-normal shadowing is too simple. Therefore, a wall blockage model is developed to characterize the impact of blockages based on the stochastic geometry. Furthermore, the mathematical expression of coverage probability for the case of impenetrable blockages is derived, which employs a path loss model incorporating both the blockage-based and distance-based path loss.