No description available.

No description available.

No description available.

No description available.

No description available.

Subsynchronous resonance (SSR) is one of the major obstacles for the wide spread of high degrees (60% and higher) of series capacitor compensation. Recently, a new obstacle, namely Subsynchronous Interaction (SSI) has been added to the list after the Zorillo Gulf wind farm incident in Texas in October 2009. SSI is due to the interaction between large Doubly Fed Induction Generator (DFIG)-based wind farms and series capacitor compensated transmission systems. In integrated power systems incorporating series capacitor compensated transmission lines and high penetration of wind energy conversion systems, especially DFIG-based wind farms, SSR and SSI could occur concurrently as a result of some system contingences. Therefore, mitigating SSR and SSI is an important area of research and development targeting at developing practical and effective countermeasures. This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the potential use of offshore and DFIG-based wind farms for simultaneous mitigation of SSR and SSI. This is achieved through introducing supplemental control signals in the reactive power control loops of the grid side converters of the DFIG wind turbines or the HVDC onshore Modular Multilevel Converter (MMC) connecting the offshore wind farm to the grid. In this context, two supplemental controls designated as Supplemental Controls I and II are examined. Supplemental Control I introduces a signal in the HVDC onshore converter to damp both SSR and SSI oscillations. On the other hand, Supplemental Control II introduces a signal in the HVDC onshore converter for damping SSR oscillations and another signal in the grid side converters of the DFIG wind turbines for damping SSI oscillations. Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the presented two supplemental controls are very effective in mitigating the SSR and SSI phenomena at different system contingencies and operating conditions.

No description available.

No description available.

Fast Field-Cycling (FFC) MRI allows the magnetic field to be switched during an imaging scan. FFC-MRI can be used to exploit a characteristic of contrast agents, i.e. the variation of its spin-lattice relaxation time (T1) or rate (R1= 1/T1) with magnetic field in order to increase contrast. Contrast agents play an essential role in MRI, allowing improved diagnosis and delineation of diseased tissue. However, the R1, and hence the effectiveness of contrast agents, varies significantly with magnetic field. Thus, Fast Field-Cycling (FFC) MRI can be used to take advantage of this variation to improve image contrast, allowing more sensitive detection of the agent. In this project new contrast agents, developed by a collaborating group (Invento S.r.l., Italy) were investigated for use with FFC-MRI. R1 dispersion curves of samples containing a range of contrast agents were first obtained using both a commercial relaxometer and a home-built whole-body FFC-MRI system, and the accuracy of the home-built FFC-MRI system was verified. The magnetisation behaviour of these samples during field-cycling pulse sequences was modelled in order to predict the pulse sequence parameters necessary for maximum T1 contrast. Images were obtained, using a number of novel imaging techniques developed on the home-built whole-body FFC-MRI system, and also, using standard T1 weighted imaging on a 3 T Philips clinical MRI scanner. A new FFC-MRI imaging method, ΔR1 mapping was employed to show an increase in contrast using a novel Mn2+ based liposomal contrast agent compared with T1 weighted images at 5 mT, 59 mT and 3 T. The low concentrations of Mn2+ based liposomal contrast agents used with ΔR1 mapping indicate suitability for molecular imaging

Magnetisation Transfer Contrast (MTC) is a well-established magnetic resonance imaging (MRI) contrast-generating mechanism, and is widely used for clarifying MR-invisible macromolecular information indirectly via MR-detectable free protons using an offresonance pre-saturation radiofrequency (RF) pulse (or MT pulse). As a result of MT pulse irradiation, magnetisation between both proton pools is exchanged and the signal intensity of mobile protons is decreased in relation to the amount of macromolecules. MTC MRI is normally implemented at a fixed magnetic field; however, it may be useful to evaluate changes of the MT effect as a function of magnetic field (B0). In order to explore fielddependent MTC experiments using a single MR instrument, two techniques are required, which enable simultaneously shifting both B0 and the resonance frequency of an RF coil (f0) during MT pulse irradiation and returning them to the original condition during MR data acquisition. Switching of B0 is achieved by fast field-cycling (FFC). FFC is a novel technique allowing B0 to shift between levels rapidly during the pulse sequence. This makes it possible to perform a number of beneficial field-dependent studies and/or to provide new MR contrast mechanisms. Switching of f0 requires an actively frequencyswitchable RF coil. This coil was designed and constructed for frequencies at and below 2.5 MHz proton Larmor frequency. The design employed PIN diodes, and enabled switching f0 between five different values. Using these techniques and tools, fielddependent MTC experiments were carried out with a control sample and samples with different concentrations of agarose gel. Due to the absence of macromolecules in the control, the MT effect was almost zero, whereas the MT effect observed in agarose samples increased with increasing concentration of macromolecules. Furthermore, MT effects ((for a given set of MT pulse conditions) were larger at higher B0.