Multi-port DC-DC Power Converter for Renewable Energy Application

In recent years, there has been lots of emphasis put on the development of renewable energy. While considerable improvement on renewable energy has been made, there are some inherent limitations for these renewable energies. For example, for solar and wind power, there is an intermittent nature. For the fuel cell, the dynamics of electro-chemical reaction is quite slow compared to the electric load. This will not be acceptable for modern electric application, which requires constant voltage of constant frequency. This work proposed and evaluated a new power circuit that can deal with the problem of the intermittent nature and slow response of the renewable energy. The proposed circuit integrates different renewable energy sources as well as energy storage. By integrating renewable energy sources with statistical tendency to compensate each other, the effect of the intermittent nature can be greatly reduced. This integration will increase the reliability and utilization of the overall system. Moreover, the integration of energy storage solves the problem of the slow response of renewable energy. It can provide the extra energy required by load or absorb the excessive energy provided by the energy sources, greatly improving the dynamics of overall system. To better understand the proposed circuit, "Dual Active Bridge" and "Triple Active Bridge" were reviewed first. The operation principles and the modeling were presented. Analysis and design of the overall system were discussed. Controller design and stability issues were investigated. Furthermore, the function of the central controller was explained. In the end, different simulations were made and discussed. Results from the simulations showed that the proposed multi-port DC-DC power converter had satisfactory performance under different scenarios encountered in practical renewable energy application. The proposed circuit is an effective solution to the problem due to the intermittent nature and slow response of the renewable energy

Investigation of Voltage Stability in Three-Phase Unbalanced Distribution Systems with DGs

Smart grids draw lots of attention and interests and they are fundamentally changing traditional power grids. One of the key aspects of smart grid is that more distributed generators (DGs) are connected in distribution systems. Distribution systems have changed from passive to active. Stability problems become important issues, one of which is voltage stability problems. To analyze voltage stability problems, many methods are proposed for transmission systems. However, because distribution systems are very different from transmission system, the methods for transmission systems cannot be directly applied to distribution systems. Therefore, effective methods of analyzing voltage stability problems for distribution systems are needed. The main focus of this dissertation is on three-phase unbalanced distribution systems with DGs. Firstly, improvements were made to an existing three-phase continuation power flow (CPF) method so that the maximum loading factor of distribution systems can be found accurately. Various distribution system components and DGs in PQ mode and PV mode with reactive power were modeled. Comparisons with Matpower software were made to validate the correctness of the implemented three-phase CPF program. Secondly, to provide more detailed voltage stability analysis and determine the weak buses of distribution systems, a new voltage stability analysis method, the CPF scan method, was proposed. The weak buses found by this method are the buses that have higher impact on the maximum loadability or the maximum total real load power that the system can support. Extensive case studies were performed and the impact of different distribution components were investigated. Lastly, to determine whether a distribution will experience voltage stability problems and to determine the weak buses, a measurement-based three-phase voltage stability index was proposed. This voltage stability index provides not only a system-wide index but also an individual index for each bus/phase. These proposed methods were applied to 8-bus system and a modified IEEE 13-node test feeder with DG to study the performance of the methods and investigate the impact on weak buses of different factors in distribution systems. The case studies showed that the proposed two methods, CPF scan and VSI, can successfully identify the impact of certain distribution system components. For more complicated components, such as untransposed lines and DG in PV mode, more research is needed. Also the CPF scan results shows good applications to distribution system operation and planning. The applications of the new proposed methods are not limited to identifying the weak buses. These methods have a great potential to be extended to voltage stability preventive and corrective control

A retrospective study of magnetic resonance-guided focused ultrasound ablation for uterine myoma in Taiwan

AbstractObjectiveTo report our experiences with 40 patients who were treated with magnetic resonance-guided focused ultrasound surgery (MRgFUS) for uterine fibroids and their 6-month follow-up status.Materials and MethodsA total of 40 patients with uterine fibroids underwent MRgFUS from January 2009 to November 2011. The Uterine Fibroid Symptoms and Quality of Life Questionnaire was used to determine the patients' Symptom Severity Scores (SSS) prior to and 6 months after treatment. The nonperfused volume (NPV) values and NPV ratio were obtained immediately at the end of the treatment and at 6 months follow-up.ResultsNo procedure-related complications were noted throughout the 6-month follow-up period among the 40 patients who underwent MRgFUS for uterine fibroids. The mean reduction in SSS in our patients after 6 months was 43.7%, and the mean reduction of fibroid volume was 31.7%. In addition, the mean reduction of NPV and mean NPV ratio was 52.7% and 33.3%, respectively.ConclusionThe results obtained from this study demonstrated that MRgFUS can be safely and effectively used to ablate uterine fibroids to produce a significant decrease in mean fibroid volume and improve SSS for up to 6 months after treatment

Xanthohumol, a Prenylated Flavonoid from Hops (Humulus lupulus), Prevents Platelet Activation in Human Platelets

Xanthohumol is the principal prenylated flavonoid in the hop plant (Humulus lupulus L.). Xanthohumol was found to be a very potent cancer chemopreventive agent through regulation of diverse mechanisms. However, no data are available concerning the effects of xanthohumol on platelet activation. The aim of this paper was to examine the antiplatelet effect of xanthohumol in washed human platelets. In the present paper, xanthohumol exhibited more-potent activity in inhibiting platelet aggregation stimulated by collagen. Xanthohumol inhibited platelet activation accompanied by relative [Ca2+]i mobilization, thromboxane A2 formation, hydroxyl radical (OH●) formation, and phospholipase C (PLC)γ2, protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and Akt phosphorylation. Neither SQ22536, an inhibitor of adenylate cyclase, nor ODQ, an inhibitor of guanylate cyclase, reversed the xanthohumol-mediated inhibitory effect on platelet aggregation. Furthermore, xanthohumol did not significantly increase nitrate formation in platelets. This study demonstrates for the first time that xanthohumol possesses potent antiplatelet activity which may initially inhibit the PI3-kinase/Akt, p38 MAPK, and PLCγ2-PKC cascades, followed by inhibition of the thromboxane A2 formation, thereby leading to inhibition of [Ca2+]i and finally inhibition of platelet aggregation. Therefore, this novel role of xanthohumol may represent a high therapeutic potential for treatment or prevention of cardiovascular diseases