A New Hybrid Multi-Level Voltage-Sourced Converter with DC Fault Blocking Capability

Voltage-Source Converters have brought numerous advantages to HVDC transmission. However, they suffer from high losses and are usually weak against faults on the DC-side. In this paper, a new topology which brings together some concepts from traditional Current Source Converters and multi-level converters, is presented. Two stacks of H-bridge cells alternate to construct the converter voltage using director switches made of IGBTs in series. The resulting converter generates AC current with low harmonic content and with low loss. Furthermore, the converter is still very responsive in case of a fault. This paper first explains the composition and the working of this converter, then detailed simulations at 20 MW illustrate the performances and low losses of this converter under normal conditions. The ability of this topology to deal with abnormal conditions is also demonstrated, especially its ability to keep control of the current despite the collapse of the DC bus voltage, e.g. a DC-side fault

Esophageal dysmotility after laparoscopic gastric band surgery

Background The effect of the laparoscopic adjustable gastric band (LAGB) on the esophagus has been the subject of few studies despite recognition of its clinical importance. The aim of this study was to investigate the frequency and clinical effect of esophageal dysmotility and dilatation after LAGB. Methods We undertook a retrospective analysis of 50 consecutive patients with no dysmotility on perioperative video contrast swallow who underwent primary LAGB operation. All patients had serial focused postoperative contrast studies for band adjustments at least 6 months post-LAGB. Clinical and radiological outcomes were assessed. Results Median follow-up time was 18 months (range 7–39 months), and the median number of contrast swallows per patient was 5. The mean excess weight loss (EWL) overall was 47 % (standard deviation (SD) 22.3). Radiological abnormalities were recorded in 17 patients (34 %, 95 % confidence interval (CI) 21–49 %), of whom 15 had radiological dysmotility and 7 had esophageal dilatation (five patients had both dysmotility and dilatation). Of these 17 patients, six (35 %) developed significant symptoms of dysphagia, gastroesophageal reflux disease (GERD) or regurgitation requiring fluid removal. In comparison, 12 of 33 (36 %) patients without radiological abnormalities developed symptoms requiring fluid removal (p = 1.00). Patients with radiological abnormalities were significantly older than those without these abnormalities. Symptoms were alleviated by removing fluid in most patients. Conclusions The LAGB operation results in the development of radiological esophageal dysmotility in a significant proportion of patients. It is not clear if these changes are associated with an increased risk of significant symptoms. Fluid removal can reverse these abnormalities and their associated symptoms

Biological Invasions and Ecological Restoration in South Africa

This chapter provides an overview of the researchers and research initiatives relevant to invasion science in South Africa over the past 130 years, profiling some of the more recent personalities, particularly those who are today regarded as international leaders in the field. A number of key points arise from this review. Since 1913, South Africa has been one of a few countries that have investigated and implemented alien plant biological control on a large scale, and is regarded as a leader in this field. South Africa was also prominent in the conceptualisation and execution of the international SCOPE project on the ecology of biological invasions in the 1980s, during which South African scientists established themselves as valuable contributors to the field. The development of invasion science benefitted from a deliberate strategy to promote multi-organisational, interdisciplinary research in the 1980s. Since 1995, the Working for Water programme has provided funding for research and a host of practical questions that required research solutions. Finally, the establishment of a national centre of excellence with a focus on biological invasions has made a considerable contribution to building human capacity in the field, resulting in advances in all aspects of invasion science—primarily in terms of biology and ecology, but also in history, sociology, economics and management. South Africa has punched well above its weight in developing the field of invasion science, possibly because of the remarkable biodiversity that provided a rich template on which to carry out research, and a small, well-connected research community that was encouraged to operate in a collaborative manner