Investigations into the upgrading of transmission lines from HVAC to HVDC.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.Emanating from the proceedings of CIGRE 2004, a new idea for higher power transmission by recycling and up rating high voltage alternating current transmission lines for high voltage direct current application was presented at the HYDC working group session. To date, there is no known application of the idea. Globally, transmission congestion, power transfer bottlenecks with restricted and limited power transfers and unobtainable servitudes challenge electric power utilities. The literature review shows that since the early sixties, several authors have studied this proposal. However, no applications were done. Admittedly, early HYDC technology was troubled by problems with multi-terminal designs, external insulation breakdown in the presence of DC stress and mercury valve rectifiers struggled with arc backs. To date, power electronic and external insulation technology has grown and matured for confident application both in point to point and multi-terminal application. The economic costs of introducing the DC technology are also more affordable given reducing prices due to higher volume of purchases. With promising developments in insulation and power electronic technology and driven by South Africa's surging growth in the consumption of electrical energy; the subject of upgrading HYAC transmission for HYDC application is revisited. For the research, the emphasis is beyond FACTS and towards a solution that could develop into a new supergrid that could overlay the existing national grid. Thus, the solution is prepared specifically for the case of recycling existing assets for higher power transfers. The working environment is defined by the difficulty in acquiring new powerline servitudes, transmission congestion in complex networks, the need for electrical islands within complex interconnections, and the need for enhanced power system stability and to promote new ancillary services energy management. The focus of this research study was to determine the technical feasibility of upgrading of existing HYAC circuits for HYDC application. It is assumed that the transmission line will remain as is in structure, layout and mechanical design. The changing of external line insulators using live line technology is an accepted modification to the original HYAC line, if required. From the study, we conclude that not all HYAC lines are recommended for upgrade to HYDe. We introduce boundary conditions as a first step towards checking on the suitability of the proposed upgrade from HVAC to HYDC mode. Emanating from this study, the first paper published introduced the initial boundary conditions as being only those lines where the "unused gap" between surge impedance loading and conductor current carrying capability is appreciable and large; generally three to four times surge impedance loading. In the case where the unused gap is the smallest or negligible, then we do nothing. In between, where the unused gap is about two to three times the surge impedance loading, then we can consider active or passive compensation using the HVAC FACTS technology options as proposed by EPRl. Having determined the candidate transmission line configuration for the proposed upgrade to HYDC application, we select the DC operating voltage as based on the voltage withstand capability of external insulation for varying environmental conditions. In addition, the DC voltage will generate allowable electrical fields and corona effects within and outside the transmission servitude. The optimum DC operating voltage would satisfy the conditions of minimum transmission power losses and volt drop for the case of maximum power transfers; within the limits of electrical fields and corona effects

Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.

BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700

Mean-variance Trading Portfolio Selection for A Class of Energy Retailers

Due to the volatile price of various energy products, energy retailers in many countries are facing the risk of going bankrupt. This paper focuses on a class of energy retailers that trade energy products including the electricity option, the natural gas option and the white certificate. From the perspective of such energy retailers, this paper studies a portfolio selection strategy that can achieve the maximized asset value and mitigate the potential risk of purchasing energy products at high prices. Firstly, a class of linear ordinary differential equations (ODEs) and stochastic differential equations (SDEs) are used to model the dynamic time-varying price of electricity option, natural gas option and white certificate accurately. Secondly, based on the mean-variance model, the portfolio selection strategy problem of energy retailers trading these three products is formulated as a stochastic optimal control problem. Then, the linear-quadratic (LQ) control method is used to solve the problem analytically with mathematical theorem, and the obtain controller is indeed the desired optimal trading strategy. Finally, a series of examples demonstrating the correctness of the proposed portfolio selection strategy are provided

Mean-variance Trading Portfolio Selection for A Class of Energy Retailers

Due to the volatile price of various energy products, energy retailers in many countries are facing the risk of going bankrupt. This paper focuses on a class of energy retailers that trade energy products including the electricity option, the natural gas option and the white certificate. From the perspective of such energy retailers, this paper studies a portfolio selection strategy that can achieve the maximized asset value and mitigate the potential risk of purchasing energy products at high prices. Firstly, a class of linear ordinary differential equations (ODEs) and stochastic differential equations (SDEs) are used to model the dynamic time-varying price of electricity option, natural gas option and white certificate accurately. Secondly, based on the mean-variance model, the portfolio selection strategy problem of energy retailers trading these three products is formulated as a stochastic optimal control problem. Then, the linear-quadratic (LQ) control method is used to solve the problem analytically with mathematical theorem, and the obtain controller is indeed the desired optimal trading strategy. Finally, a series of examples demonstrating the correctness of the proposed portfolio selection strategy are provided

Kuluttajabarometri maakunnittain 2000, 2. neljännes

Suomen virallinen tilasto (SVT

Use of failure-to-rescue to identify international variation in postoperative care in low-, middle- and high-income countries: a 7-day cohort study of elective surgery

This was an investigator-initiated study funded by Nestle Health Sciences through an unrestricted research grant and by a National Institute for Health Research (UK) Professorship held by R.P. The study was sponsored by Queen Mary University of London