Practical considerations for optimal conductor reinforcement and hosting capacity enhancement in radial distribution systems

The high penetration level of distributed generation (DG) units may lead to various problems and operational limit violations in electric power distribution systems if it exceeds a particular limit known as the system’s hosting capacity (HC). In this paper, the problem of selecting the optimal conductor for a real radial distribution system in Egypt is investigated using a recent meta-heuristic algorithm, known as salp swarm optimization. First, a constrained optimization problem is introduced to minimize the combined annual cost of energy losses and the investment cost of the conductors while complying with the system voltage limits and conductor thermal capacities. The results obtained show the effectiveness of the algorithm in satisfying the objective function and constraints. However, the optimization results also show that a reduction in the size of some existing conductors should take place, although this is not allowed by the utilities because of practical reasons such as load growth, variations in loading scenarios, and the possibility of connecting DG units with uncertain penetration levels and locations. Hence, a practical feeder reinforcement approach is proposed to maintain the constraints while considering these uncertainties. Further, a novel feeder reinforcement index is proposed to assist the distribution system operators and planners to determine the feeders that first need to be reinforced. The results obtained show that the proposed reinforcement approach attains a better level of HC than can be obtained with the conventional conductor selection approach under the same testing conditions

Large-scale unit commitment under uncertainty: an updated literature survey

The Unit Commitment problem in energy management aims at finding the optimal production schedule of a set of generation units, while meeting various system-wide constraints. It has always been a large-scale, non-convex, difficult problem, especially in view of the fact that, due to operational requirements, it has to be solved in an unreasonably small time for its size. Recently, growing renewable energy shares have strongly increased the level of uncertainty in the system, making the (ideal) Unit Commitment model a large-scale, non-convex and uncertain (stochastic, robust, chance-constrained) program. We provide a survey of the literature on methods for the Uncertain Unit Commitment problem, in all its variants. We start with a review of the main contributions on solution methods for the deterministic versions of the problem, focussing on those based on mathematical programming techniques that are more relevant for the uncertain versions of the problem. We then present and categorize the approaches to the latter, while providing entry points to the relevant literature on optimization under uncertainty. This is an updated version of the paper "Large-scale Unit Commitment under uncertainty: a literature survey" that appeared in 4OR 13(2), 115--171 (2015); this version has over 170 more citations, most of which appeared in the last three years, proving how fast the literature on uncertain Unit Commitment evolves, and therefore the interest in this subject

Pooled analysis of WHO Surgical Safety Checklist use and mortality after emergency laparotomy

Background The World Health Organization (WHO) Surgical Safety Checklist has fostered safe practice for 10 years, yet its place in emergency surgery has not been assessed on a global scale. The aim of this study was to evaluate reported checklist use in emergency settings and examine the relationship with perioperative mortality in patients who had emergency laparotomy. Methods In two multinational cohort studies, adults undergoing emergency laparotomy were compared with those having elective gastrointestinal surgery. Relationships between reported checklist use and mortality were determined using multivariable logistic regression and bootstrapped simulation. Results Of 12 296 patients included from 76 countries, 4843 underwent emergency laparotomy. After adjusting for patient and disease factors, checklist use before emergency laparotomy was more common in countries with a high Human Development Index (HDI) (2455 of 2741, 89.6 per cent) compared with that in countries with a middle (753 of 1242, 60.6 per cent; odds ratio (OR) 0.17, 95 per cent c.i. 0.14 to 0.21, P <0001) or low (363 of 860, 422 per cent; OR 008, 007 to 010, P <0.001) HDI. Checklist use was less common in elective surgery than for emergency laparotomy in high-HDI countries (risk difference -94 (95 per cent c.i. -11.9 to -6.9) per cent; P <0001), but the relationship was reversed in low-HDI countries (+121 (+7.0 to +173) per cent; P <0001). In multivariable models, checklist use was associated with a lower 30-day perioperative mortality (OR 0.60, 0.50 to 073; P <0.001). The greatest absolute benefit was seen for emergency surgery in low- and middle-HDI countries. Conclusion Checklist use in emergency laparotomy was associated with a significantly lower perioperative mortality rate. Checklist use in low-HDI countries was half that in high-HDI countries.Peer reviewe

Global variation in anastomosis and end colostomy formation following left-sided colorectal resection

Background End colostomy rates following colorectal resection vary across institutions in high-income settings, being influenced by patient, disease, surgeon and system factors. This study aimed to assess global variation in end colostomy rates after left-sided colorectal resection. Methods This study comprised an analysis of GlobalSurg-1 and -2 international, prospective, observational cohort studies (2014, 2016), including consecutive adult patients undergoing elective or emergency left-sided colorectal resection within discrete 2-week windows. Countries were grouped into high-, middle- and low-income tertiles according to the United Nations Human Development Index (HDI). Factors associated with colostomy formation versus primary anastomosis were explored using a multilevel, multivariable logistic regression model. Results In total, 1635 patients from 242 hospitals in 57 countries undergoing left-sided colorectal resection were included: 113 (6·9 per cent) from low-HDI, 254 (15·5 per cent) from middle-HDI and 1268 (77·6 per cent) from high-HDI countries. There was a higher proportion of patients with perforated disease (57·5, 40·9 and 35·4 per cent; P < 0·001) and subsequent use of end colostomy (52·2, 24·8 and 18·9 per cent; P < 0·001) in low- compared with middle- and high-HDI settings. The association with colostomy use in low-HDI settings persisted (odds ratio (OR) 3·20, 95 per cent c.i. 1·35 to 7·57; P = 0·008) after risk adjustment for malignant disease (OR 2·34, 1·65 to 3·32; P < 0·001), emergency surgery (OR 4·08, 2·73 to 6·10; P < 0·001), time to operation at least 48 h (OR 1·99, 1·28 to 3·09; P = 0·002) and disease perforation (OR 4·00, 2·81 to 5·69; P < 0·001). Conclusion Global differences existed in the proportion of patients receiving end stomas after left-sided colorectal resection based on income, which went beyond case mix alone

Egyptian grid code of wind farms and power quality

Compliance with interconnection requirements is mandatory for grid interconnected wind farms. However, there are many barriers on the track. Diversity, ambiguity, consecutive drafts and updated codes, and the different sets of connection requirements are some of these obstacles. This chapter demonstrates the recent challenges concerning the wind farms-grid interconnection regulations. The general values and impacts for wind farms-grid interconnection are presented based on the recently published Egyptian grid code. The authorities and responsibilities of the grid operators, conditions of the connection, reconnection and disconnection of farms, active and reactive power control are introduced and discussed. Besides, the various requirements concerning with quality of power (especially in weak grids), such as flickers, harmonics, imbalance and voltage fluctuations are discussed in details

State-of-the-art of hosting capacity in modern power systems with distributed generation

Currently, renewable energy is rapidly developing across the world in response to technical, economic and environmental developments, as well as political and social initiatives. On the other hand, excessive penetration of distributed generation (DG) systems into electrical networks may lead to various problems and operational limit violations, such as over and under voltages, excessive line losses, overloading of transformers and feeders, protection failure and high harmonic distortion levels exceeding the limits of international standards. These problems occur when the system exceeds its hosting capacity (HC) limit. The HC is a transactive approach that provides a way for the distribution network to be integrated with different types of energy systems. Accordingly, HC assessment and enhancements become an essential target for both distribution system operators and DG investors. This paper provides, for the first time, a systematic and extensive overview of the HC research, developments, assessment techniques and enhancement technologies. The paper consists of four HC principal sections: historical developments, performance limits, perceptions and enhancement techniques. Besides, practical experiences of system operators, energy markets and outcomes gained from real case studies are presented and discussed. It was concluded that success in integrating more distributed generation hinges on accurate hosting capacity assessment

Load frequency control of interconnected power systems using hybrid algorithm based particle swarm and grey wolf optimizers

This study introduces a new hybrid optimization technique into the research field of load frequency control. The new technique is a hybrid technique that combines two metaheuristic-based algorithms: Particle Swarm Optimizer (PSO) and Grey Wolf Optimization (GWO). This new technique facilitates the selection of the best gain values of the controller used in the power system under study. The controller utilized in this study is the classical proportional-integral-derivative (PID) controller. This classical controller is selected in this study to make a reliable comparison with other applied techniques. The study's main goal is to retain the system frequency and tie-line power within permissible limits after applying a load disturbance to one of the system areas. The system under test is built as a three area network with thermal power generation units. The hybrid PSO-GWO (HPSOGWO) algorithm is applied to the system under test. The results obtained are verified by comparing them with other techniques, including the bacterial foraging technique (BFOA) and harmony search technique (HS). The results show that the HPSOGWO algorithm can preserve the frequency and tie-line power within the permissible bounds faster and with better transient specifications than that obtained using the other algorithms under comparison. The three area system is simulated in MATLAB environment for an easier interface

A probabilistic approach for maximizing the islanding success in microgrids

With Increasing electricity demand, microgrids nowadays play an important role in smart grids which demand a systematic approach for its optimal design and performance enhancement. Microgrids development is propitious for the electric energy industry from several aspects including environmental aspects, improvement of system operation and power quality, cost saving and other market related issues. In the IEEE Standard 1547.4, splitting large distribution networks into a number of smaller networks can provide enhanced operation and control. In this paper, a distribution test system is considered for optimum design of microgrids. The design considers maximizing the success indicant of the constructed microgrids. The islanded operation of microgrids requires serving important loads when the main supply fails. Thus, the success of the islanding process can be a challenging issue for planners to enhance the distribution system reliability. The indicant formulated in this paper incorporates the crucial conditions for successful operation of microgrids such as power imbalance and voltage constraints. A radial distribution system of 69 buses is chosen for this research. The selected system has a typical fuel mix of distributed generators such as biomass generators, photovoltaic units and wind turbines. The design problem is solved using the recently developed back tracking search optimization algorithm. The intermittent nature of distributed generators is taken into consideration through a probabilistic approach