Power conversion system for high altitude wind power generation with medium voltage AC transmission

High Altitude Wind Power (HAWP) generating system provides clean energy at low cost and high capacity factor due to reduced size of the turbine and high speed streamlined wind at high altitude. An air-borne wind turbine (AWT) at high altitude extracts kinetic energy from wind using buoyancy provided by the blimp/aerostat. The generated electrical power is then transmitted to the ground based station (without any power conditioning) using the transmission lines (tether). The power conversion system (PCS) for harnessing HAWP is proposed in this paper. The proposed PCS consists of a three-level neutral point clamped (NPC) rectifier, a three-level NPC DC–DC converter followed by a two-level inverter. Modelling, design and control of the PCS are presented and discussed. The PCS provides generation side maximum power-point tracking (MPPT) using sensorless optimal torque control technique. The DC–DC converter provides electrical isolation as well as voltage step-down functions. A modified proportional resonant (PR) control which can selectively eliminate lower order current harmonics of the grid-connected inverter is also presented. The proposed control scheme of the PCS is evaluated through simulation studies using software programs like PSIM and MATLAB. A scaled-down 1 kW laboratory prototype of the complete PCS is designed, built and tested. The experimental test results obtained validate the proposed control scheme for efficient power generation from high altitude wind and interface to the grid/load.     

Krill herd algorithm for optimal location of distributed generator in radial distribution system

Distributed generator (DG) is recognized as a viable solution for controlling line losses, bus voltage, voltage stability, etc. and represents a new era for distribution systems. This paper focuses on developing an approach for placement of DG in order to minimize the active power loss and energy loss of distribution lines while maintaining bus voltage and voltage stability index within specified limits of a given power system. The optimization is carried out on the basis of optimal location and optimal size of DG. This paper developed a new, efficient and novel krill herd algorithm (KHA) method for solving the optimal DG allocation problem of distribution networks. To test the feasibility and effectiveness, the proposed KH algorithm is tested on standard 33-bus, 69-bus and 118-bus radial distribution networks. The simulation results indicate that installing DG in the optimal location can significantly reduce the power loss of distributed power system. Moreover, the numerical results, compared with other stochastic search algorithms like genetic algorithm (GA), particle swarm optimization (PSO), combined GA and PSO (GA/PSO) and loss sensitivity factor simulated annealing (LSFSA), show that KHA could find better quality solutions.     

Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern

Organic photovoltaics (OPVs) have acquired huge attention over the past years as potential renewable energy sources, adding attractive features such as aesthetics, semi-transparency, flexibility, large area printability, improved low-light performance, and cost-effectiveness to the well-known Si-based photovoltaics. Steady improvements in OPV power conversion efficiencies are continuously reported, notably for bulk heterojunction solar cells based on conjugated polymer:fullerene blends. However, apart from efficiency and cost, the stability of organic solar cell devices is of particular concern. Among the different factors contributing to OPV instability, gradual loss of the optimum phase-separated nanomorphology of the photoactive layer blend is a critical parameter. In this paper, we present the results of ‘shelf-life’ accelerated lifetime tests performed for devices containing a range of functionalized poly(3-alkylthiophene) (P3AT) donor polymers upon prolonged thermal stress. By the incorporation of functional moieties on the side chains of P3HT-based copolymers, a remarkable improvement of the intrinsic stability of the active layer blend morphology is accomplished, even for fairly low built-in ratios (5–15%) and without crosslinking to covalently anchor the polymer and/or fullerene molecules. Moreover, these alterations do not influence the initial power conversion efficiencies to a large extent. As such, the presented approach can be regarded as an attractive paradigm for OPV active layer stability.     

Optimal capacity design of battery and hydrogen system for the DC grid with photovoltaic power generation based on the rapid estimation of grid dependency

This paper presents a fast and novel method to determine the optimal capacity of a battery and a hydrogen system for a grid-connected photovoltaic (PV) system based on the required grid dependency (GD) and the minimum Levelized Cost of Energy (LCE). The GD is calculated from the weather data at 9 locations throughout Japan during 25 years considering different sizing of PV, battery, and hydrogen system. Based on the results, the relationship between the GD and the capacities of the devices and the weather parameter is established. The results show that the GD depends on the annual total solar insolation and the devices’ capacities as well. The empirical GD formula is then examined at a different location in Japan. The proposed GD calculation agrees well with the GD obtained from the real 25 – year weather data and shows great advantages over the conventional method in the simplicity and calculation time. In addition, the optimal capacity of the system can be obtained directly from the GD formula and the objective function of the LCE.     

A direct method for assessing distance-protection behavior during power swings

The aim of this work is to include the distance-protection behavior in direct methods for transient stability assessments. In this way the direct methods assess an electric-power system’s behavior more realistically and hence more reliably. In order to consider the delayed tripping zones of distance-protection relays, the time component was added to the Lyapunov energy function. The main innovation is that the dwell time of the post-fault impedance trajectory inside various tripping zones is calculated based on the speed of the transformation between the kinetic and the potential energy parts of the Lyapunov function. It enables the identification of unwanted trips during power swings. The tripping-zone settings can be revised accordingly. The method was verified by a comparison between the direct method and the well-known, time-domain, numerical-simulation method on a single-machine, infinite-bus test system where the results have to be identical. The application of the proposed method on a multi-machine power system gives good results.     

Observer backstepping control of DFIG-Generators for wind turbines variable-speed: FPGA-based implementation

In this paper, we present a new contribution for the control of Wind-turbine energy systems, a nonlinear robust control of active and reactive power by the use of the Adaptative Backstepping approach based in double-fed asynchronous generator (DFIG-Generator).Initially, a control strategy of the MPPT for extraction of maximum power of the turbine generator is presented. Thereafter, a new control technique for wind systems is presented. This control system is based on an adaptive pole placement control approach integrated to a Backstepping control system. The stability of the system is shown using Lyapunov technique. Using the FPGA to implement the order gives us a better rapidity. A Benchmark was realized by a prototyping platform based on DFIG-generator, FPGA and wind-turbine; the experimental results obtained show the effectiveness and the benefit of our contribution.     

Day‐Ahead Global Solar Radiation Forecasting Using Binary Trees and Factor Analysis

A method is proposed for forecasting global solar radiation. The method is based on weather information using binary trees and factor analysis. The feature of this method is that it is possible to use a simple linear forecasting equation. The method has been tested on meteorological and global solar radiation data obtained at several observation sites, and the results show that it is a promising means of maintaining the balance between demand and supply of electric power in power systems of the near future with a large number of photovoltaic systems installed.     

SAR of a birdcage coil with variable number of rungs at 300 MHz

The specific absorption rate produced in a rat’s brain phantom inside of quadrature birdcage coil as a function of the rung number was studied at 300 MHz. Electromagnetic field simulations and specific absorption rate and loss return responses were performed using a rat’s brain phantom weighing 100 mg. To assure optimal simulations and to evaluate coil performance, S-parameters were simulated and compared with experimentally data. Simulations showed that magnetic field uniformity improves and that electric field is increased with the number of rungs. Specific absorption rate and temperature values obtained from axial bi-dimensional maps increase as the number of rungs grows. These results corroborated very well with published data. A quadrature 16-rung birdcage coil was developed for comparison and phantom images were acquired to show its feasibility. The presented approach yields information on specific absorption rate allowing to previously develop RF coils and their possible effects on the biological sample.     

Capturing what human eyes perceive: A visual hierarchy generation approach to emulating saliency-based visual attention for grid-like urban street networks

Visual hierarchy is an important notion in urban imagery research. As the skeletons of cities, urban streets attract more attention from urban residents and street network hierarchies are important references for urban planning and urban studies. However, due to the characteristic of over-regularization, it is often difficult for humans to differentiate visual salience for grid-like street networks, resulting in the hierarchies of grid-like streets yielded by existing methods being prone to cause visual cognitive confusion. Therefore, in this study, we proposed a novel model to quantify the extent to which a street attracts human visual attention through emulating the visual attention mechanism that can capture the focus of relatively significant elements at different levels of perception. Using the natural street (also known as the stroke) as the sensor unit, the comprehensive visual salience (CVS) index combining the geometric competitive factors of natural streets at the local scale and psychological competitive factors of natural streets at the global scale is designed. Finally, the visual salience of the urban natural streets is ranked by these CVS scores and the visual hierarchy is derived by the head/tail breaks scheme. The model was applied to eight typical grid-like street networks and the results show that the performance of visual discrimination on street hierarchies is greatly improved. Our hierarchy generation method could effectively detect visually prominent streets for grid-like street networks and generate the visual hierarchies of grid-like street networks that conform to the hierarchies perceived by human eyes. These results would provide helpful suggestions in practical urban street network applications.     

Estimating stranded coal assets in China's power sector

China has suffered overcapacity in coal power since 2016. With growing electricity demand and an economic crisis due to the Covid-19 pandemic, China faces a dilemma between easing restrictive policies for short-term growth in coal-fired power production and keeping restrictions in place for long-term sustainability. In this paper, we measure the risks faced by China's coal power units to become stranded in the next decade and estimate the associated economic costs for different shareholders. By implementing restrictive policies on coal power expansion, China can avoid 90% of stranded coal assets by 2025.