With the growing deployment of microgrids, it has become urgent to investigate the microgrid behavior during transient faults and subsequent islanding conditions. The load type and the manner in ...which distributed generations (DGs) are controlled can have substantial impacts on the dynamic performance of microgrids. In this paper, impacts of different control schemes of the inverter-based DG and microgrid load types on the microgrid stability subsequent to fault-forced islanding are investigated. A microgrid model, simulated on Matlab/Simulink software, is analyzed including a mix of synchronous and inverter-based DG and a combination of passive RLC and induction motor (IM) loads. Simulation results show that in the presence of IM loads, the microgrid may lose its stable operation even if the fault is isolated within a typical clearing time. The critical clearing time of a microgrid is highly dependent on the microgrid control strategy, DG interface control, and load type. Induction motor loads can prove problematical to microgrid transient stability, particularly in situations in which the voltage dip can cause the induction motor to "pull out".
The fifth-order accurate Weighted Essentially Nonoscillatory space discretization developed by Jiang and Shu (WENO-JS) is studied theoretically. An exact Nonlinear Spectral Method (NSM) is developed ...based on an innovative yet simple methodology. The NSM explains the behaviour of nonsmooth solutions because it is valid for arbitrary modified wave numbers (MWN). The NSM clarifies the effects of the time integration methods and the Courant number. The mode isolation assumption, extensively used to analyse WENO-JS, is elucidated, and several novel findings are presented. The improved performance of the combination of WENO-JS with the forward Euler time integration method, compared to the Linear Fifth-Order Upwind discretization, is illustrated. The overdamping of the combination of WENO-JS with the popular third-order total variation diminishing Runge-Kutta method is discovered. Thus, the NSM covers several gaps in the current literature.
Keeping the generators operating during transient grid faults becomes an obligation for the bulk wind generation units connected to the transmission network, and it is highly desired for distribution ...wind generators. A proposed scheme is implemented to keep the wind-power DFIG operating during transient grid faults. Challenges imposed on the generator configuration and the control during the fault and recovering periods are presented. A comprehensive time domain model for the DFIG with the decoupled dq controller is implemented using Matlab/Simulink software. Intensive simulation results are discussed to ensure the validity and feasibility of the proposed fault ride through technique. The scheme protects the DFIG components, fulfills the grid code requirements and optimises the hardware added to the generator.
Bacillus cereus is considered the most potent bacterial strain in terms of the increment in induced proteins during thermal treatment at 52 °C for 90 min. Protein production in food-born ...microorganism (Bacillus cereus) recovered from contaminated food was investigated in response to heat shock treatment. Bacterial tolerance towards pH, salinity, and temperature at various levels was also investigated. Heat-shock proteins (HSPs) produced when exposed to 52 °C for up to 60 minutes led to significant differences (30%) above the untreated control (37 °C), and the maximum difference was recorded at 52°C at 90 minutes. ISSR detected a higher number of bands/primer than RAPD (13.7 vs. 12.7, respectively), and more polymorphic bands (10.7 vs. 8.4 bands/primer, respectively). The untreated bacterial strain did not grow at pH levels lower than 3, whereas the thermally treated strain grew significantly at pH two. A consistent increase in HSPs was observed, with a gradual increase in salinity of less than 16%. Surprisingly, the gradual increase in temperature did not induce tolerance against higher temperatures. However, a significant growth rate was noticed in response to heat-shocked treatments. The untreated Bacillus cereus demonstrated antibiotic resistance to gentamycin and clindamycin (1.54 and 1.65 cm, respectively), much lower than the corresponding inhibition areas with preheat-treated test pathogen which were recorded (2.37 and 2.49 cm, respectively).
Abstract
A two-dimensional numerical model for the solution of the nonlinear shallow water equations (NSWEs) using the discontinuous Galerkin finite element method (DGFEM) is presented. A new ...adaptation of the thin-film approach is developed for the wetting/drying treatment. The model is applied to a number of test cases that can be characterized as swash flows, or as cases that are particularly useful for swash flow modeling. The DGFEM model shows robustness and provides accurate predictions of water depth, velocities, and shoreline movement. For the case of bore collapse on a plane beach the model performs well against a state-of-the-art finite volume swash code. The new wetting/drying algorithm is tested against a previous algorithm within the same framework for simulating a solitary wave propagating on a beach with bottom friction, showing a noticeable improvement in the shoreline prediction. The model is also tested against a more subtle test case, including generation of subharmonic edge waves, in order to test the effectiveness of DGFEM in reproducing second-order effects. The model simulates the excitation and development of the subharmonic edge waves when compared with the analytical solutions in the literature. Overall, it is shown here for the first time that the DGFEM technique can be used to simulate accurately a wide range of swash zone flows and therefore swash zone processes.
Careful design of the high-tech semi-circular breakwaters (SBW), which are used for protection against sea waves, is essential to avoid the failure of these large-scale projects. An improved design ...tool adopting numerical simulations, complemented with experimental measurements is introduced. Unstructured boundary-fitted grids are used to model the curved SBW boundary accurately and resolve the flow adjacent to it. Experimental measurements based on Particle Image Velocimetry (PIV) are introduced and the numerically simulated velocity fields are validated. For the first time, the strong free surface deformation upstream of the SBW, termed trough-breaking, is reproduced numerically. The phenomenon is investigated in detail and its impact on several SBW advantages and stability is emphasized. The improved accuracy due to unstructured boundary-fitted grids complemented by the PIV data set, offers valuable prospects for performing complex designs and exploring future applications.
Article Highlights
An efficient design tool for the high-tech semi-circular breakwater is introduced adopting numerical simulations and experimental measurements.
The numerical results agree well with the experimental measurements. Hence, this experimental data set should be used extensively for validation in future studies.
The important flow phenomenon of “trough-breaking” is reproduced numerically for the first time, and its impact on the semi-circular breakwater features is clarified.
Background
Surgical mortality data are collected routinely in high‐income countries, yet virtually no low‐ or middle‐income countries have outcome surveillance in place. The aim was prospectively to ...collect worldwide mortality data following emergency abdominal surgery, comparing findings across countries with a low, middle or high Human Development Index (HDI).
Methods
This was a prospective, multicentre, cohort study. Self‐selected hospitals performing emergency surgery submitted prespecified data for consecutive patients from at least one 2‐week interval during July to December 2014. Postoperative mortality was analysed by hierarchical multivariable logistic regression.
Results
Data were obtained for 10 745 patients from 357 centres in 58 countries; 6538 were from high‐, 2889 from middle‐ and 1318 from low‐HDI settings. The overall mortality rate was 1·6 per cent at 24 h (high 1·1 per cent, middle 1·9 per cent, low 3·4 per cent; P < 0·001), increasing to 5·4 per cent by 30 days (high 4·5 per cent, middle 6·0 per cent, low 8·6 per cent; P < 0·001). Of the 578 patients who died, 404 (69·9 per cent) did so between 24 h and 30 days following surgery (high 74·2 per cent, middle 68·8 per cent, low 60·5 per cent). After adjustment, 30‐day mortality remained higher in middle‐income (odds ratio (OR) 2·78, 95 per cent c.i. 1·84 to 4·20) and low‐income (OR 2·97, 1·84 to 4·81) countries. Surgical safety checklist use was less frequent in low‐ and middle‐income countries, but when used was associated with reduced mortality at 30 days.
Conclusion
Mortality is three times higher in low‐ compared with high‐HDI countries even when adjusted for prognostic factors. Patient safety factors may have an important role. Registration number: NCT02179112 (http://www.clinicaltrials.gov).
Higher in low‐income countries
Islanding detection is an essential protection requirement for distribution generation (DG). Antiislanding techniques for inverter-based DG are typically designed and tested on constant RLC loads ...and, hence, do not take into account frequency-dependent loads. In this paper, a new antiislanding technique, based on simultaneous P - f and Q - V droops, is designed and tested under different islanding conditions considering different load types. The behavior of frequency-dependent static loads during islanding operation is discussed. The developed antiislanding technique is designed to fully address the critical islanding cases with frequency-dependent static loads and RLC-based loads. The performance of the proposed method is tested to accommodate load switching disturbances and grid voltage distortions. Theoretical investigation coupled with intensive simulation results using the Matlab/Simulink software is presented to validate the performance of the proposed antiislanding technique. Unlike previously proposed methods, the results show that the proposed islanding detection method is robust to frequency-dependent loads and system disturbances.
In this paper, a new passive islanding detection method for grid-connected inverter-based distributed-generation (DG) systems is proposed. A statistical signal-processing algorithm known as ...estimation of signal parameters via rotational invariance techniques is used to extract new features from measurements of the voltage and frequency at the point of common coupling as islanding indicators. The new features are defined based on a damped-sinusoid model for power system voltage and frequency waveforms, and include modal initial amplitudes, oscillation frequencies, damping factors, and initial phases. A set of training cases generated on the IEEE 34-bus system was used to train a naïve-Bayes classifier that discriminates islanding and nonislanding events. Cross-validation was used to evaluate the performance of the proposed islanding detection method. The results show that by using the new features extracted from ESPRIT, the classifier is capable of discriminating islanding and nonislanding events with an accuracy close to 100%.
Power variation, voltage fluctuation, and resultant flicker are critical issues related to the integration of directly grid-connected constant-speed wind turbines. This study introduces a methodology ...to enhance the quality of the power and voltage, and minimise the flicker produced from constant-speed direct-connected wind turbines. The method uses an electrolyser/fuel cell combination to be connected to the point of common coupling via limited rating converters. The control scheme is designed to keep the power ramp rate and voltage fluctuation within their limits and minimise the rating of the converters used to interface the electrolyser and the fuel cell. The incorporation of STATCOM and its effect on the generated power and flicker level have been investigated. The simulation results are carried out to demonstrate and validate the proposed technique.
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