Dynamic Stability Improvement of Decentralized Wind Farms by Effective Distribution Static Compensator

Dynamic instability of decentralized wind energy farms is a major issue to deliver continuous green energy to electricity consumers. This instability is caused by variations of voltage and frequency parameters due to intermittencies in wind power. Previously, droop control and inverter-based schemes have been proposed to regulate the voltage by balancing reactive power, while inertial control, digital mapping technique of proportional-integral-differential (PID) controller and efficiency control strategy have been developed to regulate the frequency. In this paper, voltage stability is improved by a new joint strategy of distribution static compensator (DSTATCOM) six-pulse controller based reactive power management among decentralized wind turbines and controlled charging of capacitor bank. The frequency stability is ensured by a joint coordinated utilization of capacitor bank and distributed wind power turbines dispatching through a new DSTATCOM six-pulse controller scheme. In both strategies, power grid is contributed as a backup source with less priority. These new joint strategies for voltage and frequency stabilities will enhance the stable active power delivery to end users. A system test case is developed to verify the proposed joint strategies. The test results of the proposed new schemes are proved to be effective in terms of stability improvement of voltage, frequency and active power generation.     

Online oscillation detection in the presence of signal intermittency

A novel online detector for multiple oscillations in process industry is proposed. This article is motivated by the fact that it is still an open problem to design a real-time monitor which is suitable for detecting multiple oscillations with signal intermittency and non-linear/non-stationary properties. The proposed approach of Intrinsic Time-scale Decomposition (ITD) with Robust Zero crossing intervals Clustering (ITD-RZC) (i) provides an experimental statistic to remove noisy zero-crossings intervals (ZCIs), (ii) develops a novel adaptive robust K-means ZCI clustering which enables the reconstructed Proper Rotation Component (PRC) to encapsulate the sporadic oscillation and (iii) proposes an effective online-cluster-updating mechanism for real-time intermittent oscillations detection. ITD-RZC approach is computationally efficient and capable of preserving nonlinear features of the process variables which facilitates subsequent oscillation diagnosis. Simulation examples and industrial cases study are provided to demonstrate the effectiveness of the proposed online oscillation detector.     

Drought refuges affect algal recolonization in intermittent streams

Drought refuges enable the biota of intermittent streams to survive dry periods, but their roles in sustaining algal assemblage structure and productivity are not known. This study determined the importance of two types of drought refuge (dry biofilm, permanent pools) for benthic algal regrowth in four intermittent streams in the Grampians National Park, Victoria, Australia. Dry biofilm was removed by scrubbing randomly chosen stones from two dry riffles in each of two streams with permanent pools and two streams without permanent pools. Algae were sampled from scrubbed and control stones at one, six and sixteen weeks after flow resumed. Removal of dry algal biofilm resulted in lower algal densities on stones from both stream types, and scrubbed stones had similar densities of algae after one week, regardless of the presence or absence of permanent pools. Algal density on stones exposed to both refuge types was more than an order of magnitude higher than the density on stones exposed to either one or neither refuge. The effects of biofilm removal on algal density were still apparent after six weeks of flow and assemblage composition differed between streams with and without permanent pools. Dry biofilm was an important drought refuge for algae in both stream types, but epilithic algal density in streams with permanent pools developed more rapidly than streams that were completely dry over summer. Loss of permanent pools from intermittent streams due to water abstraction may substantially reduce algal productivity after flow resumes, which may limit the supply of autochthonous carbon. Copyright © 2004 John Wiley & Sons, Ltd.     

考虑间歇效应的循环荷载下软黏土刚度软化特性

天然软土地基在地铁运营期受到周期性振动和间歇的反复作用,间歇期内软土力学特性的变化将影响其刚度软化,但以往关于交通荷载下软土刚度软化的研究大多忽视发车间歇的影响.为此,考虑不同动应力比、单位间歇时长和振动时长,设计不排水连续和停振循环三轴试验,对试样采用真实K₀固结,研究原状淤泥质软黏土在列车非连续荷载下的长期刚度软化规律.通过对比连续振动和间歇振动试验结果,发现间歇对刚度软化有削弱作用,可减少大循环周数下残余软化指数,缩短快速软化期经历的循环周数.基于试验结果深入分析间歇的影响机制,发现加载初期软土受激振作用影响,土体结构动态调整滞后至间歇期,导致孔压持续增长,软土软化,平均软化速率提高;一定循环周数后,软土由于在前期间歇和振动阶段集中对结构进行调整,土体结构提前进入稳定状态,间歇期消散部分孔压,恢复部分刚度,导致下一次振动软化速率降低,间歇期恢复的刚度随振动次数增加而累积,致使最终残余软化指数显著提高;同等条件下,间歇时长越长,初期软化速率越大,软化速率随振动次数增加的衰减也越显著,导致进入稳定期所需的振动周数越少,整体软化程度削弱越明显.     

Theoretical investigation of a semiconductor ring laser driven by Chua’s oscillator

The modeling and numerical investigation of the dynamical behavior of a semiconductor ring laser (SRL) driven by Chua’s oscillator are reported. By increasing the coupling strength between the SRL and Chua’s oscillator at a fixed bias current, the SRL exhibits an intermittency route to anti-phase chaos. However, for a fixed value of the coupling strength, we report a period-doubling route to out-of-phase and anti-phase chaos when varying one of the parameters of the Chua’s oscillator are reported. We also demonstrate that a SRL driven by the chaotic output of Chua’s oscillator generates a more complex chaos compared to the one found in a SRL subject to a sinusoidally modulated current. This new way of modulation of semiconductor lasers would not only bring a general benefit in the physical equipment and reduce their cost but could have an impact for some relevant engineering applications.     

间歇混沌合成1/f噪声的相关特性分析

本文利用非线性随机微分方程来合成间歇混沌信号,针对该信号表现出的1/f噪声特征,在不同消失矩的小波基下进行相关特性分析.仿真结果发现,在功率谱的中间频段内,该信号的功率谱密度表现出典型的1/f噪声特性,其小波变换系数方差与相应的小波尺度呈对数线性关系;且在该频段内,部分尺度下该间歇性信号的小波变换系数的相关性随小波基的消失矩的增大而减小,在另一部分尺度下该相关性则随着消失矩的增大而增大.实验结果表明,随小波消失矩的增大,并非在所有尺度下小波变换对该间歇性信号均具有去相关作用.论文讨论了小波变换系数的方差和尺度的关系,详细分析了小波变换系数的相关性随小波消失矩的变化趋势.     

Experimental research on intermittent heat pump drying with constant and time-variant intermittency ratio

The aim of this article was to estimate the effectiveness of intermittent drying with constant and time-variant intermittency ratio, compared with continuous drying. Intermittent drying was accomplished by periodically changing the heat input and air velocity. To achieve accurate control of drying temperature, a new parallel conversion controller was used. Based on the well-controlled heat pump drying system, the drying kinetics of green soybean were analyzed using seven intermittent drying profiles. The increase of intermittency ratio caused decrease of effective drying time and increase of specific moisture extraction rate (SMER). SMERs of intermittent drying were 1.21–4.94 and 0.93–5.40 times more than the continuous drying for different initial moisture content (23 and 28%), respectively. Changes of drying rate (DR) and SMER became relatively moderate when intermittent drying with time-variant intermittency ratio was used. A comprehensive evaluation parameter, based on DR and SMER, was proposed and calculated. Experimental results indicated that intermittent drying (α?=?5/6) and intermittency drying with time-variant ratio gave the best two comprehensive performances.     

The character of power output from utility‐scale photovoltaic systems

Power produced by utility‐scale solar photovoltaic (PV) systems has fluctuations on both short and long time scales. Power spectral density (PSD) analysis provides information on the character of these power fluctuations. Examination of the correlation and step size of the power output between several PV sites within a multi‐site system allows assessment of geographic diversification for addressing intermittency. Both techniques provide insight into the characteristics of firm power and/or demand response required to accommodate large‐scale PV deployment. Copyright © 2007 John Wiley & Sons, Ltd.     

Drop break-up in intermittent turbulence: Maximum stable and transient sizes of drops

A multifractal method describing the fine-scale structure of turbulence, including its intermittency, is applied to derive the drop breakage functions for drops whose diameter falls within the inertial subrange of turbulence, including effects of the viscosity of the dispersed phase. The model predicts well the transient drop size distributions of dispersions undergoing breakage at long stirring times. Concepts of quasi-stable and asymptotically stable drop sizes are presented and discussed in relation to the exponent on the Weber number. The functions for drop break-up in the viscous subrange are proposed and discussed.