Stability analysis of power systems described with detailed models by automatic method

The power system stability analysis is approached taking into explicit account the dynamic performance of generators internal voltages and control devices. The proposed method is not a direct method in the usual sense since conclusion for stability or instability is not exclusively based on energy function considerations but it is automatic since the conclusion is achieved without an analyst intervention. The stability test accounts for the nonconservative nature of the system with control devices such as the automatic voltage regulator (AVR) and automatic generation control (AGC) in contrast with the well-known direct methods. An energy function is derived for the system with machines forth-order model, AVR and AGC and it is used to start the analysis procedure and to point out criticalities. The conclusive analysis itself is made by means of a method based on the definition of a region surrounding the equilibrium point where the system net torque is equilibrium restorative. This region is named positive synchronization region (PSR). Since the definition of the PSR boundaries have no dependence on modelling approximation, the PSR test conduces to reliable results.     

High-temperature stability of nano-grained B2-structured RuAl-based intermetallics by mechanical alloying

A series of nano-grained B2-structured RuAl-based intermetallics and corresponding composites are synthesized by mechanical alloying. These RuAl-based materials include stoichiometric RuAl, B2-structured ternary (Ru,Ni)Al and (Ru,Ir)Al, eutectic RuAl–Ru and particle reinforced RuAl–ZrO₂ composite. The accumulation of impurities, mainly Fe, in grain boundaries and the structural evolution during grain growth are found to be the controlling factors for the grain growth stagnation in nano-grained B2-structured binary RuAl and ternary (Ru,Ni)Al and (Ru,Ir)Al intermetallics. Ternary alloying element forming iso-structured pseudo-binary compounds, eutectic in situ and ceramic particle reinforced composites are explored to be the effective routes to enhance the stability of single-phase RuAl. The thermal stability of microstructure in various RuAl-based materials are found to decrease in a sequence from pseudo-binary RuAl–IrAl, RuAl–NiAl, eutectic RuAl–Ru composites, ceramic particle reinforced RuAl–ZrO₂ composites to binary stoichiometric RuAl.     

Error evaluation of free-form surface based on distance function of measured point to surface

As free-form surface is widely used in engineering, it is urgently needed to develop advanced methodology of detecting and evaluating the profile error. To this end, the semantic of profile tolerance in ASME Y14.5.1M is reviewed and the mathematical definition of profile tolerance is discussed. Subsequently, a mathematical model for error evaluation is built. This mathematical model is augmented based on distance function by considering the second-order terms in the computation of the distance from point to surface. Then, a profile error evaluation algorithm, which combines Differential Evolution (DE) algorithm and Nelder–Mead (NM) algorithm, is developed to solve this model. The proposed model and optimization algorithm are validated with simulation results from a case study. Additionally, the model is superior to Least-squares (LS) model in simplicity, efficiency and robustness.     

Evaluating one- and two-source energy balance models in estimating surface evapotranspiration from Landsat-derived surface temperature and field measurements

This article compares one- and two-source energy balance (OSEB and TSEB) models in the estimates of surface energy components using Landsat imagery and surface measurements acquired from an experimental field at Yucheng Station in Northern China. Compared to surface measurements, similar performance between the TSEB and OSEB models has been observed for estimated surface net radiation and soil heat flux. The root mean square difference (RMSD) is within 14–39 W m^?2 in both the TSEB and OSEB models. The residual energy (E _R) correction method yields the best agreement in comparisons of the sensible (H) and latent (LE) heat fluxes estimated using both the TSEB and OSEB models to the eddy covariance (EC) system measurements. The TSEB model is shown to greatly outperform the OSEB model in reproducing surface H and LE measurements. Cirrus clouds are likely responsible for the surface temperature retrieved from the enhanced thematic mapper plus (ETM+) sensor being lower than air temperature on days of the year (DOYs) 178 and 218 of 2009. This atmospheric stability is contrary to the unstable atmosphere that the EC measurements observe. If data on these two days are excluded and the E _R correction method is applied, when comparing the estimated H and LE to the EC measurements, RMSD is within 55 W m^?2 in the TSEB model and is larger than 97 W m^?2 in the OSEB model.     

Scenarios for biofuel demands,biomass production and land use – The case of Denmark

The land potential for producing biomass for bioenergy purposes has been highly debated in recent years. The present paper analyses the possibilities and consequences for land use and agricultural production of biofuel production in Denmark based on domestic wheat and rape under specific scenario conditions for the period 2010–2030. The potential is assessed for a situation where policy targets for renewable energy carriers in the transport sector is reached using biofuels, and where second generation ethanol increasingly substitutes first generation ethanol.Three scenarios are developed and evaluated: a baseline, an alternative scenario allowing continuous growth in the now dominant livestock branch and a biofuel scenario assuming that efforts to achieve self-sufficiency in biofuel displaces part of the domestic production of fodder.Results show that the biofuel demand could be met in 2020; but only if current rape oil production is used to satisfy local bio-diesel demand. It would also imply that the Danish bio-diesel export currently supplying a minor part of the German fuel market would seize. In 2030, however, only about 60 percent of the biofuel demand would be covered by self-sufficiency. If biofuels were to displace animal production to make up for this, a reduction of the pig production between 10 and 20 percent would result. Efficiency increases across production branches would allow the animal production to continue un-affected if about half of the rape oil produced for other purposes is utilized.     

基于PnET-CN模型的亚热带人工针叶林生产力与固碳潜力模拟研究

A generalized, lumped-parameter ecological model PnET-CN was calibrated and validated for a subtropical coniferous plantation in southem China. PnET-CN model describes the biogeochemical cycles of carbon(C)and nitrogen(N)and can assist in estimating carbon sequestration potential. For validation of PnET-CN, data from coniferous forest plantations in southern China was used. Simulated daily gross primary productivity(GPP)from 2005 to 2007 agreed well with observations(R2=0.56, S.D.=0.009). Simulations of monthly soil res-piration(R3)from 2005-2007 agreed well with R8 observations(R2=0.67, S.D. =0.03). Simu-lated annual net primary productivity(NPP)from 1998-2006 was 803±33 gCm-2a-1, about 4% higher than NPP observation(752±51 gCm-2a-1). Simulations of annual NEP from 2005-2007 only overestimate 9 gCm-2a-1(4%), 4 gCm-2a-1(1%)and 34 gCm-2a-1(8%)compared to NEP observations, respectively. Simulated annual foliar N concentration(FolNCon)(1.09%)is 10% lower than observed monthly FolNCon(0.87%-1.58%). Simulated annual N leaching(0.26 gNm-2)is about 10% lower than leaching observation(0.29 gNm-2). PnET-CN model valida-tion indicates that PnET-CN is capable to simulate daily GPP, annual NPP, annual NEP, monthly R,, annual FolNCon and annual nitrate N leaching for subtropical coniferous planta-tions in southem China. The results obtained from the validation test revealed that PnET-CN model can be used to simulate carbon sequestration of planted coniferous forests in southern China to a high level of precision. Sensitivity analysis suggests that great care should be taken in developing generalizations as to how forests will respond to a changing climate. PnET-CN performed satisfactorily in comparison to other models that have already been calibrated and validated in coniferous planted subtropical forests in China. Based on PnET-CN validation and its comparison to other models, future improvement of PnET-CN should focus on seasonal foliar N dynamics and the effects of water stress on autotrophic respirations in subtropical coniferous plantations in southern China.     

A Breakpoints based Spatio-temporal Analysis of Tibetan Plateau Vegetation

Due to the fragile ecosystem and unique geographical environment on the TP，the vegetation strongly responds to climatic shifts.Therefore，it is of great significance to discuss the spatiotemporal trend shift of vegetation，to evaluate the climate change of the plateau and to predict regional ecological development.Using the GIMMS NDVI3g dataset from 1982 to 2012 to extract the NDVI information of the TP，as well as establishing seasonal trend model to classify research through the seasonal trend analysis and breakpoints detection method，reveals the spatiotemporal pattern of the trend shifts of plateau vegetation at both ends of the breakpoints combining the classification of land cover.The results shows that conclusions.(1) The seasonal trend model can effectively identify the breakpoints of vegetation time series，moreover the time span of the breakpoints were large and the spatial heterogeneity were strong.(2) The trend of vegetation degeneration in the western part of the Tibetan Plateau was small，vegetation degeneration in the south and northeast regions was obvious，and vegetation in the central and eastern regions has improved.58.93% of the vegetation status tends to be stable.The area where the vegetation status changes significantly accounts for about 32.3% of the entire plateau.(3) In the area where the vegetation status is generally or significantly changed，the vegetation improvement of monotonous trend and interruption trend were more than that of degradation，and the degenerative situation in the reverse trend were more than the improvement.Monotonous trend changed in 3.14% of the regions，58.36% of the regions occurred interruption trend changes，and 38.50% of regions occurred reverse trends.The time distribution of the monotonous trend and the interruption trend were more concentrated，while the reverse trend covered the entire time series.(4) The vegetation improvement and degradation in different land cover types were various conditions.The type with the highest rate of improvement was desert(53.30%)，and the type with the highest rate of degradation was sparse vegetation(60.14%).Overall，the vegetation in Tibetan plateau tends to be greening，but the spatial heterogeneity remains significant.     

The Evaluation of Efficiency of Color Metrics in Monitoring Robiuia PseudoacaciaPhenology based on RGB Images

The widely application of digital camera，especially the appearance of time\|lapse camera，inspired the monitoring vegetation seasonal dynamic using time\|series RGB imagery.Extract critical temporal stage of vegetation dynamic is the most useful aspect.Color index including Green Excess Index(ExG)，Green Chromatic Coordinate(Gcc)，Green Red Vegetation Index(GRVI)，and Hue based on HSL(Hue) are the most widely used metrics.However，their efficiency for specific plants may differ.In this study，the efficiency of four color indices mentioned above were tested taking RGB imagery of Robiuia Pseudoacacia as data source.The critical timing point of plant growth and senescence reflected by greenness of leaves were used to pick out the most efficiency color index.The results showed that the best SOS(Start of the Seasonality)，EOS(End of the Seasonality) can be extracted using ExG following the