Generic Regular Decompositions for Parametric Polynomial Systems

This paper presents a generalization of our earlier work in [19]. In this paper, the two concepts, generic regular decomposition (GRD) and regular-decomposition-unstable (RDU) variety introduced in [19] for generic zero-dimensional systems, are extended to the case where the parametric systems are not necessarily zero-dimensional. An algorithm is provided to compute GRDs and the associated RDU varieties of parametric systems simultaneously on the basis of the algorithm for generic zero-dimensional systems proposed in [19]. Then the solutions of any parametric system can be represented by the solutions of finitely many regular systems and the decomposition is stable at any parameter value in the complement of the associated RDU variety of the parameter space. The related definitions and the results presented in [19] are also generalized and a further discussion on RDU varieties is given from an experimental point of view. The new algorithm has been implemented on the basis of DISCOVERER with Maple 16 and experimented with a number of benchmarks from the literature.Comment: It is the latest version. arXiv admin note: text overlap with arXiv:1208.611

Hierarchical Comprehensive Triangular Decomposition

The concept of comprehensive triangular decomposition (CTD) was first introduced by Chen et al. in their CASC'2007 paper and could be viewed as an analogue of comprehensive Grobner systems for parametric polynomial systems. The first complete algorithm for computing CTD was also proposed in that paper and implemented in the RegularChains library in Maple. Following our previous work on generic regular decomposition for parametric polynomial systems, we introduce in this paper a so-called hierarchical strategy for computing CTDs. Roughly speaking, for a given parametric system, the parametric space is divided into several sub-spaces of different dimensions and we compute CTDs over those sub-spaces one by one. So, it is possible that, for some benchmarks, it is difficult to compute CTDs in reasonable time while this strategy can obtain some "partial" solutions over some parametric sub-spaces. The program based on this strategy has been tested on a number of benchmarks from the literature. Experimental results on these benchmarks with comparison to RegularChains are reported and may be valuable for developing more efficient triangularization tools

Research on grid-connected and islanding control strategy of AC/DC hybrid microgrid in a small plant

LAUREA MAGISTRALECon l’esaurirsi dei combustibili fossili e il deterioramento delle condizioni ambientali, sempre maggiore attenzione ricade sulla questione dell’approvvigionamento energetico e sulla sua primaria importanza. Le microreti in particolare hanno conosciuto un importante sviluppo in quanto sono considerate un sistema in grado di raggruppare efficacemente nuove fonti energetiche, dispositivi di accumulo e carichi. Le microreti ibride CA/CC contengono sia sbarre in CA che in CC. Rispetto alle tradizionali microreti in CA e alle più recenti microreti in CC, le microreti ibride riescono efficacemente a ridurre il numero di stadi di conversione in cascata, migliorare la qualità della fornitura e l’utilizzazione dell’energia. Inoltre, la gestione delle microreti ibride può essere significativamente più flessibile. Per queste ragioni, le microreti ibride sono considerate il futuro delle microreti. Questa tesi è focalizzata sulle strategie di controllo delle microreti ibride CA/CC. Vengono discusse diverse strategie di controllo in diversi modi operativi delle microreti ibride e, in particolare, si propone una nuova strategia di controllo basata su statismi generalizzati per l’intera rete che consente la gestione in isola delle porzioni in CA e CC. In accordo con quanto previsto dalla strategia di controllo proposta, viene inoltre presentata la ricerca svolta sulle transizioni morbide (cosiddette “smooth transition”) tra differenti modi operativi. In primo luogo, la struttura di una microrete ibrida CA/CC che comprende un impianto fotovoltaico, batterie, celle a combustibile e carichi a potenza costante viene introdotta considerando un piccolo impianto; un modello di simulazione viene definito tramite PSCAD/EMTDC. In seconda battuta, vengono analizzate le strategie di controllo delle microreti ibride CA/CC nei diversi modi operativi. Per risolvere i problemi delle strategie di controllo tradizionali, quali distribuzione squilibrata del carico, forte dipendenza dalla struttura della microrete, dimensionamento eccessivo del convertitore di interconnessione ecc., si propone una nuova strategia di controllo basata su statismi generalizzati per l’intera rete (indifferentemente in CA o CC). Quando la microrete opera connessa alla rete di distribuzione, si sfrutta quest’ultima per mantenere un funzionamento stabile del sistema e sfruttare al massimo le fonti rinnovabili. Quando operata in isola, tutta la generazione distribuita controllabile fornisce supporto al sistema. La strategia di controllo proposta migliora la distribuzione del carico e rende il sistema più affidabile e espandibile; inoltre, realizza la cosiddetta connessione “plug-and-play” per la generazione distribuita. I risultati delle simulazioni svolte in ambiente PSCAD/EMTDC mostrano che, applicando la strategia di controllo proposta, la microrete ibrida CA/CC riesce a operare in modo stabile ed efficiente. Infine, i metodi per ottenere una transizione morbida tra i diversi modi operativi sono studiati. Viene analizzata la struttura dei regolatori PI e viene discussa la causa dell’impatto delle transizioni tra modi operativi sul controllo della rete. Viene proposto un metodo per ottenere transizioni morbide basato su un segnale di passaggio all’operazione in isola. Rispetto ai metodi tradizionali per ottenere transizioni morbide, i risultati di simulazione provano che il metodo proposto permette di semplificare significativamente la taratura dei guadagni dei regolatori PI e di ottenere prestazioni migliori. Durante la transizione tra due modi operativi, la durata del transitorio è minore e l’effetto sulle grandezze elettriche è più contenuto. L’effetto del carico sulla transizione morbida tra modi operativi è studiato considerando il metodo proposto.With the depletion of fossil energy and the deterioration of environmental, the issue of energy has received increasing attention as the primary issue. As an effective way to combine distributed generators(DGs), energy storage system(ESS) and loads, microgrid has developed a lot. The AC/DC hybrid microgrid contains both AC and DC buses. Compared with the traditional AC microgrid and novel DC microgrid, it can effectively avoid repeated power conversion, improve power quality and energy utilization. What’s more, the operation can be more flexible. Hybrid microgrid has become the future of microgrid. This thesis focuses on the control strategy of AC/DC hybrid microgrid. It discusses about different control strategies in different operation modes of hybrid microgrid, especially, the novel entire network droop control strategy for AC-DC mixed islanding mode is proposed. According to the proposed control strategy, research on smooth transition is also conducted. Firstly, an AC/DC hybrid microgrid structure including PV cell, battery, fuel cell and constant power load is proposed in a small plant and an electromagnetic transient simulation model is established based on PSCAD/EMTDC. Secondly, the control strategies of AC/DC hybrid microgrids in different operating modes are analyzed. To solve the problems of traditional control strategy such as unreasonable load distribution, heavy dependence on microgrid structure, excessive transmission power of interlink converter(ILC), etc., a novel entire network droop control strategy is proposed. In grid-connected mode, the microgrid is supported by utility grid to maintain stable operation of the system and make maximum use of renewable energy. In islanding mode, all controllable DGs provide support for the system. The proposed control strategy improves load sharing and makes the system more reliable and expandable. The "plug and play" of DGs is realized. The simulation results based on PSCAD/EMTDC show that by applying this control strategy, the AC/DC hybrid microgrid can operate stably and efficiently. Finally, the smooth transition method between different operating modes is studied. The structure of PI regulator is analyzed and the reason of transition impact is discussed. A smooth transition method based on islanding signal is proposed. Compared with the traditional smooth transition method, the simulation results show that the proposed smooth transition method can greatly simplify the setting of the PI regulator’s parameters and achieve better smooth transition effects. During mode transition, the transient time is shorter and the electrical impact is smaller. The influence of load on smooth transition is also studied based on the novel smooth transition method

SPATIAL-TEMPORAL PATTERN OF VEGETATION INDEX CHANGE AND THE RELATIONSHIP TO LAND SURFACE TEMPERATURE IN ZOIGE

The Zoige wetland is the largest alpine peat wetland in China, and it has been degrading since 1960s. MODIS Enhance Vegetation Index (EVI) and Land Surface Temperature (LST) products in late august from 2000 to 2014 were employed to explore vegetation index and land surface temperature change tendency and to perform Temperature Vegetation Dryness Index (TVDI). The correlation between the annual mean of EVI and annual mean of LST was also calculated at pixel scale. The main purpose of this study is to explore the relationship between wetland degradation and climate change. The main conclusions are as follows: (1) Average EVI in Zoige plateau tended to be decreasing from 2000 to 2014, especially after 2007. In wetland areas, the annual mean of EVI were negative, while the slope were positive. It showed that the water storage of wetlands in Zoige plateau had been decreasing in the past 15 years and will keep decreasing in the future. (2) Overall, LST in the whole Zoige plateau had been increasing since 2000. While the minimum TVDI increased from 2000 to 2008 and then decreased. The change of TVDI suggested that drought should be a main factor that lead to wetland degradation in Zoige. (3) The uneven distribution of the correlation between EVI and LST suggested that LST is also one of the main reasons of wetland degradation

Spatiotemporal analysis of vegetation variability and its relationship with climate change in China

This paper investigated spatiotemporal dynamic pattern of vegetation, climate factor, and their complex relationships from seasonal to inter-annual scale in China during the period 1982–1998 through wavelet transform method based on GIMMS data-sets. First, most vegetation canopies demonstrated obvious seasonality, increasing with latitudinal gradient. Second, obvious dynamic trends were observed in both vegetation and climate change, especially the positive trends. Over 70% areas were observed with obvious vegetation greening up, with vegetation degradation principally in the Pearl River Delta, Yangtze River Delta, and desert. Overall warming trend was observed across the whole country (\u3e98% area), stronger in Northern China. Although over half of area (58.2%) obtained increasing rainfall trend, around a quarter of area (24.5%), especially the Central China and most northern portion of China, exhibited significantly negative rainfall trend. Third, significantly positive normalized difference vegetation index (NDVI)–climate relationship was generally observed on the de-noised time series in most vegetated regions, corresponding to their synchronous stronger seasonal pattern. Finally, at inter-annual level, the NDVI–climate relationship differed with climatic regions and their long-term trends: in humid regions, positive coefficients were observed except in regions with vegetation degradation; in arid, semiarid, and semihumid regions, positive relationships would be examined on the condition that increasing rainfall could compensate the increasing water requirement along with increasing temperature. This study provided valuable insights into the long-term vegetation–climate relationship in China with consideration of their spatiotemporal variability and overall trend in the global change process

{\pi}-{\pi} Interaction-facilitated formation of interwoven trimeric cage-catenanes with topological chirality

Catenanes as interlocked molecules with a nonplanar graph have gained increasing attention for their unique features such as topological chirality. To date, the majority of research in this field has been focusing on catenanes comprising monocyclic rings. Due to the lack of rational synthetic strategy, catenanes of cage-like monomers are hardly accessible. Here we report on the construction of an interwoven trimeric catenane that is composed of achiral organic cages, which exhibits topological chirality. Our rational design begins with a pure mathematical analysis, revealing that the formation probability of the interwoven trimeric catenane surpasses that of its chain-like analogue by 20%; while driven by efficient template effect provided by strong {\pi}-{\pi} stacking of aromatic panels, the interwoven structure emerges as the dominant species, almost ruling out the formation of the chain-like isomer. Its topological chirality is unambiguously unravelled by chiral-HPLC, CD spectroscopy and X-ray diffraction. Our probability analysis-aided rational design strategy would pave a new venue for the efficient synthesis of topologically sophisticated structures in one pot

Wind Speed Multi-Mode Ensemble Forecasting for Wind Farms Based on Machine Learning

[Introduction] With the extensive construction of wind farms, the combination of researches on different machine learning algorithms and meteorological forecasting modes has received widespread attention. [Method] This paper was based on the spatial distribution characteristics of wind energy resources in Hubei Province, and utilized representative stations in combination with experimental data analysis to conduct in-depth discussions on the results. [Result] The wind farms in operation and under construction in Hubei Province are all located in the "Three Zones and One Area", including the north-south wind zone from Jingmen to Jingzhou in the central part of Hubei Province, the east-west wind zone from Zaoyang to Yingshan in the north of Hubei Province, certain lake islands and zones along the lake, as well as some high mountainous areas in the southwest and southeast of Hubei Province. This research uses four different numerical forecasting products, namely CMA-WSP, CMA-GD, WHMM, and EC, to compare with the measured wind speeds and investigated the applicable range of these four numerical modes. [Conclusion] By analyzing the performance of five ensemble forecasting methods based on machine learning and the mean method, we identified suitable algorithm and forecasting model combinations, providing references for improving the accuracy of ensemble forecasting

Automated cropping intensity extraction from isolines of wavelet spectra

Timely and accurate monitoring of cropping intensity (CI) is essential to help us understand changes in food production. This paper aims to develop an automatic Cropping Intensity extraction method based on the Isolines of Wavelet Spectra (CIIWS) with consideration of intra- class variability. The CIIWS method involves the following procedures: (1) characterizing vegetation dynamics from time–frequency dimensions through a continuous wavelet transform performed on vegetation index temporal profiles; (2) deriving three main features, the skeleton width, maximum number of strong brightness centers and the intersection of their scale intervals, through computing a series of wavelet isolines from the wavelet spectra; and (3) developing an automatic cropping intensity classifier based on these three features. The proposed CIIWS method improves the understanding in the spectral–temporal properties of vegetation dynamic processes. To test its efficiency, the CIIWS method is applied to China’s Henan province using 250 m 8 days composite Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) time series datasets. An overall accuracy of 88.9% is achieved when compared with in-situ observation data. The mapping result is also evaluated with 30 m Chinese Environmental Disaster Reduction Satellite (HJ-1)-derived data and an overall accuracy of 86.7% is obtained. At county level, the MODIS-derived sown areas and agricultural statistical data are well correlated (r2 = 0.85). The merit and uniqueness of the CIIWS method is the ability to cope with the complex intra-class variability through continuous wavelet transform and efficient feature extraction based on wavelet isolines. As an objective and meaningful algorithm, it guarantees easy applications and greatly contributes to satellite observations of vegetation dynamics and food security efforts