A thorough and exhaustive presentation of theoretical analysis and practical techniques for the small-signal analysis and control of large modern electric power systems as well as an assessment of ...their stability and damping performance. Such systems may contain many hundreds of synchronous generators and high voltage power electronics equipment known as FACTS Devices. The book describes new techniques not only for the tuning and analysis of stabilizers for systems with many generators and FACTS Devices but also for their coordination. Of practical interest, these techniques are illustrated with relevant examples based on a multi-machine power system containing FACTS Devices for operating conditions ranging from light to peak load. By introducing new analytical concepts, using examples, and by employing production-grade software, practical insights are provided into the significance and application of various analytical techniques.
This paper summarizes the work done by the Task Force on Assessing the Need to Include Higher Order Terms for Small-Signal (Modal) Analysis. This Task Force was created by the Power System Dynamic ...Performance Committee to investigate the need to include higher order terms for small signal (modal) analysis. The focus of the work reported here is on establishing and documenting the practical significance of these terms in stability analysis using the method of Normal Forms. Special emphasis was placed on determining and describing conditions when higher order terms need to be included to accurately describe modal interactions. Test cases were developed on a standard test system to demonstrate the application of appropriate indices to detect the occurrence of nonlinear interaction and hence the need for higher order terms in stability analyzes. The use of the higher order terms in the site selection for a damping controller is also documented.
The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better ...understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.
Several methods for the design of compensation for power system stabilizers (PSSs) are used in practice. The object of this paper is to reconcile the methods and explore their relative advantages and ...disadvantages. Three methods are investigated: the GEP and the P-Vr frequency response approaches, and the method of residues. It is shown the phase response of a modified GEP transfer function (TF) agrees closely with that of the P-Vr TF, thus providing the basis for the design of a robust PSS. Residues yield only a limited number of phase angles that can be used with confidence for design purposes and are consistent with the P-Vr phase response. The remaining residues for rotor modes are affected by variability of participation factor angles and interactions from other machines. Unlike other methods, the P-Vr approach yields magnitude and phase information that simplifies the synthesis of the PSS TF and yields a robust stabilizer.
In this paper it is shown that interactions occur between stabilizers in multimachine power systems, the stabilizers being power system stabilizers (PSSs), FACTS device stabilizers (FDSs) or both. ...The interactions, which are identified and quantified, may enhance or degrade the damping of certain modes of rotor oscillation. In particular, interactions between PSSs are found to adversely affect the damping of inter-area modes. The analysis of interactions also provides a practical means for quantifying and assessing simultaneously the relative effectiveness of both PSSs and FDS in damping the rotor modes of oscillation. This is achieved using a stabilizer damping contribution diagram. A theoretical basis is given for the analysis of interactions and the effectiveness of stabilizers; the practical significance and applications are illustrated using a case study on a 3-area, 400 state system having 28 generator groups and a number of FDSs. For systems of more than 600-700 states the modified Arnoldi method is used for eigenanalysis-based calculations.
Tundra Trait Team Bjorkman, Anne D.; Myers-Smith, Isla H.; Elmendorf, Sarah C. ...
Global ecology and biogeography,
December 2018, Volume:
27, Issue:
12
Journal Article
Peer reviewed
Open access
Motivation
The Tundra Trait Team (TTT) database includes field‐based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used ...to address theoretical questions about plant strategy and trade‐offs, trait–environment relationships and environmental filtering, and trait variation across spatial scales, to validate satellite data, and to inform Earth system model parameters.
Main types of variable contained
The database contains 91,970 measurements of 18 plant traits. The most frequently measured traits (> 1,000 observations each) include plant height, leaf area, specific leaf area, leaf fresh and dry mass, leaf dry matter content, leaf nitrogen, carbon and phosphorus content, leaf C:N and N:P, seed mass, and stem specific density.
Spatial location and grain
Measurements were collected in tundra habitats in both the Northern and Southern Hemispheres, including Arctic sites in Alaska, Canada, Greenland, Fennoscandia and Siberia, alpine sites in the European Alps, Colorado Rockies, Caucasus, Ural Mountains, Pyrenees, Australian Alps, and Central Otago Mountains (New Zealand), and sub‐Antarctic Marion Island. More than 99% of observations are georeferenced.
Time period and grain
All data were collected between 1964 and 2018. A small number of sites have repeated trait measurements at two or more time periods.
Major taxa and level of measurement
Trait measurements were made on 978 terrestrial vascular plant species growing in tundra habitats. Most observations are on individuals (86%), while the remainder represent plot or site means or maximums per species.
Software format
csv file and GitHub repository with data cleaning scripts in R; contribution to TRY plant trait database (www.try-db.org) to be included in the next version release.
Opioid use in chronic pain treatment is complex, as patients may derive both benefit and harm. Identification of individuals currently using opioids in a problematic way is important given the ...substantial recent increases in prescription rates and consequent increases in morbidity and mortality. The present review provides updated and expanded information regarding rates of problematic opioid use in chronic pain. Because previous reviews have indicated substantial variability in this literature, several steps were taken to enhance precision and utility. First, problematic use was coded using explicitly defined terms, referring to different patterns of use (ie, misuse, abuse, and addiction). Second, average prevalence rates were calculated and weighted by sample size and study quality. Third, the influence of differences in study methodology was examined. In total, data from 38 studies were included. Rates of problematic use were quite broad, ranging from <1% to 81% across studies. Across most calculations, rates of misuse averaged between 21% and 29% (range, 95% confidence interval CI: 13%-38%). Rates of addiction averaged between 8% and 12% (range, 95% CI: 3%-17%). Abuse was reported in only a single study. Only 1 difference emerged when study methods were examined, where rates of addiction were lower in studies that identified prevalence assessment as a primary, rather than secondary, objective. Although significant variability remains in this literature, this review provides guidance regarding possible average rates of opioid misuse and addiction and also highlights areas in need of further clarification.
This paper summarizes a set of six benchmark systems for the analysis and control of electromechanical oscillations in power systems, recommended by the IEEE Task Force on Benchmark Systems for ...Stability Controls of the Power System Dynamic Performance Committee. The benchmark systems were chosen for their tutorial value and particular characteristics leading to control the system design problems relevant to the research community. For each benchmark, the modeling guidelines are provided, along with eigenvalues and time-domain results produced with at least two simulation softwares, and one possible control approach is provided for each system as well. Researchers and practicing engineers are encouraged to use these benchmark systems when assessing new oscillation damping control strategies.
For the purposes of planning it is necessary to determine, in a transmission system, which are the best signals for estimating the parameters of the electromechanical modes, given the characteristics ...of the modal estimator. Closed-form mathematical expressions are derived in another paper for the Power Spectral Densities (PSDs) of the candidate signals at selected locations due to random fluctuations in the system loads. The expressions for the PDSs comprise the separate identifiable contributions of every system mode to the PSD of the signal within a specified frequency band. Also, an expression has been derived which provides a measure of the contribution of the mode to the PSD. The measure is called the Modal Power Contribution which reveals the suitability of a given signal for modal estimation, especially when modes are closely-spaced. The paper demonstrates the practical application of the theoretical analysis.
The impact of increasing the amount of wind generation on the damping performance of the New Zealand power system is assessed. A comparison is made between the damping performance of (i) a base case ...scenario which has no wind generation (except for existing wind farms) and (ii) a corresponding scenario in which wind generation is introduced to the system by displacing an equivalent amount of synchronous generation. The damping performance of the pre- and post-wind scenarios is compared, based on the eigenvalues of a linearised model of the system. Small-signal models of the New Zealand power system are developed as well as generic small-signal models of a number of different types of wind energy converter (WEC) technologies. The sensitivity of the system damping performance to the type of WEC technology, type of voltage control, variation in the wind farm power output and level of system loading is assessed.
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