[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

National Institute of Standards and Technology における超伝導研究及び生活

Differential-Difference Equations. By Richard Bellman and Kenneth L. Cooke. Pp. xvi, 465. 1963. (Academic Press)

Much has been written about theory and practice in the law, and the tension between practitioners and theorists. Judges do not cite theoretical articles often; they rarely "apply" theories to particular cases. These arguments are not revisited. Instead the Essay explores the working and interaction of theory and practice, practitioners and theorists. The Essay starts with a story about solving a legal issue using our intellectual tools - theory, practice, and their progenies: experience and "gut." Next the Essay elaborates on the nature of theory, practice, experience and "gut." The third part of the Essay discusses theories that are helpful to practitioners and those that are less helpful. The Essay concludes that practitioners theorize, and theorists practice. They use these intellectual tools differently because the goals and orientations of theorists and practitioners, and the constraints under which they act, differ. Theory, practice, experience and "gut" help us think, remember, decide and create. They complement each other like the two sides of the same coin: distinct but inseparable.

Of Theory and Practice

(1990). ENERGY FUNCTION ANALYSIS FOR POWER SYSTEM STABILITY. Electric Machines & Power Systems: Vol. 18, No. 2, pp. 209-210.

Energy function analysis for power system stability

Day-ahead wind speed forecasting using f-ARIMA models

This paper conducts an analysis of subsynchronous resonance (SSR) phenomena in doubly-fed induction generator (DFIG)-based wind farms interconnected with series compensated networks. A dynamic model is developed to analyze the induction generator effect (IGE) and torsional interaction (TI) in such systems. A test system derived from the IEEE first benchmark model is considered for the analysis. The effect of two factors namely: 1) series compensation level and 2) wind speed on the IGE and TI are studied. In addition, impact of the inner current converter controller parameters and turbine parameters on SSR is also addressed. Small signal (eigenvalue) analysis is conducted to assess the damping of network and torsional modes followed by dynamic (time domain) simulations. The major contribution of this paper is the analytical investigation on SSR phenomena presented in DFIG-based wind farms interconnected with series compensated networks. The paper clearly demonstrates that IGE instead of TI is the major reason for SSR in such systems.

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Modeling of DFIG-Based Wind Farms for SSR Analysis

This paper explores the task of real-time identification of dynamic events leading to a layer of situational awareness that can become a reality due to increased penetration of phasor measurement units in transmission systems. Two underlying models for this task-data driven and physics based-are explored with examples. Challenges, advantages, and drawbacks of each model are discussed based on the availability of data, attributes of such data, and processing options. Potential applications of the task to improve security of power system protection and anomaly detection in the case of a cyberattack are conceptualized. Some known issues in data communications are discussed vis-a-vis the requirements imposed by the proposed task.

Real-Time Identification of Dynamic Events in Power Systems Using PMU Data, and Potential Applications—Models, Promises, and Challenges

In this paper, a systematic analysis of hourly wind speed data obtained from four potential wind generation sites in North Dakota is conducted. The power spectra of the data exhibited a power law decay characteristic of 1/fα processes with possible long range correlations. The temporal scaling properties of the records were studied using the sophisticated multifractal detrended fluctuation analysis MFDFA. It is seen that the records at all four locations exhibit similar scaling behavior which is also reflected in the multifractal spectrum determined under the assumption of a binomial multiplicative cascade model.

A multifractal description of wind speed records

This paper develops a framework for analysis of harmonics in a doubly fed induction generator (DFIG) caused by nonsinusoidal conditions in rotor and unbalance in stator. Nonsinusoidal rotor voltages are decomposed into harmonic components and their corresponding sequences are identified. Induced harmonics in stator are analyzed and computed, from which the torques produced by these interactions between stator and rotor harmonic components can be found. During unbalanced stator conditions, symmetric component theory is applied to the stator voltage to get positive-, negative-, and zero-sequence components of stator and rotor currents. The steady-state negative-sequence equivalent circuit for a DFIG is derived based on the reference frame theory. Harmonic currents in the rotor are computed based on the sequence circuits. In both scenarios, the harmonic components of the electromagnetic torque are calculated from the interactions of the harmonic components of the stator and rotor currents. Three case studies are considered, namely: 1) nonsinusoidal rotor injection; 2) an isolated unbalanced stator load scenario; and 3) unbalanced grid-connected operation. The analysis is verified with results from numerical simulations in Matlab/Simulink. For illustration, the second case is verified using experiments. The simulation results and experimental results agree well with the results from analysis.

/pdf/harmonic-analysis-of-a-dfig-for-a-wind-energy-conversion-8jtrt1jwa3.pdf

Rajesh Kavasseri

Papers

Day-ahead wind speed forecasting using f-ARIMA models

Modeling of DFIG-Based Wind Farms for SSR Analysis

Real-Time Identification of Dynamic Events in Power Systems Using PMU Data, and Potential Applications—Models, Promises, and Challenges

A multifractal description of wind speed records

Harmonic Analysis of a DFIG for a Wind Energy Conversion System