123,565 research outputs found
Using Effective Generator Impedance for Forced Oscillation Source Location
Locating the sources of forced low-frequency oscillations in power systems is
an important problem. A number of proposed methods demonstrate their practical
usefulness, but many of them rely on strong modeling assumptions and provide
poor performance in certain cases for reasons still not well understood. This
paper proposes a systematic method for locating the source of a forced
oscillation by considering a generator's response to fluctuations of its
terminal voltages and currents. It is shown that a generator can be represented
as an effective admittance matrix with respect to low-frequency oscillations,
and an explicit form for this matrix, for various generator models, is derived.
Furthermore, it is shown that a source generator, in addition to its effective
admittance, is characterized by the presence of an effective current source
thus giving a natural qualitative distinction between source and nonsource
generators. Detailed descriptions are given of a source detection procedure
based on this developed representation, and the method's effectiveness is
confirmed by simulations on the recommended testbeds (eg. WECC 179-bus system).
This method is free of strong modeling assumptions and is also shown to be
robust in the presence of measurement noise and generator parameter
uncertainty.Comment: 13 page
Generating realistic scaled complex networks
Research on generative models is a central project in the emerging field of
network science, and it studies how statistical patterns found in real networks
could be generated by formal rules. Output from these generative models is then
the basis for designing and evaluating computational methods on networks, and
for verification and simulation studies. During the last two decades, a variety
of models has been proposed with an ultimate goal of achieving comprehensive
realism for the generated networks. In this study, we (a) introduce a new
generator, termed ReCoN; (b) explore how ReCoN and some existing models can be
fitted to an original network to produce a structurally similar replica, (c)
use ReCoN to produce networks much larger than the original exemplar, and
finally (d) discuss open problems and promising research directions. In a
comparative experimental study, we find that ReCoN is often superior to many
other state-of-the-art network generation methods. We argue that ReCoN is a
scalable and effective tool for modeling a given network while preserving
important properties at both micro- and macroscopic scales, and for scaling the
exemplar data by orders of magnitude in size.Comment: 26 pages, 13 figures, extended version, a preliminary version of the
paper was presented at the 5th International Workshop on Complex Networks and
their Application
Automating property-based testing of evolving web services
Web services are the most widely used service technology that drives the Service-Oriented Computing~(SOC) paradigm. As a result, effective testing of web services is getting increasingly important. In this paper, we present a framework and toolset for testing web services and for evolving test code in sync with the evolution of web services. Our approach to testing web services is based on the Erlang programming language and QuviQ QuickCheck, a property-based testing tool written in Erlang, and our support for test code evolution is added to Wrangler, the Erlang refactoring tool.
The key components of our system include the automatic generation of initial test code, the inference of web service interface changes between versions, the provision of a number of domain specific refactorings and the automatic generation of refactoring scripts for evolving the test code. Our framework provides users with a powerful and expressive web service testing framework, while minimising users' effort in creating, maintaining and evolving the test model. The framework presented in this paper can be used by both web service providers and consumers, and can be used to test web services written in whatever language; the approach advocated here could also be adopted in other property-based testing frameworks and refactoring tools
Heat Transfer and Pressure Drop in a Developing Channel Flow with Streamwise Vortices
Experiments to assess the heat transfer and pressure-drop effects of delta-wing
vortex generators placed at the entrance of developing channel flows are reported in
this study. The experimental geometry simulates common heat exchanger
configurations and tests are conducted over a velocity range important to heating, air
conditioning and refrigeration. An innovative liquid-crystal thermography technique
is used to determine the local and average Nusselt numbers for an isoflux channel
wall, and conventional methods are used to determine the Fanning friction factor.
Vortex generators with aspect ratios of A = 2 and A = 4 are studied at attack angles
of a. = 20?? to 45????. The results indicate that the streamwise vortices generated by a
delta wing can enhance local Nusselt numbers by more than 200% in a developing
channel flow. Under some conditions, the spatially average Nusselt number nearly
doubled for a heat transfer area that was 37 to 63 times the wing area. The Fanning
friction factor increased by a few percent to nearly 60%, depending on the Reynolds
number.Air Conditioning and Refrigeration Project 4
The RANLUX generator: resonances in a random walk test
Using a recently proposed directed random walk test, we systematically
investigate the popular random number generator RANLUX developed by Luescher
and implemented by James. We confirm the good quality of this generator with
the recommended luxury level. At a smaller luxury level (for instance equal to
1) resonances are observed in the random walk test. We also find that the
lagged Fibonacci and Subtract-with-Carry recipes exhibit similar failures in
the random walk test. A revised analysis of the corresponding dynamical systems
leads to the observation of resonances in the eigenvalues of Jacobi matrix.Comment: 18 pages with 14 figures, Essential addings in the Abstract onl
Applied Koopman Operator Theory for Power Systems Technology
Koopman operator is a composition operator defined for a dynamical system
described by nonlinear differential or difference equation. Although the
original system is nonlinear and evolves on a finite-dimensional state space,
the Koopman operator itself is linear but infinite-dimensional (evolves on a
function space). This linear operator captures the full information of the
dynamics described by the original nonlinear system. In particular, spectral
properties of the Koopman operator play a crucial role in analyzing the
original system. In the first part of this paper, we review the so-called
Koopman operator theory for nonlinear dynamical systems, with emphasis on modal
decomposition and computation that are direct to wide applications. Then, in
the second part, we present a series of applications of the Koopman operator
theory to power systems technology. The applications are established as
data-centric methods, namely, how to use massive quantities of data obtained
numerically and experimentally, through spectral analysis of the Koopman
operator: coherency identification of swings in coupled synchronous generators,
precursor diagnostic of instabilities in the coupled swing dynamics, and
stability assessment of power systems without any use of mathematical models.
Future problems of this research direction are identified in the last
concluding part of this paper.Comment: 31 pages, 11 figure
ChimpCheck: Property-Based Randomized Test Generation for Interactive Apps
We consider the problem of generating relevant execution traces to test rich
interactive applications. Rich interactive applications, such as apps on mobile
platforms, are complex stateful and often distributed systems where
sufficiently exercising the app with user-interaction (UI) event sequences to
expose defects is both hard and time-consuming. In particular, there is a
fundamental tension between brute-force random UI exercising tools, which are
fully-automated but offer low relevance, and UI test scripts, which are manual
but offer high relevance. In this paper, we consider a middle way---enabling a
seamless fusion of scripted and randomized UI testing. This fusion is
prototyped in a testing tool called ChimpCheck for programming, generating, and
executing property-based randomized test cases for Android apps. Our approach
realizes this fusion by offering a high-level, embedded domain-specific
language for defining custom generators of simulated user-interaction event
sequences. What follows is a combinator library built on industrial strength
frameworks for property-based testing (ScalaCheck) and Android testing (Android
JUnit and Espresso) to implement property-based randomized testing for Android
development. Driven by real, reported issues in open source Android apps, we
show, through case studies, how ChimpCheck enables expressing effective testing
patterns in a compact manner.Comment: 20 pages, 21 figures, Symposium on New ideas, New Paradigms, and
Reflections on Programming and Software (Onward!2017
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