1,721,115 research outputs found
Some meaningful examples of sequence theory use limitation
No full textSequence theory [1] is one of the most important tool for power system engineers. In fact, both in planning and operating activities, power flow and short circuit studies are always based on the knowledge of the sequence impedances.
Consequently, positive, negative and zero sequence impedances are the first step for power flow and short circuit analyses and are also necessary for setting the distance relays.
Notwithstanding, it is also true that by considering the physical reality of the power networks, it can be questionable to assume purely three-phase configurations and perfectly symmetrical ones, so to use sequence theory based modelling.
It is worth remembering that the use of zero, positive-negative sequence impedances Z0, Z1, Z2, is only exact if the system is symmetric since the application of voltage phasors of a sequence causes the circulation of current phasors only of the same sequence so that it is possible to compute the ratios between voltage and current phasors. The paper presents three meaningful examples of sequence theory use limitations
Image correlation-based identification of fracture parameters for structural adhesives
No full textWe consider the identification problem of a cohesive model using kinematic full-field data obtained via digital
image correlation during the fracture test of an adhesive joint. A criterion is presented for selecting measurements
that exhibit optimal features for identification purposes. The proposed selection scheme is constructed using the
sensitivity information and its performances are evaluated at varying noise-to-signal ratio
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Availability analysis of HVDC-VSC systems: A review
No full textThis work stems from the worldwide increasing need to precisely consider, in the design
phase of an HVDC project, the availability of the HVDC system. In this paper, an overview of the
availability assessment methods for HVDC-VSC transmission systems is presented. In particular, the
state of the art of the procedures to estimate the availability of both the HVDC link reparable
components and the conversion system on the basis of the converter configuration is given.
The theoretical fundamentals of each method, together with their practical applications, have
been described, in order to highlight the limits and the potentialities of each approach. The authors
aim at giving a guide to choosing the best computation approach on the basis of the specific needs
of the users and at summarizing all the key aspects which can be taken into account during the
availability assessment of HVDC-VSC links
Analytical formulation for estimating power low frequency oscillations between two overhead lines operating at different frequencies
Full text linkThis paper derives an analytical formulation to estimate the period and the amplitude of power low frequency oscillations originated by the mutual coupling of two power systems operating at two slightly different frequencies. This is the typical case of the power system restoration tests, in which the restoration backbone involves two overhead lines installed on the same tower. These oscillations have been detected during several real-world power system restoration tests with a non-negligible amplitude. In some cases, the active power exchanged between the two systems could damage the control and protection equipment of the elements involved in the test, leading to the failure of the restoration test itself. For these reasons, the analytical prediction of the period and the magnitude of these oscillations could constitute a useful and reliable tool for the planning and the simulation of the restoration tests
On the CIM Modelling Approach for the Frequency Estimation of a Power System Restoration Process
No full textThis paper aims at presenting and discussing modeling and simulation issues regarding the bottom-up power system restoration process. In particular, it compares different modelling approaches for the main regulation sets of the black-start generator focusing on the frequency behaviour. The regulation sets proposed by the Common Information Models (CIMs) library are compared with the actual ones installed on real power plants. The results are thus compared with on-field recordings performed during a real power system restoration test. These results highlight how, for systems which present a low power frequency characteristic, i.e. an islanded system during the restoration process, the adoption of the CIMs seems to be not always correct to estimate the frequency transient: in some cases, the real model has to be adopted
Reliability assessment of a multi-state hvdc system by combining markov and matrix-based methods
Full text linkThe purpose of this paper is to highlight that, in order to assess the availability of different HVDC cable transmission systems, a more detailed characterization of the cable management significantly affects the availability estimation since the cable represents one of the most critical elements of such systems. The analyzed case study consists of a multi-terminal direct current system based on both line commutated converter and voltage source converter technologies in different configurations, whose availability is computed for different transmitted power capacities. For these analyses, the matrix-based reliability estimation method is exploited together with the Monte Carlo approach and the Markov state space one. This paper shows how reliability analysis requires a deep knowledge of the real installation conditions. The impact of these conditions on the reliability evaluation and the involved benefits are also presented
Lessons Learnt from Modelling and Simulating the Bottom-Up Power System Restoration Processes
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