1,720,956 research outputs found
Supraharmonic Emissions in Future Electricity Networks with Solar PV Systems
No full textThe world is experiencing a shift from centralised to decentralised power generation as a pivotal strategy for achieving net-zero emission targets. As a result, solar photovoltaic (PV) systems have become the leading distributed energy resource (DER) connected to the grid. The integration of new DER systems presents several technical challenges, one of which is the power quality (PQ) issues introduced by associated power electronic interfaces. Conventional PQ studies have concentrated on voltage and current waveform distortion in the frequency range of 0–2 kHz, however, special attention has been paid to conducted emissions above 2 kHz over the past decade since their systematic knowledge is not completely developed yet. Waveform distortion in the 2–150 kHz frequency range is referred to as “supraharmonic emissions” or “high-frequency harmonic emissions”.This research study aims to identify the supraharmonic emission characteristics from rooftop solar PV systems containing identical PV inverters and their propagation characteristics within low voltage (LV) networks, focusing on different supraharmonic phenomenon appearing within the 2-25 kHz frequency range.The first part of this study is aimed at characterising supraharmonic emissions based on measurements of an LV installation with a rooftop solar PV system, consists of six (6) identical inverters. There are three measurement methods stipulated in international regulations, i.e. IEC 61000-4-7 (informative), IEC 61000-4-30 (informative), and CISPR 16-1-1 (normative). However, those regulations do not include prescriptive measurement and analysis techniques for the entirety of this range (2-150 kHz). Thus, the measurement methodology in IEC 61000-4-7 which is intended for emissions in the 2-9 kHz range, is utilised to analyse measurements up to 25 kHz in this study maintaining the consistency between 2-9 kHz and 9-25 kHz ranges.Two supraharmonic emission phenomena have been identified in two distinct frequency ranges: (a) resonance related emissions (2-5 kHz) due to the front-end filter of PV inverters, and (b) switching frequency emissions (15-17 kHz) due to the high frequency switching PWM pulses. Simultaneous measurements at different locations of the same LV installation showed that the emissions associated with resonance propagates within the installation, however, they do not propagate into the substation level where electricity is supplied to the installation. In addition, three (3) methods have been proposed to represent the phase angle for grouped supraharmonic emissions to be incorporated with harmonic summation techniques, which will assist in understanding the phase angle diversity of supraharmonic emissions.The second part of the study is aimed at analysing supraharmonic emissions based on a generic MATLAB/Simulink model. The model is developed using the commonly adopted voltage oriented control (VOC) under synchronous reference frame, often referred to as direct-quadrature (d-q) reference frame, identifying the boundary up to which low-frequency harmonic models are capable for supraharmonic analysis. The model is capable of representing the resonance and switching frequency supraharmonic emission phenomena identified, and is validated based on the measurements used in the first part of the study. The impact of carrier waveform phase shift in PWM signal generation is investigated within a multiple solar PV inverter system, which can be used to mitigate switching frequency harmonic flow into the grid.The study uniquely delves into characterising these emissions using advanced methods, such as phase angle representation, and improving understanding of the complex interactions that occur at the micro-level of inverters. Thus, the results from this research study are of significant importance to characterising and developing systematic knowledge on supraharmonic emissions from rooftop solar PV inverter systems.</p
Characterising Non-Intentional Supraharmonic Emissions from Inverters in Power Grids: Review and Challenges
No full textSupraharmonic emissions, referred to as voltage/current waveform distortions in the 2–150 kHz range, have been identified as an emerging power quality concern. With the increased number of non-linear devices connected to the power grid, such as photovoltaic inverter systems, supraharmonic disturbances are expected to increase. Despite being a source of supraharmonic emissions, power electronic equipment has become a ubiquitous technology due to recent advancements. Similarly, researchers around the world have started studying these emissions; however, complete systematic knowledge concerning supraharmonic emissions is yet to be achieved. This paper uniquely delves into characterising emissions using existing knowledge, significantly improving the understanding of their complex micro-level interactions and highlighting emerging challenges. The paper presents a comprehensive summary integrating existing studies on supraharmonic emissions in five key areas: emissions, propagation and attenuation, measurement techniques, modelling and simulation, and mitigation.</p
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Characterisation of supraharmonic emissions based on phase angle representation methods
No full textPower electronic interfaces in solar PV inverters generate high-frequency harmonic emissions, also referred as ’supraharmonic’ emissions, due to the fast switching (above 2 kHz) required to create low distortion output waveforms. Measuring and evaluating these emissions is a new challenge for power quality experts, as present standards (IEC, CISPR) do not include prescriptive measurement and analysis techniques for the entirety of this range (2–150 kHz). This paper investigates supraharmonic emissions from identical solar PV inverter systems highlighting distinct frequency zones to distinguish phenomenon types. The existing high-frequency harmonic measurement method given in IEC 61000-3-6 with some enhancements has been used to characterise supraharmonic emissions. This paper proposes three methods to represent the phase angle for grouped supraharmonic emissions that assist in understanding their diversity. In terms of aggregation of emissions, field measurements indicate that a summation exponent (α) equal to 1 should be applied for resonant emissions, while for switching frequency emissions, circulation between sources leads to harmonic cancellation.</p
Characterisation of Harmonic Resonance Phenomenon of Multi-Parallel PV Inverter Systems: Modelling and Analysis
No full textSolar PV inverters require output filters to reduce unwanted harmonics in their output, where LCL filters are a more economical choice than larger inductance-only filters. A drawback of these filters is that they can introduce power quality disturbances, especially at higher frequencies (above 2 kHz). This paper investigates and characterises the resonance phenomenon introduced by different filter types, i.e., LC or LCL, and identifies their behavioural change when combined with multiple parallel grid-tied PV inverter systems. MATLAB/Simulink modelling aspects of PV inverter systems related to resonance phenomenon are presented, including establishing resonance at a specific frequency where potentially large variations in the parameter selection across manufacturers may exist. In addition, a method is developed to establish output filter frequency response through measurements, which is used to develop validated solar PV harmonic models for high-frequency analysis. The low-frequency harmonic models can be used up to the resonant frequency where the current flowing through the filter capacitor is insignificant compared to the current flowing into the electricity network.</p
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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