1,720,970 research outputs found
Modelling and fault ride-through control of grid supporting inverter-based microgrid
No full textThesis submitted in the fulfilment of the requirements of the degree of Doctor of Engineering in Electrical Engineering in the Faculty of Engineering and the Built Environment at the Durban University of Technology, 2021.This thesis is focused on modeling and fault ride-through control, local load power delivery, and grid power exchange of power electronic interfaced Distributed Energy Resources (DERs) for grid supporting microgrids. Active and reactive power regulations are the requirements for a grid-supporting system operating as a current source, while frequency and voltage magnitude regulation in the grid-supporting system acting as a voltage source. Consequently, these are put into consideration as the primary control requirements for the inverter-based microgrid. To that end, two discrete-time models of a grid-feeding system and grid-forming system were developed to serve as controls for a single DER operating in grid-connected mode and islanded mode, respectively. Consequently, for the first set of mathematical models: grid feeding and grid forming were interfaced with a droop control to allow for parallel operation of additional DERs for power coordination within the microgrid for grid-connected and islanded operation. However, virtual impedance was incorporated into the grid-supporting system's droop control operating as a voltage source to emulate the link feeder's physical impedance to the main grid. Based on the developed grid supporting models, the microgrid primary control schemes effectively delivered power to the host grid and simultaneously contributed to the grid's frequency and voltage regulation. Furthermore, to ensure grid code compliance and ensure the microgrid provides ancillary services to the host grid, such as fault ride-through and reactive power compensation for voltage recovery, a novel technique is proposed in the microgrid's secondary control. The secondary control realizes the fault ride-through for the grid supporting system using a delay signal cancellation algorithm for negative sequence detection. The proposed control scheme actualizes grid code requirements by providing a secondary voltage control, which is active and more prominent in the transient period of faults without mode switching. The strategy's performance is further enhanced with an IGBT-Diodes switched AC reactor to improve the voltage and prevent the transient overcurrent in the microgrid during the grid fault. This ensures a continued supply of the microgrid's local sensitive load while meeting the grid code requirement. Similarly, the active power injection into the main grid is limited to maximize reactive power injection into the main network to support the grid voltage sag. The detection algorithm using the delayed signal cancellation algorithm is implemented to detect the instance of fault in 1.6% of the half-cycle under grid disturbance/fault to activate the proposed secondary control. This effectiveness and fault ride-through compliance of the developed control models were tested on an inverter-based microgrid system with an ideal voltage source DERs. Finally, to accommodate for the grid dynamics introduced to the DC link parameters of an ideal voltage source DER such as PV, the models were also implemented and assimilated for a solar PV sourced DER used with a grid supporting inverter-based microgrid. The injection of active power into the main grid is constrained by systematically shifting the MPPT operating point based on voltage sag depth to maximize reactive power injection to support the grid voltage sag. The strategy developed in the PV sourced system also ensured that the DC-link voltage and AC grid current raises are suppressed while meeting microgrid load requirements. The models' implementation, DER primary control, and proposed secondary control schemes are established through detailed time-domain simulation studies using MATLAB Simscape Electrical™ and Control System™.Research and Postgraduate Support Directorate
Adaptive Delay Compensation Frameworks for Distributed Real-Time Co-Simulation in Power Systems
No full textThis dissertation presents a model-free, adaptive delay prediction and compensation framework for geographically distributed real-time power system co-simulation environments. Communication delays—both constant and real-time-varying—significantly degrade the accuracy, fidelity, and stability of co-simulated systems, particularly in dynamic and transient analyses of partitioned power systems. To address this, a predictor-based framework is developed that compensates for delays without requiring system models, computationally intensive signal transformations, or manual intervention.
The proposed solution leverages a Damping Impedance Method as the interface algorithm, combined with a sliding-mode control-inspired predictor system. Both single-parameter and multi-parameter predictor configurations are implemented, with the multi-parameter design providing an additional degree of freedom for delay compensation and extending the open-loop stability region. An adaptive, delay-aware tuning mechanism further enhances implementation by making predictor parameters dynamic functions of observed real-time delays. Validation was conducted through offline simulations and hardware-in-the-loop (HIL) experiments, incorporating typical three-phase AC systems, partitioned IEEE test systems (power system transmission-distribution co-simulation), and a real-time partitioned military-based 600 V DC microgrid.
Performance metrics such as state-tracking error, coupling error, subsystem frequency discrepancies, and a proposed residual energy metric were utilized to quantitatively assess fidelity and energy consistency across subsystems. Results show a significant improvement in operational stability and coupling fidelity with the implemented prediction systems over traditional approaches based on coupling signal transformation. Unlike conventional methods that rely on RMS, dynamic phasors, wave variable transformation, or synchronous reference frame transformations, the proposed model-free framework operates directly in the time domain, reducing complexity while improving robustness under time-varying delays.
Additionally, a Deep Deterministic Policy Gradient (DDPG) reinforcement learning agent is integrated into the co-simulation platform for real-time energy management across partitioned military-based 600 V DC microgrid with vehicle-to-grid (V2G) and vehicle-to-vehicle (V2V) capabilities. By formulating the problem as a Markov Decision Process, the distributed agent dynamically optimizes power generation, storage, and distribution, minimizing operational costs and adapting to load variations. Experimental results demonstrate that the DDPG agent effectively manages energy flows, maintains critical load supply, and enhances overall system resilience.
The developed co-simulation platform with adaptive delay prediction and compensation thus provides a scalable and practical solution for improving accuracy, stability, and energy management in distributed real-time power system simulations. Future research will focus on expanding the framework’s design flexibility and scaling to larger, multi-partitioned systems with simultaneous real-time simulator exchanges
A new technique for improvement differential relay performance in power transformers
No full textTransformer protection devices are often used to identify internal or external transformer problems and act to either prevent damage or unnecessarily disconnect power transformers. This study proposes a new differential element that combines harmonic restraint, security, and reliability with harmonic blocking speed to improve the relay performance in a power transformer. Under high load, a negative-sequence differential element adds more sensitivity for internal turn-to-turn failures. External fault detection monitoring enhances security in an external problem involving current transformer (CT) saturation. Furthermore, overcurrent elements may be configured to vary dynamically in operation is provided. This element enhances protection coordination for various operating conditions without requiring modifications to the transformer group settings. The balance of the paper discusses the use of an under-load tap changer using a time-synchronized phasor monitoring system to reduce loop current and losses in parallel transformer applications
Performance evaluation of percentage differential relays on power transformer and reliability assesment in HVDC grid protection scheme
No full textPercentage differential relays remain the most
sensitive protection tool applied as backup protection on power
transformers, busbar, and generators. Relays sometimes do
mis-operate with the current transformer being affected by
external fault leading to saturation, and the subsidence current
present after clearing external faults. The cause of misoperation of percentage differential relays cannot be ignored
that it entirely depends on magnitudes more than directionality
for tripping decisions. This paper covers evaluating differential
element performance, analysis of transformer inrush current,
internal faults, external faults, and overexcitation conditions.
The accurate computing of current transformers is also
included. This protection only applies to 10MVA and above on
transformers; however, it is not limited to transformers, but
also transmission lines, busbars, and generators. The balance
of the paper is on reliability assessment based on the HVDC
grid protection scheme operatio
Fault Analysis of Injection SubstationUsing Symmetrical Component MethodA Case Study of Mofor Injection Substation, Delta State, Nigeria
Get PDFThis study centers on the fault analysis of 15MVA Mofor Injection Substation, which is an Injection Substation located in Warri, Delta state, Nigeriawhich gets its source from PTI transmission station. Mofor Injection substation has two outgoing feeders which are Orhuwhorun feeder and Ekete feeder. The analysis was carried out and deductions were made considering the various faults which occurred during the period of assessment and their associated fault current was calculated using symmetrical Component method of Fault analysis. Amodel of the distribution network was made using Electrical Transient Analyzer Program(ETAP); the value of real and reactive powers and voltage magnitudes in the whole network wereobserved. Thedata obtained from the injection substation indicates that Orhuwhorun feeder has a higher frequency of fault and from results obtained from Symmetrical method of fault analysis revealed that double Line to ground fault has the highest fault current and could cause adverse damages to equipments’and as such must be avoided. The fault current calculated from Symmetrical component method of Fault analysis was validated with computer program MATLAB as results agreed closely since error was below 0.1%.This paper analyzes several faults from an injection substation. The distribution network was modelled in ETAP, observing values for active and reactive power and voltage magnitude
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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