156 research outputs found
Parameter identification of unsymmetrical transmission lines using fault records obtained from protective relays
No full textDuring a fault, protection relays of the faulty line as well as adjacent lines react and record measurement data. The records of an adjacent healthy line can be used for line parameter identification. The special feature of the proposed method is the consideration of unsymmetrical transmission lines and the application of a high-accuracy signal modeling technique. Derived from a π line model, an estimation equation for determining the different impedances is developed. This equation contains the signal models of measurements as well as their time derivatives, which can be estimated very precisely compared to the classical approach based on filtering. By means of a simple simulation model, the accuracy of the proposed algorithm is compared to an approach previously proposed which is based on deconvolution and filtering methods as well as on the representation of a transmission line using time-varying phasors.Roberto Schulze, Peter Schegner Rastko Živanovi
Enhanced segmentation of disturbance records by adaptive thresholding
No full textTo analyse power system fault events captured by disturbance recorders the sampled analogue waveforms (voltages and currents) have to be segmented into system states (pre-fault, fault,...). In this paper a combination of wavelet transform (WT) and linear prediction coding (LPC) is applied to the symmetrical space phasor signals of the voltage and current waveforms. The signals obtained contain impulses when the system state changes abruptly. A threshold for segmentation is calculated using median filtering with adaptive window length. The segment borders are finally merged and validated.Stachel Philipp, Schegner Peter and Zivanovic, Rastko;http://www.pscc2008.org
Survey of harmonic current unbalance in public low voltage networks
No full textThe paper presents a survey of harmonic current unbalance in public low voltage networks based on measurements of 130 German grids. To ensure a representative set of sites, the selection is based on a classification schema that considers different consumer, generation and network configurations. New individual and aggregated harmonic unbalance indices, which have been recently introduced starting from a harmonic and interharmonic unbalance theory, are applied for the analysis. The results show that fundamental as well as harmonic currents have unbalances higher than 10% and the unbalance levels increase further with the harmonic order. Exceptional high current unbalance levels have been found in some networks with shopping centers and office buildings as unbalance issues have not been taken properly into account at the design stage. The analysis suggests that harmonic studies in public LV networks have to take into account harmonic current unbalances in order to obtain realistic results
Identification of the resonant-grounded system parameters by evaluating fault measurement records
No full textCopyright © 2004 IEEEThe operation of a resonant-grounded network during an earth-fault condition depends on the three basic parameters: damping, detuning, and unbalance factor. These parameters are influenced by the environmental conditions (e.g. humidity, temperature, and pollution), and the network topology. Accurate values of these parameters during an earth-fault condition are required to examine the operation of the compensation system. The fault records could be used for that purpose. The recorded neutral-to-ground voltage signals have been parameterized (using damping and detuning as parameters) according to the mathematical model of the transient process. Iteratively reweighted least squares algorithm has been used to fit the model. This algorithm is the major improvement over the classical least squares approach. It is able to filter out noise more efficiently. As a direct result, very accurate parameter identification has been achieved. This paper concludes with the practical examples.Rastko Zivanovic´, Peter Schegner, Olaf Seifert, and Georg Pil
Time–frequency multiresolution of fault-generated transient signals in transmission lines using a morphological filter
No full textThe ongoing transformation of electrical power systems highlights the weaknesses of the protection schemes of traditional devices because they are designed and configured according to traditional characteristics of the system. Therefore, this work proposes a new methodology to study the fault-generated high frequency transient signals in transmission lines through multiresolution analysis. The high frequency components are determined by a new digital filtering technique based on mathematical morphology theory and a spectral energy index. Consequently, wide spectra of signals in the time–frequency domain are obtained. The performance of this method is verified on an electrical power system modeled in ATP-Draw, where simulation and test signals are developed for different locations, fault resistances, inception angles, high frequency noises, sampling frequencies, types of faults, and shapes of the structuring element. The results show the characteristics of the fault such as the traveling wave frequency, location, and starting time.Fil: Quispe, Juan Carlos. Universidad Nacional de San Juan; Argentina. Universidad Tecnológica del Perú; PerúFil: Morales Garcia, John Armando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Energía Eléctrica. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Energía Eléctrica; ArgentinaFil: Orduna, Eduardo. Universidad Nacional de San Juan; ArgentinaFil: Liebermann, Carlo. Technische Universität Dresden; AlemaniaFil: Bruhns, Michael. Technische Universität Dresden; AlemaniaFil: Schegner, Peter. Technische Universität Dresden; Alemani
Simulation and Management of Distributed Generating Units using Intelligent Techniques
No full textDistributed generation is attracting more attention as a viable alternative to large centralized generation plants, driven by the rapidly evolving liberalization and deregulation environments. This interest is also motivated by the need for eliminating the unnecessary transmission and distribution costs, reducing the greenhouse gas emissions, deferring capital costs and improving the availability and reliability of electrical networks. Therefore, distributed generation is expected to play an increasingly important role in meeting future power generation requirements and to provide consumers with flexible and cost effective solutions for many of their energy needs. However, the integration of these sources into the electrical networks can cause some challenges regarding their expected impacts on the security and the dynamic behaviour of the entire network. It is essential to study these issues and to analyze the performance of the expected future systems to ensure satisfactory operation and to maximize the benefits of utilizing the distributed resources.
The thesis focuses on some topics related to the dynamic simulation and operation of distributed generating units, specifically fuel cells and micro-turbines. The objective of this dissertation is to put emphasis on the following aspects:
Dynamic modelling of fuel cells: Analyzing electrical power systems requires suitable dynamic models for all components forming the system. Since fuel cell units represent new promising sources, the research ascribes special consideration to developing models that describe their dynamic behaviour. It is envisaged to develop a simple and flexible model for stability studies and controller-design purposes in addition to an exhaustive nonparametric model for detailed analysis of the fuel cells.
Simulation of a large number of DG units incorporated into a multi-machine network: With large numbers of distributed sources, it is expected that decentralized generation impacts the dynamic behaviour of the high voltage network. Therefore, it is intended to investigate the case, where several fuel cells and micro-turbines are integrated into the distribution system of a multi-machine network. This can help in studying the operation of the entire network and highlighting the mutual impact of the high-voltage and low-voltage networks on each other.
Dynamic modelling and simulation of hybrid fuel cell/micro-turbine units: The hybrid configuration of fuel cells and micro-turbines exhibits many advantages enabling this technology to represent a considerable percentage of the next advanced power generation systems. The dynamic performance of such units, however, is still not fully understood. Hence, it is desirable for understanding their behaviour to highlight the dynamic interdependencies between the fuel cell and the micro-turbine, the overall system transient performance, and the dynamic control requirements.
Dynamic equivalents of distribution power networks: The need for fast and simplified analysis of interconnected power networks obligates developing robust dynamic equivalents for certain electrical power subsystems. Nonparametric dynamic equivalents will avoid the identification of complicated mathematical models, which would adequately reflect the performance of the replaced network under various operating conditions. For distribution systems, the equivalent model has to take into consideration the characteristics of distributed generating units which are mostly connected to the network through inverters and in some cases their operating principles are not based on the electromechanical energy conversion mechanism.
Impact of distributed generation on the stability of power systems: The existence of distributed sources with large numbers can impact the stability of the power system considerably. Angle-stability, frequency stability as well as voltage stability can be affected when the power from these units increases. It is essential to study this impact to ensure secure operation of the power system. Therefore, it is envisaged to study the performance of a hypothetical network and to demonstrate different stability classes at different penetration levels of the distributed generating units.
Online management of fuel cells and micro-turbines for residential applications: The optimal management of the power in distributed generation for residential applications can significantly reduce the operating cost and contribute towards improving their economic feasibility. The management process, however, has to be accomplished in the online mode and to account for all decision variables that affect the setting values. Therefore, it is aimed to develop an online intelligent strategy to manage the power generated in fuel cells and micro-turbines when used to supply residential loads in order to minimize the daily operating cost and achieve an overall reduction in the electricity price
Modellbasierte Berechnung der frequenzabhängigen Stromverteilung in räumlich ausgedehnten Erdungssystemen
No full textDie Auslegung von Erdungsanlagen nach der DIN EN 50522 basiert im Wesentlichen auf der Bewertung der bei Erdfehler auftretenden Erdungs- bzw. Berührungsspannungen. Deren Berechnung setzt die Kenntnis der Fehlerstromaufteilung (bzw. der wirksamen Reduktionsfaktoren) voraus, da nur der Stromanteil, der als Erdungsstrom über der Erdungsimpedanz wirksam wird, tendenziell gefährliche Potentialanhebungen verursacht. Maßgebend für die Verteilung der Ströme im Erdungssystem sind die induktiven Kopplungen zwischen dem fehlerstromführenden Leiter und den zum Sternpunkt zurückführenden Strompfaden. Daraus geht hervor, dass sowohl die räumliche Anordnung der Leiter als auch die Frequenz der Fehlerstromanteile die vorherrschenden Verhältnisse entscheidend beeinflussen. Beide Einflüsse werden in den Standards und der Fachliteratur bis dato nur bedingt betrachtet.
Diese Arbeit befasst sich daher mit der rechnerischen Bestimmung und Bewertung der Potentialanhebungen, welche sich im Falle eines Erdfehlers in Abhängigkeit von den Frequenzanteilen des Fehlerstroms in unterschiedlich aufgebauten Erdungssystemen ergeben. Wesentlicher Bestandteil der Ausführungen ist die Entwicklung eines Modells zur Berechnung der frequenzabhängigen Stromverteilung und den daraus resultierenden Erdungsspannungen in verbundenen Erdungsanlagen. Dieses bildet, in Erweiterung zu bestehenden Ansätzen, insbesondere den Einfluss der dreidimensionalen Anordnung der Leiter auf deren magnetische Kopplungen ab. Durch theoretische Betrachtungen und begleitende Messungen werden die Grenzen derartiger rechnerischer Bewertungsverfahren aufgezeigt.
Anhand einfacher Modellanordnungen wird beispielhaft der Einfluss der räumlichen Struktur des Erdungssystems auf die frequenzabhängige Fehlerstromverteilung, die wirksamen Erdungsimpedanzen und die resultierenden Erdungsspannungen analysiert. Dabei werden explizit Erdfehler an zusammengeschlossenen Hoch- und Niederspannungs-Erdungsanlagen untersucht. Die Ergebnisse zeigen, dass rein zweidimensionale Berechnungsverfahren den theoretischen Worst-Case hinsichtlich der Erdungsspannung nicht abdecken. Für die praktische Anwendung des Modells werden sowohl Vereinfachungen als auch zusätzliche Sicherheitsaufschläge abgeleitet.
Weiterhin erfolgt die modellbasierte Untersuchung einpoliger Fehler in voll- und teilverkabelten Netzen. Im Fokus steht dabei die potentialanhebende Wirkung der Frequenzanteile des einpoligen Fehlerstroms. Es wird aufgezeigt, dass infolge der induktiven Kopplungen ein hoher Anteil des Rückstroms in den beidseitig geerdeten Schirmen der fehlerstromführenden Kabel fließt. Bedingt durch die Frequenzcharakteristik des Reduktionsfaktors wirken die höherfrequenten Anteile des Erdfehlerstroms in vollverkabelten Netzgebieten in deutlich geringerem Maße potentialanhebend als der Grundschwingungsanteil. Daher lassen sich insbesondere für die Erdungssysteme von Kabelnetzen mit Resonanz-Sternpunkterdung vereinfachte Bewertungsmethoden ableiten.
In Summe liefert diese Arbeit einen Beitrag dazu, wie der rechnerische Nachweis zulässiger Berührungsspannungen geführt und das Prozedere zur Auslegung von Erdungsanlagen vereinfacht werden kann.The design of earthing installations in European high voltage grids has to comply with the standard EN 50522. The determining quantities for the risk assessment are the touch and step voltages occurring during an earth fault. Since the direct estimation of these voltages is complicated, earthing installations are usually assessed by means of the earth potential rise. For the calculation of the earth potential rise the knowledge of the fault current distribution (or the effective reduction factors) is required, since only the proportional current to earth raises the potential of the earthing installations. The inductive coupling between the conductor carrying the fault current and the current paths leading back to the neutral point is decisive for the distribution of the currents in the earthing system. This indicates that both the spatial arrangement of the conductors and the frequency of the fault current components have a determining influence on the prevalent conditions. To date, both influences have only been considered to a minor extent in the standards and technical literature.
This thesis relates to the calculation and evaluation of the earth potential rises in differently structured earthing systems. Thereby, the varying potential raising effect of the harmonics in the earth fault current is examined. For this purpose, a model for the calculation of the frequency-dependent current distribution in interconnected earthing installations is developed. In extension to existing approaches, this model particularly represents the influence of the three-dimensional arrangement of the conductors on their magnetic coupling. Through theoretical considerations and accompanying measurements, the limits of such calculational evaluation methods are demonstrated.
Based on simple model arrangements, the influence of the spatial structure of the earthing system on the frequency-dependent fault current distribution (reduction factors), the effective impedances to earth and the resulting earth potential rise is analysed exemplarily. Thereby, earth faults at interconnected high-voltage and low-voltage earthing installations are investigated explicitly. The results show that two-dimensional calculation methods do not cover the theoretical worst case regarding the earth potential rise. For the practical application of the model, simplifications as well as additional safety surcharges are derived.
Furthermore, model-based investigations of single-pole faults in fully and partially cabled grids are carried out. The focus is on the potential-raising effect of the harmonic components of the single-pole fault current. It is shown that, due to inductive coupling, a high percentage of the return current flows via the shields of the cables carrying the fault current. Due to the frequency characteristic of the reduction factor, in fully cabled grid areas higher-frequency components of the earth fault current have a significantly lower potential-raising effect than the fundamental component. Particularly for the earthing systems of cable grids with resonant earthing, simplified assessment methods can thus be derived.
In summary, this thesis provides a contribution on how to proof permissible touch voltages by calculation and how to simplify the procedure for the assessment of earthing installations in perspective
Voltage Stability and Reactive Power Provision in a Decentralizing Energy System: A Techno-economic Analysis
No full textElectricity grids require the ancillary services frequency control, grid operation, re-establishment of supply and voltage stability for a proper operation. Historically, conventional power plants in the transmission grid were the main source providing these services. An increasing share of decentralized renewable energy in the electricity mix causes decreasing dispatch times for conventional power plants and may consequently lead to a partial replacement of these technologies. Decentralized energy sources are technically capable of providing ancillary services. This work focuses on the provision of reactive power for voltage stability from decentralized sources. The aim is to answer the question of how voltage stability and reactive power management can be achieved in a future electricity system with increasing shares of decentralized renewable energy sources in an economical and efficient way. A methodology that takes reactive power and voltage stability in an electricity system into account is developed. It allows for the evaluation of the economic benefits of different reactive power supply options. A non-linear and a linearized techno-economic grid model are formulated for this purpose. The analysis reveals an increasing importance of reactive power from the distribution grid in future development scenarios, in particular if delays in grid extension are taken into account. The bottom-up assessment indicates a savings potential of up to 40 mio. EUR per year if reactive power sources in the distribution grid provide reactive power in a controlled manner. Although these savings constitute only a small portion of the total cost of the electricity system, reactive power from decentralized energy sources contributes to the change towards a system based on renewable energy sources. A comparison of different reactive power remuneration mechanisms shows that a variety of approaches exist that could replace the inflexible mechanisms of obligatory provision and penalized consumption of reactive power that are mostly in place nowadays
Series arc faults in low-voltage AC electrical installations
No full textThis thesis provides a complete analysis of all the topics related to series arc faults in low-voltage electrical installations. The part played by series arc faults in the outbreak of hazardous electrical fires is explained by using fire statistics, performing an electrical and thermal characterization of the phenomenon, and comparing with other electrical faults their ability to ignite an insulation material. The analysis demonstrates that the series arc fault generates a heat stress that largely exceeds the ignition threshold of insulation polymers. Other electrical faults such as bolted short-circuit, overload, earth leakage, and glowing fault result in much lower risks of fire ignition. The principles of arc fault detection are explained and the contribution of the active protection AFDD (arc fault detection device) is taken into account in this analysis. The option of achieving a passive protection using appropriate insulation materials is also explored. The performance requirements described in the product standard of AFDD are reviewed and recommendations for improving the standards are proposed.Diese Dissertation ist eine vollständige Analyse der Problematik des seriellen Störlichtbogens in der Niederspannungselektroinstallation. Der Beitrag von seriellen Störlichtbögen in der Entstehung von Elektrobränden wird mit Hilfe der Brandstatistik, der Durchführung der elektrischen und thermischen Charakterisierung des Phänomens und des Vergleichs der Fähigkeit von den anderen elektrischen Fehlern ein Isolierungspolymer zu entzünden, erklärt. Die Analyse beweist, dass der serielle Störlichtbogen eine thermische Beanspruchung erzeugt, die die Zündgrenzen der Isolierungspolymere weitaus überschreitet. Andere elektrische Fehler wie Kurzschlüsse, Überlast, Erdschlüsse und glühende Kontaktstellen ergeben ein wesentlich niedrigeres Brandrisiko. Die Funktionsweise vom Störlichtbogenschutzschalter (auch Brandschutzschalter oder AFDD genannt) wird erklärt und der Beitrag von diesem aktiven Schutz wird in der Analyse berücksichtigt. Der passive Schutz mit geeigneten Isolationsmaterialien wird auch untersucht. Die Anforderungen, die in der Produktnorm für Störlichtbogenschutzschalter beschrieben sind, werden überprüft und Verbesserungen für diese Normen werden vorgeschlagen
Analysis of the Impact of Reactive Power Control on Voltage Stability in Transmission Power Grids
No full textVoltage stability is the ability of a power system to maintain acceptable voltages at all buses under normal and abnormal operating conditions. The scarcity of reactive power or reactive power imbalance is the main reason of voltage instabilities. The energy supply of Germany will be dominated by renewable energy sources (RES) within energy transition actions leading to a decrease in conventional power plants especially in the transmission grid. Since the conventional power plants are still main reactive power sources, key challenge in the future will be the provision of system services such as control of reactive power to maintain voltage stability in the transmission networks.
A wide range of technology that is able to provide the required dynamic reactive power compensation is already available. However, in order to find the most effective implementation, it is necessary to investigate and compare these different technologies for a voltage stable grid operation. The main challenge regarding comparative studies that incorporate reactive power compensation devices is the development and the implementation of reliable comparison strategies.
This thesis analyses the impact of reactive power on the voltage stability phenomena both in long-term and short-term time frames under various grid situations. Voltage
stability margins in the long-term time frame are quantified by gradually increasing the reactive power injection into certain buses until the voltage collapses. Voltage stability in the short-term time frame is assessed by applying grid faults and simulating the time-domain grid response. A transient voltage severity ratio (TVSR) is developed in this thesis as the main comparison metric to evaluate and compare the performance of investigated compensation devices. Additionally, TVSR and other developed comparison metrics in this thesis are used to determine the optimal settings of the controller parameters of compensation devices. As a result, the submitted thesis shows a practical and reliable approach to analyze the performance of different compensation devices under different grid situations
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