Nu för tiden kablifieras nätet, vilket innebär att allt fler luftledningar ersätts med markkabel, en följd av stormar som åstadkommit stora och långa avbrott i nätet. Egenskaperna i elnätet förändras på grund av kablifieringen och beräkningarna för reläskyddsinställningarna för de riktade jordströmsskyddet som görs idag behöver därför ses över om de fortfarande kan utföras på samma sätt som tidigare. Detta för att säkerställa korrekt och selektiv felbortkoppling i händelse av fel. I föreliggande rapport förklaras i förenklade drag hur resistiva nollföljdsströmmar rör sig i nätet och hur de mäts in av jordfelsskydd. Ett fiktivt 22 kV nät har studerats och jordfel på två olika ledningar i detta nät analyseras och diskuteras. Beräkningar görs med olika storlekar på nollpunktsmotstånd samt olika höga resistiva nollföljdsbidrag från ledningarna vilket ger olika resistiva nollföljdsströmmar i nätet. Detta görs i både normaldrift och i reservdrift för att kunna studera vilka skillnader som faktiskt kan uppkomma. Diskussion sker om vad som hänt om hänsyn tagits till snedavstämning, obalans, vinkelfel och mätfel, vilket beräkningarna i detta arbete inte gör. Slutsatsen som dras av den utförda studien är att det är svårt att ställa in riktade jordströmsskydd samt nollpunktsspänningsskydd för att kunna säkerställa korrekt bortkoppling vid både normal- och reservdrift. Detta på grund av de olika resistiva nollföljdsströmmarna som uppkommer i nätet. / Today more and more overhead lines are replaced by underground cables as a result of storms which have caused major interruptions in the power grid. Because of the cabling, the characteristics of the power grid change and the calculations for the relay protection settings that are made today need to be reviewed if they still can be performed in the same way as before. This to ensure correct and selective fault disconnection. This report, in a simplified way, explains how resistive zero-sequence currents flow in the grid and how they are measured by earth fault protection. A fictitious 22 kV grid has been studied and earth faults on two different cables in the grid are analyzed and discussed. Calculations are made with different sizes of the zero-point resistance resulting in different resistive zero-sequence currents in the grid. This is done both in normal operation and reserve power operation to be able to study differences that can occur. Discussions are made regarding misalignment, imbalance, angle error and measurement error, which have not been included in this work. The conclusion from the study performed is that it is difficult to parameterize a directional earth fault protection and a zero-point voltage protection correctly to be able to secure a correct disconnection in both normal and backup operation. This is because of the various resistive zero-sequence currents that arise in the grid.

Alternative energy resources (such as photovoltaics, fuel cells, wind turbines, micro-turbines, and internal combustion engines) and energy storage systems (such as batteries, supercapacitors, and flywheels) are increasingly being connected to the utility grid, creating distributed energy resources which require the implementation of an effective distributed power management strategy. Parallel-connected power converters form a critical component in such a distributed energy resources system. This dissertation addresses small-signal modeling and analysis of parallel-connected power converter systems operating in distributed energy environments. This work focuses on DC-DC and DC-AC power converters. First, this work addresses the small-signal modeling and analysis of parallel-connected power converters in a battery/supercapacitor hybrid energy storage system. The small-signal model considers variations in the current of individual energy storage devices and the DC bus voltage as state variables, variations in the power converter duty cycles as control variables, and variations in the battery and the supercapacitor voltages and the load current as external disturbances. This dissertation proposes several different control strategies and studies the effects of variations in controller and filter parameters on system performance. Simulation studies were carried out using the Virtual Test Bed (VTB) platform under various load conditions to verify the proposed control strategies and their effect on the final states of the energy storage devices. Control strategies for single DC-AC three-phase power converters are also identified and investigated. These include a novel PV (active power and voltage) control with frequency droop control loop, PQ (active power and reactive power) control, voltage control, PQ control with frequency droop control, and PQ control with voltage and frequency droop control. Small-signal models of a three-phase power converter system with these control strategies were developed, and the impact of parameter variations on the stability of a PV controlled converter were studied. Moreover, a small-signal model of parallel-connected three-phase DC-AC power converters with individual DC power supplies and network is proposed. The simulations carried out in stand-alone and grid-connected modes verify the combined control strategies that were developed. In addition, a detailed small-signal mathematical model that can represent the zero-sequence current dynamics in parallel-connected three-phase DC-AC power converters that share a single DC power source is presented. The effects of a variety of factors on the zero-sequence current are investigated, and a control strategy to minimize the zero-sequence current is proposed. Time-domain simulation studies verify the results. Simulations of a parallel-connected DC-AC power converter system with nonlinear load were carried out. The active power filter implemented in this system provides sharing of harmonic load between each power converter, and reduces harmonic distortion at the nonlinear load by harmonic compensation.

No description available.

Ett viktigt uppdrag för Svenska kraftnät (SvK) som den systemansvariga myndigheten i Sverige är att driva elnätet på det bästa möjliga sättet. För att kunna garantera den önskade 99,9% tillför- litligheten behövs konstant underhåll och diagnostisering av kraftöverföringssystemets komponenter, varav elkrafttransformatorer är en viktig beståndsdel. Nollföljdsimpedansen behövs för att bestämma hur stora felströmmar som uppkommer när fel inträffar i systemet. Nollföljdsimpedansen är en av tre följder (sekvenser) som används för att analysera trefaskomponenter och är en storhet som inte kan bestämmas analytisk på ett enkelt sätt. Dess värde brukar fås från tester som görs av tillverkaren när transformatorn fabriceras. Denna information saknas dock för flera transformatorer som finns driftsatta i nätet. I detta examensarbete utvecklas empiriska modeller för att estimera nollföljdsimpedansen hos tre- och fembenta elkrafttransformatorer med tre (YY∆) och fyra lindningar (YYY∆), av vilka en av lindningarna är en utjämningslindning. En utjämningslindning är en deltakopplad lindning som icke är åtkomlig och som är avsedd för att minska och balansera nollföljdsimpedansvärdena för intilliggande lindningar. Nollföljdsimpedansen kan endast skattas eftersom en direkt mätning inte är möjlig hos dessa transformatorer. De erhållna modellerna bildar en estimeringsalgoritm, där olika transformatorstorheter kan användas som indata för att skatta nollföljdsimpedansvärdet. Metodiken för att ta fram modellerna bygger på en analys av korrelationen mellan nollföljdsim- pedansen och andra kända och beräknade storheter, där en linjär regression baserad på minsta kvadratmetoden resulterar i en approximativ trendlinje. Approximationen förbättras med en bisquare algoritm, där avvikande värden ges mindre vikt. Denna process utförs för alla transformatorer i stu- dien där generella anpassningsmodeller tas fram. Processen upprepas för särskilda undergrupper med syfte att erhålla modeller med högre anpassningsgrad. Indelning i undergrupper baseras på kopp- lingsart för transformatorernas sekundärlindning som består av antingen fullindad eller autokopplad lindning, samt kärnkonstruktion som består av tre- eller fem ben. Tillförlitligheten för varje modell analyseras med hänsyn till deras anpassningsgrad, statistisk relevans samt kvaliteten av data som användes för att utveckla modellen. Endast modeller med måttlig anpassningsgrad (goodness of fit) och som baseras på ett tillräckligt stor antal enheter för att betraktas som statistisk representativ för målgruppen, inkluderas i algoritmen. Generellt sätt följer data en normalfördelning som tyder på få extrema värden. Följaktligen, anses data ha en bra distribution för att avgöra dess trend. Modellernas prestanda för transformatorer med tre lindningar testades mot ett förväntat värde för nollföljdsimpedansen, baserat på en skattning som för närvarande används av Svk. Testresultaten vissa modeller medan för andra inträffar avvikelser. I detta projekt används LAB som utvecklingsplattform för alla analyser / An important comission for Svenska kraftnät (SvK) as the Swedish authory in charge of the power distribution network is to operate the grid in the most efficient way. In order to ensure 99,9% uptime, constant maintenance and fault diagnostics are required for the entire network of which power transformers are an important component. The zero-sequence impedance is a way to determine the maximum fault currents a transformer can accept before being damaged. The zero- sequence impedance is one of three sequences used to analyze three-phase systems and is a quantity which is not easily obtained analytically. Its value is usually determined in tests carried out during the fabrication process. This information is not available for several transformers currently in service on the power grid. The focus of this thesis is to develop empirical models to estimate the value of the zero-sequence impedance for three- and five limb power transformers with three (YY∆) and four windings (YYY∆) including a stabilizing winding. A stabilizing winding is a delta-connected winding which is not brought out; its purpose is to reduce and balance the zero-sequence impedance in adjacent windings. The value for the zero-sequence impedance can only be estimated since a direct measurement on the winding is not possible for this type of transformers. The resulting models are part of an algorithm where different transformer quantities are used as inputs to estimate the value of the zero-sequence impedance for a given transformer. The method used to obtain these models is based on an analysis of the correlation between the zero-sequence impedance and other known and calculated quantities where a linear regression based on the least-squares method produces an approximative trend line. This approximation is made more robust with a bisquare-weight algorithm where less weight is given to diverging data points. This process is carried out for all the transformers included in this study where general models are obtained. The process is then repeated for specific subgroups of transformers with the goal of deriving more accureate models for these subgroups. The subdivision criteria is based in coupling- and core type which includes an0- and yn0-couplings and three- and four limb cores respectively. The reliability of each model is analayized with respect to their goodness of fit, statistical relevance and the quality of the data used to develop the model. Only models with a substantial goodness of fit and which were based on a sufficiently large number of transformers to be considered statistically representative of the target group are included in the final algorithm. In general, all the data analyzed follows a normal distribution which indicates that there are few extreme values. Consequently, the data used in this study is condsidered to be appropriately distributed to determine its trend. The performance of the models for transformers with three windings was tested against the expected values for the zero-sequence impedance based on the approximation algorithm which is currently being used by Svk. The test results from some of the models match the expected values while oth deviations. MATLAB is used in this projetct as the development platform for all analyses

This dissertation studies modeling and control issues of parallel three-phase pulse-width modulated (PWM) converters. The converters include three-phase boost rectifiers, voltage source inverters, buck rectifiers and current source inverters. The averaging of the parallel converters is performed based on a generic functional switching unit, which is called a phase leg in boost rectifiers and voltage source inverters, and a rail arm in buck rectifiers and current source inverters. Based on phase-leg and rail-arm averaging, the developed models are not only equivalent to the conventional three-phase converter models that are based on phase-to-phase averaging, but they also preserve common-mode information, which is critical in the analysis of the parallel converters. The models reveal such parallel dynamics as reactive power circulation and small-signal interaction. A unique feature of the parallel three-phase converters is a zero-sequence circulating current. This work proposes a novel zero-sequence control concept that uses variable zero-vectors in the space-vector modulation (SVM) of the converters. The control can be implemented within an individual converter and is independent from the other control loops for the converter. Therefore, it greatly facilitates the design and expansion of a parallel system. Proper operation of the parallel converters requires an explicit load-sharing mechanism. In order to have a modular design, a droop method is recommended. Traditionally, however, a droop method has to compromise between voltage regulation and load sharing. After parametric analysis, a novel droop method using a gain-scheduling technique is proposed. The numeric analysis shows that the proposed droop method can achieve both good voltage regulation and good load sharing. An interleaving technique is often used in parallel converter systems in order to reduce current ripples. Because of its symmetrical circuit structure, the parallel three-phase converter system can reduce both differential-mode and common-mode noise with a center-aligned symmetrical SVM. Based on the concept that a symmetrical circuit can reduce common-mode dv/dt noise, a conventional three-phase, four-leg inverter is modified so that its fourth leg is symmetrical to the other three legs. The common-mode dv/dt noise can be practically eliminated with a new modulation strategy. Meanwhile, with a modified control design, the new four-leg inverter still can handle low-frequency common-mode components that occur due to unbalanced and nonlinear load. / Ph. D.

This master´s thesis deals with the indication and localization of earth faults in overhead lines. Earth fault is the most frequently occurring type of fault in medium voltage overhead lines – it covers approx. 95% of all faults and is very difficult to indicate and localize them correctly and in time with currently available methods on the market. Therefore is very important to study earth fault and its indication, localization. The thesis consists of a theoretical and a practical part. The theoretical part deals with faults in overhead networks with different type of neutral grounding, mainly with one phase to the ground fault in the compensated, ungrounded, solidly grounded and via resistance grounded networks. Most of the theoretical part is dedicated to one phase to the ground fault in the compensated and ungrounded networks, where this type fault is called the earth fault. In the compensated and ungrounded networks is described in details behavior – voltage and current relations during both steady state and transient state earth fault. The theoretical part is further dedicated to detection methods of earth faults and their preconditions for use. There is described also in details the complete procedure of earth fault detection, which includes indication, unhealthy feeder determination and exact position or line section localization. End of the theoretical part is then focused on determination of accuracy requirements for measurement of basic quantities and computation of other parameters. The practical part deals with a work at medium distribution network model, which includes familiarization with the model, detailed verification of its functionality and behavior during the earth fault, obtaining faults records and algorithmization of methods: method of qu – diagram and method of first half - period, which are able to detect unhealthy feeder. This part of the thesis was put together based on a demand of company Mega, corp., which wanted to verify function of both above mentioned and by them not yet tested methods.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nowadays, the quality of the electric energy emerges as an important subject for both the consumers and the power utilities. In this context, it is recognized the existence of national and international guide lines towards the establishment of boundary or reference values for the power indexes so as to guarantee the desired standard of voltage quality. Amongst these indicators, the harmonic distortions appear as a quite important matter and the mitigation techniques to eliminate or to reduce the level of distortions have been largely discussed in the literature. The use of classical passive and active filters are the most traditional solutions, however, some authors have pointed the possibility of using a different approach based on electromagnetic devices. Focusing this subject, this thesis investigates the use of two distinct filtering devices for harmonic filtering. The first proposal is related to the well known zero sequence harmonic components and the second to the positive and negative components. The work considers the electromagnetic filter physical principles, modeling, computational implementation, investigation of ideal and non ideal operational conditions, laboratory scale prototypes and validation tests. / A qualidade da energia elétrica se apresenta como uma área de grande interesse para os consumidores e concessionários de energia. Buscando meios para regulamentar o assunto, reconhece-se, na atualidade, a existência de recomendações, normas e critérios nacionais e internacionais voltados para a orientação dos valores limites ou de referência para os diversos indicadores que caracterizam o padrão de qualidade desejado. Dentro deste contexto, as distorções harmônicas nos sinais de tensão e corrente surgem como uma das questões mais preocupantes. Almejando, pois a preservação da qualidade do suprimento no que tange às distorções harmônicas, assim como para os demais itens de definição da qualidade, faz-se, muitas vezes necessário a utilização de técnicas para a mitigação dos fenômenos prejudiciais ao atendimento dos padrões definidos. Focando a questão do controle das distorções harmônicas, surge esta proposta de tese, a qual enfoca a utilização de duas metodologias distintas, baseadas em arranjos eletromagnéticos para a filtragem de correntes harmônicas. Uma das propostas destina-se a filtragem das correntes harmônicas de seqüência zero e outra voltada para as componentes de seqüência positiva e negativa. O trabalho apresenta os princípios de funcionamento, modelagens, análises computacionais e validação experimental dos dispositivos supramencionados. Os estudos são iniciados através da operação dos filtros sob condições ideais, seguido de investigações associados com situações não ideais de funcionamento. / Doutor em Ciências

Durant ces dernières années, l'intérêt pour les technologies des véhicules à faibles émissions de carbone a fait un bond important à travers l'Union européenne (UE) et au-delà, encouragé en cela par les gouvernements et les constructeurs automobiles. De grands espoirs ont été mis plus récemment dans les véhicules électriques (VE) et les véhicules électriques hybrides (VEH) en tant que technologies clés pour atténuer le changement climatique, améliorer la sécurité énergétique et favoriser une nouvelle branche de l'industrie dans le secteur automobile. Ainsi, l'électrification des transports a été considérée comme une stratégie clé pour réduire les émissions de CO2 dans le secteur des transports. Le principal défi est d’augmenter l’autonomie des véhicules (ce qui a toujours été au coeur de la concurrence des industries du transport), ainsi que la durée de vie des volumineuses et coûteuses batteries. Par conséquent, ceci indique que le rendement du convertisseur de puissance est un des points clés à développer pour les générations des véhicules électriques à venir. L’autre paramètre influant est la qualité de la tension et du courant (en particulier la suppression des harmoniques basses fréquences) qui permet de réduire la taille des filtres d'entrée et de sortie de ces convertisseurs. L'objectif de cette thèse est de parvenir à un meilleur rendement en proposant de nouvelles structures de convertisseur de puissance et des commandes vectorielles modifiées ; le choix de deux onduleurs alimentant un moteur ouvert aux deux extrémités. Après l'analyse étape par étape, modèle théorique, simulation et enfin une mise en oeuvre expérimentale, il a été constaté que les nouvelles méthodes proposées sont compétitives et peuvent s’appliquer aux cas des VEH et des VE afin d’apporter des caractéristiques supérieures en termes d’efficacité et de qualité de tension et de courant. / Over the last decade, the interest for low-carbon vehicle technologies has surged among both governments and automotive manufacturers across and beyond the European Union (EU). Great hopes have been put, first, on biofuel vehicles and more recently on electric vehicles (EVs) and hybrid electric vehicles (HEVs) as key technologies to mitigate climate change, enhance energy security and nurture new industry branches within the automotive sector. So electrification of vehicles has been seen as a key strategy to reduce CO2 emissions from the transport sector. The main challenge toward EVs and HEVs is to keep driving for longer distance (which has been always fields for competition among traction industries) as well as lifetime battery cells as storage system. As a result, these indicate importance of power converter efficiency as a key gate for next generations of these up-coming vehicles. The next parameter is the quality of output voltage/current (especially by suppressing low-order harmonics) to reduce the size of filtering. The aim of this thesis is to achieve better efficiency and output voltage/current Total Harmonic Distortion (THD) by proposing novel power converter and associated Pulse Width Modulation (PWM) methods while imposing modification on power converter topology. As a result, dual-inverter is proposed to supply open-end motor from both sides. To this aim, three PWM methods are suggested as: The first one, Modified Space Vector Modulation (MSVM) for dual-inverter supplied by single dc source, improves efficiency by 4-5% (while having lower switching losses), and reduces Common Mode Voltage (CMV) levels by 66%, as well. The voltage/current harmonics are analytically analyzed which shows mainly better performance. Effective switching frequency is also reduced by 66% due to the reduction of number of commutations. In the second one, Near State PWM (NSPWM) is adapted for dual-inverter supplied by single dc source in order to eliminate triplen harmonics (therefore Zero Sequence Voltage, ZSV) and improve efficiency (by 3-4%) compared to Space Vector Modulation (SVM). Additionally due to avoiding use of zero vectors, CMV is improved by 66%. While having 8 commutations instead of 12 in SVM, effective switching frequency is improved by 33%. And finally, the third proposed method deals with NSPWM for dual-inverter supplied by two isolated dc sources wherein efficiency and CMV levels show the same performance as previous one. However, in this method, voltage THD is highly reduced compared to SVM. Triplen harmonics of the output voltage are inherently suppressed by the structure. These 3 proposed methods are analytically studied and their performances are step by step simulated in Matlab/Simulink environment. Then the methods are implemented in dualinverter fed open-end motor in laboratory setup; and the results are compared with these of SVM. Finally, it is found that novel proposed methods are so competitive solutions to be applied in HEVs and EVs and bring superior efficiency and voltage/current harmonic features.

This thesis is engaged in a theoretical calculation of short-circuit currents in electric power system in compliance with the norm ČSN 60909-0 and first of all with calculation of dynamical and thermal effects of short-circuit currents in HV switching stations in accordance with the norm ČSN 60865-1. Next the thesis is engaged in a production of programs for simulation of mechanical effects of short-circuit currents with respect to a dimensioning of constructional elements of switching station and afterwards in a practical simulation of selected parts of the real switching station type „H“ in Slavkov u Brna.

Parallellgående kraftledningar inducerar spänning i varandra som i sin tur driver en ström. I denna rapport utreds hur osymmetriska kortslut-ningar på en kraftledning påverkar strömmen i en parallellgående kraft-ledning på grund av ömsesidig induktion och vilka faror inducerad spän-ning och den av inducerade spänningen drivna strömmen kan leda till, ur hänsyn till leveranssäkerhet samt underhållsarbetare på parallell kraft-ledning.Målet med examensarbetet är att öka noggrannheten för felströmsberäk-ningar på Vattenfalls elnät samt att öka kunskapen för vattenfallpersonal om fenomenet av ömsesidig koppling och de faror den kan innebära.Rapporten kunde konstatera att ömsesidig koppling kan leda till faror för leveranssäkerheten av elnätet och att det finns ett behov av analys av specifika fall av parallella kraftledningar i Vattenfalls elnät. / Parallel overhead power lines induce voltage in one another, which in turn drives a current. In this report an investigation is made in how asymmetrical short circuits on an overhead power line influence the cur-rent in a parallel overhead power lines due to the inductive coupling be-tween the power lines and which hazards it can cause for the electrical delivery dependability as well as high voltage workers.The aim with this bachelor thesis is to increase the accuracy in fault cur-rent calculation in the Vattenfall electricity grid as well as to increase the knowledge within Vattenfall of the phenomena of inductive coupling and the hazards which comes with it.The report could ascertain that inductive coupling can lead to danger for electrical delivery dependability and that there exists a need of analysis on specific parallel overhead power lines within the Vattenfall electricity grid.