ACTIVE CURRENT INJECTION METHOD FOR LIMITING GROUND FAULT CURRENT HARMONICS IN UNDERGROUND COAL MINES

Current practice in U.S. underground coal mine high-voltage distribution systems is to attempt to limit ground fault current to 25 Amperes and de-energize the circuit at 10 Amperes. However, the significant amount of system capacitance due to the use of shielded cables can cause ground fault current to be two or three times the intended ground fault limit. Consequently, this practice can cause several issues such as ground fault currents significantly exceeding the neutral grounding resistor current limit, loss of relay selectivity in the distribution system, and transient overvoltages in certain ground fault situations. These issues are solved to some extent by using a resonance grounded system, currently used in some other countries. However, a shortcoming of traditional resonance grounded systems is the inability to deal with the harmonic components existing in ground fault current. With the increasing use of nonlinear sources such as variable frequency drives, the proportion of harmonic components in ground fault current can be significant. Consequently, although the fundamental component can be almost fully compensated in a traditional resonance grounded system, the harmonic components can still be large enough to maintain arcing and cause personal injury and equipment damage. In this dissertation, a novel method is developed to perform real-time prediction of the harmonics in ground fault currents. Methods for neutralizing the ground fault current harmonics and identifying ground fault location are also developed. Results indicate that the combination of traditional high-resistance grounding and active current injection to neutralize harmonics in the ground fault has the potential to significantly reduce the total ground fault current and reduce arc and flash hazards during ground faults in high voltage distribution systems

Analysis and synthesis of concepts for hybrid power electronic earth fault compensators for medium voltage grids

In den vergangenen Jahren ist das Energieübertragungssystem mit der Zunahme der Produktion angewachsen. Jedes Jahr werden daher neue Übertragungsnetze in Betrieb genommen und die bereits bestehenden Übertragungsnetzwerke werden erweitert. In der Vergangenheit wurde der Strom in großen Kraftwerken zentral erzeugt und vom Hochspannungsnetz übertragen. Jetzt werden jedoch zunehmend auch große Mengen der elektrischen Energie vom NS- und MS-Netz übertragen. Die Anlagen zur Nutzung der erneuerbaren Energien sind grundsätzlich auf der Mittelspannungseben angeschlossen. Die modernen Netze müssen somit nicht nur mit einer schwankenden Stromerzeugung sondern auch mit verschiedenen Fehlern umgehen können. Mit wachsender Netzausdehnung steigt auch die Wahrscheinlichkeit für einen Fehlereintritt. Folglich müssen neue Verfahren entwickelt werden, um die Zuverlässigkeit und Stabilität der Netze auch im Fehlermodus zu verbessern. Derzeit werden oft kompensierte MS-Netze zum Schutz vor einphasigen Erdfehlern verwendet, wobei der Neutralleiter entweder über eine Drossel oder einen Widerstand mit der Erde verbunden ist. Damit kann der Fehlerstrom begrenzt und die Netze im Fehlerfall weiter betrieben werden. Gleichwohl haben auch die modernen passiven Kompensationsanlagen Probleme mit der Abstimmgenauigkeit, den Abmessungen sowie aufgrund der Komplexität des Antriebssystems. Moderne leistungselektronische Kompensationsanlagen werden zunehmend in MS-Netzen eingesetzt, um die Blindleistung zu kompensieren und nichtlineare Lastströme zu filtern. Sie können außerdem verwendet werden, um den Fehlerstrom zu kompensieren und eine optimale Ausnutzung der Übertragungskapazitäten der Leitungen zu ermöglichen. Da diese innovativen leistungselektronischen Kompensationsanlagen bei relativ hohen Frequenzen arbeiten, können außerdem wertvolle Materialien wie Kupfer und Stahl, die für die 50-Hz-Drosseln notwendig sind, eingespart werden. Diese Arbeit widmet sich der Entwicklung eines Hochleistungs-MS-Wechselrichters sowie dessen zur Kompensation notwendigen Steuerungssystems. Der Kompensator dient dabei zur Eliminierung des einpoligen Erdfehlerstromes (Grund- und Oberschwingungskomponenten) und kann daher im Übertragungssystem als Äquivalent der Petersonspule oder des Widerstands betrachtet werden. Der auf der Hilbert-Transformation basierende Steueralgorithmus wird ebenfalls erörtert.In the last years, the power generation systems have increased constantly with the increase in production. Every year new distribution networks are put into operation. The already existing networks are expanded. Moreover, in the past the power had been generated centrally in large power plants and transmitted by the high-voltage transmission grid, now vast amounts of the electric energy are handled by the low- and medium-voltage grid. The renewable energy sources are basically united in medium voltage grids. The modern grids has to be able to handle the fluctuating power generation and various sort of faults. With the growing grids the fault chance increases. Consequently, the new methods have to be developed to improve the reliability and stability of the grids in fault modes. Currently, to protect from one-phase ground faults the compensated networks are used with the neutral connected with the ground through the reactor or resistor. It allows to limit the fault current and the networks be able to be operated. Nevertheless, the modern compensation devices have the problems with the tuning accuracy, dimensions and the complexity due to the drive system. The modern power electronic devices are used in MV grids to compensate the reactive power (STATCOMs) and to filter the non-linear loads currents. They could be used to compensate the fault current and to allow the optimal utilization of lines as well. Moreover, since these converters operate at relatively high frequencies, valuable materials like copper and steal, used for 50 Hz reactors, can be saved. This work is dedicated to the development of a high-power medium-voltage power converter and its control system. This converter is used to compensate the one-phase ground fault current (main and high frequency components) and therefore is considered as the equivalent of the reactor or resistor in the classical system. The control algorithm based on the Hilbert transformation is proposed as well

Protection Against Ungrounded Single Phase Open Circuit Faults in 3-Phase Distribution Transformers

This thesis explores the impacts and behavior of 3-phase distribution transformers when subject to ungrounded single phase open circuit faults. A simple 3-phase system is modeled using MATLAB Simulink and operation under fault conditions are simulated and studied. Simulation results are confirmed via lab experimentation. Finally, a robust detection and protection method using neutral current injection (as proposed in industry literature) is built and demonstrated. Electric utility operating experience has demonstrated that all too often, loads on 3-phase distribution transformers are not adequately protected against an ungrounded single phase open circuit fault (commonly called “single phasing”). This type of fault is amongst the least understood and hence the least protected against. This is especially true at end of transmission system radial feeds where 3-phase transformers can re-create the opened phase voltage due to a variety of effects including magnetic coupling, voltage loops and loading effects. Operating experience in the nuclear power industry has shown that the results can be catastrophic especially considering the impacts to motor loads. Impacts can result in unavailability of emergency loads, tripping of motor protection circuits or even motor damage and failure

Designing antennas and RF components for upper millimeter frequencies using advanced substrate technology

Abstract. When shifting towards high frequency range, integration in the RF-front end becomes crucial. The ongoing planning of 6G communications systems causes a need to explore the possibilities beyond current 5G systems. To address the compactness and smaller sizes of the RF circuit components, the Integrated Passive Devices (IPD) multilayer technology provides us one solution to this problem. There are options already being tested in terms of implementing on-chip components, especially Antenna-in-Package (AiP) designs with a variety of different substrates. Among these technologies, Low Temperature Co-Fired Ceramic (LTCC) can be seen as a choice offering the freedom of multiple metal layers. IPD can be used for providing AiP solutions, as well as passive components such as baluns, filters, and power dividers. The main target of this thesis is to explore the possibilities and limitations for high frequency designs offered by IPD technology developed by (VTT) Technical Research Centre of Finland. The technology has already been tested at 20 GHz, but the focus was to reach the D-band (110–170 GHz) frequency range and subsequently up to even G-band (220–330 GHz). The technology utilizes 3 metal layers and a high resistivity silicon substrate (a lossy material). Starting off with simple transmission line structures (microstrip lines, strip lines and coplanar waveguides), the designs up to 330 GHz, provided information on the possibilities offered by this technology. After that, different AiP options were simulated with frequencies ranging from D- band to G- band. In addition to single elements, also antenna arrays were studied. Additionally, bandpass filters were designed. The dielectric thickness and the width and thickness of 3 the metal layers play a pivotal role in defining the performance of all the RF components designed using this technology. Furthermore, the size and pitch of the RF probe pads used to excite the structures show an impact on the overall behavior of the transmission lines.Antennien ja RF-komponenttien suunnittelu ylemmille millimetritaajuuksille edistynyttä substraattitekniikkaa käyttäen. Tiivistelmä. Siirryttäessä korkeammille taajuuskaistoille RF-etupään integrointi on entistä tärkeämpää. Käynnissä oleva kuudennen sukupolven (6G) viestintäjärjestelmien suunnittelu edellyttää nykyisiä 5G-järjestelmiä edistyksellisempien teknologisten mahdollisuuksien tarkastelua. Entistä pienempien RF-piirikomponenttien toteuttaminen vaatii uusia teknisiä ratkaisuja, ja yksi mahdollisuus komponenttien pienentämiseen on käyttää integroituihin passiivirakenteisiin (Integrated Passive Devices, IPD) pohjautuvaa monikerrosteknologiaa. Eri vaihtoehtoja on jo testattu sirulle sijoitettavien komponenttien toteuttamiseen eri substraattimateriaaleilla, etenkin paketoitujen antenniratkaisujen (Antenna-in-Package, AiP) suunnittelemiseksi. Eräs vaihtoehto IPD:lle on matalan lämpötilan yhteissintrattava keraamiteknologia (Low Temperature Co-Fired Ceramic, LTCC), joka mahdollistaa useamman metallikerroksen hyödyntämisen suunniteltaessa AiP-rakenteita sekä muita passiivikomponentteja (kuten symmetrointimuuntajia, suodattimia sekä tehonjakajia). Tämän opinnäytetyön päätavoitteena on tarkastella Valtion teknillisen tutkimuskeskuksen (VTT:n) kehittämän IPD-teknologian mahdollisuuksia ja rajoitteita korkean taajuuden rakenteiden suunnitteluun. IPD-teknologiaa on tähän mennessä testattu 20 GHz:n taajuudelle asti, mutta tässä työssä tarkoituksena on tutkia teknologiaa 110–170 GHz:n taajuuksille (D-kaista) sekä myöhemmin aina 220–330 GHz:iin saakka (G-kaista). Teknologia hyödyntää kolmea metallikerrosta sekä häviöllistä korkean ominaisvastuksen piisubstraattia. Yksinkertaisten siirtojohtorakenteiden (mikroliuskajohto, liuskajohto, koplanaarijohto) suunnittelu aina 330 GHz:n taajuudelle asti antoi tietoa teknologian mahdollisuuksista, minkä jälkeen erilaisia AiP-rakenteita simuloitiin D- ja G-kaistoilla. Yksittäisten antennielementtien ohella tarkasteltiin antenniryhmiä. Työssä suunniteltiin myös kaistanpäästösuodattimia. Käytettävissä olevien metalli- ja substraattikerrosten paksuudella sekä niiden mahdollistamilla liuskanleveyksillä on keskeinen rooli IPD-teknologialla suunniteltujen komponenttien suorituskyvyn kannalta. Lisäksi RF-mittapäiden kontaktikohtien koko ja välimatka vaikuttavat siirtojohtojen ominaisuuksiin

Fault Location in Grid Connected Ungrounded PV Systems Using Wavelets

Solar photovoltaic (PV) power has become one of the major sources of renewable energy worldwide. This thesis develops a wavelet-based fault location method for ungrounded PV farms based on pattern recognition of the high frequency transients due to switching frequencies in the system and which does not need any separate devices for fault location. The solar PV farm used for the simulation studies consists of a large number of PV modules connected to grid-connected inverters through ungrounded DC cables. Manufacturers report that about 1% of installed PV panels fail annually. Detecting phase to ground faults in ungrounded underground DC cables is also difficult and time consuming. Therefore, identifying ground faults is a significant problem in ungrounded PV systems because such earth faults do not provide sufficient fault currents for their detection and location during system operation. If such ground faults are not cleared quickly, a subsequent ground fault on the healthy phase will create a complete short-circuit in the system, which will cause a fire hazard and arc-flashing. Locating such faults with commonly used fault locators requires costly external high frequency signal generators, transducers, relays, and communication devices as well as generally longer lead times to find the fault. This thesis work proposes a novel fault location scheme that overcomes the shortcomings of the currently available methods. In this research, high frequency noise patterns are used to identify the fault location in an ungrounded PV farm. This high frequency noise is generated due to the switching transients of converters combined with parasitic capacitance of PV panels and cables. The pattern recognition approach, using discrete wavelet transform (DWT) multi-resolution analysis (MRA) and artificial neural networks (ANN), is utilized to investigate the proposed method for ungrounded grid integrated PV systems. Detailed time domain electromagnetic simulations of PV systems are done in a real-time environment and the results are analyzed to verify the performance of the fault locator. The fault locator uses a wavelet transform-based digital signal processing technique, which uses the high frequency patterns of the mid-point voltage signal of the converters to analyze the ground fault location. The Daubechies 10 (db10) wavelet and scale 11 are chosen as the appropriate mother wavelet function and decomposition level according to the characteristics of the noise waveform to give the proposed method better performance. In this study, norm values of the measured waveform at different frequency bands give unique features at different fault locations and are used as the feature vectors for pattern recognition. Then, the three-layer feed-forward ANN classifier, which can automatically classify the fault locations according to the extracted features, is investigated. The neural network is trained with the Levenberg-Marquardt back-propagation learning algorithm. The proposed fault locating scheme is tested and verified for different types of faults, such as ground and line-line faults at PV modules and cables of the ungrounded PV system. These faults are simulated in a real-time environment with a digital simulator and the data is then analyzed with wavelets in MATLAB. The test results show that the proposed method achieves 99.177% and 97.851% of fault location accuracy for different faults in DC cables and PV modules, respectively. Finally, the effectiveness and feasibility of the designed fault locator in real field applications is tested under varying fault impedance, power outputs, temperature, PV parasitic elements, and switching frequencies of the converters. The results demonstrate the proposed approach has very accurate and robust performance even with noisy measurements and changes in operating conditions

Methodology for 3D full-wave simulation of electrostatic breakdown across an air gap

The Rompe-Weizel SPICE model is used to obtain the time dependent arc resistance during simulation of air gap discharge. The SPICE model is solved using a circuit simulator, and the accompanying 3D model is solved using the transmission-line matrix time domain numerical method. Transient co-simulation is a new technique that is used to solve both circuit and 3D models at the same time. Transient co-simulation with the Rompe-Weizel SPICE model is first validated for different arc lengths using a simple geometry of a rod discharging to a ground plane. Validation is achieved by comparing the discharge currents from simulation with measurement. Next, a new simulation setup that uses a circuit switch along with the Rompe-Weizel model to capture the full physics of the Secondary ESD is tested. This simulation setup is tested by using an adjustable spark gap structure to generate Secondary ESD and validating it with measurements of the voltage across the gap and the discharge currents. Finally, the methodology is tested for practical usage by simulating the Secondary ESD in an actual smartphone product that is susceptible to secondary breakdown. The system level simulation predicts the coupling from ESD to a victim trace in the smartphone. Measurements performed at several stages of modeling the smartphone validate the simulation results. Using this novel methodology, the user can simulate secondary discharge in products to predict ESD damage and disruption on a system level --Abstract, page iv

Improved test methods for determining lightning-induced voltages in aircraft

A lumped parameter transmission line with a surge impedance matching that of the aircraft and its return lines was evaluated as a replacement for earlier current generators. Various test circuit parameters were evaluated using a 1/10 scale relative geometric model. Induced voltage response was evaluated by taking measurements on the NASA-Dryden Digital Fly by Wire F-8 aircraft. Return conductor arrangements as well as other circuit changes were also evaluated, with all induced voltage measurements being made on the same circuit for comparison purposes. The lumped parameter transmission line generates a concave front current wave with the peak di/dt near the peak of the current wave which is more representative of lightning. However, the induced voltage measurements when scaled by appropriate scale factors (peak current or di/dt) resulting from both techniques yield comparable results

Improved transient earth fault clearing on solid and resistance earthed MV netwworks

Includes bibliographical references.The aim of this thesis is to endeavour to develop, through a literature study, a method or methods whereby transient earth faults on neutral earthed MV networks may be cleared without customer supply interruptions, without compromising public safety and without compromising network integrity. In order to propose such a method, or methods, it is important to understand the various earthing practices employed in MV networks in terms of network behaviour under earth fault conditions, as this may influence network component insulation rating requirements, as well as the way in which such a system may function