Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Antennas parameterization for the detection of partial discharges

Partial discharge (PD) detection is a widely extended technique for electrical insulation diagnosis. Ultrahigh-frequency detection techniques appear as a feasible alternative to traditional methods owing to their inherent advantages such as the capability to detect PDs online and to locate the piece of equipment with insulation problems in substations and cables. In this paper, four antennas are thoroughly studied by means of their theoretical and experimental behavior when measuring electromagnetic pulses radiated by PD activity. The theoretic study of the band of frequencies in which the pulse emits and the measurement of the parameters S11 are complemented with the frequency response and wavelet transform of a set of 500 time signals acquired by the antennas, and the results are analyzed in detail.This work was supported by the Spanish Science and Technology Ministry under Contracts DPI 2009-14628-C03-02 and TEC 2011- 29006-C03-03.Publicad

Partial discharges location in power transformers using piezoceramic sensors

The detection and the spatial localization of partial discharges in high-voltage electrical machines are considered as an effective method in predictive maintenance that can provide valuable information on the health of the insulation system and allow to determine accurately the location of the risky insulation elements, which in turn will avoid any premature equipment’s deterioration by scheduling preventive maintenance action. After confirming in a previous published paper the efficiency of a new generation of piezoceramics sensors (high temperature ultrasonic transducers) to detect and characterize partial discharges, we are going to investigate, in this work, a second potential of this technology to locate the partial discharge sources by relying on its ability to detect acoustic signals emitted by partial discharge sources. We will present experimental results, demonstrating the effectiveness of these sensors to locate partial discharges sources and, we will also present an algorithm for calculating the partial discharge foci, based on the acoustic wave flight time

Online condition monitoring of MV cable feeders using Rogowski coil sensors for PD measurements

Condition monitoring is a highly effective prognostic tool for incipient insulation degradation to avoid sudden failures of electrical components and to keep the power network in operation. Improved operational performance of the sensors and effective measurement techniques could enable the development of a robust monitoring system. This paper addresses two main aspects of condition monitoring: an enhanced design of an induction sensor that has the capability of measuring partial discharge (PD) signals emerging simultaneously from medium voltage cables and transformers, and an integrated monitoring system that enables the monitoring of a wider part of the cable feeder. Having described the conventional practices along with the authors’ own experiences and research on non-intrusive solutions, this paper proposes an optimum design of a Rogowski coil that can measure the PD signals from medium voltage cables, its accessories, and the distribution transformers. The proposed PD monitoring scheme is implemented using the directional sensitivity capability of Rogowski coils and a suitable sensor installation scheme that leads to the development of an integrated monitoring model for the components of a MV cable feeder. Furthermore, the paper presents forethought regarding huge amount of PD data from various sensors using a simplified and practical approach. In the perspective of today’s changing grid, the presented idea of integrated monitoring practices provide a concept towards automated condition monitoring.fi=vertaisarvioitu|en=peerReviewed

Online condition monitoring of MV cable feeders using Rogowski coil sensors for PD measurements

Condition monitoring is a highly effective prognostic tool for incipient insulation degradation to avoid sudden failures of electrical components and to keep the power network in operation. Improved operational performance of the sensors and effective measurement techniques could enable the development of a robust monitoring system. This paper addresses two main aspects of condition monitoring: an enhanced design of an induction sensor that has the capability of measuring partial discharge (PD) signals emerging simultaneously from medium voltage cables and transformers, and an integrated monitoring system that enables the monitoring of a wider part of the cable feeder. Having described the conventional practices along with the authors' own experiences and research on non-intrusive solutions, this paper proposes an optimum design of a Rogowski coil that can measure the PD signals from medium voltage cables, its accessories, and the distribution transformers. The proposed PD monitoring scheme is implemented using the directional sensitivity capability of Rogowski coils and a suitable sensor installation scheme that leads to the development of an integrated monitoring model for the components of a MV cable feeder. Furthermore, the paper presents forethought regarding huge amount of PD data from various sensors using a simplified and practical approach. In the perspective of today's changing grid, the presented idea of integrated monitoring practices provide a concept towards automated condition monitoring.This work is done under the project Smart Condition Monitoring of Power Grid that is funded by the Academy of Finland (Grant No. 309412)

Development of a quantitative health index and diagnostic method for efficient asset management of power transformers

Power transformers play a very important role in electrical power networks and are frequently operated longer than their expected design life. Therefore, to ensure their best operating performance in a transmission network, the fault condition of each transformer must be assessed regularly. For an accurate fault diagnosis, it is important to have maximum information about an individual transformer based on unbiased measurements. This can best be achieved using artificial intelligence (AI) that can systematically analyse the complex features of diagnostic measurements. Clustering techniques are a form of AI that is particularly well suited to fault diagnosis. To provide an assessment of transformers, a hybrid k-means algorithm, and probabilistic Parzen window estimation are used in this research. The clusters they form are representative of a single or multiple fault categories. The proposed technique computes the maximum probability of transformers in each cluster to determine their fault categories. The main focus of this research is to determine a quantitative health index (HI) to characterize the operating condition of transformers. Condition assessment tries to detect incipient faults before they become too serious, which requires a sensitive and quantified approach. Therefore, the HI needs to come from a proportionate system that can estimate health condition of transformers over time. To quantify this condition, the General Regression Neural Network (GRNN), a type of AI, has been chosen in this research. The GRNN works well with small sets of training data and avoids the needs to estimate large sets of model parameters, following a largely non-parametric approach. The methodology used here regards transformers as a collection of subsystems and summarizes their individual condition into a quantified HI based on the existing agreed benchmarks drawn from IEEE and CIGRE standards. To better calibrate the HI, it may be mapped to a failure probability estimate for each transformer over the coming year. Experimental results of the research show that the proposed methods are more effective than previously published approaches when diagnosing critical faults. Moreover, this novel HI approach can provide a comprehensive assessment of transformers based on the actual condition of their individual subsystems

Multichannel instrumentation system with optical fiber heterodyne interferometer: application to the detection and localization of ultrasound acoustic emissions generated by partial discharges

Mención Internacional en el título de doctorLa monitorización en línea de los transformadores resulta imprescindible para garantizar su funcionamiento seguro y estable. Para este propósito, uno de los métodos más fiables es la observación de la actividad de descargas parciales. Éstas son un tipo de descargas producidas por rupturas localizadas en el aislamiento eléctrico que lo cortocircuitan parcialmente. Su actividad está involucrada en el proceso de degradación del sistema de aislamiento y proporciona información acerca del estado interno de los transformadores, la cual se utiliza para su mantenimiento predictivo y diagnóstico. Las descargas parciales generan emisiones de ultrasonidos que se utilizan para medir su actividad mediante técnicas acústicas. De este modo, un fallo en el aislamiento se manifiesta como una fuente de emisión acústica. La monitorización acústica de descargas parciales no sólo detecta la actividad, sino que además aprovecha este comportamiento para localizar los fallos, siempre y cuando se utilicen varios sensores espacialmente distribuidos. Para ello es imprescindible disponer de sistemas de instrumentación multicanal capaces de obtener medidas de los tiempos de llegada de las señales acústicas a los múltiples sensores y de utilizarlas en un algoritmo de localización para encontrar las posiciones de las fuentes de emisión acústica. La monitorización acústica de descargas parciales se realiza normalmente con sensores piezoeléctricos colocados en las paredes del tanque del transformador. Sin embargo, este método presenta inconvenientes derivados de que la instalación de los sensores en el exterior: las señales detectadas son débiles y están distorsionadas. Además, se producen múltiples caminos de llegada de las señales acústicas a los sensores y reflexiones en las paredes del tanque, lo cual dificulta la localización de descargas parciales. Con el fin de evitar estos inconvenientes, en la última década se han propuesto nuevos sensores que pueden instalarse en el interior del tanque del transformador. Estos sensores emplean tecnología de fibra óptica para poder soportar las condiciones que se presentan en dicho entorno. Se han propuesto varios tipos: sensores basados en cavidades Fabry-Perot, sensores interferométricos de fibra óptica y sensores basados en redes de Bragg en fibra. Con el desarrollo de estos sensores internos de fibra óptica también se ha despertado un interés creciente en investigar nuevos sistemas de instrumentación multicanal para detectar y localizar las descargas parciales con sensores de fibra óptica. Esta tesis está dedicada a ambos temas. El primer aporte principal es el diseño y desarrollo de un sistema de instrumentación interferométrico multicanal para la monitorización acústica de descargas parciales y su localización usando sensores de fibra óptica. El diseño del sistema multicanal está basado en un interferómetro heterodino de fibra óptica siguiendo un esquema de Mach-Zehnder con múltiples brazos de medida en paralelo y un brazo de referencia común. Este sistema interferométrico multiplexado se ha demostrado con una implementación de cuatro canales en la que se obtiene muy alta resolución en la medida de presión acústica de ultrasonidos (1 Pa @ 150 kHz). También se ha demostrado experimentalmente la capacidad que tiene para localizar las fuentes de emisión acústica por medio de la medida simultánea con varios sensores. Adicionalmente, en estos experimentos de localización se ha añadido ruido sintético a la medida de los tiempos de llegada con el fin de evaluar las prestaciones del sistema. En los resultados de localización se obtiene la posición de la fuente de emisión acústica con una desviación típica menor que 1 cm en cada uno de los ejes (~1% de la dimensión normalizada del tanque). El segundo aporte principal es la investigación sobre una nueva sonda de fibra óptica y su desarrollo, la cual es capaz de medir descargas parciales acústicamente en el interior y es compatible con el sistema interferométrico multicanal. El diseño de la cabeza del sensor está basado en una bobina de fibra óptica compuesta por múltiples capas. Este diseño es compacto, robusto y de bajo coste. Se obtuvo una sensibilidad adecuada para la detección acústica de descargas parciales utilizando 17 m de fibra dispuestos en una bobina con 5 capas. La sonda de fibra óptica, junto con el interferómetro heterodino, tiene la capacidad de detectar las emisiones acústicas generadas por descargas parciales a 150 kHz con una resolución de 1 Pa dentro de un rango de 17 kPa. Se ha realizado una caracterización completa de la sonda de fibra óptica en una plataforma diseñada para ensayos acústicos de ultrasonido, en la cual se pueden reproducir emisiones acústicas con características similares a las generadas por las descargas parciales. También se ha caracterizado la sonda de fibra óptica en experimentos de alta tensión, en los cuales se han generado diversos tipos de descargas parciales en aceite de transformador y bajo diferentes condiciones. En estos experimentos se midió con la sonda en instalaciones eléctricas de alta tensión en AC, con descargas parciales internas y superficiales, y también en instalaciones eléctricas de alta tensión DC. Por lo tanto, se ha demostrado su efectividad para detectar descargas parciales en un rango amplio de condiciones. Es importante resaltar que en los experimentos de alta tensión se demostró por primera vez la detección acústica de descargas parciales internas con un sensor de fibra óptica.On-line monitoring of power transformers is essential to guarantee their stable and safe operation. Partial discharges monitoring is one of the reliable methods used for this purpose. Partial discharges are local electrical breakdowns that only partially break the insulation. Their activity is involved in the degradation process of the transformers and provides information about the internal conditions of the insulation system, which is used for the diagnosis of power transformers. Partial discharges generate ultrasound acoustic emissions that are used for the acoustic measurement of their activity. In other words, a fault in the insulation behaves as a localized acoustic emission source. The acoustic monitoring of partial discharges exploits this characteristic in order to localize faults in the insulation system. The localization of partial discharges is only possible using a multichannel system able to obtain the times of arrival of the acoustic signals at the multiple sensors and use them in a computational localization algorithm. The conventional acoustic monitoring of partial discharges is done with piezoelectric sensors mounted on the transformer tank walls. However, the method suffers from some problems due to the external installation of the sensors. The detected signals are weak and distorted. There are multiple paths to the sensors and reflections at the tank walls. In the last decade, new sensors have been proposed for their internal installation within the transformer tank in order to avoid such problems. These sensors use optical fiber technology to withstand the harsh environment inside the transformers. A variety of them have been proposed: sensors based on Fabry-Perot cavities, fiber-optic interferometric sensors and sensors based on fiber Bragg gratings. With the development of these fiberoptic internal sensors a new research interest has also arisen about the development of new multichannel instrumentation systems for the localization of partial discharges using fiber-optic sensors. This dissertation is devoted to these both topics. A major contribution is the design and development of a multichannel interferometric instrumentation system for acoustic monitoring of partial discharges and their localization by using fiber-optic sensors. The design of the multichannel system is based on a fiber-optic heterodyne interferometer that is configured as a Mach-Zehnder scheme with multiple sensing arms in parallel and a common reference arm. This interferometric multiplexing system has been demonstrated with four channels in which a high resolution is obtained in the measurement of acoustic pressure (1 Pa @ 150 kHz). The capability of this system for localizing the acoustic emission sources by means of the simultaneous measurement with multiple sensors has also been demonstrated experimentally. In addition, in these experiments synthetic noise was added to the measurements of the times of arrival in order to evaluate the performance of the system. The results of the acoustic source localization show a standard deviation better than 1 cm in each axis (~1% of the normalized dimension of the tank). The second major contribution is the research and development of a fiber-optic probe, compatible with the multichannel interferometric system. The design of the fiberoptic sensing head is based on a coil of optical fiber with multiple layers. It is compact, rugged and cost-effective. An adequate sensitivity was achieved with 17 m of fiber disposed in a coil with 5 layers. Combined with the heterodyne interferometer the fiberoptic interferometric sensor is able to detect the acoustic emission of partial discharges at 150 kHz with resolution of 1 Pa in a range of up to 17 kPa. The complete characterization of the fiber-optic probe was performed in an acoustic test bench that was designed for ultrasound tests with acoustic emissions that reproduce the characteristics of those generated by partial discharges. The fiber-optic sensing probe has also been characterized in high voltage experiments, where different types of partial discharges were generated in transformer oil and in different conditions. It was tested in high voltage AC setups, with internal partial discharges, with surface partial discharges, and also in high voltage DC setups, thus its effectiveness for detecting partial discharges under a wide range of conditions was demonstrated. It is worth mentioning that the acoustic detection of internal partial discharges in a transformer with a fiber-optic sensor was demonstrated for the first time.El presente trabajo de investigación ha sido financiado por el Ministerio de Ciencia e Innovación dentro del proyecto coordinado “Sistema de instrumentación multicanal para la medida de descargas parciales en campo mediante la integración de sensores acústicos y eléctricos. Aplicación a su localización y análisis en transformadores de alta potencia,” (DPI2009-14628-C03-0, coordinador Prof. José Antonio García Souto), y el subproyecto “Interferómetro multicanal con fibra óptica para medida de descargas parciales en el interior de transformadores. Nueva instrumentación de ultrasonidos para detección y localización continua en campo,” (No. DPI2009-14628-C03-01, I.P. Prof. José Antonio García Souto). Las medidas acústicas de DP en experimentos con HVAC fueron realizadas en el Laboratorio de Investigación y Ensayos en Alta Tensión (LINEALT) de la Universidad Carlos III de Madrid, en colaboración con el Grupo de diagnóstico de máquinas eléctricas y materiales aislantes (DIAMAT), Departamento de Ingeniería Eléctrica, dentro del contexto del proyecto coordinado (DPI2009-14628-C03-0). Las medidas acústicas de DP en experimentos con HVDC fueron realizadas en colaboración con el Departamento de Ingeniería Eléctrica de la Universidad de Bolonia. Estas fueron llevadas a cabo durante una estancia de investigación de I. Búa-Núñez y una visita corta del autor, bajo la supervisión del Prof. Andrea Cavallini en 2013. En colaboración con INESC – Porto, Portugal, durante una estancia de investigación del autor en 2013, bajo la supervisión del Prof. José Luis Santos se realizó una investigación enfocada en los sistemas rápidos de interrogación de sensores FBG para la medida de ultrasonidos. En colaboración con el Applied Optics Group de University of Kent, UK, bajo la supervisión del Prof. David A. Jackson y durante una estancia de investigación de tres meses realizada por el autor en 2014, se realizó una investigación enfocada al desarrollo de un interferómetro heterodino de FO para la medida simultánea con cuatro canales.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: José Miguel López Higuera.- Secretario: María Carmen Vázquez García.- Vocal: Brian Culsha

Comparative evaluation of acoustic and electric signals of partial discharges

Failures of power electric components such as transformers and outages can lead to a huge economical loss in the electric power grid. One of the main parts of a power electric components is the insulation system, namely, insulation oil, impregnated pressboard and paper. Several methods exist for diagnostics of these insulation materials. Partial discharge (PD) measurement known as one of the main non-destructive monitoring systems of the insulation materials. However, it has been mainly done off-line in maintenance periods, and the existing on-line methods generally provide less information due to environment electric noises. In contrast to electric PD measurement system, the acoustic emission (AE) measurement system is well known for its immunity against environment electrical noises. In this thesis comparative evaluation of acoustic and electric signals of PD events generated in oil impregnated pressboard and papers is investigated. The thesis is focused on the characteristic of PD activity and the consequence of that on the electric and AE signal. PD classification is defined by using the relation between acoustic and electric signals of PD events. Although the sensitivity of the AE sensors has been improved over the years, but the detection of the acoustic signals from PD activity in power equipment mainly transformers remain the main challenge of acoustic measurement. Lack of information regarding evaluation of electric PD signals and AE signals beside the mechanical attenuation are two main disadvantages of AE measurement method. Due to mechanical and electrical mechanism of waves generated during PD activities, the mechanical and electrical behaviour of the waves is discussed in more detail to have better understanding about the electric and acoustic signals. PD sources were generated at different electrode configurations such as needle-plane and electrode ball arrangement within a sample in the tank to investigate different types of PD. Electric characteristics of PD and different PD measuring technics such as electric, UHF and acoustic beside the mechanical behaviour of the acoustic waves are also discussed. The corona in oil results regarding the relation between AE and electric PD signals shows the correlated behaviour between AE and PD apparent charge magnitude. However, in surface discharges these behaviours are uncorrelated. In this regards the surface discharge is studied in more detail, leading to the first results of PD with very low acoustic (no acoustic) activity. Regarding these results two different categories in term of AE signals of PDs are defined, silent PD and non-silent PD. Silent PDs are those PD activities without or with very low acoustic signal and non-silent PDs are with acoustic signal. The existence of the silent PD is validated via oscilloscope and digital signal processing (DSP) devices. Also, with different innovative methods and arrangements such as needle plane and ball electrodes with and without oil gap, the probable reasons of creation this phenomenon (silent PD) is investigated. It is found that the carbonization patterns start with non-silent PD and remain the same during silent PD activities even with very high electric apparent charges. It means the development in carbonization traces produce electric and AE signals and in contrast no changes in carbonization traces produce only electric signals with no AE signal. These results verify the advantages of using acoustic technics and electric measurement in terms of PD classification and localization.Ausfälle von Komponenten in elektrischen Energiesystemen wie Transformatoren können zu einem enormen wirtschaftlichen Verlust im Energiesystem führen. Einer der Hauptbestandteile der Komponenten in elektrischen Energiesystemen ist das Isoliersystem, nämlich Öl, imprägniert Pressboard und Papier. Es gibt mehrere Methoden zur Diagnose dieser Isoliermaterialien. Die Messung der Teilentladung (TE) ist als eines der wichtigsten zerstörungsfreien Überwachungssysteme für Isoliermaterialien bekannt. Jedoch wird dies in Wartungsperioden hauptsächlich offline durchgeführt, und die existierenden Online-Verfahren liefern im Allgemeinen weniger Informationen aufgrund von elektromagnetischen Störungen. Im Gegensatz zum elektrischen TE-Messsystem ist das Schallemissionsmesssystem für seine Immunität gegen elektrische Umgebungsgeräusche bekannt. In dieser Arbeit wird die vergleichende Auswertung von akustischen und elektrischen Signalen von TE-Ereignissen untersucht, die in ölimprägnierten Pressboard und Papieren erzeugt werden. Sie konzentriert sich auf die Charakteristik der TE-Aktivität und deren Einfluss auf akustische Signale. Die TE-Klassifizierung wird definiert, indem die Beziehung zwischen akustischen und elektrischen Signalen von TE-Ereignissen verwendet wird. Obwohl die Empfindlichkeit der akustischen Sensoren im Laufe der Jahre verbessert wurde, bleibt die Erkennung der akustischen Signale von TE-Aktivität das Hauptproblem bei Komponenten in elektrischen Energiesystemen, hauptsächlich Transformatoren. Fehlende Informationen zur Auswertung von elektrischen TE-Signalen und akustischen Signalen sind neben der mechanischen Dämpfung zwei Hauptnachteile der akustischen Messung. Wegen der mechanischen und elektrischen Mechanismen von Wellen, die während der TE-Aktivitäten erzeugt werden, wird deren Verhalten ausführlicher diskutiert, um ein besseres Verständnis über die elektrischen und akustischen Signale zu erhalten. An verschiedenen Elektrodenkonfigurationen innerhalb einer Probe im Öltank werden TE-Quellen an verschiedenen Elektrodenkonfigurationen wie Spitze-Platte und Elektrodenkugelanordnung innerhalb einer Probe im Tank erzeugt, um verschiedene Arten von TE zu untersuchen. Neben dem mechanischen Verhalten der akustischen Wellen werden auch elektrische Eigenschaften von TE und verschiedene TE-Messtechniken wie elektrisch, UHF und akustisch behandelt. Die Ergebnisse bezüglich des Verhältnisses zwischen AE- und elektrischen TE-Signalen für Korona im Öl zeigen das korrelierte Verhalten zwischen AE- und TE-Signalen. Bei Oberflächenentladungen sind diese Verhaltensweisen jedoch unkorreliert. Die Oberflächenentladung wird genauer untersucht, was zu den ersten Ergebnissen von TE mit sehr geringer akustischer (keine akustischen Signale) Aktivität führt. In Bezug auf diese Ergebnisse werden zwei verschiedene Kategorien in Bezug auf elektrische und AE-Signale von TE definiert, stille TE und nicht-stille TE. Stille TE sind elektrische TE-Signale ohne oder mit sehr geringer akustischer Aktivität, und nicht-stille TE sind elektrische TE-Signale mit akustischer Aktivität. Die Existenz der stillen PD wird über Oszilloskope und digitale Signalverarbeitungsgeräte (DSP) validiert. Auch mit verschiedenen innovativen Methoden und Anordnungen wie Nadel und Kugelelektroden mit und ohne Ölspalt werden die wahrscheinlichen Entstehungsursachen dieses Phänomens (Silent TE) untersucht. Es wurde festgestellt, dass die Karbonisierungsmuster mit nicht-stiller TE beginnen und während stiller TE-Aktivitäten selbst bei sehr hohen scheinbaren elektrischen Ladungen unverändert bleiben. Dies bedeutet, dass bei der Entwicklung der Karbonisierungsspuren elektrische und AE-Signale erzeugt werden und im Gegensatz dazu ohne Änderungen der Karbonisierungsspuren nur elektrische Signale (ohne AE-Signale) erzeugt werden. Diese Differenzierung ist nur möglich bei gleichzeitigem Einsatz der akustischen Technik und elektrischen Messung im Hinblick auf die TE-Klassifizierung und Lokalisierung