Partial Discharge in Electronic Equipments

Tato disertační práce se věnuje studiu částečných výbojů (PD) způsobených poklesem spolehlivosti a životnosti elektronických zařízení a systémů. Diagnostika PD je dnes známá metoda pro vysoké napětí u vysoko-výkonných zařízení. V případě elektronických zařízení PD testování není ale běžně používáná metoda, přestože je zde také potenciál pro vysoké elektrické zatížení vzhledem k velmi krátké vzdálenosti. Tato práce je zaměřena na vyšetřování PD činnosti u elektronických zařízení. Bylo navrženo a provedeno pracoviště pro diagnostiku PD v elektronických zařízeních. Pracovní frekvence se pohybuje od několika stovek Hz až 100 kHz. Maximální amplituda PD testovaného napětí je vyšší než 10 kV. Navzdory jednoduché konstrukci toto zařízení přináší vysokou spolehlivost měření. Více než 300 PD testů bylo provedeno na různých elektronických zařízeních a elektronických součástí,např. na planárních transformátorech a elektronických komponentách používaných při vysoko-napěťových měničíchThis dissertation thesis is devoted to study of partial discharge (PD) caused decrease of reliability and lifetime of electronic equipments and systems. PD diagnostic is nowadays well known method for high voltage high power equipments but in case of electronic devices PD testing it is not used routinely despite that there is also a potential for high electric load due to extremely short distances. The risk of PD caused failure is here extremely high because of high working frequency and consequently high repetition rate of PD events. Therefore, this work is focused on investigation of PD activity in electronic equipments. The workplace for PD diagnostic in electronic devices based on switched power supply was designed and made. Working frequency ranges from several hundreds of Hertz up to 100 kHz. The maximal amplitude of PD testing voltage is higher than 10 kV. Despite the simple design this equipment brings high repeatability and reliability of measurement. More than 300 PD tests were made on different electronic devices and electronic components, on planar transformers, and on components for voltage gate drivers for use in high voltage power converters. Possibilities of PD tools in investigation and engineering ofd insulation systems were demonstrated.

Design of a fast computer-based partial discharge diagnostic system

Partial discharges cause progressive deterioration of insulating materials working in high voltage conditions and may lead ultimately to insulator failure. Experimental findings indicate that deterioration increases with the number of discharges and is consequently proportional to the magnitude and frequency of the applied voltage. In order to obtain a better understanding of the mechanisms of deterioration produced by partial discharges, instrumentation capable of individual pulse resolution is required. A new computer-based partial discharge detection system was designed and constructed to conduct long duration tests on sample capacitors. This system is capable of recording large number of pulses without dead time and producing valuable information related to amplitude, polarity, and charge content of the discharges. The operation of the system is automatic and no human supervision is required during the testing stage. Ceramic capacitors were tested at high voltage in long duration tests. The obtained results indicated that the charge content of partial discharges shift towards high levels of charge as the level of deterioration in the capacitor increases

Phase Resolved Partial Discharge Evaluation at dc Voltage

Partial discharge diagnostics are common for the condition assessment of high voltage alternating current (ac) and direct current (dc) equipment. For ac applications the phase resolved partial discharge evaluation method has prevailed [1]. Even though, numerous methods for dc partial discharge diagnostics have been proposed no agreed standard exists. This is primarily because of the fact that these methods are not as easily applicable as the well-known, intuitive, pattern based phase resolved method. Due to the often assumed unavailability of a useable phase angle information [2]–[4] this method has never been transferred to dc partial discharge diagnostics. However, as a dc voltage is usually not generated but obtained from a rectification process a natural ripple, which provides a phase information, will always be available. Hence, a novel dc partial discharge evaluation method that uses a phase resolved approach is introduced in this thesis. The introduction of a ripple on the dc voltage, which, in this thesis, is obtained from a half-wave rectification, might alter the discharge behaviour. Consequently, the maximum tolerable ripple below which a dc discharge behaviour is maintained has to be evaluated. Such an evaluation has never been carried out for a partially rectified voltage. Hence, for the first time, four characteristic discharge quantities have been evaluated with respect to a varying ripple. The outcome is a maximum tolerable ripple of 3 % for negative corona, 2 % for positive corona, 0.8 % for negative and positive surface discharges as well as 0.4 % for internal discharges. These newly defined values have then been used as a basis to obtain characteristic phase resolved partial discharge patterns at dc voltages. The novel patterns have been explained through the underlying physical phenomena. Therefore, for the first time, this work successfully demonstrated that a phase resolved partial discharge evaluation can be conducted at dc voltages, while the general dc discharge behaviour is maintained. This newly proposed recognition method has the potential to change dc partial discharge diagnostics in practical as well as in laboratory applications

X-Rays from long laboratory sparks in air

This collective book provides a review of research concentrated on runaway electron beams and X-rays in an inhomogeneous electric field with different gases at increased pressure. Attention is also given to supershort avalanche electron beam (SAEB) in the optimal conditions. New experimental techniques and equipment, including those with picosecond time resolutions, were required for diagnostics of electrical and optical signals. The book consists of twenty-four chapters, some of which were written jointly by researchers of different teams. Some chapters consider the range of SAEB applications.Postprint (published version

The Different Stages of PRPD Pattern for Positive Point to Plane Corona Driven by a dc Voltage Containing a Ripple

The different phase resolved partial discharge patterns, which can be recognized when a positive dc voltage containing a ripple due to a half-wave rectification is used are presented in this paper. Results are given from corona onset to the appearance of breakdown streamers. The outcomes are five different patterns, which represent the four stages of positive corona and their transition zones. Three of them are clearly identifiable with respect to patterns from other discharge sources that have already been reported for dc voltages containing a ripple. For an assessment of pattern formation discharge pulses as recorded at the test object are presented. Finally, the appearance of patterns is explained with support of the measured discharge pulses and the underlying physical processes

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

Reliability of Interfaces for HVDC Cable Accessories

HVDC cable systems are widely used in modern transmission networks. This project investigates the dielectric withstand properties of the interfaces existing in the accessories (joints, termination). Interfaces are a weak point as cavities exist at the contact between the cable and the accessory body. Interfaces have been studied as the mating of two elastic surfaces using elastic-plastic mechanics. Their withstand properties are greatly improved by using rubbery material (e.g., silicon rubber) and oiling the surfaces as the number and size of the cavities is reduced. However, thermal aging of the surfaces (with loss of elasticity) and the migration of the lubricant might lead to conditions where the interfaces are again the weak link of the system. The goal of this thesis is to understand breakdown mechanisms of XLPE/XLPE and XLPE/LDPE interfaces through the analysis of partial discharges, space charge accumulation and leakage current measurement. Space charge is measured through the Thermal Step and the Pulsed Electro-Acoustic methods (TSM and PEA). TSM highlights that there exists a critical voltage at which the space charge shifts from homo- to hetero-charge. The conductivity characteristics of the XLPE insulation of HVDC cable are investigated under different applied mechanical stress and the breakdown voltage which could be affected by crosslinking byproducts within the insulation of the material. Partial Discharge is a key tool to ensure the reliability and life extension of High Voltage electrical equipment. For cable interfaces, it is generally assumed that surface partial discharges create carbonized tracks at the contact between the cable insulation and the accessory body leading to breakdown. In this thesis, it was not possible to reproduce this condition. Breakdown occurred through a mechanism that remains to be unraveled. A possible connection with the shifts mentioned above, from homo to hetero charge at the interface, seems to exist

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