Partial Discharges (PD) E-Book

Norasage Pattanadech

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley - IEEE
• Sprache: Englisch

Seitenzahl: 509

Veröffentlichungsjahr: 2023

Beschreibung

PARTIAL DISCHARGES (PD) -- DETECTION, IDENTIFICATION AND LOCALIZATION Explore state-of-the-art partial discharge measurement techniques In Partial Discharges (PD) -- Detection, Identification and Localization, a team of distinguished electrical engineers delivers a comprehensive treatment of the behavior, modeling, measurement, monitoring, localization, and evaluation of partial discharges. It includes coverage of all major advancements in the field that have occurred over the last few decades. It also discusses partial discharge phenomena, detection methods, and strategies for analyzing and processing collected data. Mechanisms of insulation failure are explored, as is the denoising of partial discharge measurement data and the localization of partial discharge in large, high-voltage equipment. Non-electric principles and procedures are discussed, and the book offers a variety of tables, figures, and photographs to illustrate the concepts discussed within. Partial Discharges(PD) also provides: * A thorough introduction to the physical behavior of partial discharges, including their causes and classification * Comprehensive modeling of partial discharge behavior, including classical and dipole discharges * Practical discussions of the measurement of partial discharges, including the electrical method, partial discharge decoupling, and pre- and post-processing of partial discharges * In-depth examinations of the monitoring of partial discharge behavior, including methods and realization Perfect for electrical engineers engaged in electrical power engineering, Partial Discharges (PD) will also earn a place in the libraries of research and development specialists employed in the manufacturing, quality testing and operation of electrical systems.

Leseprobe

Partial Discharges (PD)

Detection, Identification, and Localization

Norasage Pattanadech

King Mongkut Institute of Technology Ladkrabang

Bangkok, Thailand

Rainer Haller

University of West Bohemia

Plzeň, Czech Republic

Stefan Kornhuber

University of Applied Science

Zittau, Germany

Michael Muhr

Graz University of Technology

Graz, Austria

This edition first published 2023© 2023 John Wiley & Sons Ltd

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

The right of Norasage Pattanadech, Rainer Haller, Stefan Kornhuber, and Michael Muhr to be identified as the authors of this work has been asserted in accordance with law.

Registered Office(s)John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USAJohn Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.

Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some content that appears in standard print versions of this book may not be available in other formats.

Trademarks: Wiley and the Wiley logo are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries and may not be used without written permission. All other trademarks are the property of their respective owners. John Wiley & Sons, Inc. is not associated with any product or vendor mentioned in this book.

Limit of Liability/Disclaimer of WarrantyWhile the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

Library of Congress Cataloging‐in‐Publication Data applied for:

Hardback ISBN: 9781119568452

Cover Design: WileyCover Image: Courtesy of Authors

Author Biographies

Dr. Norasage Pattanadech received his PhD in Engineering Sciences Electrical Engineering from the Institute of High Voltage Engineering and System Management, Graz University of Technology, Austria. He has more than 20 years of experience in the field of High‐Voltage Testing and Analysis, especially in condition monitoring of high voltage equipment. He has served on IEC TC42 MT 23 and MT 14 committees. He received a Japanese patent in 2021 for a breaker with electrode controlling current. He is the author or co‐author of more than 100 publications and four books on electrical engineering and PD measurement.

Rainer Haller received his diploma and PhD in High Voltage Engineering from the High Voltage Department at the University of Technology in Dresden (Germany). During this time, he was mainly engaged in PD measurement on high voltage insulation as well as in the development of appropriate PD measuring technique. At the same time, he participated on activities of the IEC–WG “High Voltage Test and Measurement Techniques” in cooperation with CESI (Italy). He joined a manufacturer of high voltage equipment in 1986 and was mainly responsible for the development of high voltage testing equipment as well as power transformers. In 1991, he became a professor at the University of Applied Sciences in Regensburg (Germany), where he was engaged in High Voltage Engineering and Electrical Power Engineering. In 2006, he joined the University of West Bohemia in Pilsen (Czech Republic) and is currently the head of the High Voltage Section of the Regional Innovation Center for Electrical Engineering (RICE). Dr. Haller is author and co‐author of more than 100 scientific papers, numerous lectures, and one monography. Currently, he is actively engaged as lecturer and researcher on High Voltage Engineering as well as a member in international organizations like CIGRE.

Stefan Kornhuber studied Electrical Power Engineering at the Graz University of Technology. In 2005, he received his diploma degree and in 2007 his doctoral degree with the main research topic on Temperature Measurement and Uprating of OHTLs. Until 2006, he was with Test Institute for High Voltage Engineering Graz GmbH, with the main research topics in high voltage testing, simulation, and investigation of stresses of transients in electrical power networks. From 2006 to 2013 he was with Lemke Diagnostics and Doble Lemke. In 2013 he joined ABB AG Power Transformers – Engineering Solutions in Halle, Germany, as Head of Condition Management for Power Transformers and was later responsible for on‐site and local high voltage test field and systems.

In 2014, Dr. Kornhuber became chair in High Voltage Engineering and Theoretical Electrical Engineering at the University of Applied Science Zittau/Görlitz. The main research topics are outer and inner electrical interfaces of polymeric materials, test and measuring methods, and methods for technical diagnostics. He is a member of working groups at CIGRE, IEC, and DKE and is a convenor of CIGRE D1.58 and IEC TC 112 WG3. In 2021, he received the CIGRE Technical Council SC D1 Award and in 2022 the IEC 1906 Award. Since the beginning of 2023 he has been convening the German CIGRE SC D1 mirror committee and the IEEE DEIS Outdoor Insulation Technical Committee.

Michael Muhr received his diploma degree in 1971, PhD in 1978, and the Habilitation 1983 from Graz University of Technology (TU Graz). In 1990, he was appointed to the head of the Institute of High Voltage Engineering at TU Graz, and in 1996 he was appointed as full professor of High Voltage Engineering.

He was managing director of the Test Institution of High Voltage Engineering of TU Graz. From 2003 to 2007, he was head of the Senate, and from 2007 to 2011 vice‐rector for Academic Affairs of TU Graz.

Dr. Muhr has edited more than 190 publications and reports and supervised more than 160 diploma and 50 doctoral thesis. He has received recognitions and awards (Dr. h. c. from University in Pilsen), cooperated with many institutes in Europe and Overseas, and is a member in the national and international societies ÖVE, DKE, IEEE, IEC, and CIGRE.

Foreword

Partial discharges (PDs) igniting in the bulk dielectric of high‐voltage (HV) equipment may cause irreversible insulation deterioration and hence initialize an ultimate spark breakdown. Although this has been known since the beginning of the last century, the detection of partial discharges in HV equipment became more crucial in the 1960s, when organic insulation materials were more widely introduced in the HV industry, such as epoxy resin and polyethylene. These dielectrics are very sensitive to PD events. Hence, detection, measurement, and localization of partial discharges became an indispensable tool for quality assurance tests of HV equipment. These topics were extensively addressed by F.H. Kreuger in a textbook titled Discharge Detection in High Voltage Equipment, first published in 1964, and updated in 1989.

Due to the fast‐growing practical experiences in PD measurements as well as the development of modern PD measuring systems, the advancements achieved within the following years were summarized by D. König and Y.N. Rao and published in a monography titled Partial Discharges in Electrical Power Apparatus, edited in 1993. The latest edition of a textbook traces back to the year 2010, when T.S. Ramu and H.N. Nagamani published the book Partial Discharge Based Condition Monitoring of High Voltage Equipment.

Since that time, greater advancements have been achieved – on one hand, the further growing practical experiences and, on the other hand, the use of advanced digital signal processors and computer‐aided PD measuring systems to acquire, visualize, and classify the captured PD data. However, these achievements have not yet published in an updated textbook but rather in numerous technical papers and conference papers as well as in specific chapters of various textbooks dealing with HV measuring and test techniques, and thus, are not easily accessible to technicians and engineers engaged in the field of PD measurements.

Therefore, I appreciate very much the authors' decision to write this textbook, as well as their outstanding work addressing the state‐of‐the art in detection, identification, and location of partial discharges. In this context, it is worth mentioning that the authors are well‐acknowledged experts in the field of HV measurement and test techniques, including the specifics of PD measurements and diagnosis tests. A great deal of work on this subject has been done by them in various national and international societies and organizations, such as IEC, CIGRE, IEEE, and DKE. Moreover, one author, Michael Muhr, was appointed for a long time as chairman of the CIGRE AG HV Test Techniques and at present he is convenor of IEC TC 42 working groups: “IEC 60270 – Partial discharge measurements” and “IEC 62748 – Measurement of partial discharges by electromagnetic and acoustic methods.” At this point I would also like to mention that the authors have been known to me for more than a half century, and several topics covered in this textbook have been investigated and discussed together, particularly when participating in the annual meetings of the above‐mentioned organizations.

As outlined by the authors, the primary objective of the book is to present the current status of the knowledge regarding the detection and measurement of partial discharges as well as the procedures available to localize, identify, and classify harmful PD defects. The introducing chapters address the very complex physics of partial discharges in gaseous inclusions and their modeling, while the chapters that follow treat the fundamentals of PD tests and the specific aspects to be considered under on‐site condition, such as the denoising of the captured PD transients. Moreover, the book provides valuable information on the opportunities and limitations of alternative approaches, such as the use of ultrasonic and electromagnetic PD detection methods. A specific chapter is dedicated to the challenges of PD measurements under direct voltages, which is especially of interest for quality assurance tests of high‐voltage direct current (HVDC) equipment, increasingly used to transmit and distribute renewable energy.

The book is primarily intended for researchers, engineers, and technicians dealing with the development, design, and manufacturing and quality assurance test of HV equipment. For me, this is also the ultimate book for the maintenance staff performing PD tests to ascertain the insulation integrity of HV apparatus after manufacturing and repair. Moreover, this book would be of great interest to students educated in electrical engineering, particularly if interested in more in‐depth studies of the very complex PD phenomena in gaseous inclusions. The book offers readers real‐work experience, problem description, and solutions, while teaching them about the nowadays available tools for PD detection, as well as for localization, identification, and classification of the captured and acquired PD transients. Experts may use the knowledge provided by this book as they consider upgrading the current standard IEC 60270. The textbook includes full‐color photos and illustrations, forms, and tables to complement the topics covered in the individual chapters. There is also an extensive reference list, supporting the readers interested in more in‐depth studies of PD phenomena in dielectric bounded air gaps, as representative for gaseous inclusions embedded in the bulk dielectric of HV equipment.

Eberhard Lemke

Prof. Dr.‐Ing. habil. Dr. h. c.

Dresden, Germany

1Introduction

This book is a comprehensive introduction of partial discharges (PDs) – detection, identification, and localization. Dielectric insulations used in high‐voltage equipment have to counter the effect of PDs caused by inhomogeneous field configurations or inhomogeneous dielectric material. These continuous stress from PDs can increase, damage the insulation, and lead to power outages within a short time. Therefore, detection, identification, and localization of PDs are required to evaluate the dielectric insulation performance of electrical power equipment. Moreover, PD measurement is used for quality assurance during high‐voltage tests in order to confirm that the high‐voltage equipment will operate with high reliably and at high efficiency by which the PD level obtained from the factory test must be within the limits of the equipment standards. In addition, PD measurement is efficient and important for diagnosis and research.

PD phenomena have been recognized since the beginning of high‐voltage technology in the twentieth century. Building on loss angle measurements in the 1930s, Frederik Hendrik Kreuger introduced modern PD testing in the 1960s through charge‐based measurement. This moved quickly to an international IEC standard (IEC 60270). Since then, PD measurement has occupied an important place in research and industrial application as a nondestructive high‐voltage test of the insulation systems of power equipment. With the progress in microelectronics and computer development, new methods have become common practice, such as phase‐resolved PD measurement. Furthermore, considerable progress has been made in PD detection techniques such as acoustic method and the UHF method to apply in field tests.

In the standard (IEC 60270), the measurement of PDs is a charge‐based measurement. The values are expressed in Coulomb (C). PDs can also be detected through electromagnetic (EM) waves, light, heat, pressure, noise, and chemical changes in the dielectric materials. All of these physical and chemical processes can be used to measure the discharge phenomena. Above all, the acoustical and UHF range measurements range (IEC TS 62478) have prevailed in the PD detection in recent years. There are also different recommendations for PD detection by these methods in the equipment standards.

Another important aspect is the behavior of PDs governed by direct current (DC) voltage. DC is coming up more and more, especially for long transmission lines, but there is a lack of knowledge and testing about the behavior of PDs under DC stress. Therefore, there is a great deal of catching up to do in research and evaluation in this field. This is reflected in numerous publications, recommendations, regulations, and standards published.

1.1 Overview

PD has been presented in the chapters of many books in recent years. However, the last book that dealt exclusively with PD was published years ago. The authors' intention in writing this book is to provide a far more up‐to‐date discussion of the topic, beginning with the physical behavior and classification of various PD types in Chapter 2, before proceeding to discuss modeling of PD behavior in Chapter 3. There follows a discussion of the well‐known classical (PD) model using capacitors (and resistors), followed by discussion of the dipole model, which may bring new and interesting aspects to this field. Chapter 4 describes the measurement of PDs with decoupling, acquisition, processing, and pattern classification – one of the important chapters in this book. Another important concept is the denoising for PD measurement, which is very significant, especially for onsite PD measurement and the PD monitoring.

Chapter 5 deals with EM methods and Chapter 6 with nonelectrical methods of PD detection. It includes EM and acoustical methods but also optical and chemical methods. Especially acoustical detection and the PD detection with UHF are widely used in on‐site PD testing and diagnosis. Besides the detection of PDs, the location of the PDs is very important. On the one hand, the levels or patterns of PD are informative, but on the other hand it is the PD source location that indicates how dangerous PD is. An evaluation of PDs is discussed as well: Are they harmless or harmful? And what are the limiting values given in the standards? The chapter wraps up with an outlook for the industry going forward.

The primary objective of this book is to present the current status of the knowledge in PD detection, identification, and localization in both research and industrial application. Chapter 7 deals with the localization of PD effects, which is important for knowing the location of PDs. Despite all the advances provided by new technologies and new methods of PD detections, phenomena associated with signal propagations and PD activities remain problematical with regard to the various insulation systems in high‐voltage equipment (Chapters 9 and 10), as these phenomena cannot be fully explained by traveling wave theory or high‐frequency signal analysis. Therefore, difficulties persist in the analysis and evaluation of PD test results. The parameters measured at the terminals do not directly correlate with the phenomena occurring at the PD site. For example, while PD phenomena are measured at the terminals of the test sample as a transient mechanism, the test parameters do not depend on the PD phenomena solely occurring at the breakdown location, but also on the setup of the test sample, and therefore standard PD measurement only quantifies what is called the apparent charge (Chapter 11). Moreover, the measurement of PDs is different, depending on types of power apparatus, because of the inherent capacitive and/or inductive behavior present. Thus, the test sample must be considered electrically as a lump or distributed parameter.

Besides universities and research institutes, there are several national and international societies that deal with PD questions. These include the International Electrotechnical Commission (IEC), Institute of Electrical and Electronics Engineers (IEEE), Conseil International des Grands Réseaux Électriques (in English, International Council for Large Electric Systems, CIGRE), and Deutsche Elektrotechnische Kommission (in English, German Electrotechnology Commission, DKE). Moreover, the contemporary issues of both PD phenomena and PD measurement are discussed, not only for practical applications but also to provide theoretical background about PD behavior. The reflection on the fundamental knowledge of PD is more and more important to thoroughly understand PD phenomena and their influence on insulation materials and systems. It is especially important to research DC stress and PD behavior.

1.2 Acknowledgments

We would like to thank all our expert colleagues and friends who have brought us more thoroughly into to the field of partial discharges and deepened our knowledge, expertise, and interest of it through conversations, discussions, publications, and development in this field.

Participation in many international (IEC, CIGRE) working groups, committees, meetings, and conferences has also helped us to bring the field of PDs close enough that we could devote ourselves to the work of writing this book. We therefore want to thank everyone from universities, industry, and economy once again, but we would particularly like to give our special thanks to our excellent experts and colleagues Eberhard Lemke and Detlev Gross.

1.3 Users

This book is intended for all interested readers a monography, a reference book, or a state‐of‐art report. Therefore, this book can be of interest to:

Electrical engineers engaged in the manufacturing of high‐voltage equipment, such as transformers, generators, switchgears, cables, and so on. Engineers working in utilities will also find it useful.

Students in universities specializing in high‐voltage engineering.

Personnel in research and development employed in design of power apparatus, testing high‐voltage apparatus, or working in independent test laboratories with concerning with standardization.

August 2022

Norasage Pattanadech

King Mongkut's Institute of Technology Ladkrabang – Bangkok/Thailand

Rainer Haller

University of West Bohemia Pilsen/Czech Republic

Stefan Kornhuber

University of Applied Science Zittau/Germany

Michael Muhr

Graz University of Technology Graz/Austria

2Physical Behavior of Partial Discharges

2.1 Introduction

A partial discharge (PD) is defined as

“… localized electrical discharge that only partially bridges the insulation between the conductors and which can or cannot occur adjacent to a conductor. Partial discharges are in general consequences of local electrical stress concentration in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having duration of much less than 1 μs” [1].

This statement is focused on pulse‐like behavior of partial discharges that are generated by ionization processes within the insulation. However, this definition does not consider that in many cases of electrical insulation, there are already charge carriers (mainly ions) below the ionization level1 due to natural radioactivity or cosmic radiation. For example, under (normal) atmospheric air conditions, the number of charged ions of ~(102–103) 1/m3 may be expected [2].

If within the insulation any electric field is acting, those charge carriers move according to physical law toward their counter‐electrodes. If there are any insulating interfaces within or near the field‐space, the moving carriers could accumulate on their surface for a certain time.

According to the Shockley theorem, each movement of charge carriers leads to an equivalent current in a connected outer circuit [3]. Below the already mentioned physical defined level of self‐sustaining ionization process, the value of such currents is minimal, in the range of (p … n) A. In the case of alternating current (AC) electric fields, as is common practice for quality test procedures under AC conditions, the just‐mentioned effect of accumulated charges does not play any significant role due to the recombination processes at polarity change of outer electric field. But in case of applied unipolar voltage stress as in a DC or impulse field, some charge accumulation could be considered on existing interfaces within the electrode system. From a physical point of view, such charge is acting as an additional field source and, therefore, could have certain influence on initiating self‐sustaining ionization processes by influencing the original (background) field within the electrode system. In addition, there is evidence that pulse‐less discharges may occur even at AC electric fields [4]. Nevertheless, the majority of PD phenomena is based on the pulse character, as mentioned, and will therefore be described in later chapters.

For local occurrence of any partial discharge, which is ignited within a certain part of electrical insulation, two basic requirements must be fulfilled:

Local electrical field strength

E

loc

(given by voltage stress) in that part must exceed the (intrinsic) dielectric strength

E

d

of the insulation material and, therefore, fulfill the equation:

(2.1)

Free charge carrier(s), mainly electron(s), enable any ionization process to be initiated.

As defined, a partial discharge leads to a limited breakdown within a volume‐part of insulation necessarily associated with an ionization process of gas molecules.2 This event can take place not only in ambient air or other gases but also in gas‐filled cavities of solid insulation or in micro bubbles and water vapor of insulation liquids. Likewise, partial discharges may occur on interfaces between different dielectrics as pressboard barrier in liquids, insulator surfaces in gases, or interfacial discharges in slots of solids. Those partial discharges have a typical characteristic related to the connected surface: They “glide” over the participated surface led by electrical field strength, and therefore, they are often called gliding (or creeping) discharges. A simplified overview about such possible “PD‐sources” is shown in Figure 2.1, whereby at “homogeneous” air insulation3 those parts are mainly located near electrodes,4 but at other insulation type the condition acc. to (2.1) may be fulfilled also within the insulation by possible imperfections like bubbles, impurities, etc. Note, that the physical nature of partial discharges is mainly based on physics of gas discharges, discussed in this chapter.

Any partial discharge process is dependent on parameters, which are influenced not only by magnitude of electrical field strength E in the insulation caused by electrode configuration (field type, type of imperfect size, and its location) but also by structure and type of insulating material (dielectric parameters, dielectric (intrinsic) field strength), evidence of interfaces, and even by ambient conditions (pressure, humidity, pollution). The requirement of available charged carriers is mainly influenced by type and duration of stressed voltage (alternating, direct, impulse or combined), the type of insulation material (gaseous, liquid, solid, hybrid), and the evidence of any other sources of charged carriers caused by cosmic, natural, or artificial radiation.

Figure 2.1 Typical PD sources within the insulation (schematically) (a) external, (b) internal, (c) gliding‐type.

Partial discharges caused by self‐sustaining ionization process are accompanied with a large variety of physical phenomena like light emission, mechanical and chemical reactions, as well as acoustic phenomena. All these phenomena will be applied for recognition, detection, and interpretation of PD measurement. It is obvious, that the PD behavior is several for different types of insulating material, electrical field configuration and imperfections, therefore, for a common description a certain classification of PD phenomena seems to be necessary. There are various criteria for such a classification: the location of PD source (imperfection) relating to the insulation (external, internal, surface); the insulating material (gaseous, liquid, solid, hybrid); the initiating electrical field (AC, DC, impulse); physical phenomena (electrical, optical, chemical, mechanical, acoustical); or even the electrical equipment in which the PD occur (e.g. switchgear, transformer, rotating machines, cables, insulators). For this chapter, the first classification will be preferred.

2.2 External Discharges

As defined, external PD occur “outside” of any insulation equipment preferable on sharp edges or points, but also on long electrodes with small curvature (e.g. ropes on overhead lines) or on surfaces of solid insulation (e.g. insulators). Such PD are typical gas discharges and may occur if electrical field strength E is high enough to initiate a self‐sustaining ionization process, that value commonly termed as inception value.

The physics of gas discharges were intensively investigated already in last centuries [5–9], covering a large variety of characteristic discharges like glow‐, Townsend‐, streamer‐, leader discharges, etc., sometimes also characterized as Corona discharges. Such PDs occur, if the electrical field has a certain degree of nonuniformity, which might be characterized, for example, by the field efficiency factor η. The field efficiency factor η is defined by the ratio of electric field strength E within an electrode arrangement (voltage U, gap distance d) as average value Emean divided by maximum value Emax[10]:

(2.2)

For any evidence of (stable) PD the value of η should be, for example, in atmospheric air in the range or less than 0.2, mainly dependent on pressure and may be different for various gas type.

Each gas discharge is characterized by movement of charge carriers within the insulation gap, caused by formation of electron avalanches and drift of ions of both polarities. As already mentioned, each movement of charge carriers leads to an equivalent current in a connected outer circuit. Due to a several mobility of electrons and ions and their equivalent current is different as shown by theoretical calculation and practical measurement [11, 12] (Figure 2.2). A typical PD current is a very fast pulse characterized by a rise time in the nanosecond (ns) range ,  shown in the red dotted ovals) and with longer duration time of ns to μs range  shown in green dotted ovals). That time behavior corresponds in the frequency domain with equivalent spectra, which reach up to a few GHz, depending on discharge type (Figure 2.3) [14].

Figure 2.2 Time behavior of PD current pulse (acc. to [11, 12]).

For better explanation of PD behavior, a simple tip‐to‐plane arrangement with η < 0.2 in air is considered (Figure 2.4). The expected discharge current will be detected by measuring resistance (e.g. using an oscilloscope).

If the voltage U and, therefore, the electrical field strength E, reaches its inception value, a self‐standing ionization process is initiating. That may be described with the generation of electron avalanches caused by collisions of accelerated electrons with neutral gas molecules [7]. Likewise, ions of both polarities will be generated. Note that electrons will be delivered not only by collision actions but also by secondary effects such as impact of ions with the electrode surface or by high‐energized photons [9]. The generated space charge carriers – electrons and ions – have different drift velocity (e.g. the mobility of electrons is much higher than for ions). Therefore, the further development of discharge depends on the polarity of the tip, which influences the movement of the generated charge carriers and should be discussed separately.

Figure 2.3 Frequency spectra of PD measured and calculated by several authors, after [14] (a, b, c) (1,2 – PE‐cable core, 3,4 – rod‐plane‐arrangement, 5 – calculated for cable core). Behavior of simulated PD current pulses based on (d) time and (e) frequency.

Figure 2.4 Tip‐to‐plane arrangement in air.

2.2.1 Tip with Negative Polarity

If the tip polarity is negative, the generated positive ions are collected in a (critical) area near the tip, but the negative ions, mostly produced by attachment of electrons, are in certain distance from the positive ones, forming a kind of dipole field (Figure 2.5a, b). The dipole field is superimposed with the background (Laplacian‐) field Eg(x) and might be termed as Poisson‐field E(x) as schematically shown in Figure 2.5c. The area for ionization activities is given by the field‐dependent ionization coefficient αe and the exceeding of Eover Ei, the minimal necessary ionization field strength [5]. On one hand, the field strength is increased within a nearest distance to the tip forcing the ionization activity in that area (αe > 0), but likewise, at a certain distance near the tip, the formed negative ions reduce the field strength magnitude (αe