
Partial Discharges (PD)
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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.
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Persons
Norasage Pattanadech, PhD, is Associate Professor in the Electrical Engineering Department at King Mongkut Institute of Technology in Bangkok, Thailand.
Rainer Haller, PhD, Professor on High Voltage Engineering at Faculty of Electrical Engineering at the University of West Bohemia, Pilsen, Czech Republic.
Stefan Kornhuber, PhD, is Professor for High Voltage Engineering at the University of Applied Science, Germany. His research interests include outer and inner electrical interfaces of polymeric materials and the diagnosis.
Michael Muhr, PhD, is Emeritus Professor at the Institute of High Voltage Engineering and System Management at the Graz University of Technology in Austria.
Content
Author Biographies xi
Foreword xiii
Symbols and Abbreviations xv
1 Introduction 1
1.1 Overview 2
1.2 Acknowledgments 3
1.3 Users 3
2 Physical Behavior of Partial Discharges 5
2.1 Introduction 5
2.2 External Discharges 7
2.2.1 Tip with Negative Polarity 10
2.2.2 Tip with Positive Polarity 11
2.3 Internal Discharges 17
2.3.1 Discharges in Liquid Insulation 17
2.3.2 Discharges in Solid Insulation 18
2.4 Gliding Discharges 24
2.5 PD Quantities 24
References 29
3 Modeling of PD Behavior 33
3.1 Introduction 33
3.2 Network-Based Model 33
3.3 Field-Based Model 42
3.3.1 Stages of PD Behavior Modeling for DC Conditions 48
3.3.1.1 Stage 1: Inception of Ionization Processes 48
3.3.1.2 Stage 2: Establishment of an Electrical Dipole 49
3.3.1.3 Stage 3: Dissipation of the Electrical Dipole 49
3.3.2 Extended Modeling Parameters 49
3.3.3 Summary 52
References 53
4 Measurement of Partial Discharges 55
4.1 Introduction 55
4.2 Signal Properties 57
4.2.1 Device Under Test 57
4.2.2 High Voltage Circuit 58
4.3 Coupling Methods 59
4.3.1 Capacitive Coupling with Measuring Impedance 60
4.3.2 Inductive Coupling with High-Frequency Current Transformer 65
4.4 Signal Processing 68
4.4.1 Full Analog Processing 68
4.4.2 Semi-Digital Processing 68
4.4.3 Full Digital Processing 69
4.5 Measurement Principles 70
4.5.1 Narrow-Band Measurement 72
4.5.2 Wide-Band Measurement 76
4.5.3 Time Domain Integration 79
4.5.4 Radio Interference Voltage (RIV) Measurement 83
4.5.5 Synchronous Measurement for Multichannel Application 84
4.6 Noise Suppression and Reduction 86
4.6.1 Introduction 86
4.6.2 Noise Sources 87
4.6.2.1 Main Sources of Conducted Coupled Noise 87
4.6.2.2 Blocking Impedance and Filters 88
4.6.2.3 Electrodes and Wire 88
4.6.2.4 Coupling Capacitor 88
4.6.2.5 Floating Potential Elements 88
4.6.2.6 Pulse-Shaped and Harmonic Noise 89
4.6.2.7 Noise via Grounding System or Wire Loops 89
4.6.2.8 Mains Plug and Background Noise of the Measurement Instrument 89
4.6.3 Denoising Methods 89
4.6.3.1 Shielding 90
4.6.3.2 Filters 90
4.6.3.3 Balanced Bridge Measurements 90
4.6.3.4 Windowing 92
4.6.3.5 Gating 93
4.6.3.6 Clustering 93
4.7 Visualization and Interpretation of PD Events 96
4.7.1 Introduction 96
4.7.2 Classical Methods 97
4.7.3 Advanced Methods 99
4.7.4 Pulse Sequence Analysis 103
4.8 Artificial Intelligence and Expert Systems 104
4.8.1 Introduction 104
4.8.2 Artificial Intelligence and Artificial Neural Networks 105
4.8.2.1 Learning Process 107
4.8.2.2 ANN Architecture 108
4.8.2.3 Common Principles 108
4.8.2.4 Applications for PD Classification and Localization 110
4.8.2.5 Basic Principles of PD Recognition 110
4.8.3 Expert System 116
4.8.3.1 Introduction 116
4.8.3.2 Application for PD Diagnostic 118
4.9 Calibration 119
4.9.1 Calibration of PD Measuring Circuit 119
4.9.2 Performance Test of PD Calibrators 122
References 124
5 Electromagnetic Methods for PD Detection 129
5.1 Introduction 129
5.2 PD Measurement by HF and VHF Sensors 129
5.2.1 PD Measurement by cc 129
5.2.1.1 Theory 129
5.2.1.2 cc Characteristics and Installation Aspect for PD Measurement 133
5.2.1.3 cc Installation 134
5.2.1.4 cc for PD Measurement 134
5.2.2 PD Measurement by Inductive Couplers 137
5.2.2.1 PD Measurement by High-Frequency Current Transformers 137
5.2.2.2 PD Measurement by Rogowski Coil 141
5.2.3 PD Measurement by DCS 144
5.2.3.1 Theory 144
5.2.3.2 DCS Structure and Characteristic 144
5.2.3.3 Application of DCS for Cable and Joint PD Measurement 146
5.3 PD Measurement by UHF Method 146
5.3.1 Theory 146
5.3.1.1 General Idea 146
5.3.1.2 Propagation and Attenuation of UHF Signal 147
5.3.1.3 UHF Signal Attenuation 150
5.3.2 UHF Sensors 150
5.3.3 UHF PD Measurement System 154
5.3.3.1 Sensitivity Verification for GIS PD Measurement 157
5.3.3.2 Determination of PD Measurement by UHF PD Technique 160
5.3.4 Application of UHF PD Measurement 161
5.3.4.1 PD Detection by UHF in GIS and GIL 161
5.3.4.2 UHF PD Detection in Transformers 161
5.3.4.3 Application of UHF PD Detection to Other High-Voltage Equipment 161
References 164
6 Non-electrical Methods for PD Measurement 167
6.1 Introduction 167
6.2 Optical PD Measurement 167
6.2.1 Theory 167
6.2.2 Principle for Optical PD Measurement Technique 169
6.2.3 Application of Optical PD Measurement 171
6.2.3.1 Insulators, Transformer Bushings, Surge Arrestors, Transmission Lines, and Fittings 171
6.2.3.2 Rotating Machines 172
6.3 Acoustic Emission PD Measurement 172
6.3.1 Theory 172
6.3.2 Acoustic Receivers and Acoustic Sensors 175
6.3.2.1 Hand-Held AE PD Receivers 175
6.3.2.2 Instrument-Based AE PD Detection 177
6.3.3 Acoustic Noises in AE PD Measurement 183
6.3.4 General Idea for AE PD Measurement 184
6.3.4.1 Sensitivity Check for AE PD Measurement 184
6.3.4.2 AE PD Measurement 184
6.3.5 Application of Acoustic PD Measurement for High-Voltage Apparatus 184
6.3.5.1 Detection of Corona and Surface Discharge from Outdoor Insulators or High- Voltage Conductors 184
6.3.5.2 PD Detection in Transformers 185
6.3.5.3 PD Detection by AE PD Measurement Technique in GIS and GIL 185
6.3.5.4 PD Detection in Rotating Machine by AE PD Measurement Technique 187
6.3.5.5 PD Detection for Other High-Voltage Equipment 188
6.4 Chemical Byproducts 188
6.4.1 Theory 188
6.4.2 Dissolved Gas Analysis for Liquid Insulation 188
6.4.2.1 Dissolved Gas Generation in Liquid Insulation 188
6.4.2.2 Application of DGA for PD Analysis 189
6.4.3 Decomposition Gas Analysis 194
6.4.3.1 Decomposition SF 6 Analysis for Gas-Insulated High-Voltage Equipment 194
6.4.4 Ozone Measurement and Analysis for Air-Cooled Hydrogenerators 195
References 195
7 PD Localization 199
7.1 Introduction 199
7.2 The Complexity of PD Localization 199
7.3 Classification of PD Localization 200
7.3.1 PD Localization for the Internal Insulation 200
7.3.2 PD Localization for the External Insulation 200
7.4 PD Localization Techniques for the Internal Insulation 200
7.4.1 Pulse Time Arrival Method 201
7.4.1.1 Concept of Pulse Time Arrival Method 201
7.4.1.2 Application of the Pulse Time Arrival Method for PD Localization in High Voltage Equipment 201
7.4.2 Auscultatory Method 205
7.4.2.1 Concept of Auscultatory Method 205
7.4.2.2 Application of the Auscultatory Method for PD Localization in a Transformer 206
7.4.3 Triangulation Method 206
7.4.3.1 Concept of Triangulation Method 206
7.4.3.2 Application of the Triangulation Method for PD Localization in a Transformer 210
7.4.4 Bouncing Particle Localization Method 216
7.5 PD Localization Techniques for the External Insulation 217
7.5.1 Application of the Corona Camera 217
7.5.2 Application of the Airborne Acoustic Probe 217
References 218
8 PD Measurement Under Direct and Impulse Voltage Stress Conditions 221
8.1 Introduction 221
8.2 PD Measurement at Direct Voltage Conditions 222
8.3 PD Measurement at Impulse Voltage Conditions 229
8.3.1 PD Measurement at Classical Impulse Voltage Conditions 230
8.3.2 PD Measurement at Repetitive Pulse Voltage Conditions 233
References 236
9 Monitoring of PD Behavior 239
9.1 Introduction 239
9.2 PD Monitoring 239
9.2.1 Off-Line and On-Line PD Measurement 240
9.2.1.1 Off-Line PD Monitoring 240
9.2.1.2 On-Line PD Monitoring 242
9.2.2 PD Monitoring System 247
9.2.2.1 PD Sensor 247
9.2.2.2 Data Acquisition 249
9.3 Application of PD Monitoring 254
9.3.1 Application of PD Monitoring for the Existing High-Voltage Equipment 254
9.3.2 Application of PD Monitoring for the New Equipment Supporting Smart Grid 255
9.4 Challenges for PD Monitoring in Future 256
References 258
10 Evaluation of PDs 261
10.1 Introduction 261
10.2 In-House and On-Site PD Testing 262
10.2.1 In-House PD Testing 263
10.2.2 On-Site PD Testing 263
10.3 How to Evaluate PD Test Results 264
10.4 Effect of PD on Insulation Degradation 264
10.5 Integrity of PD Measurement 266
10.6 PD Quantity 267
10.6.1 Discharge Magnitude 267
10.6.2 PDIV, PDEV, and Other PD Quantities 267
10.6.3 PD Quantity as Criteria for Evaluation of Insulation Condition 268
10.7 PD Patterns 270
10.7.1 Analysis of PD Patterns 270
10.7.2 PD Patterns as Criteria for Evaluation of Insulation Condition 271
10.8 PD Signal in Time Domain and Frequency Domain Analysis 275
10.9 PD Source as Criteria for Evaluation of Insulation Condition 280
10.10 Noise Patterns and Noise Reduction 280
10.10.1 Noise Patterns 280
10.10.2 Noise Reduction 281
10.11 Effective Evaluation of PD Phenomena 283
References 284
11 Standards 285
11.1 Standards 285
11.2 Technical Brochures 287
11.3 Books 289
References 289
12 Conclusions and Outlook 291
Index 293
2
Physical 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 Eloc (given by voltage stress) in that part must exceed the (intrinsic) dielectric strength Ed 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 ?. The field efficiency factor ? 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 ? 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 ? < 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...
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