Measurement of Partial Discharges in Power Transformers using Electromagnetic Signals

Copyright © 2012 by Sebastian Coenen. All rights reserved.

D 93 (Dissertation Universität Stuttgart)

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Acknowledgements

I want to express my deepest gratitude to Prof. Stefan Tenbohlen for his encouraging and cooperative way of supervising this thesis. His challenging academic guidance to new questions and topics made the achievements of this work possible. I appreciated that he also ensured a close connection between this research and practical application. Many thanks to Prof. dr. Johan J. Smit for his co-examination and interest in this work.

I am very much obliged to the Workshop for preparing the laboratory set-ups. My special thank to Erwin Beck for sharing his extensive experience in mechanical production and his constant willingness to manufacture precise constructions out of simple drawings and coffee break discussions. I like to thank the colleagues of the administrative department, Hermine Lwowski, Nicole Schärli and Dr.-Ing. Ulrich Schärli for their help in organisational matters and their personal encouragement, support and advice beyond the professional.

Without the contributions of all students listed in the appendix the extent of this thesis would not have been possible. Special thanks to my former students and colleagues Maximilian Heindl, Michael Beltle, Andreas Müller and Martin Reuter for their exceptional great help during research, development and on-site measurements. Many thanks to all academic colleagues for providing a marvellous work climate.

I am very grateful to Dr.-Ing. Sacha M. Markalous for illuminating discussions round the clock and his academic guidance. I want to express my thanks to the Doble Lemke GmbH for supporting my work in many respects. My special thanks to Richard Heywood and his team of Doble Power Test, UK, where I spent a very nice time sharing their extensive knowledge in transformer condition assessment.

For several on-site measurements I wish to convey my gratitude to the Siemens AG in Nürnberg. I enjoyed working with the test field department as well as the TLM business unit. Special thanks to Uwe Thiess for his commitment for practically relevant measurements. My gratitude also to the respective departments of EnBW and RWE offering interesting case study objects.

I feel deeply grateful to my beloved wife and daughters. They relieved my daily strain, helped me to relax and made it all possible with their confidence, patience and love.

Table of Contents

List of abbreviations and symbols

Abstract

Kurzfassung

1     Introduction and Objectives of the Research

1.1  Introduction

1.2  Objective and Structure

2     Fundamentals of Partial Discharge Detection in Transformers

2.1  Definition of the PD Phenomenon

2.2  PD Detection Methods

2.2.1  Electrical method

2.2.2  Acoustic method

2.2.3  Electromagnetic method

3     UHF Probes

3.1  Performance Check for Verification of UHF System Functionality

3.1.1  Single Port Performance Check

3.1.2  Dual Port Performance Check

3.2  Characterisation of UHF Probe by the Antenna Factor

3.2.1  Transverse-EM cell

3.2.2  Determining the antenna factor

3.2.3  Influence of oil on antenna factor

3.2.4  Resulting sensitivity for UHF probe within oil

3.3  Mechanical Properties

3.4  UHF Top Hatch Probes

3.5  Conclusions for UHF Probes

4     Fundamentals of UHF Signals in Transformers

4.1  Laboratory Investigations on UHF Signal Propagation

4.1.1  Radiation behaviour of PD sources

4.1.2  Influence of windings on UHF signals of PD sources

4.1.3  Influence of windings on UHF signals of a signal generator

4.2  Attenuation of UHF Signals inside Transformers

4.2.1  Different UHF quantities

4.2.2  Attenuation of UHF Signals

4.3  Correlation between UHF Signal Quantities and IEC60270 Quantities

4.3.1  Simultaneous measurements of UHF and IEC quantities

4.3.2  Correlation between UHF and IEC quantities

4.4  Sensitivity Check

4.4.1  Sensitivity Check in laboratory environment with portable PD source

4.4.2  Sensitivity Check for transformers in the field with PD source probe

4.5  Differences between UHF Signals Quantities and IEC60270 Quantities

4.5.1  Movable PD source with constant geometry and measuring setup

4.5.2  Change of apparent charge depending on PD location

4.5.3  Simultaneous UHF and IEC measurements

4.6  Gating

4.7  Conclusions for UHF Signals in Transformers

5     Case Studies for PD Detection by UHF Signals

5.1  Performance Check and Identification of On-site UHF disturbances

5.2  Detection of PD and Resilience against External Corona Discharge

5.3  Dual Port Performance Check and UHF Monitoring

5.4  Gating of IEC Measurements by UHF Signals

5.5  Conclusions for PD detection by UHF Signals

6     PD Localisation by UHF and Acoustic Measurements

6.1  Techniques for PD Localisation

6.1.1  PD localisation with two sensors in the same position

6.1.2  PD Localisation with two sensors at different positions

6.1.3  PD localisation with acoustic sensor array

6.2  Case Study for State of the Art PD Localisation

6.3  Case Study for PD Localisation with two UHF Probes

6.4  Case Study for PD Localisation with Acoustic Sensor Array

6.5  Conclusion for PD Detection and Localisation by different Techniques for On-site Measurements

7 Conclusions and Recommendations for Further Research

8 Appendixes

9 References

List of abbreviations and symbols

Latin symbols:

Greek symbols:

Abbreviations:

Abstract

The most important task for insulation systems used in high voltage technology is the control of high electrical field strengths. For the examination and diagnosis of the insulation quality within power transformers the measurement of partial discharges (PD) is utilized. PD result from localised excessive electrical field strengths or from local reduction of the electrical strength of an insulation system. The occurrence of PD during acceptance tests refers to design or constructional faults. PD occurrence during diagnostic measurements indicates weakening by age or damage due to overstresses of the insulation system whilst power transformers are in service.

With the application of conventional PD measurement according to IEC60270, the information gained from the measurements can be very limited or even useless because of possible high interference levels on-site. Hence there is a need to investigate and develop new measurement techniques. The current thesis deals with the fundamentals of the unconventional “UHF method”, measuring electromagnetic (EM) signals emitted by PD in the ultrahigh frequency range (UHF: 300-3000MHz).

Suitable UHF probes are necessary for any UHF PD measurements on transformers, but up to now there is no standard available for the probes or the UHF method. The current thesis describes in detail the electrical and mechanical characteristics of the UHF probe developed, and thereby improves the comparability with measurement results of other probes. The presented methods for the characterisation of the probes, by the determination of the antenna factor for measurement conditions under oil, can be transferred to arbitrary UHF probes.

This work reviews fundamental knowledge of the EM waves arising inside power transformers to determine the achievable results from UHF PD measurements. The EM waves result from fast current flow through structures in the direct environment of PD sources. On their way to the UHF probes, waves have to pass a set of obstacles in the form of active parts of a transformer. These influences, and the rather small signal attenuation, are presented on the basis of exemplary measurements on transformers and experimental setups in HV laboratory.

The Performance Check developed allows verification of the functionality of the UHF PD measurement system. Such functional inspection is required, since calibration of that method is not possible.

Results from laboratory and on-site measurements show that a general ratio cannot be determined between the measured UHF quantities and the measured quantities of conventional measurements according to IEC60270. Thus the development of a method to confirm the achievable UHF sensitivity, a so-called Sensitivity Check, is not possible. However during several on-site measurements, UHF measurements proved to be generally responsive and could be used as stand-alone measurements or to support the IEC60270 measurement method by the so-called Gating procedure. The analysis and evaluation of UHF PD measurements take place in the same way as the conventional method, e.g. by phase resolved measurements of the occurring signals. However an experimental setup shows that not all PD sources emit corresponding UHF signals to all occurring signals that are measurable by the conventional means. Hence such analyses of the PD source might be hindered by exclusive UHF measurement.

The most important criterion for the evaluation of a new measurement method is their applicability under real test conditions for transformer measurements in the field. Several case studies document and confirm the characteristics of the UHF measurement method. It is possible to identify disturbances such as strong UHF transmitters, e.g. walkie-talkies, and to eliminate the disturbances by adequate signal filtering. A crucial advantage of UHF measurement is their resilience against external corona discharge signals, which usually represent the largest disturbance potential for conventional measurements. A further case study shows the possibility of trend analyses by monitoring UHF signals.

Finally the thesis identifies future applications and combinations of options for the UHF measurement method at on-site measurements. It is a suitable, fast and economic application to support acoustic measurement methods locating PD sources. Furthermore, propagation time differences measured with UHF signals can be used to locate PD defects. A method is presented by only using two UHF probes. In addition, a new acoustic localisation approach using a unified linear sensor array is illustrated. The potential and limits of PD measurement techniques used during this work leads to the conclusion, that a combination of all procedures promises best chance for successful PD measurements.

Kurzfassung

Die wichtigste Aufgabe der in der Hochspannungstechnik eingesetzten Isolationssysteme ist die Beherrschung hoher elektrischer Feldstärken. Zur Überprüfung und Diagnose der Isolationsqualität bei Transformatoren wird die Messung von Teilentladungen (TE) durchgeführt. TE entstehen durch Feldstärkeüberhöhungen oder durch lokale Minderung der elektrischen Festigkeit eines Isolationssystems. Das Auftreten von TE bei Abnahmeprüfungen weist auf Design- oder Fertigungsfehler hin und zeigt bei Messungen im Betrieb altersbedingte Schwächung oder Schädigung des Isolationssystems an.

Bei Anwendung der konventionellen Teilentladungsmessung nach IEC60270 kann das Messergebnis durch den hohen Störpegel Vor-Ort nur beschränkt aussagekräftig oder unter Umständen unbrauchbar sein. Dadurch ergibt sich der Bedarf neue Messmethoden zu erforschen und zu entwickeln. Die vorliegende Arbeit beschäftigt sich grundlegend mit der unkonventionellen „UHF Methode“, die von Teilentladungen ausgesendete elektromagnetische Signale im ultrahohen Frequenzbereich (UHF: 300 – 3000MHz) erfasst.

Geeignete UHF Sensoren stellen die Voraussetzung für TE-Messungen an Transformatoren dar, entziehen sich bisher aber jeglichen Normen und Standards. Die vorliegende Arbeit beschreibt detailliert die elektrischen und mechanischen Eigenschaften eines entwickelten UHF Sensors und ermöglicht dadurch die Vergleichbarkeit mit anderen Sensoren. Die vorgestellten Methoden zur Charakterisierung der Sensoren, durch die Bestimmung des Antennenfaktors für Messbedingungen unter Öl, lassen sich auf beliebige UHF Sensoren übertragen.

Um die erzielbaren Messergebnisse bewerten zu können, vermittelt diese Arbeit grundlegende Kenntnisse über die auftretenden elektromagnetischen Wellen im Transformator. Sie entstehen durch schnellen Ladungsdurchfluss von Strukturen in direkter Umgebung von Teilentladungsquellen. Auf ihrem Weg zu den UHF Sensoren passieren die Wellen eine Reihe von Hindernissen in Form der aktiven Teile eines Transformators. Die zu erwartenden Beeinflussungen und geringen Signaldämpfungen werden anhand von Beispielmessungen an Transformatoren und Versuchsaufbauten im Hochspannungslabor dargestellt.

Der Performance Check als entwickelte Methode zur Funktionsüberprüfung erlaubt Rückschlüsse auf die Aussagekraft der nachfolgenden Messungen. Die UHF Messmethode erfordert eine Möglichkeit der Funktionsüberprüfung, da eine Kalibrierung nicht möglich ist. Messergebnisse im Labor und Vor-Ort zeigen, dass keine allgemeingültige Beziehung zwischen den Messgrößen der UHF Messung und den Messgrößen der konventionellen Messung nach IEC60270 herstellbar ist. Somit ist die Entwicklung einer belastbaren Methode zum Empfindlichkeitsnachweis der UHF Methode, eines sogenannten Sensitivity Checks, nicht möglich. Vor-Ort Messungen zeigen allerdings die generelle Empfindlichkeit der UHF Methode, die als eigenständige Messung verwendbar ist oder beim sogenannten Gating auch zur Unterstützung der IEC60270 Messmethode verwendbar ist. Die Bewertung einer UHF TE Messung erfolgt z.B. durch phasenaufgelöste Messungen der auftretenden Signale. Allerdings zeigt ein Versuchsaufbau, dass nicht alle TE Quellen zu allen konventionell messbaren Signalen entsprechende UHF Signale aussenden und die Analyse solcher TE Quellen durch alleinige UHF Messungen beeinträchtigt sein kann.

Wichtigstes Kriterium für die Bewertung einer neuen Messmethode ist ihre Anwendbarkeit unter realen Prüfbedingungen bei Vor-Ort-Messungen. Mehrere Fallbeispiele dokumentieren und bestätigen die zuvor analysierten Eigenschaften der UHF Messmethode. Es ist möglich, Störungen dieser Methode durch starke UHF Sender wie z.B. Sprechfunkgeräte zu identifizieren und Störfolgen zu eliminieren. Ein entscheidender Vorteil der UHF Messmethode ist ihre Störfestigkeit gegen Signale externer Korona Entladungen, die in der Regel das größte Störpotential für konventionelle Messungen darstellt. Ein weiteres Fallbeispiel zeigt die Möglichkeit des Monitorings und einer Trenderfassung mit Hilfe von UHF Signalen.

Zum Schluss der Arbeit werden weiterführende Anwendungs- und Kombinationsmöglichkeiten der UHF Messmethode vorgestellt. Sie ist durch schnelle und kostengünstige Anwendung geeignet um akustische Messverfahren zur Lokalisierung von Teilentladungsquellen zu unterstützen. Weiterhin können gemessene Laufzeitunterschiede bei UHF Signalen zur örtlichen Eingrenzung von Fehlstellen verwendet werden. Vorgestellt wird eine Methode zur Ortung von Teilentladungen mit lediglich zwei UHF Sensoren. Weiterhin wird eine neue akustische Ortungsmethode mit Hilfe eines Sensorarrays präsentiert. Eine Aufstellung während dieser Arbeit gesammelten Vor- und Nachteile aller verwendeten TE Messprinzipien schließt mit dem Fazit, dass eine Kombination aller Verfahren die größte Aussicht auf eine erfolgreiche TE Messung verspricht.

1  Introduction and Objectives of the Research

1.1  Introduction

The reliability of electrical energy networks depends on the quality and availability of electrical equipment such as power transformers. Furthermore, power transformers represent the most expensive assets for utilities. Due to the liberalised electric energy market, utility companies are driven to greater cost effectiveness and are changing their strategies from time-based maintenance (TBM) to condition-based maintenance (CBM).

Determining the condition of the insulation of oil/paper-insulated transformers during their full operation or at least on-site becomes more and more important because of the increasing number of transformers reaching their technical life expectancy. Local failures of their insulation may lead to catastrophic breakdowns and might cause high outage and penalty costs. To prevent these destructive events, several diagnostic methods are applied to power transformers:

• Basic electrical off-line measurements like winding resistance, insulation resistance, capacitance and dissipation factor.
• Basic oil analyses with on-site sampling determining, e.g., the breakdown voltage, dissolved gas analyses and moisture content [Koch, 2008].
• Permanent online monitoring of voltage and current, oil temperature [Cardillo, 2007] and hydrogen content, tap changer position, cooling unit, bushing capacitance.
• Advanced diagnostic after fault indication, like transmission ratio, short circuit impedance, frequency response analysis (FRA) [Wimmer, 2010], Furanes in oil and partial discharge measurements.

The current work deals with a diagnostic method of Partial Discharge (PD) measurements. Power transformers are tested on PD activity before commissioning and also during service.

So-called conventional PD measurements according to IEC60270 [IEC60270, 2000] might show certain drawbacks for online measurement. For example, external corona discharges produce similar measurement readings and might therefore not allow a decision as to whether or not a transformer has internal PD. The conventional measurement of apparent charge of PD according to IEC60270 is based on signal decoupling with a high-voltage (HV) capacitor. That is common measurement technology within quality assurance laboratories for the production of HV equipment [IEC60270, 2000].

In recent years, a growing demand for on-site/online PD diagnostics with high measurement sensitivity has lead to the development of innovative advanced PD decoupling and measurement methods to overcome certain drawbacks of the conventional methods. Combining so-called UHF (Ultra High Frequencies) or acoustic sensor technology with suitable instrumentation and data processing gives a series of advantages over different HV devices such as:

• Ease of distinguishing between internal and external PD signals by using the “Faraday’s cage” shielding effect of the transformer tank for UHF detection
• Diagnostic “double checks” and possibly deeper insight into deterioration processes, e.g. through comparison with IEC60270 measurement results
• Geometric PD location by a combination of UHF and acoustic methods or the sole application of one of the methods, depending on the HV devices being tested
• Alternative PD decoupling options without galvanic contact to any HV components.

The current contribution deals with the electromagnetic (EM) method, also known as the UHF-Method. PD under oil are very fast electrical processes and radiate EM waves with frequencies up to the ultrahigh frequency range (UHF: 300 – 3000MHz). EM waves are detectable with UHF probes. The probes can be inserted into the transformer during full operation using the oil filling valve. As a result of the shielding characteristics of the transformer tank against external EM waves, normally a clear decision can be made concerning the PD activity of the test object.

1.2  Objective and Structure

The first objective of this thesis is the documentation of the electrical and mechanical properties of the developed UHF PD probe. Furthermore, characterisation of its measuring sensitivity makes further measurement results comparable.

For the verification of the sensitivity achievable with the UHF PD measuring method, the relation between unconventionally measured UHF quantities to the PD apparent charge level in picoCoulomb (pC) is the aim of the research. Thus the following questions are important:

• Can UHF signals be measured everywhere inside the transformer tank with the same sensitivity? How dependent is the signal amplitude or signal energy on the measuring location?