PARTIAL DISCHARGES IN TRANSFORMERS

676
PARTIAL DISCHARGES
IN TRANSFORMERS
WORKING GROUP
D1.29
FEBRUARY 2017

Members
J. Fuhr, Convenor CH
S. Markalous, Secretary DE
S. Coenen DE
M. Haessig CH
M. Judd GB
A. Kraetge DE
M. Krueger AT
R. Lebreton FR
E. Lemke DE
S. Okabe JP
R. Schwarz AT
U. Sundermann DE
S. Tenbohlen DE
P. Werle DE
WG D1.29
Copyright © 2017
“All rights to this Technical Brochure are retained by CIGRE. It is strictly prohibited to reproduce or provide this publication in
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PARTIAL DISCHARGES IN
TRANSFORMERS
ISBN : 978-2-85873-379-8

PARTIAL DISCHARGES IN TRANSFORMERS
ACKNOWLEDGEMENT
Members of the CIGRE working group D1.29 express their sincere thanks to colleagues and friends for
contributions and assistance in carrying out and finishing this work: Thomas Aschwanden (CH), Andrea
Cavallini (IT), Pascal Fehlmann (CH), Thomas Heizmann (CH), Jacko Koen (ZA), Lars Lundgaard (NE),
Volker Schmidt (DE), Thomas Steiner (DE).
In particular, the convenor and the working group wish to thank Joe Tusek (AU) for proofreading the
final version and for valuable comments that make this TB more understandable to a wide community
of PD interested engineers.
ISBN : 978-2-85873-379-8

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PARTIAL DISCHARGES IN
TRANSFORMERS
EXCUTIVE SUMMARY
PD measurements on transformers........................................................................................ 6
Interpretation and evaluation of PD results............................................................................. 6
Localization ........................................................................................................................... 7
Procedure for solution of PD problems................................................................................... 7
Step 1: Analysis of PD pattern and “calibration” ..................................................................... 9
Step 2: Position of acoustic and UHF probes....................................................................... 10
Step 3: Simulation of the PD source .................................................................................... 11
Keywords............................................................................................................................. 11
1 INTRODUCTION............................................................................................................... 12
2 PARTIAL DISCHARGE DETECTION SYSTEMS FOR TRANSFORMERS ....................... 13
2.1 Measuring system for detection of electric signals ........................................................ 13
Conventional PD measuring system..................................................................................... 13
Phase resolved PD data processing system......................................................................... 15
2.2 Non-standard electrical PD signal detection systems.................................................... 18
2.3 System for detection of acoustic sound waves.............................................................. 20
2.4 System for detection of UHF signals............................................................................. 22
2.5 System for detection of chemical reactions-decomposition of insulating materials......... 24
3 COMMON TYPES OF PD SOURCES IN TRANSFORMER INSULATION ........................ 25
3.1 Typical PD patterns...................................................................................................... 25
PD pattern type 1 ................................................................................................................ 26
PD pattern type 2 ................................................................................................................ 26
PD pattern type 3 ................................................................................................................ 27
PD pattern type 4 ................................................................................................................ 27
PD pattern type 5 ................................................................................................................ 28
PD pattern type 6 ................................................................................................................ 29
3.2 Origin of PD sources in the electrical insulation system of transformers ........................ 29
PD sources due to inhomogenity in materials....................................................................... 31
PD sources due to the design .............................................................................................. 32
PD sources due to the manufacturing process ..................................................................... 32
PD sources due to components ........................................................................................... 34
PD sources due to acceptance test procedure ..................................................................... 34
PD sources due to final assembly on-site............................................................................. 34
PD sources due to operation................................................................................................ 34
4 PD SIGNALS IN TRANSFORMER ELECTRICAL INSULATION SYSTEMS..................... 36
4.1 Principles of detection and quantification of electrical PD signals .................................. 36
Electrical PD signal measurable at bushings........................................................................ 36
Measurement of apparent charge ........................................................................................ 36
Error due to integration of PD pulses.................................................................................... 40
Error due to calibration......................................................................................................... 41

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PD pattern ........................................................................................................................... 42
Attenuation of electric PD signals within a transformer ......................................................... 43
Limitations of electric PD signal detection within a transformer............................................. 47
4.2 Principles of propagation, detection and quantification of acoustic PD signals............... 47
Propagation of acoustic PD signals within a transformer....................................................... 48
Attenuation of acoustic PD signals within a transformer........................................................ 50
Limitation of acoustic PD signal detection within a transformer............................................. 51
4.3 Principles of propagation, detection and quantification of UHF signals ......................... 51
Propagation of UHF signals ................................................................................................. 51
Attenuation of UHF signals .................................................................................................. 53
Attenuation due the transformer tank ................................................................................... 55
Attenuation due to Windings ................................................................................................ 55
Limitation of UHF signals ..................................................................................................... 57
4.4 Principles of PD localization in transformers ................................................................. 57
PD source localization using electric signals ........................................................................ 57
PD signals in frequency domain........................................................................................... 58
PD signals in time domain.................................................................................................... 59
PD source localization using acoustic PD signals................................................................. 60
Mathematical formulation of acoustic PD location................................................................. 61
Acoustic methods “Time Differences Approach”................................................................... 62
Acoustic methods “Pseudo-Time Approach”......................................................................... 63
PD source localization using UHF PD signals....................................................................... 64
Mathematical formulation of UHF method for PD location..................................................... 65
5 PD MEASUREMENTS ON TRANSFORMERS IN A HV LABORATORY .......................... 70
5.1 Testing procedure ........................................................................................................ 70
5.2 Test circuit for PD measurements on transformers........................................................ 70
5.3 Permissible values of apparent charge ......................................................................... 71
IEC Standard....................................................................................................................... 71
IEEE Standard..................................................................................................................... 72
Multi Terminal Calibration according to IEC 60270 ............................................................... 72
Deficiencies of the test circuit............................................................................................... 73
5.4 PD source investigation................................................................................................ 75
Characterisation of the transformer under test...................................................................... 77
Localisation by using acoustic and UHF signals ................................................................... 82
5.5 Recommended procedure for successful solution of PD problems................................ 83
Step 1.1 .............................................................................................................................. 83
Step 1.2 .............................................................................................................................. 84
Step 1.3 .............................................................................................................................. 85
Step 1.4 .............................................................................................................................. 85
Step 2 ................................................................................................................................. 85
Step 3 ................................................................................................................................. 86
6 PD MEASUREMENTS ON-SITE....................................................................................... 87
6.1 Excitation of the transformer on-site ............................................................................. 87
6.2 PD measuring system for on-site.................................................................................. 88
6.3 PD test......................................................................................................................... 89
6.4 Suppression of electromagnetic interference ................................................................ 90
Measuring circuit.................................................................................................................. 90
Measuring procedure........................................................................................................... 91
Analysing software............................................................................................................... 92

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7 RISK ASSESSMENT BASED ON PD MEASUREMENT................................................... 94
7.1 Possible criteria for evaluation of the severity of PD sources......................................... 94
Dangerous PD sources........................................................................................................ 94
Less dangerous PD sources ................................................................................................ 98
Limits in identification of dangerous PD sources................................................................... 98
7.2 PD monitoring system for transformers......................................................................... 99
8 CONCLUSIONS.............................................................................................................. 102
9 BIBLIOGRAPHY/REFERENCES.................................................................................... 103
10 ANNEXES.................................................................................................................... 111
10.1 Recorded variations of PD pattern ........................................................................... 111
Tip electrode at the ground potential .................................................................................. 111
Floating conducing particle inside insulating material.......................................................... 112
Floating conducting particle on the surface of the insulating material.................................. 113
Floating conducting particle on the surface of the insulating material.................................. 114
Void with the contact to the electrode................................................................................. 115
Floating void inside the insulating material ......................................................................... 116
Floating void inside the insulating material ......................................................................... 117
Floating void with interaction at the surface of the insulating material ................................. 118
Floating voids in the glued insulating plates........................................................................ 119
Floating voids in different glue materials............................................................................. 120
Floating voids inside insulating materials............................................................................ 122
Carbonized tracks on the surface of the insulating material ................................................ 123
Bad contact of shielding electrodes.................................................................................... 124
10.2 Case Studies ........................................................................................................... 125
Case 1: Electrical and acoustic method ............................................................................. 125
Case 2: Electrical and acoustic Method ............................................................................. 128
Case 3: Acoustic method................................................................................................... 129
Case 4: Acoustic method................................................................................................... 131
Case 5: Electrical and acoustic method ............................................................................. 133
Case 6: Electrical and UHF method................................................................................... 135
Case 7: Acoustic and UHF method.................................................................................... 138
Case 8: Acoustic and UHF method.................................................................................... 141
Case 9: UHF method......................................................................................................... 148
Case 10: UHF method....................................................................................................... 150
Case 11: UHF method....................................................................................................... 152
Case 12: Advanced PD system ......................................................................................... 154
Case 13: Advanced PD system .........................................................................................156
Case 14: Advanced PD system .........................................................................................158

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EXECUTIVE SUMMARY
This report summarises the main developments in PD measurements on transformers during the last 10 years,
covering equipment and tests in laboratories as well as on-site. There is also a detailed discussion of a possible
procedure for the successful location of PD within a transformer.
The unambiguous identification of dangerous PD sources in the electrical insulation system of transformers
remains a topic for further research.
PD MEASUREMENTS ON TRANSFORMERS
Partial Discharges (PD) are partial electrical breakdowns in weak regions of an electrical insulation system.
Generally, there are two different types of partial discharges in all electrical equipment [IEC-1999, Duval-2001]:
- PD of the corona type. In the transformer insulation system these occur in gas bubbles in oil where
gas is ionized in cold plasma of low (ambient) temperature, which will produce mostly Hydrogen (H2)
together with some Methane (CH4) and will not damage or carbonize paper insulation. PD of the corona
type is also used to describe discharges into air or gas at the terminals of the transformer under test (if
shielding electrodes are not used).
- PD of the sparking type occurring in the liquid (oil) or solid (paper) phase. Such PD are small arcs
according to the conventions of physics their temperature is very high (> 3000 °C). They produce mostly
(C2H2) and Hydrogen (H2) and will damage the paper insulation (carbonized pinholes, tracking) and the
oil (decomposition of the oil, dissolved gas in oil).
Accordingly, the continuous presence of PD activity in the solid insulation of the transformer involves a
deterioration of surrounding materials and may in the long term lead to a total breakdown of the electrical
insulation system.
PD measurements on transformers include both: I) off-line testing in the laboratory, used as a basis for quality
assurance and acceptance testing (Factory Acceptance Test: FAT) to reveal contamination, manufacturing
errors or incorrect design, and II) on-site PD diagnostic measurements (off-line or on-line, Site Acceptance
Tests: SAT), or on-line monitoring on new or service aged transformers, where it can serve as a condition
assessment tool. In this Technical Brochure (TB), the interpretation and localisation methods as well as the
suggested procedure for solution of PD problems in transformers are valid for both FAT and SAT. The content
of the TB is based on experience from PD measurements on transformers filled with mineral oil. The same
procedures are expected to be applicable to transformers filled with new alternative insulating liquids.
INTERPRETATION AND EVALUATION OF PD RESULTS
In the past, simple PD patterns (see Figure 1a) representing the superposition of the applied voltage with fast
PD pulses collected during one cycle (two channel oscilloscope) were measured. However, in the last 20 years,
the Phase Resolving Partial Discharge Analyser (PRPDA) has increasingly been used to record PD patterns
(PRPD pattern, see Figure 1b).
These patterns display PD pulses (pC values in the calibrated test circuit) measured during a pre-set acquisition
time (recommended time: 1 minute). The PRPD pattern can be displayed in a three dimensional graph (see
Figure 1b), where the amplitude of the apparent charge (vertical axis), the phase position (horizontal axis) and
the number of counts (colour) are visible.
PRPD patterns deliver information about physical processes occurring during the PD in a weak region of the
electrical insulation system. The fundamental aspects of PD initiation and the characteristic PD signatures (PD
pattern) as recorded in the laboratory in models of the insulation system of transformers are described in
[CIGRE-2000]. In this document, it was shown that typical PD patterns exist that are representative for all
electrical insulation systems. These PD patterns can be used as a basis for identification of the type of PD
source detected during dielectric tests on transformers (see Typical PD pattern). For the interpretation of PD
results and judgement of the PD severity in an electrical insulation system, the recording of PRPD patterns has

Page 7
a significant advantage when compared to the conventional measurement of the magnitude of apparent charge
(e.g. in pC).
a) simple PD pattern [Tettex-1987] b) PRPD pattern [Carlson-2010]
Figure 1: Typical PD results
In the case of several different PD sources existing in the same electrical insulation system, the identification of
the correct type of PD source is more difficult due to superimposed PD signal clusters. This circumstance may
require a combination of methods such as different HV tests (single phase tests, applied voltage etc.), new
analysing tools (e.g. relating measured PD amplitudes from 3 synchronously measured PD-detectors see
section PD pattern) or combined measurements of electric, electromagnetic and acoustic PD quantities to
distinguish between different PD sources. The objective being to separate PD pulse data coming from different
sources, such that each localised PD activity can be displayed as an individual pattern for reliable interpretation.
LOCALIZATION
Beside the measured intensity of a PD activity (pC) and the identification of the type of the PD source (PD
pattern), the PD source localisation is an essential step towards risk assessment and judgement of the severity
of a PD source.
Before opening the transformer for repair, a reliable localisation of the PD source using the best available
methods should be applied. Beside the established measurement of acoustic PD signals (see PD source
localization using acoustic PD signals) the localisation using the detection of UHF PD signals in combination
with acoustic PD signals or exclusively using UHF PD signals in oil insulated systems is advantageous (see PD
source localization using UHF PD signals). These new methods have been successfully applied to solve
several PD problems as reported in the Annex. Finally, if no acoustic signals are detectable, the analysis of
electric PD signals in the time and frequency domain, recorded at measuring taps of bushings is an alternative
method to localise PD sources hidden in the electrical insulation system of oil filled power transformers (see PD
source localization using electric signals).
PROCEDURE FOR SOLUTION OF PD PROBLEMS
Based on discussions in WG D1.29, an investigation procedure (containing a number of steps) was established
for measurements in HV laboratories and on-site to assess PD problems. A detailed description of this
procedure is given in chapter 5.5. Recommended procedure for successful solution of PD problems.
The calibration procedure in Step 1 of the procedure should be considered as a characterisation of the
transformer under test. Both the calibration coupling matrix (pC values) and the transfer function coupling matrix
are only valid for a PD source located close to the terminal where the calibrating signal was injected. For PD
sources that are located deep in the electrical insulation system, those coupling matrices are not applicable.
phase
amplitudeinpC
phase
amplitudeinpC
amplitudeinmV
time

Page 8
Therefore, the amplitude of the apparent charge is not always a meaningful criterion in determining whether the
detected PD source is harmful to the electrical insulation system of the transformer (see Measurement of
apparent charge).
The analysis of results from Step 1 usually reveals the type of the PD source (via the PD pattern) and possible
locations (via analysis of PD signals in time and frequency domain). In connection with information about the
design of the transformer under test, suitable positions for placing acoustic sensors or UHF probes can be
defined (Step 2). If possible, UHF probes (UHF sensors) should also be introduced into the tank of the
transformer (for example using an oil valve, see Non-standard electrical PD signal detection systems).
The in depth analysis of all recorded data in Step 2 reveals the location of the PD source or defines the next
steps in the investigation, such as the application of a single phase test or an applied voltage test to get more
information about the electrical field at the location of the PD source.
The shape of the PD pattern generated by electrical, acoustic and UHF signals should be similar if all recorded
PD signals belong to the same PD source. This is not valid for a strongly asymmetrical PD pattern. In the case,
where the localisation of the PD source using the analysis of acoustic PD signals and/or UHF signals is not
successful, there is the possibility of localizing the PD source directly in the active part (transformer without oil)
by injecting calibrating signals at locations suggested by results of previous investigations (see Step 3).

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STEP 1: ANALYSIS OF PD PATTERN AND “CALIBRATION”
PD pattern [Fuhr-PD test]
Analyse the PD pattern recorded at bushings (according to
IEC 60270) during three phase induced voltage test and
confirm that the detected PD activity is inside the electrical
insulation system.
U V W
U 100% 25% 17%
V 47% 100% 54%
W 18% 30% 100%
time 15.00 15:01 15:02
Coupling matrix
The calibration PD coupling matrix is used to define coupling
at each terminal, where a calibrated pulse generator is
connected to the terminal with the highest pC value recorded
in previous PD tests.
PD measurements using wide band PD sensors (up to 30 MHz) at all accessible bushings of the transformer
(HV side, LV side, neutral terminal, grounding of the core and press plate).
PD signal in time domain [Fuhr-PD test]
Coupling matrix signals in the time domain are a response to
the RLCM network of the transformer to the calibrating signal
injected at the terminal where the highest PD values were
detected (characterization of the transformer under test).
PD signal in frequency domain [Fuhr-PD test]
Coupling matrix of signals in the frequency domain as a
response to the RLCM network of the transformer to the
calibrating signal injected at the terminal where the highest
PD values were detected (characterization of the
transformer).
Recording of the PD pattern and the real PD signals in the time and frequency domain under the same testing
conditions at all bushing (where PD signals are available).
In depth analysis of all recorded data and comparison with the characteristic values of the transformer
(calibration coupling matrix, transfer function coupling matrix).
HV
LV
LV
HV

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STEP 2: POSITION OF ACOUSTIC AND UHF PROBES
Acoustic sensors [OMICRON]
Analysis of recorded acoustic signals using software (3D
model of the transformer) [Kraetge-2010]
UHF probe [OMICRON, Judd-2005]
Analysis of recorded UHF signals [Appendix-case 8]
Recording of all data described in STEP 1 with simultaneous detection of acoustic and UHF PD signals.

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STEP 3: SIMULATION OF THE PD SOURCE
[Fuhr-PD test]
Injection of calibrating signals at locations on the active
part of the transformer insulation at assumed PD source
locations. Measurement of responses at bushings, or at
leads of windings connected to external coupling
capacitance.
Comparison of PD signals in the time and frequency
domain recorded during the test with responses of the
active part (RLCM network) to excitation by the calibrating
signal at assumed PD source locations.
Measured PD signal [Fuhr-PD test] Calibrating signal [Fuhr-PD test]
The location of the real PD source is where the responses to the calibrating signal, as applied to the insulation, deliver
the closest match to measured signals in the time and frequency domain (see case studies in Annex).
KEYWORDS
Power transformer, partial discharge (PD), partial discharge sensors, partial discharge systems, partial
discharge signals, time domain partial discharge analysis, frequency domain partial discharge analysis,
apparent charge, acoustic partial discharge signals, UHF partial discharge detection, partial discharge defect
types, analysis of partial discharge pattern, localisation of partial discharge sources, on-site partial discharge
measurements, criteria for dangerous partial discharge sources, partial discharge risk assessment.

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1 Introduction
Large power transformers are amongst the most expensive and strategically important components of any
power generation and transmission system. Reliable operation of transformers is strongly dependent on the
ability of the insulation system to withstand permanent electrical stress without damage during the expected life
of typically more than 40 years. During service, the transformer’s insulation system is continuously aging,
primarily through a combination of electrical, mechanical, thermal and chemical stresses. As a result of this
"normal" aging process, weak regions with decreased dielectric strength are generated and randomly distributed
in the insulation system. Weak regions with decreased dielectric strength are potential sources of permanent
partial discharge (PD) activity which in the long run can be harmful to the safe operation of any HV component.
For this reason, partial discharge measurement is an important tool in establishing the actual condition of all HV
electrical insulation systems. PD measurement is one of the few non-destructive methods, able to detect local
defects (PD sources) in complex electrical insulation systems, such as those in power transformers. The
theoretical background, the definition of the PD measurement test circuit and the acceptance criteria based on
the amplitude of the apparent charge in pC or μV are well established and described in various standards (IEC
60076-3, IEC 60270, IEEE C57) and in the numerous publications [Schon-1986, Tettex-1994, König-1993,
Kreuger-1992, Kuffel-2000, Hauschild-2014].
There are two generic classifications of PD measurement scenarios:
Group I) Laboratory PD measurements at a transformer manufacturer’s facility where PD measurements are
carried out for quality assurance to reveal contamination, manufacturing errors or incorrect design. Here the
transformer manufacturer seeks to find the technical reasons for PD generation in their products and
subsequently minimize the likelihood and influences of PD in their products; through type tests and routine tests
before consignment to their customers (Factory Acceptance Tests).
Group II) are on-site PD diagnostic measurements or monitoring on new or service aged equipment where PD
measurements can be conducted either off-line or on-line. Typical applications of on-site PD measurements are
for commissioning of new equipment or after repair at site, or for diagnostic purposes as part of an asset
management program. Consequent application of new diagnostic methods in site acceptance tests might
comprise: shift from a time to a condition based maintenance strategy; early recognition of insulation
degradation; detection of incipient faults; reduction of the cost of outages and unplanned apparatus repairs or
replacement; staff safety and the reduction of risk to the environment; identifying bad workmanship after final
assembly; fleet characterisation; optimization of investment decisions; etc.
On-site in-service PD source detection has historically involved mainly dissolved gas analysis (DGA) based on
oil samples taken periodically from the transformer tank, with hydrogen used as a key indicator for PD activity in
the active part of a transformer. DGA results in an indirect PD measurement, being slower to identify a rapidly
evolving PD and usually unable to localise the PD for additional risk assessment. From experience with DGA a
minimum amount of gas formation is necessary to declare a PD activity to be significant [Duval-2001]. On the
other hand, there are several practical examples where no increase of combustible gases was recorded despite
a PD source being detected by electric and acoustic methods and confirmed through disassembling the
transformer. Therefore, electric, acoustic and electro-magnetic PD pulse based measurements are of a more
instantaneous and direct nature. These can successfully be used for in-depth analysis of PD activity and the
assessment of the severity and threat posed by the PD to the safe operation of a transformer.
The goal of the work of WG1.29 was to summarize the progress in PD measurements on transformers taking a
wide view. Both the application of different measuring systems and different measurement principles, as well as
the analysing software are described. A number of practical case studies are show in the Annex, where for the
successful solution of PD problems the testers have to go beyond the requirements specified in the IEC and
IEEE Standards. Generally, the investigation procedure must be adapted to the behaviour of the PD source.
Finally, an attempt has been made to define possible criteria for distinguishing between dangerous and less
dangerous PD sources in the oil impregnated electrical insulation of power transformers. This topic is of such
importance that it will likely be the subject of further research and development in the field of transformer PD
defect analysis.

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2 Partial Discharge Detection Systems for Transformers
Partial discharge is an electrical breakdown in a weak region in the electrical insulation system where the local
electrical field exceeds the dielectric strength of the specific material.
An overview about physical effects of the permanent PD activity is shown in Figure 2.1.
Figure 2.1 : Overview about physical effects of the PD [Schwarz-Siemens]
At the source, the local electrical breakdown (PD) generates a fast current impulse with short rise and fall times
(usually in the range between ns-µs, depending on the insulating medium). This fast current impulse is damped
and dispersed by the active part of the transformer. Hence the pulse measured at the bushings of the
transformer is not the original fast current pulse.
In the electrical insulation system of HV components the following measurable physical signs occur as a
consequence of the fast current impulse (PD) in a weak region:
- Electric signals are measurable at bushings of the transformer.
- Electro-magnetic transient waves (up to GHz range) are measurable via antennas.
- Acoustic sound waves are measurable via acoustic sensors on the tank of the transformer.
- Decomposition of the insulating material (oil and cellulose) is detectable by analysis of oil samples.
2.1 Measuring system for detection of electric signals
CONVENTIONAL PD MEASURING SYSTEM
All PD systems based on measurement of electric signals are detecting the circulating PD current impulses i(t)
in the parallel-connected capacitors Ck and Ct via measuring impedance Zm (see Figure 2.2). In Figure 2.2a
coupling capacitance is the capacitive bushing with Ck = C1 and Cm parallel connected to the measuring
impedance Zm. The measuring impedance Zm is either that of the RLC circuit (frequency range up to 10 MHz)
or, alternatively a high frequency current transformer (HFCT, frequency range up to 30 MHz).
Physical effects of partial discharge
High frequency wave
Pressure wave
(Sound)
Optical effects
(Light)
Chemical effects
Electrical signal
Partial dischargeHeat
Two basic kinds of detection
Electrical
- Conventional measuring method (IEC 60270)
narrow-band, limited wide-band, wide-band
- HF - Measuring technique
- UHF - Measuring technique
optical
chemical
acousticalNon
electrical
methods
(proof
procedures)
Physical effects of partial discharge
High frequency wave
Pressure wave
(Sound)
Optical effects
(Light)
Chemical effects
Electrical signal
Partial dischargeHeat
Two basic kinds of detection
Electrical
- Conventional measuring method (IEC 60270)
narrow-band, limited wide-band, wide-band
- HF - Measuring technique
- UHF - Measuring technique
optical
chemical
acousticalNon
electrical
methods
(proof
procedures)
acoustic

Page 14
If the capacitive bushing is not available (old transformers or LV bushings), an external coupling capacitance is
connected parallel to the bushing (see Figure 2.2b).
In Standards (IEC, IEEE) the measurement of apparent charge q is the required parameter. The amplitude of
apparent charge measured in pC or in µV is the integral of the PD current impulse detectable at bushings of the
transformer. The integration of PD current impulses can be performed either in the time domain (digital
oscilloscope) or in the frequency domain (quasi integration with a band pass filter). Most PD systems available
on the market according to IEC 60270 are using two different band pass filters [IEC, 2000-2].
- wide band (bandwidth 900kHz, upper frequency:1MHz)
- narrow band (∆f: 9kHz up to 30kHz, centre frequency: 50kHz ≤ 1MHz)
A detailed description and theory of the principle of the quasi integration using band pass filters is described in
numerous papers [Tettex-1994, Schon-1986].
a) coupling capacitance Ck is the bushing b) external coupling capacitance Ck
U = test voltage, Z = impedance of the connection of the test voltage to the transformer under test, C1 = capacitance of
the bushing, Ct = capacitance of the test object, Cm = capacitance of the measuring tap, Ck = coupling capacitance,
Zm = measuring impedance, PDS = measuring system
Figure 2.2 : Test circuit arrangement for electrical PD measuring system on transformers [IEC-2000-1]
The sensitivity of the common PD circuit is dependent on the value of the coupling capacitance Ck (Figure 2.3).
For Ck = Ct the measurable charge qm corresponds to 0.5 q. With the decreasing value of the coupling
capacitance (Ck < Ct) the sensitivity is approaching zero. In the test circuit for PD measurements on
transformers the coupling capacitance Ck is defined by the capacitance of the capacitive bushing C1 (Figure
2.2a). Typical values of capacitances of HV bushings are between 400-600pF. If the capacitance of the bushing
is very low, an external coupling capacitance should be connected to reach higher sensitivity in the test circuit.
Z
U
C
C
C
C Z1 m
k
t
m
PDS
}
Z
U
C
C
Z
k
t
m
PDS
}

Page 15
Figure 2.3 : Influence of the value of coupling capacitance on measurable apparent charge
[Carlson-2010]
The classical PD measuring system, which has been around for over 50 years, is presented in Figure 2.4. The
result of this measuring system is the mean value of the amplitude of apparent charge.
Figure 2.4 : Example of conventional PD system [CIGRE-2008-2]
PHASE RESOLVED PD DATA PROCESSING SYSTEM
Developments in measuring techniques and instrumentation led to the introduction of Phase Resolving Partial
Discharge Analyser (PRPDA) systems over 20 years ago. These had input circuits using the same “band pass
filters” (wideband or narrowband) as used in conventional PD measuring systems. PRPDA systems comprised
a two dimensional multichannel analyser connected to a computer which performs a statistical analysis of the
recorded data. At the specific test voltage, the integrated PD current impulses (pC or µV) are saved as a
function of the phase position and of the amplitude of apparent charge during a pre-set measurement time. The
results are finally presented as two dimensional or three dimensional PD patterns (amplitude of apparent
charge, phase position and number of counts). These are presented two dimensionally with the third dimension

Page 16
being colour, indicating the total number of PD impulses collected during the pre-set measuring time and
occurring at the same phase position and at the same amplitude. To ensure sufficient information about the PD
source is gathered, a minimum recording time equivalent to 3000 cycles is recommended (for 50 Hz the pre-set
measuring time would be 60 seconds). The main components of modern PRPDA systems are shown in Figure
2.5.
a)
b)
c)
Figure 2.5 : Main component of the PRPDA-system
a) principle of the PRPDA system [Haessig-2003]
b) analogue pre-processing and digital post processing of PD impulses [CIGRE-2008-2]
c) digital pre-processing and post processing of PD impulses [CIGRE-2008-2]
The PD pattern is the representation of the physical processes during the electrical breakdown in a weak region
of the electrical insulation. Different types of PD defects (see chapter Typical PD patterns) can be recognized

Page 17
by their specific PD pattern. Depending on the position of the PD source, the amplitude of the apparent charge
might change, but the shape of PD pattern (for example symmetrical or asymmetrical clusters of PD pulses) is
not influenced by the structure of complex electrical insulation systems. When PD activity is recoded at a
specific bushing, the type of the PD source can be identified, even if the amplitude of apparent charge is very
low. Typical examples of different presentations of PRPD patterns are shown in Figure 2.6.
a) linear unipolar b) liner bipolar c) three dimensional
d) logarithmic unipolar (increasing
amplitude from bottom to top)
e) logarithmic bipolar [OMICRON] f) linear unipolar (increasing amplitude
from top down)
Figure 2.6 : Different representations of PRPD patterns [Fuhr PD-test, OMICRON]
Registration of PRPD patterns has two advantages over measurement of mean values of the amplitude of
apparent charge as required in IEC Standards:
1. Recognition of the type of the PD source (statistical analysis of PD activity, see chapter 3.1)
2. Meaningful definition of the value of apparent charge, especially in the case when permissible values
during the delivery test are reached (see Figure 2.7)
Figure 2.7 : Definition of different values of apparent charge [Carlson-2010]
phase
amplitudeinpC
phase
amplitudeinpC
IEC pC value of
apparent charge
(average)
maximal pC value of
apparent charge
phase
pC
phase
pC
phase
pC
pC
phase

Page 18
The value of apparent charge recorded during the FAT is always an average magnitude (definition in IEEE-
2010: apparent charge level: Mean value of the apparent charge of partial discharge (PD) pulse trains
measured in terms of picocoulomb (pC) by means of PD instruments). The PRPD pattern allows the separation
of individual sources of PD that would otherwise potentially result in pulse loading of the conventional system
and would provide the wrong indication. In the example in Figure 2.7, the superimposed PD source due to the
surface discharge will be not recognised by the measurement according to IEC Standard.
Due to automated testing procedures used in a FAT, these days PRPDA systems are almost universally used in
HV laboratories. In the first instance only IEC standard pC values are recorded. For the suitable control of
transformer quality, it is recommended that PD patterns are recorded at all voltage levels when pC values
exceed the basic noise level in the laboratory (normally 5 pC in a shielded laboratory).
2.2 Non-standard electrical PD signal detection systems
When aiming to find the PD source as quickly as possible, the detailed investigation and localization of a PD
source must often go beyond the requirements specified in the IEC and IEEE standards [Fuhr-1993, Carlson-
2003].
The PD current impulses measurable at bushings of the transformer are the response of the RLCM network of
the electrical insulation system to the excitation caused by the current impulse generated during the discharge
in a weak region (PD source) of the insulation at an unknown location. Only PD sources very close to the
specific bushing or in the bushing itself are not influenced by the attenuation and reflection phenomena of the
RLCM network of the active part of the transformer.
Using this knowledge, the measurable electric signals at bushings contain information about the location of the
PD source. Therefore, a multi-terminal measurement of electric signals, in time and frequency domain, is
expected to be a promising method for localizing a PD source; such a system is shown in Figure 2.8.
Figure 2.8 : Equipment for non-standard measurement of PD signals [Haessig-2003]
High frequency measuring impedance: High frequency measuring impedance Zm, for example HF-current
transformers (HF-CT, 100 kHz - 30 MHz) should be connected to each bushing of the HV component (see
Figure 2.9).
Frequency domain measurement: A spectrum analyser is used to analyse PD signals in the frequency
domain (full span mode) and to perform quasi-integration of PD signals, to evaluate the value of apparent
Oscilloscope for measurements
in time domain
Spectrum analyzer for measurements
in frequency domain
PRPDA for recording
a PD pattern

Page 19
charge in pC using the variable band-pass filter of the spectrum analyser (zero span mode). A spectrum
analyser can be used as a front end for the PRPDA system. The analysis of the frequency spectra gives rough
information about the location of the PD source (see PD source localization using electric signals).
Time Domain measurement: A digital oscilloscope can be used as an analysing device for PD signals in the
time domain. Additionally, a digital oscilloscope should always be used to qualify the input signals for the
conventional PD system (band pass filter) so as to estimate both, the integrating error (amplitude of apparent
charge) by the deviation of the real PD signal from the idealized PD current impulse (see Error due to
integration of PD pulses) and to select correct settings of the PRPDA system (dead time to prevent double
counting of PD pulses).
a) example of the connection of high frequency measuring impedance (high frequency current transformer HF-CT)
b) typical frequency response (phase and amplitude) of the HF-CT
Figure 2.9 : High frequency measuring impedance [Haessig-2003]
Voltage divider
High frequency current transformer (HF-CT)
Trace 1 = phase [deg]
Trace 2 = amplitude [dB]
A = possible application region
B = constant amplitude region (0 dB)
2
1

Page 20
Typical result of recorded PD signals in the time and frequency domain is shown in Figure 2.10.
Calibrating signal at HV-bushing PD current impulse at HV-bushing
Figure 2.10 : Typical calibrating signal and PD current impulse in time and frequency domain
[Fuhr-PD test]
2.3 System for detection of acoustic sound waves
The acoustic PD method is based on the detection of acoustic waves emitted by each PD source. Acoustic
waves result from the pressure transient associated with the PD event (vibration in elastic medium). Because of
the short duration of PD impulses, the resulting compression waves have frequencies up to ultrasonic
frequencies (some kHz up to 1MHz). Therefore, the frequency range used for piezoelectric sensors mounted on
the outside of the transformer housing is generally from 10 to 300 kHz. The velocity of acoustic waves in oil, for
operational temperatures between 50° C and 80° C, may vary from around 1240 m/s to 1300 m/s [Howells-
1984].
There are two types of acoustic sensors available:
- wideband type
- resonant type (piezoelectric transducer with resonance frequency between 60kHz-150kHz)
In terms of sensitivity it is recommended to use resonant type acoustic sensor rather than wideband ones. In
Figure 2.11 examples of acoustic sensors are presented.
Acoustic measuring systems consist of acoustic sensors (at least 3 sensors recommended), transient recorder
and analysing software. There is no standard for measurement of acoustic signals, but there is an IEEE Guide
C57.127 [2007] describing applications of the acoustic method. It is not possible to calibrate the acoustic
system, that is amplitudes of acoustic and electric signals (amplitude of apparent charge) are not comparable.
However, for a single PD source the phase correlated occurrence of acoustic and electric signals should be
comparable (see PD source localization using acoustic PD signals).
The acoustic method is normally used for PD source localisation in combination with other PD detection
methods. Examples of measurements of acoustic impulses for localisation purposes are presented in case
studies in the Annex, where localisation approaches are described. Detailed descriptions of both the acoustic
systems and their application for localisation of PD sources can be found in [Großmann-2002, Markalous-2006]
or in the guidelines published by CIGRE working group CIGRE-2010. Figure 2.12 shows a typical acoustic PD
signal recorded on the tank wall of the transformer.
ns/div
time domain time domain
frequency domain frequency domain
ns/div
MHz/div
MHz/div
mV/div mV/div
dBm/div
dBm/div

Page 21
Acoustic sensor [OMICRON] Preamplifier and acoustic sensors [OMICRON]
Holder for acoustic sensors [Power Diagnostix] Principle of acoustic sensor [Markalous-2006]
Figure 2.11 : Example of acoustic sensor (resonant type)
Figure 2.12 : Example of typical acoustic PD signal [Markalous-2006]
200 300 400 500 600
-2
-1
0
1
2
'PD-signal'
amplitude(V)
time (λs)
sensor 6
geometric
arrival time
'PD-noise'
(structure-borne
interference)

Page 22
2.4 System for detection of UHF signals
The basis of UHF PD detection is that the very rapid formation of an electron avalanche (current pulse) in any
part of the insulation system will radiate an electromagnetic transient wave. Due to the short rise and fall times
of PD current pulses (which usually contain transients of ≤ 1 ns), the spectrum of the radiated signal extends
into the GHz. Advantages of this technique include sensitivity, due to the fact that background noise is usually
reduced compared to what is seen at lower frequencies, and its use in locating PD sources using time-of-arrival
methods due to the fact that the signal propagation velocity is well defined. The UHF method has been
investigated in many scientific papers and has become widely used in GIS monitoring applications [Judd-2001].
The UHF method is classified as a non-conventional method in the upcoming IEC 62478 standard. The last
decade has seen a continual evolution of the UHF method, including installation of detectors on power
transformers. Several examples of UHF probes developed for transformer tests are shown in Figure 2.13.
As a measuring system a wide band, high sampling rate, 4 channel (or more) oscilloscope is commonly used
when accurate timing for PD source location is needed. Alternatively, for continuous monitoring or PD pattern
analysis, dedicated UHF monitoring equipment of the kind used for GIS can be adopted. Commonly, such a
system does not digitise the ‘raw’ UHF signal (due to the very high sampling rate) but uses an RF receiver front
end to detect the amplitude of each burst of UHF signal corresponding to the individual PD pulses.
The UHF signals can be used as a trigger for the monitoring of acoustic signals. Through averaging of the UHF
triggered acoustic signals, acoustic noise and trigger jitter can be much reduced compared with using only
acoustic sensors.

Page 23
UHF probe for installation in oil drain valve [OMICRON] UHF probe installed on the tank wall [Judd-2005]
UHF probe ready for insertion through an oil drain valve
[OMICRON]
UHF probe installed through a spare gate valve for oil cooler
[OMICRON]
Internal view of the UHF probe after installation (oil was
lowered at this point during repair) [OMICRON]
Top view of a transformer showing permanent, internal UHF
probes fitted to the circular hatch plates [Judd-2005]
Figure 2.13 : Examples of UHF probes suitable for transformers

Page 24
Figure 2.14 shows typical UHF signals measured during a transformer PD test.
Figure 2.14 : Example of typical UHF PD signals
(single phase test on the transformer, PD source of 200pC) [Appendix-case 9]
2.5 System for detection of chemical reactions-decomposition of
insulating materials
Continuous PD activity in an electrical insulation system of HV components generates measurable chemical
products due to the decomposition of the insulating material involved.
In transformers the amount of gas content in the insulating oil can be determined through Dissolved Gas
Analysis (DGA) of an oil sample [CIGRE TB 409, Duval-1989, Duval-2001, Knab-1993]. DGA is performed
either through a laboratory analysis of an oil sample taken from a transformer, or through the use of a
permanent gas monitoring system (Hydrogen or multi gas sensors, see Figure 2.15). In the case where DGA
indicates PD activity inside a transformer, an on-site PD measurement can help to find the reason for the DGA
result (see chapter PD measurements on-site).
Figure 2.15 : Example of monitoring system for dissolved gases in oil [Fuhr-BKW Energy Ltd]
time in ns
amplitudeinmV
gas sensor

Page 25
3 Common types of PD sources in transformer insulation
A PD source in the electrical insulation system of a transformer results from a weak region where continuous
electrical breakdown occurs either at nominal voltage for transformers in service or at a specific test voltage
during transformer HV testing.
Two requirements must be fulfilled to initiate a PD within a weak region of an electrical insulation system:
1. Local electric field strength E in the weak region must be greater than the inception electric field of the
PD source (depends on the insulating material)
2. Free electrons must be available to initiate the electric discharge (dependent on the position of the weak
region with respect to its contact to an electrode)
For new transformers, excessive stress in a weak region can result from design flaws, contamination or
deviation from permissible tolerances in the manufacturing process, insulating material flaws, etc. Another
possibility is hidden damage to the insulation caused by previous tests like lightning impulse test or heat run
test.
For transformers in service, a combination of different stresses, like electrical, mechanical, thermal and
chemical, may cause PD sources which are in most cases detectable by DGA of the insulating oil [CIGRE TB
409, Duval-2001, Duval 1989]. PD activity in gas bubbles in oil produces mostly hydrogen (H2) together with
some methane (CH4). PD-activity in solid insulation (paper, pressboard) may produce a variety of dissolved
gases, in particular carbon monoxide (CO), carbon dioxide (CO2) and water (H2O). Arcing in oil or across
interfaces cellulose/oil is characterized by acetylene (C2H2), methane (CH4), ethane (C2H6), ethylene (C2H4) and
hydrogen (H2).
For PD sources in bushings of the transformer, or PD sources completely enclosed in solid insulation, there may
be no significant increase of dissolved gases in oil despite the on-going permanent PD activity during operation.
3.1 Typical PD patterns
As mentioned in chapter 2.1, PD patterns reflect the underlying physical phenomena (statistical behaviour) of
specific PD sources. The statistical behaviour of the PD source is mainly influenced by the availability of starting
electrons, which trigger an electric discharge in a weak region in the transformer electrical insulation system
[Fruth-1990, Fuhr-1991]. The availability of starting electrons is strongly dependent on the PD defect itself (solid,
fluid or gaseous material) and on the position of the PD source with respect to metallic electrodes. Based on the
physics of electrical discharge processes in the weak region of the insulation, it is possible to define 6 typical PD
patterns (see Figure 3.1 to Figure 3.6).
The recording and analysis of PD patterns are important tools for the judgment of the quality of all HV electrical
insulation systems for the following reasons:
- Due to the same statistical behaviour of a single PD source at the same location, PD pattern recorded
by different measuring methods would have theoretically the same shape. During the investigation
procedure the PD pattern of electric signals, acoustic signals and electro-magnetic waves (VHF, UHF)
should be compared to make sure, that all recorded signals are generated by the same PD source. For
PD sources, generating extremely asymmetric PD signal clusters (tip electrode), the recorded PD
pattern of electric signals and acoustic signals may be different.
- Theoretically the shape of the PD pattern (symmetric or asymmetric PD signal clusters) is not
influenced by the structure of the electrical insulation system, even if the amplitude of the apparent
charge will decrease for PD sources located far away from the specific bushing. As soon as the
sensitivity of the detecting system is sufficient, the typical PD pattern can be recognized.
- With increasing test voltage, the shape of PD pattern may change due to the superimposed new PD
activity triggered at higher electrical fields. Different PD sources can easily be distinguished by their PD
pattern.

Page 26
PD PATTERN TYPE 1
Conducting material directly connected to the metallic electrode (tip electrode)
PD source Schematic PD pattern Measured PD pattern
Figure 3.1 : Example of typical PD pattern type 1 [Schwarz-Siemens, Carlson-2010]
PD PATTERN TYPE 2
Conducting material without any contact to the metallic electrode (floating particles)
E
e1
E
e1
E
e1
E
e1
phase
amplitudeinpC
phase
amplitudeinpC

Page 27
PD PATTERN TYPE 3
Conducting particles laying on the surface of the insulating material surface (surface
discharge, creepage discharge)
PD PATTERN TYPE 4
Non conducting material (cavity) with direct contact to the electrode
Cavity with contact to HV-
electrode
Cavity with contact to ground electrode
E
e1
e2
E
e1
e2
E
e1
e2
E
e1
e2

Page 28
PD PATTERN TYPE 5
Non conducting material (cavity) without direct contact to the electrode
Depending on the location of the cavity and on the material of the glue, there are different PD pattern
representing a “bubble type PD pattern”
E
e1
e2
E
e1
e2
phase
amplitudeinpC
phase
amplitudeinpC
E
e1
E
e1
phase
amplitudeinpC
phase
amplitudeinpC
E
e1
e2
e2 E
e1
e2
e2
phase
amplitudeinpC
phase
amplitudeinpC

Page 29
PD PATTERN TYPE 6
Non conducting material (cavity) without direct contact to the electrode with interaction at
the surface
Examples of additional PD patterns, measured in the electrical insulation system of transformers (in HV
laboratories or on-site) and their deviations from the typical shape of PD patterns are discussed in Annex 1.
3.2 Origin of PD sources in the electrical insulation system of
transformers
Oil filled power transformers (see Figure 3.7) consist of the following main components:
- Core (stainless steel)
- Windings (different type of copper wires)
- Electrical insulation system (different type of cellulose for copper wire and pressboard barriers for the
main electrical insulation system)
- Oil for the electrical insulation system and cooling (active part is in the tank)
- On load tap changer (OLTC)
- Tank
- Bushings
The cellulose and oil are the most important materials of the electrical insulation system of power transformers
(see Figure 3.8). Soft paper tape is used to insulate copper wires of windings, shielding rings of windings and
all leads to bushings or to the tap changer. Laminated wood or pressboard is used for clamping rings of
windings and for all supporting parts of leads. Pressboard barriers are used for the main insulation between low
voltage and high voltage windings, between the phases and for the insulation against the grounded core and
tank.
PD sources in the oil impregnated electrical insulation system of the transformer can be caused by:
- inhomogeneity’s in materials
- flaws in the design
- contamination or change of permissible tolerances during manufacturing process
- failures during final assembly
In the following section, the origin of different PD sources, their typical PD pattern and the probability of reliable
detection will be discussed.
E
e1
e2
E
e1
e2
phase
amplitudeinpC
phase
amplitudeinpC

Page 30
Figure 3.7 : Example of the active part of power transformer [Siemens]
a) old design (separate press plates of windings)
[Moser-1979]
b) actual design (common press plate of windings)
[Kuechler-2009]
Figure 3.8 : Schematic presentation of the electrical insulation system of a transformer
Core Low-
voltage
High-
voltage
oil
canal
E(x,t)
Core Low-
voltage
High-
voltage
oil
canal
E(x,t)

Page 31
PD SOURCES DUE TO INHOMOGENITY IN MATERIALS
PD sources due to failure of materials are mainly caused by the manufacturing process of components, like kits
for the electrical insulation system or copper wire for the windings.
Kits for the electrical insulation system contain pressboard (transformer board) barriers with insulating spacers,
angle rings, coil collars and different "snout sections" for HV lead exits etc. In any of these components, a
metallic particle may be introduced during the manufacturing process. Especially sensitive are formed parts like
coil collars and lead exits which are exposed to elevated dielectric stress. A metallic particle hidden in such
insulating parts without contact to a metallic electrode will normally generate Type 2 PD pattern (conducting
particle without contact to an electrode). To eliminate metallic particles in cellulose insulation, a final quality
control during the manufacturing process using X-rays should be applied.
Beside PD sources caused by metallic particles hidden in the cellulose, cavities in the glue of clamping rings are
sometimes the reason for PD activity during delivery tests. The required thickness of clamping rings is reached
by gluing several pressboard plates or laminated wood together. Cavities in the glue generate Type 4 PD
patterns (cavity with contact to electrode), Type 5 (cavity without contact to electrode) or Type 6 (cavity without
contact to electrode with interaction at the surface). Depending on the location of the cavity (with or without
contact to the electrode) and on the composition of the glue, there are different PD patterns representing a
"cavity type PD source".
In some cases “diamond paper insulation" is used to reach the required short circuit stability for a specific
winding, which may introduce bubbles during the curing of the winding. Such PD sources would generate PD
pattern of Type 5 (cavity without contact to electrode).
Finally, there may be PD defects caused by the manufacturing process of copper wires (paper or enamel
insulated) used for different types of windings. There may be a "tip electrode" or small radius effects of the
copper wire. Such PD defects would deliver a PD pattern similar to Type 1 (conducting particle with contact to
electrode). To avoid such PD sources, an appropriate cross sectional profile of the copper wire should be
specified. In Figure 3.9 an example of the insufficient radius of the copper is shown.
Figure 3.9 : Checking of the radius of the copper wire [Fuhr-BKW Energy Ltd]
PD defects caused by material failures are in most cases hidden deep in the electrical insulation system. The
localisation of such PD sources tends to be time consuming due to the fact, that there are very often only weak
or no acoustic signals available. Concerning the necessary detection sensitivity, UHF PD techniques are
promising. Also more elaborated PD source localisation techniques, such as UHF or combined UHF-acoustic
methods can be helpful (see chapter Principles of PD localization in transformers). To identify and eliminate PD
problems, the active part of the transformer must be disassembled in most cases. Examples of possible PD
sources in material are shown in Figure 3.10.

Page 32
Conducting particle with contact to
electrode (copper for windings)
Cavities with or without contact to the
electrode
Conducting particle without contact to
the electrode (particles in pressboard)
Figure 3.10 : PD sources due to an inhomogeneity in the material [Carlson-2010]
PD SOURCES DUE TO THE DESIGN
Due to highly developed simulation programs for calculating and simulating electrical stresses under different
test conditions and due to a very good knowledge of the insulating materials, PD problems caused by bad
design are very rare.
PD SOURCES DUE TO THE MANUFACTURING PROCESS
As the manufacturing process of transformers relies on a large amount of manual work, there is always the
possibility of the introduction of a PD source into a well-designed electrical insulation system. Some practical
example are covered in the following section.
Core: When cutting core lamination sheets, the well-known whiskers may end up being the reason for PD,
resulting in a Type 1 (conducting particle with contact to electrode) pattern.
Other reasons for PD sources in the core may be ungrounded parts of the core or in core accessories lying in
high electrical field areas, like unshielded sharp corners of flux collectors or screws. The PD patterns will be
similar to Type 1 (conducting particle with contact to electrode) or similar to Type 2 (conducting particle without
galvanic connection to metallic electrode).
Assembly of the active part: In a polluted environment, the assembly of the main insulation between HV and
LV windings may result in the inclusion of metallic particles, which may result in Type 2 (conducting particle
without contact to electrode) PD patterns.
Generally, all connections to windings, lead exits to bushings, leads to the tap changer etc., can be a source of
PD, if the shielding of these connections are not made properly. The resulting PD patterns will be similar to Type
1 (conducting particle with contact to electrode) or to Type 2 (conducting particle without galvanic connection to
metallic electrode).
Creepage discharges along solid-liquid interfaces tend to cause damage to the solid insulation. This was
investigated using a tip electrode on the pressboard surface [Yi-2013]. In the transformer insulation system, a
creepage discharge may be generated by conducting particles laying on the surface of the insulating material,
or at so called “triple points” with excessive field stress in the main insulation system. Typical triple points in a
barrier system of a transformers exist at locations where the pressboard barriers, the distance ledges and the
insulating oil join together. Such PD defects would generate PD patterns similar to Type 3 (conducting particles
laying on the surface of the insulating material).
Final assembly of the transformer: It is well known, that proper drying and impregnation of the oil/cellulose
electrical insulation system have a significant influence on the voltage withstand ability of the insulation during
the enhanced dielectric stress in delivery tests. Figure 3.11 shows the changes in the PD patterns due to the
different levels of impregnation of the cellulose insulation.

Page 33
Figure 3.11 : PD activity at different impregnating condition [Lebreton-GE]
In case of local moisture in the cellulose, bubbles (cavities) may be generated after a certain time of the applied
test voltage due to the dielectric losses in the insulation. Also, during the oil filling process, bubbles (cavities) in
the oil may be introduced as a result of insufficient vacuum. In both cases, PD patterns similar to Type 5 (cavity
without contact to electrode) are typical.
From the shape of a PD pattern alone, it is not possible to distinguish whether cavities are in the oil or in the
solid insulation. Bubbles in the oil normally disappear after circulating the oil or after repositioning the
transformer in the laboratory.
Finally, with the installation of the bushings or with closing the tank, metallic particles may fall onto the active
part of the transformer and cause PD, resulting in a Type 2 (conducting particle without galvanic connection to
metallic electrode) PD pattern. An overview of PD sources generated by the manufacturing or installation
process are presented in Figure 3.12.
Conducting particle with contact to
electrode (core cutting ->angels hair)
Cavities without contact to the
electrode (local moisture, oil filling)
the electrode (particle from the
assembly)
Figure 3.12 : Possible PD sources in the electrical insulation due to manufacturing [Carlson-2010]

Page 34
PD SOURCES DUE TO COMPONENTS
In some cases, PD activity detected during the delivery test may be caused by PD sources in bushings or in the
tap changer. Normally, the quality of both components should be confirmed by the manufacturer before
assembly.
PD SOURCES DUE TO ACCEPTANCE TEST PROCEDURE
The PD measurement is the only non-destructive method able to detect local defects in an electrical insulation
system. Therefore, the PD test should be always the last test in the factory acceptance test (FAT) procedure.
After lightning impulse tests, small carbonized tracks may be generated which can be detected by a sensitive
PD measurement, resulting in Type 2 (conducting particle without galvanic connection to metallic electrode) PD
patterns.
During the heat run test, unexpected hot spots may generate bubbles in oil, which can be recognized in PD
measurements in the form of Type 5 (cavity without contact to electrode) PD patterns. Classical PD sources
identified during FAT are presented in Figure 3.13.
Cavities without contact to the
electrode (bubbles in oil due to the
heat run test)
the electrode (carbonized tracks due
to lightning impulse test)
Figure 3.13 : Possible PD sources in electrical insulation system due to FAT testing [Carlson-2010]
PD SOURCES DUE TO FINAL ASSEMBLY ON-SITE
For the transport of transformers, HV bushings must normally be disassembled and large power transformers
are usually transported without oil. In such cases, the final critical assembly of the transformer occurs at site. As
discussed above, the assembly of bushings and the oil filling process may introduce PD sources as shown in
Figure 3.132.
It was demonstrated that PD measurement on site (see chapter PD measurements on-site) is a very effective
method to check the quality of the final assembly. A mobile external voltage source (excitation up to 120% Un) is
needed to perform such on-site PD tests with the highest detection sensitivity.
PD SOURCES DUE TO OPERATION
During the operation of the transformer, its technical life is largely influenced by electrical, thermal, mechanical
and chemical processes. Not only each process alone, but also their combination can generate weak regions
within the transformer electrical insulation system. These areas can be exposed to elevated dielectric stresses,
which in the long run can be harmful to the safe operation of the transformer. For transformers in service, DGA
is the most efficient method to identify both the continuous PD activity and problems associated with hot spots in
the electrical insulation system. In case of assumed PD activity, a sensitive on-site PD test using an external
voltage source should be performed to investigate and localise the PD source (see chapter Recommended
procedure for successful solution of PD problems).

Page 35
The origins of PD sources which may be caused by the operation of the transformer are as follows:
- Bubbles (cavities) caused by local hot spots (corona type PD ->increasing H2 content)
- Bubbles due to dielectric losses at locations of moisture (corona type PD ->increasing H2 content)
- Loss of interfacial withstand capability due to decomposition of the cellulose (sparking type PD ->
increasing C2H2 content)
- Loss of dielectric strength through aging of material (sparking type PD ->increasing C2H2 content)
- Loss of dielectric strength through lightning and switching incidents
- Conducting particles from the cooling system or particles floating in the oil
- Change of electrical field stress due to a deformation of the windings caused by a short circuit
Theoretically, all types of PD defects discussed above may be generated during the operation of a transformer.

Page 36
4 PD signals in transformer electrical insulation systems
As discussed in the chapter Partial Discharge Detection Systems for Transformers, PD in a transformer’s
electrical insulation system is an electrical discharge in a weak region where the electrical field strength exceeds
the material limits for any reason. All PD sources in the electrical insulation system of the transformer generate
three different signals, which are measurable at the bushings or at the walls of the tank.
- Electrical current impulses measurable at bushings of the transformer
- Acoustic signals detectable at the tank walls
- Electro-magnetic-waves (EM-waves) measurable with UHF probes (antennas) installed in openings
of the tank (for example oil drain valves).
4.1 Principles of detection and quantification of electrical PD signals
The fast electric signals (time scale ns to µs) generated during the discharge are not directly detectable at the
measuring tap of bushings. These fast current impulses are damped and dispersed by their propagation through
the RLCM network of the transformer.
The rise time of the PD impulse at the original location is strongly dependent on the material where the PD
activity takes place. For electrical breakdowns in gases, the typical rise time is in the ns-range, for breakdowns
in oil, rise times may vary between ns to µs, depending on following factors: (1) on the type of physical process,
(2) on the set-up and (3) on the measuring system [CIGRE-2000, Judd-1998].
ELECTRICAL PD SIGNAL MEASURABLE AT BUSHINGS
Theoretically, all information about the location of the PD source is available from the PD current signals
detectable at the bushings. For real conditions, however, with distorted PD signals recorded at bushings, there
exists no de-convolution method to get a unique solution for the location of the PD source. Typical electrical PD
signals in the time domain, recorded at the HV bushings and at the neutral terminal, for a PD source close to the
HV bushings are shown in Figure 4.1.
The analysis of electrical PD signals in the time and frequency domain as measured at several bushings of a
transformer (multi terminal measurement) and successfully used for localization of a hidden PD source is
discussed in section PD source localization using electric signals.
MEASUREMENT OF APPARENT CHARGE
The amplitude of the “apparent charge” qm measured in pC or in µV during a PD test on a transformer is the
integral of the PD current impulse, detected in a classical PD test circuit employing a parallel coupling
capacitance Ck (capacitive bushing or external coupling capacitor) and the capacitance of the transformer (see
chapter Measuring system for detection of electric signals). The question of how the recorded charge qm, as
measured at the terminals of the HV equipment relates to the size, shape and location of the PD source in the
insulation system is under debate. There are still contradictory arguments as to what the measured PD signals
represent.
The globally accepted capacitive equivalent circuit, the so called “abc-model” as shown in Figure 4.2a, was
introduced many years ago [Gemant-1932, Kreuger-1989]. According to this model, the measurable charge qm ,
which flows into the terminals during the breakdown process in the capacitance c, is linked to the true charge q1
via the series capacitances b1 and b2. The measurable charge qm is defined as an apparent charge [IEC 60270-
2000]. Due to the unknown location of the PD source, the capacitances b1 and b2 cannot be determined and
consequently the true charge cannot be calculated. In Figure 4.2b the application of the “abc-model” to the PD
test circuit for a transformer is shown.

Page 37
More than two decades ago, Pederson and his co-workers [Pederson-1986, Pederson-1987] argued that the
term “apparent charge” is misleading from a physical point of view, because it is deduced from a capacitive
equivalent circuit (“abc-model”). The main argument is that the discharge in a cavity (PD source) in the
insulation system cannot be represented by the discharge of a capacitance. Pedersen proposed a dipole model
where the discharge in the cavity is directly related to the electric field between the electrodes (terminals of the
HV component).
Figure 4.1 : Measurable PD signals at bushings of the transformer (RLCM-network) [Fuhr-2005]
layout of windings

Page 38
a) “abc-model” representing PD activity in voids in solid dielectrics [Lemke-2012]
b) Application of the “abc-model” to the insulation system of a transformer [Carlson-2010]
Figure 4.2 : Presentation of the “abc-model” (a) and its application to the electrical insulation system of
a transformer (b)
A detailed discussion of the dipole model theory would be beyond the scope of this brochure. Nevertheless, the
basic difference between both equivalent circuits is given in Figure 4.3 and in Figure 4.4 [Lemke-2012].
Analogous to the capacitive equivalent circuit (Figure 4.3), the measurable charge qm at the terminals of a HV
component is much smaller that the true charge at the PD source location. Whereas in the dipole model (Figure
4.4), the measurable charge qm at the terminals of a HV component is identical with the true charge at the PD
source location. A detailed discussion of both models can be found in [Taylor-2013] and in [Lemke-2013].
Beside the lack of knowing the origin of the recorded PD signals, the measured amplitude of the apparent
charge qm is additionally influenced by other factors.
qm
q1
BU = bushing (coupling capacitance Ck)
HV = high voltage
NT = neutral terminal
C2,3 = active part of the transformer (with oil)
C1 = PD source (void)
Ct = test object capacitance (C’2 und C’3)
q1 = true charge due to the breakdown in the void
qm = measurable charge at the terminal (BU)
Zm = measuring impedance
qm

Page 39
Figure 4.3 : Capacitive equivalent circuit
Figure 4.4 : Dipole-model equivalent circuit [Lemke-2012]
An important scaling rule was derived based on a detailed physical analysis of the “abc-model” for a cavity
discharge in solid isolation [Boggs-1990]. Due to the decreasing series capacitance b1 and b2 in the “abc-model”
(Figure 4.2), the detection sensitivity, i.e. measurable charge qm at the electrode, decreases as the inception
voltage increases for a cavity of the same size (capacitance c) located in the same position. For power
transformers, the measurable apparent charge qm, generated by the same PD defect (same cavity size and
location) would have a tendency to be smaller and the inception voltage would be higher for an insulation
system with a higher nominal voltage (for example 400 kV), than for transformers with lower nominal voltage (for
example (220 kV). Based on this fact, the recommended acceptance criteria in the international standards for
maximal allowable apparent charge [pC] in power transformers should be adapted to the nominal voltage of the
transformer.
Furthermore, as disused in the chapter Measuring system for detection of electric signals, the measurable
apparent charge qm is influenced by the value of the coupling capacitance (in most cases the capacitance of the
bushing) in the test circuit.

Page 40
Regardless of arguments about the relationship between the true charge and measurable apparent charge qm,
and irrespective of the issue of the correct detection of the amplitude of apparent charge qm, the detectable PD
current pulses at the bushings of a transformer during the PD test, need to be analysed to determine how
harmful the PD may be to the insulation system of the HV component.
ERROR DUE TO INTEGRATION OF PD PULSES
The theory behind quasi integration in the frequency domain, using band pass filters, is valid for PD impulses as
shown in Figure 4.5a, i.e. also for slow impulses influenced by the transfer through the active part of the
transformer. The recorded amplitude of the apparent charge will be identical for all three impulses if the centre
frequency of the band pass filter of the PD system is set to the frequency range where the amplitude in the
frequency spectrum of the recorded impulse is constant (see Figure 4.5b) [Schon-1986]. To be precise, the
correct quasi integration in the frequency domain is valid only for the frequency range F(f) / F(0) = 1. For
impulse trace 3, the frequency range for the correct measurement of the apparent charge is typically below
1MHz.
In practice, the real PD signals measurable at the bushings of a transformer (see Figure 4.6) do not correspond
to the theoretical PD current pulses as required for correct integration using a band pass filter. There is a risk of
an integration error, i.e. an amplitude error, due to the deviation of the real PD signal shape from the idealized
PD pulse.
a) different shape of PD current impulses [Schwarz-Siemens]
b) frequency spectra for correct setting of the band pass filter [Koenig-1992]
Figure 4.5 : Principle of quasi integration of PD pulses in the frequency domain
1 original impulse
2 pulse distortion due to oscillatory circuit
3 pulse distortion due to damped capacitive circuit
i
t
1
2
3 〉
⁄
<
0
)( dttiq
321 qqq <<

Page 41
Figure 4.6 : Example of real PD signals measured at bushing of the transformer
ERROR DUE TO CALIBRATION
According to theory, the measurable value of apparent charge in the external test circuit corresponds to the
charge transferred during a voltage drop ΔU t between capacitance Ct (transformer) and Ck (bushing) in the
external test circuit (see Figure 4.7).
Figure 4.7 : Standardized measuring circuit for PD detection on transformers [Carlson-2010]
In a complex electrical insulation system like that of the transformer, the values for ΔUt (measurable at the
external test circuit) are in the mV range, while the magnitude of ΔU1 (at the location of the PD source) may be
in the kV range. Due to the unknown relationship between the true charge and measurable apparent charge the
calibration of the external measuring circuit is only valid for PD sources located close to the calibrated bushing.
Idealized PD current impulse [Koenig-1993] Real PD signals measured at bushings [Fuhr-PD
test]
U} = test voltage source (G)
Z = voltage source connection
Ck = coupling capacitance
C1 = capacitance of the bushing
Ct = test object capacitance
Cm = capacitive tap
Zm = measuring impedance
PDS = measuring system
ik,t
= displacement currents
U1
= voltage across Ck and Ct
i(t) = circulating current
q = apparent charge
A, B = bushings of the transformer

Page 42
For all PD sources hidden deep in the insulation system of the transformer, i.e. far away from the calibrated
bushing, an error of more than 50% in the measured apparent charge may occur. Therefore, the calibration
procedure by injecting a calibrating signal at one bushing, validates the entire measurement chain consisting of
coupling capacitance (bushing), measuring impedance, coaxial cables and the input of the measuring system.
When considering the limitations in the correct measurement of the magnitude of the apparent charge, the
recommended limits in international Standards, which are based on experience, are not a reliable criterion for
deciding if the PD source is dangerous to the electrical insulation system of a transformer.
PD PATTERN
Developments in oscilloscopes made it possible to show the amplitude of the apparent charge and its
dependence on the phase position of the applied test voltage (Figure 4.8a). The phase position of the apparent
charge enabled external and internal PD sources to be distinguished. Some typical PD sources, like cavities
and floating metallic particles, were also able to be recognized using this simple technique. These first PD
patterns were summarized in a CIGRE paper [Electra Report-1969].
Using a two-dimensional multi-channel analyser (introduced in 1990 for PD measurement), the statistical
analysis of the recorded values of apparent charge (via band pass filters) became possible and a Phase
Resolved Partial Discharge Analyser (PRPDA system) was introduced (see Phase resolved PD data
processing system). A typical PD pattern is shown in Figure 4.8b, where the third dimension is represented by
the colour.
Finally, Figure 4.8c shows the analysis of recorded amplitudes of apparent charge vs number of counts (pulse
height analysis).
a) simple PD pattern (pC values
recorded during one cycle)
b) advanced PD pattern (pC value
recorded during several cycles)
c) pulse height analysis
Figure 4.8 : Example of different types of PD pattern [OMICRON]
These representations are used as “pattern” of the recorded PD activity.
In some cases, especially when measuring old transformers on-site, the recorded PD pattern consists of the
superposition of several PD sources, originating either from the investigated phase itself, or from the coupling of
external noise, or from a PD activity of the other phases. The interpretation of superimposed PD pattern, i.e. the
identification of different types of PD sources, requires experience. There are some tools which may help to
solve the problem when analysing superimposed PD signals (see Figure 4.9). Using a synchronous
measurement at three bushings, a star diagram can be generated where recorded pulses from all three phases
are presented [Plath-2002, Kraetge-2010]. From the relationship of pulses measured at the three bushings, the

Page 43
separation of different PD sources can be achieved and external noise signals can even be removed (see
Analysing software).
To date, there exists no automatic recognition of different PD sources hidden in a PD record.
Figure 4.9 : Example of 3 Phase Amplitude Relation Diagram (3PARD) [OMICRON]
ATTENUATION OF ELECTRIC PD SIGNALS WITHIN A TRANSFORMER
Damping phenomena of PD signals within windings of transformers and their analysis were investigated in 1970
[Raju-1973] by injection of calibrating signals at different positions in the winding model and by recording the
time domain signals with an oscilloscope at the end of the winding via a coupling capacitance Ck. Results are
shown in Figure 4.10 [FGH-1984].
Figure 4.10 : Components of the PD signal recorded at bushings of a transformer [FGH-1984]
a) recorded PD signal at bushing, b) capacitive component,
c) traveling wave component, d) oscillating component

Page 44
Further investigations of the damping phenomena were performed at the University of Stuttgart [Coenen-2012,
Siegel-2014].
In the first experiment [Coenen-2012], a model of a disc winding with a grounded metal sheet core inside the
winding was used. A constant PD source was simulated using an Ogura needle (tip radius of 3 µm). This PD
source was moved along the winding (Figure 4.11).
Bipolar PD patterns were recorded using a commercially available PD system in accordance with IEC 60270
(see Conventional PD measuring system) at four different positions of the “tip electrode” (0 mm, 100 mm, 200
mm and 500 mm) using a constant test voltage of 28 kV ac applied to the disc winding (see Figure 4.12).
As expected, by increasing the distance of the constant PD source from the coupling capacitance Ck there is a
significant decrease in the measured amplitude of apparent charge. In this experiment the low values of
apparent charge are caused by both, the attenuated PD signal by the winding structure and the selected centre
frequency of the band pass filter at 4 MHz. For slow PD signals, the centre frequency of 4 MHz is too high to
perform a correct quasi integration of circulating currents in the test circuit.
The phase position of the recorded PD signals did not change with the position of the constant PD source due
to the fact, that the statistical behaviour of one specific PD source is not influenced by the structure of the
winding.
The recorded PD pattern associated with Figure 4.11 were distorted by a “double counting” of positive and
negative magnitude of apparent charge due to the oscillatory nature of the signals measured at the coupling
capacitance Ck. In such cases, the recoding of unipolar PD pattern would offer a clearer picture. Theoretically,
the movable PD source should correspond to the PD pattern Type 1 (tip electrode at ground, see chapter
Typical PD patterns) and the shape of PD pattern should be independent of the position of the PD source.
Figure 4.11 : Constant PD source inside a model of HV disc winding [Coenen-2012]

Page 45
Figure 4.12 : PD pattern recorded at four different positions,
Test voltage =28kV ac, band pass filter: fc =4MHz, ∆f =1MHz [Coenen-2012]
In the second experiment [Siegel-2014], a layer winding inside a tank filled with oil was used. In this set-up, the
constant PD source was simulated using a surge arrester movable on the tank wall. The goal of this
investigation was to demonstrate the influence of the selected centre frequency of the band pass filter
(according to IEC 60270) on the measurement of the amplitude of apparent charge. To record the amplitude of
apparent charge at centre frequencies higher than 1 MHz, a coupling impedance with a pass band of 15 MHz
was used. The frequency spectra of the artificial PD source were recorded, as shown in Figure 4.13. Then the
corresponding amplitudes of the apparent charge were recorded at three different centre frequencies, as shown
in Figure 4.14.
Figure 4.13 : Frequency spectra at different positions along the model winding [Siegel-2014]
100 k 1 M 10M
-110
-100
-90
-80
-70
-60
-50
-40
frequency / Hz
powerlevel/dBm
2,5 cm
10 cm
40 cm
80 cm
IEC broadband IEC narrowband increased frequency

Page 46
Figure 4.14 : Recorded amplitude of apparent charge at three different centre frequencies [Siegel-2014]
Summarising the results from both laboratory experiments on winding models, the following should be
considered when analysing PD data:
- The shape of the PD pattern (not the amplitude) is nearly constant for all four positions of the PD source
due to the same statistical behaviour of the PD source (Figure 4.12).
- Frequency spectra are changing with the position of the PD source with respect to the measuring
terminal as specific resonances exist (Figure 4.13) at each location.
- Due to the slower rise time of the PD signals at the measurement terminal with increasing distance of
the PD source from the measuring terminal, the amplitude of the apparent charge decreases for
positions further away from the terminal, especially for a PD system using a higher centre frequency (>
1 MHz) for the band pass filter (Figure 4.14).
In Figure 4.14 the highest amplitude of the apparent charge was recorded for the artificial PD source
positioned at 80 cm from the measuring point. According to the theory, the highest value of apparent charge
should be recorded for PD sources close to the measuring point, i.e. for the PD source positioned at 2.5 cm.
This result confirms the experience that the most sensitive measurement of apparent charge can be
achieved if the centre frequency of the band pass filter is near a resonant frequency of the measuring chain
(Figure 4.15). The resonant frequency of a measuring chain depends upon: type of bushing, measuring
impedance, and input impedance of the PD measuring system. The position of this resonance can be
determined (a) by injecting a calibrating impulse at the specific bushing and (b) by recording the frequency
spectrum at the measuring impedance (Figure 4.15, for details see section PD source investigation).
Figure 4.15 : Example of a resonance frequency in a measuring chain [Fuhr-PD test]
0 10 20 30 40 50 60 70 80
0
100
200
300
400
500
PD position along the winding /cm
apparentchargeQIEC
/pC
IEC broadband
IEC narrowband
increased frequency

Page 47
LIMITATIONS OF ELECTRIC PD SIGNAL DETECTION WITHIN A TRANSFORMER
In general, electrical PD signals can be detected at the bushings of a transformer. PD detection using transient
earth voltage (TEV) is not considered here, as in the case of power transformers this method does not reveal
sufficient detection sensitivity.
As discussed above, the main limiting factor for the correct measurement of electrical PD signals is the
decreasing sensitivity to measurable PD current pulses in the external test circuit with increasing distance of the
PD source to the specific bushing. This can be partly compensated by:
- Amplification of the PD system
- Ratio between coupling capacitance and test object capacitance (Ck > Ct)
- Multi-terminal measurement (HV bushings, LV bushings, neutral terminal)
- Measurement at a resonance frequency of the measuring chain (see Figure 4.15)
With increasing amplification in the PD measurement system, external noise will also be amplified and
superimpose on the measurable PD signals. Special noise suppression techniques may be needed to enable
sensitive measurements in a noisy environment [Kraetge-2011].
The ratio between coupling capacitance and test object capacitance is (Ck > Ct) relates to the capacitance of the
bushing. To increase the sensitivity of the measuring circuit, external capacitors may be connected to the
bushing.
By employing multi-terminal measurements, there is always at least one bushing, which shows the highest
sensitivity to the detectable PD current pulses.
The measurement of apparent charge at a resonance frequency of the measuring chain (see Figure 4.15)
combined with multi-terminal measurement is considered the most promising method to detect all PD sources
hidden in the electrical insulation system of the transformer (see PD source investigation).
4.2 Principles of propagation, detection and quantification of acoustic
PD signals
A theoretical overview of acoustic and UHF signal propagation and measurement is schematically presented in
the Figure 4.16.

Page 48
Figure 4.16 : Signal transfer path for acoustic and UHF signals [Schwarz-Siemens]
Acoustic PD measurements rely on the fact that acoustic signals are emitted by each internal PD source.
Because of the short duration of the PD impulses, the resulting compression waves have frequencies up to
ultrasonic frequencies. The frequency spectrum of acoustic waves typically lies in the range between
20 kHz up to 1 MHz [Markalous-2006].
PROPAGATION OF ACOUSTIC PD SIGNALS WITHIN A TRANSFORMER
In transformers, mechanical waves generated by PD sources propagate at the origin as a spherical pressure
(longitudinal) wave through the oil until reaching windings, barriers and finally the tank wall. For the detection of
acoustic signals, piezoelectric sensors, fixed externally at suitable positions on the tank wall, are used. Figure
4.17 shows a possible propagation path for an acoustic wave.
Figure 4.17 : Propagation of acoustic sound waves [Markalous-2006]
In most cases, the acoustic sensor does not directly detect the acoustic PD signal propagating through the oil.
The ultrasonic waves strike the tank and create an alternative propagation path via the tank wall with higher
wave speeds than that in oil. Using arrival times of the signals which have travelled partly as structure borne
waves and not those with a direct oil path, in combination with the average sound velocity for oil, would result in
an incorrect distance being determined between the PD source and the acoustic sensor. In cases where there is
no direct oil path signal, there is limited accuracy in the location of PD sources [IEEE Guide-2003].
The correct determination of the signal arrival time is an important part of the location process (see chapter
Principles of PD localization in transformers). This is particularly important when structure borne path
signals are present which can disguise the onset of the direct (oil) path signal [Lundgaard-1989, Phung-1991].
Transformer acoustic signal propagation phenomena were investigated in many laboratory experiments [Phung-
1991] by moving an acoustic sensor gradually on the steel plate from a position perpendicular to an acoustic
source in oil, to other positions along a metal plate (Figure 4.18).
It was found that, while only longitudinal waves (particle motion in the direction of propagation) exist in oil, metal
plates support generally both longitudinal and transversal waves (motion transverse to the direction of
propagation; also called shear wave) [Lundgaard-1989, Phung-1991, IEEE Guide-2003]. Depending on the
angle Ψ (angle measured from the normal to the tank wall to the position of the acoustic sensor, see Figure
4.18) three regions can be distinguished. Within the range ≤13.7° both waves can be generated, while the
conversion of the longitudinal oil wave to the longitudinal wave in the plate is more efficient than to the shear
type. Between 13.7° to 25.9° only the shear wave is stimulated and above 25.9° the oil tank interface might
result in total reflection [Phung-1991].
Within an incident angle of 30°, it could be difficult to distinguish between the two waves [Lundgaard-1989]. It
was pointed out that in this range the estimated arrival time can be regarded as direct sound. The systematic
measuring system

PARTIAL DISCHARGES IN TRANSFORMERS

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