CLC/TS 60034-27:2011
(Main)Rotating electrical machines - Part 27: Off-line partial discharge measurements on the stator winding insulation of rotating electrical machines
Rotating electrical machines - Part 27: Off-line partial discharge measurements on the stator winding insulation of rotating electrical machines
Provides a common basis for measuring techniques and instruments, the arrangement of test circuits, normalization and testing procedures, noise reduction, the documentation of test results, the interpretation of test results with respect to partial discharge off-line measurements on the stator winding insulation of rotating electrical machines.
Drehende elektrische Maschinen - Teil 27: Off-line-Teilentladungsmessungen an der Statorwicklungsisolation drehender Maschinen
Machines électriques tournantes - Partie 27: Mesures à l'arrêt des décharges partielles effectuées sur le système d'isolation des enroulements statoriques des machines électriques tournantes
Présente des modalités communes concernant les techniques de mesure et les appareils de mesure, la mise en place des circuits d'essai, les procédures de normalisation et les modes opératoires d'essai, la réduction du bruit, la documentation des résultats des essais, l'interprétation des résultats des essais pour les besoins des mesures à l'arrêt des décharges partielles dans les systèmes d'isolation des enroulements statoriques des machines électriques tournantes.
Električni rotacijski stroji - 27. del: Meritve delne izpraznitve na izolaciji statorskih navitij neaktivnih električnih rotacijskih strojev (IEC/TS 60034-27:2006)
Ta del IEC 60034 zagotavlja splošno podlago za merilne tehnike in instrumente, postavitev preskusnih vezij, postopke za normalizacijo in preskušanje, zmanjševanje hrupa, dokumentiranje preskusnih rezultatov in razlago rezultatov preskusa glede na nesprotne meritve delne izpraznitve na izolaciji statorskih navitij neaktivnih električnih rotacijskih strojev, kadar so preskušani z izmenično napetostjo do 400 Hz. Ta standard velja za električne rotacijske stroje, ki imajo palice, ali za tuljave s predhodno oblikovanimi navitji s prevodnim premazom. Običajno velja za stroje z oceno napetosti 6 kV in višjo. Metode merjenja, opisane v tem dokumentu, lahko prav tako veljajo za stroje brez prevodnega premaza. Vendar se lahko rezultati razlikujejo in niso zajeti v tem standardu.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-april-2011
(OHNWULþQLURWDFLMVNLVWURMLGHO 0HULWYHGHOQHL]SUD]QLWYHQDL]RODFLMLVWDWRUVNLK
QDYLWLMQHDNWLYQLKHOHNWULþQLKURWDFLMVNLKVWURMHY ,(&76
Rotating electrical machines - Part 27: Off-line partial discharge measurements on the
stator winding insulation of rotating electrical machines (IEC/TS 60034-27:2006)
Drehende elektrische Maschinen - Teil 27: Off-line-Teilentladungsmessungen an der
Statorwicklungsisolation drehender Maschinen (IEC/TS 60034-27:2006)
Machines électriques tournantes - Partie 27: Mesures à l'arrêt des décharges partielles
effectuées sur le système d'isolation des enroulements statoriques des machines
électriques tournantes (CEI/TS 60034-27:2006)
Ta slovenski standard je istoveten z: CLC/TS 60034-27:2011
ICS:
29.160.01 Rotacijski stroji na splošno Rotating machinery in
general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
TECHNICAL SPECIFICATION
CLC/TS 60034-27
SPÉCIFICATION TECHNIQUE
February 2011
TECHNISCHE SPEZIFIKATION
ICS 29.160
English version
Rotating electrical machines -
Part 27: Off-line partial discharge measurements on the stator winding
insulation of rotating electrical machines
(IEC/TS 60034-27:2006)
Machines électriques tournantes - Drehende elektrische Maschinen -
Partie 27: Mesures à l'arrêt des décharges Teil 27: Off-line-Teilentladungsmessungen
partielles effectuées sur le système an der Statorwicklungsisolation drehender
d'isolation des enroulements statoriques Maschinen
des machines électriques tournantes (IEC/TS 60034-27:2006)
(CEI/TS 60034-27:2006)
This Technical Specification was approved by CENELEC on 2011-01-25.
CENELEC members are required to announce the existence of this TS in the same way as for an EN and to
make the TS available promptly at national level in an appropriate form. It is permissible to keep conflicting
national standards in force.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Management Centre: Avenue Marnix 17, B - 1000 Brussels
© 2011 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TS 60034-27:2011 E
Foreword
The text of the Technical Specification IEC/TS 60034-27:2007, prepared by IEC TC 2, Rotating
machinery, was submitted to the formal vote and was approved by CENELEC as CLC/TS 60034-27 on
2011-01-25.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following date was fixed:
– latest date by which the existence of the CLC/TS
has to be announced at national level (doa) 2011-07-25
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the Technical Specification IEC/TS 60034-27:2006 was approved by CENELEC as a
Technical Specification without any modification.
- 3 - CLC/TS 60034-27:2011
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year
IEC 60060-1 - High-voltage test techniques - EN 60060-1 -
Part 1: General definitions and test
requirements
IEC 60060-2 - High-voltage test techniques - EN 60060-2 -
Part 2: Measuring systems
IEC 60270 2000 High-voltage test techniques - Partial EN 60270 2001
discharge measurements
SPÉCIFICATION CEI
TECHNIQUE
IEC
TS 60034-27
TECHNICAL
Première édition
SPECIFICATION
First edition
2006-12
Machines électriques tournantes –
Partie 27:
Mesures à l’arrêt des décharges partielles
effectuées sur le système d’isolation des
enroulements statoriques des machines
électriques tournantes
Rotating electrical machines –
Part 27:
Off-line partial discharge measurements
on the stator winding insulation of rotating
electrical machines
© IEC 2006 Droits de reproduction réservés ⎯ Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in any
utilisée sous quelque forme que ce soit et par aucun procédé, form or by any means, electronic or mechanical, including
électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
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Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
CODE PRIX
XA
PRICE CODE
Commission Electrotechnique Internationale
International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
Pour prix, voir catalogue en vigueur
For price, see current catalogue
TS 60034-27 © IEC:2006 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11
1 Scope.15
2 Normative references .15
3 Terms and definitions .15
4 Nature of PD in rotating machines .19
4.1 Basics of PD .19
4.2 Types of PD in rotating machines .21
4.3 Pulse propagation in windings .23
5 Measuring techniques and instruments.25
5.1 General .25
5.2 Influence of frequency response of measurement system.25
5.3 Effects of PD coupling units.27
5.4 Wide-band and narrow band measuring systems.29
6 Visualization of measurements .33
6.1 General .33
6.2 Minimum scope of PD data presentation.33
6.3 Additional means of PD data representation .35
7 Test circuits.39
7.1 General .39
7.2 Individual winding components .39
7.3 Complete winding.43
8 Normalization of measurements .47
8.1 General .47
8.2 Individual winding components .49
8.3 Complete windings .49
9 Test procedures .53
9.1 Acquiring PD measurements on windings and winding components.53
9.2 Identifying and locating the source of partial discharges.61
10 Interpretation of test results.63
10.1 General .63
10.2 Interpretation of PDIV, PDEV and Q .65
m
10.3 PD pattern recognition.69
11 Test report.73
Annex A (informative) On-line partial discharge measurements.79
Annex B (informative) Non-electrical methods of PD detection and methods for
localization .83
Annex C (informative) External noise, disturbance and sensitivity.85
Annex D (informative) Methods of disturbance suppression .91
Annex E (informative) Interpretation of PD magnitude data and phase resolved PD
patterns .103
TS 60034-27 © IEC:2006 – 5 –
Bibliography.111
Figure 1 – Frequency response of a PD pulse and coupling units of various time
constants .27
Figure 2 – Typical pulse responses of wide band and narrow band PD systems.31
Figure 3 – PD magnitude as a function of the normalized test voltage Q =f(U/U ) .35
m max
Figure 4 – Example of a φ-q-n partial discharge pattern where the PD was measured in
series with the test object in accordance with Figure 5b, with colour code for the pulse
number H(n) .37
Figure 5 – Basic test circuits in accordance with IEC 60270.41
Figure 6 – Test circuit for PD measurement (S1.1) on complete winding .43
Figure 7 – Normalization of the test circuit for measurement S1.1.51
Figure 8 – Test voltage applied to the test object during PD measurement .57
Figure 9 – Example for identification and localization of PD sources .71
Figure C.1 – Recharging of the test object by various current components.87
Figure D.1 – Without window masking.91
Figure D.2 – With window masking .91
Figure D.3 – Pulse currents through the measuring circuit .93
Figure D.4 – Example of noise rejection.99
Figure D.5 – Example of cross-talk rejection .101
Figure E.1 – Example PD patterns .105
Table 1 – Connection diagram S1 for open star point.45
Table 2 – Connection diagram S2 for closed star point .45
Table 3 – Connection diagram E1 for open star point.47
Table 4 – Connection diagram E2 for closed star point .47
Table E.1 – Risks associated with the main PD sources in rotating machines .107
TS 60034-27 © IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ROTATING ELECTRICAL MACHINES –
Part 27: Off-line partial discharge measurements on the stator
winding insulation of rotating electrical machines
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote interna-
tional co-operation on all questions concerning standardization in the electrical and electronic fields. To this
end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Techni-
cal Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”).
Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject
dealt with may participate in this preparatory work. International, governmental and non-governmental organiza-
tions liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Or-
ganization for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all inter-
ested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinter-
pretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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tween any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equip-
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6) All users should ensure that they have the latest edition of this publication.
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members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and ex-
penses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publica-
tions.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In excep-
tional circumstances, a technical committee may propose the publication of a technical speci-
fication when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
TS 60034-27 © IEC:2006 – 9 –
IEC 60034-27, which is a technical specification, has been prepared by IEC technical commit-
tee 2: Rotating machinery.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
2/1384/DTS 2/1395A/RVC
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60034 series, under the general title Rotating electrical machines,
can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
TS 60034-27 © IEC:2006 – 11 –
INTRODUCTION
For many years, the measurement of partial discharges (PD) has been employed as a sensi-
tive means of assessing the quality of new insulation as well as a means of detecting local-
ized sources of PD in used electrical winding insulation arising from operational stresses in
service. Compared with other dielectric tests (i.e. the measurement of dissipation factor or
insulation resistance) the differentiating character of partial discharge measurements allows
localized weak points of the insulation system to be identified.
The PD testing of rotating machines is also used when inspecting the quality of new assem-
bled and finished stator windings, new winding components (e.g. form-wound coils and bars,
HV bushings, etc.) and fully impregnated stators.
In connection with the servicing and overhaul of rotating machines, the measurement of par-
tial discharges can also provide information on:
– points of weakness in the insulation system;
– ageing processes;
– further measures and intervals between overhauls.
Although the PD testing of rotating machines has gained widespread acceptance, it has
emerged from several studies that not only are there many different methods of measurement
in existence but also the criteria and methods of analysing and finally assessing the measured
data are often very different and not really comparable. Consequently, there is an urgent need
to give some guidance to those users who are considering the use of PD measurements to
assess the condition of their insulation systems.
Partial discharge testing of stator windings can be divided into two broad groups:
a) off-line measurements, in which the stator winding is isolated from the power system and
a separate power supply is employed to energize the winding;
b) on-line measurements, in which the rotating machine is operating normally and connected
to the power system.
Both of these approaches have advantages and disadvantages with respect to one another. A
brief discussion of the merits of on-line testing, as well as the drawbacks, is provided in
Annex A. However, while acknowledging the extensive world-wide use of on-line methods and
their proven value to industry, this technical specification is confined to off-line techniques.
This approach is considered necessary to render this specification sufficiently concise to be of
use by non-specialists in the field of PD testing.
TS 60034-27 © IEC:2006 – 13 –
Limitations:
When stator windings are being tested different types of PD measuring instruments will inevi-
tably produce different results and consequently PD measurements will only be comparable
under certain conditions. Therefore, absolute limits for the windings of rotating machines, for
example as acceptance criteria for production or operation, are difficult to define. This is
mainly due to pulse propagation phenomena, specific difficulties with calibration and the indi-
vidual frequency response characteristics of stator windings and PD measuring systems.
In addition, the degree of deterioration, and hence the risk of insulation system failure, de-
pends on the specific type of PD source and its location within the stator winding insulation,
both of which can influence the test results very significantly.
Empirical limits verified in practice can be used as a basis for evaluating test results. Fur-
thermore, PD trend evaluation and comparisons with machines of similar design and similar
insulation system measured under similar conditions, using the same measurement equip-
ment, are recommended to ensure reliable assessment of the condition of the stator winding
insulation.
Users of PD measurement should be aware that, due to the principles of the method, not all
insulation-related problems in stator windings can be detected by measuring partial dis-
charges (e.g. insulation failures involving continuous leakage currents due to conductive
paths between different elements of the insulation or pulseless discharge phenomena).
For testing individual winding components, the limitations due to pulse propagation phenom-
ena need not be considered when interpreting the results of measurements.
TS 60034-27 © IEC:2006 – 15 –
ROTATING ELECTRICAL MACHINES –
Part 27: Off-line partial discharge measurements on the stator
winding insulation of rotating electrical machines
1 Scope
This part of IEC 60034 which is a technical specification provides a common basis for
– measuring techniques and instruments,
– the arrangement of test circuits,
– normalization and testing procedures,
– noise reduction,
– the documentation of test results,
– the interpretation of test results
with respect to partial discharge off-line measurements on the stator winding insulation of
rotating electrical machines when tested with alternating voltages up to 400 Hz. This technical
specification applies to rotating machines having bars or form wound coils with conductive
slot coating. This is usually valid for machines with voltage rating of 6 kV and higher. The
measurement methods described in this specification may also be applied to machines with-
out conductive slot coating. However, results may be different and are not covered by this
specification.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60060-2, High-voltage test techniques – Part 2: Measuring systems
IEC 60270:2000, High-voltage test techniques – Partial discharge measurements
3 Terms and definitions
For the purposes of this document, the general terms and definitions for partial discharge
measurements given in IEC 60270 apply, together with the following.
3.1
off-line measurement
measurement taken with the rotating machine at standstill, the machine being disconnected
from the power system
NOTE The necessary test voltage is applied to the winding from a separate voltage source.
TS 60034-27 © IEC:2006 – 17 –
3.2
on-line measurement
measurement taken with the rotating machine in normal operation
3.3
stress control coating
paint or tape on the surface of the groundwall insulation that extends beyond the conductive
slot portion coating in high-voltage stator bars and coils
NOTE The stress control coating reduces the electric field stress along the winding overhang to below a critical
value that would initiate PD on the surface. The stress control coating overlaps the conductive slot portion coating
to provide electrical contact between them.
3.4
conductive slot coating
conductive paint or tape layer in intimate contact with the groundwall insulation in the slot
portion of the coil side, often called semiconductive coating
NOTE This coating provides good electrical contact to the stator core.
3.5
resistance temperature detector
RTD
a temperature detector inserted into the stator winding, usually between the top and bottom
bar or embedded coil sides in a given slot
3.6
slot discharges
discharges that occur between the outer surface of the slot portion of a coil or bar and the
grounded core laminations
3.7
internal discharges
discharges that occur within the insulation system
3.8
surface discharges
discharges that occur on the surface of the insulation or on the surface of winding compo-
nents in the winding overhang or the active part of the machine winding
3.9
pulse height distribution
the number of pulses within a series of equally-spaced windows of pulse magnitude during a
predefined measuring time
3.10
pulse phase distribution
the number of pulses within a series of equally-spaced windows of phase during a predefined
measuring time
3.11
partial discharge pattern
PD distribution map of PD magnitude vs a.c. cycle phase position, for visualization of the PD
behaviour during a predefined measuring time, in which specific PD parameters are used for
graphical representation
TS 60034-27 © IEC:2006 – 19 –
3.12
coupling device
usually an active or passive four-terminal network that converts the input currents to output
voltage signals
NOTE These signals are transmitted to the measuring instrument by a transmission system. The frequency re-
sponse of the coupling device is normally chosen at least so as to efficiently prevent the test voltage frequency and
its harmonics from reaching the measuring instrument.
3.13
PD coupling unit
a high voltage coupling capacitor of low inductance design and a low voltage coupling device
in series
3.14
largest repeatedly occurring PD magnitude
Q
m
the largest magnitude recorded by a measuring system which has the pulse train response in
accordance with 4.3.3 of IEC 60270, or the magnitude associated with a PD pulse repetition
rate of 10 pulses per second (pps), which can be directly inferred from a pulse height distribu-
tion
3.15
normalized quantity number
NQN
normalized area under a straight line fitted to the pulse counts in each magnitude window of a
pulse height analysis, in which the pulse counts are expressed as a logarithm of the pulses
per second and the pulse magnitude window is a linear scale
4 Nature of PD in rotating machines
4.1 Basics of PD
Generally, partial discharges (PD) can develop at locations where the dielectric properties of
insulating materials are inhomogeneous. At such locations, the local electrical field strength
may be enhanced. Due to local electrical over-stressing this may lead to a local, partial
breakdown. This partial breakdown does not result in a total breakdown of the insulation sys-
tem. PD in general requires a gas volume to develop, for example in gas filled voids embed-
ded in the insulation, adjacent to conductors or at insulation interfaces.
A partial discharge can occur when the local field strength of each inhomogeneity exceeds its
breakdown field. This process may result in numerous PD pulses during one cycle of the ap-
plied voltage.
The amount of charge transferred in the discharge is closely related to the specific properties
of the inhomogeneity such as the dimensions, the actual breakdown voltage and the specific
dielectric properties of the materials involved, for example surface properties, kind of gas, gas
pressure, etc.
TS 60034-27 © IEC:2006 – 21 –
Stator winding insulation systems for high voltage machines will normally have some PD ac-
tivity, but are inherently resistant to partial discharges due to their inorganic mica compo-
nents. However, significant PD in these machines is usually more a symptom of insulation
deficiencies, like manufacturing problems or in-service deterioration, rather than being a di-
rect cause of failure. Nevertheless, depending on the individual processes, PD in machines
may also directly attack the insulation and thus influence the ageing process. The time to fail-
ure may not correlate with PD levels, but depends significantly on other factors, for example
operating temperature, wedging conditions, degree of contamination, etc.
The measurement and the analysis of the specific PD behaviour can be efficiently used for
quality control of new windings and winding components and for early detection of insulation
deficiencies caused by thermal, electrical, ambient and mechanical ageing factors in service,
which might result in an insulation fault.
4.2 Types of PD in rotating machines
4.2.1 General
Partial discharges may develop throughout the stator winding insulation system due to spe-
cific manufacturing technologies, manufacturing deficiencies, normal in-service ageing, or
abnormal ageing. Machine design, the nature of the materials used, manufacturing methods,
operating conditions, etc. can profoundly affect the quantity, location, characteristics, evolu-
tion and the significance of PD. For a given machine, the various PD sources may be identi-
fied and distinguished in many cases by their characteristic PD behaviour.
4.2.2 Internal discharges
4.2.2.1 Internal voids
Although manufacturing processes are designed to minimize internal voids, inevitably there is
some void content in a resin impregnated mica tape insulation system that is normally used in
high voltage rotating machines. Actually, the mica in the insulation system prevents the partial
discharges from developing into a complete breakdown. As long as internal voids are small
and do not significantly enlarge, operational reliability is not reduced.
4.2.2.2 Internal delamination
Internal delamination within the main insulation can be caused by imperfect curing of the insu-
lation system during manufacturing or by mechanical or thermal over-stressing during opera-
tion. Large voids may develop over a large surface resulting in discharges of relatively high
energy, which may significantly attack the insulation. In particular, delamination will reduce
the thermal conductivity of the insulation, which might lead to accelerated ageing or even a
thermal runaway. Thus, delamination needs careful consideration when PD activity is being
assessed.
TS 60034-27 © IEC:2006 – 23 –
4.2.2.3 Delamination between conductors and insulation
Delamination at the interface of the copper conductor and the main insulation that usually
results from excessive thermal cycling is dangerous since the turn or strand insulation of the
conductors can be severely damaged.
4.2.3 Slot discharges
Slot discharges in high voltage machines will develop when the conductive slot portion coat-
ing is damaged due to bar/coil movement in the slot or slot exit area, for example by a loss of
wedging pressure due to settlement, erosion of the material, abrasion, chemical attack or
manufacturing deficiencies. High-energy discharges will develop when serious mechanical
damage is already present, which may result in additional damage to the main insulation and
eventually in an insulation fault. In the early stage, slot discharges are rather vibration spark-
ing than being classical partial discharges. This vibration sparking may also occur at low po-
tential sites, for example close to the star point of the winding. Though the absolute time be-
tween detection of this phenomenon and final insulation failure is unknown, but could be
short, reliable detection at an early stage is necessary to initiate appropriate remedial action.
4.2.4 End-winding surface discharges
Partial discharges in the end-winding area may occur at several locations with high local elec-
tric field strengths. Such discharges usually occur at interfaces between different elements of
the stator winding overhang. If the stress control coating of the end-winding becomes ineffec-
tive because of poorly designed interfaces, contamination, porosity, thermal effects, etc. reli-
able field grading is no longer assured and surface discharges will develop, which may gradu-
ally erode the materials. This is normally a very slow failure mechanism, even though the PD
behaviour might be subjected to relatively fast changes due to surface effects. In addition, PD
may occur between phases, for example due to inadequate interface clearance, at elements
of the overhang support system, or as phase to ground discharges on the end-winding sur-
face.
4.2.5 Conductive particles
Conductive particles, especially small particles, for example due to contamination of the wind-
ing, may result in a strong local concentration of partial discharges. This may result in a ‘pin-
hole’ in the insulation.
4.3 Pulse propagation in windings
At its origin a partial discharge current can be characterized as a transient pulse with a rise
time of only a few nanoseconds. For these short PD pulses with a high frequency spectrum,
the stator windings represent objects with distributed elements in which travelling wave, com-
plex capacitive and inductive coupling, and resonance phenomena occur. Therefore, PD pulse
propagation phenomena need to be considered. Due to the attenuation, distortion, reflection
and cross-coupling of travelling wave signals, the form and magnitude of the PD signal re-
corded at the terminals of the winding differ from those at the point where it originates. With
that in mind, the following points are very important for interpreting PD measurements taken
on rotating machines:
TS 60034-27 © IEC:2006 – 25 –
– the transmission function from the PD source to the PD sensor is unknown and depends
on the specific design of the machine which determines the frequency response of the sta-
tor winding. Therefore, the energy at the source of the PD, which can be taken as a meas-
ure of the erosion of the insulation, cannot be measured directly;
– the individual high frequency transmission behaviour of a stator winding produces PD sig-
nals at the terminals that are a characteristic of the machine being tested and of the loca-
tion of the PD source;
– very high frequency components of PD signals are subject to considerable attenuation
when travelling through the winding and, depending on the origin of the PD, might not be
detectable at the terminals of the test object
As a consequence of the above-mentioned phenomena not only the particular stator winding
design but also the specific frequency response of the PD detection system, including cou-
pling devices, will significantly influence the characteristics of the signal detected at the ter-
minals of the winding.
5 Measuring techniques and instruments
5.1 General
In line with IEC 60270, this clause deals solely with electrical methods of measuring partial
discharges because the electrical, conductive measurement of partial discharges is the most
commonly used method of assessing the winding insulation of rotating machines. Non-
electrical methods of measurement and localization are listed in Annex B.
Partial discharge measuring systems can be divided into subsystems: coupling device, trans-
mission system (for example, connecting cable or optical link) and measuring instrument. In
general, the transmission system does not contribute to the circuit characteristics, apart from
some possible signal attenuation, and will thus not be taken into consideration.
5.2 Influence of frequency response of measurement system
The frequency response of the PD detection system, including the PD coupling unit, deter-
mines how much energy of the PD signal from the winding can be detected. Thus, the fre-
quency response of the system, especially the type of coupling unit being used, has a consid-
erable impact on the overall sensitivity of detection. Due to the different values of lower cut-
off frequency, the following qualitative relationships are basically applicable when testing
complete windings:
– measurement in the lower frequency range ensures good sensitivity not only for partial
discharges in bars/coils close to the sensor but also for those that originate from further
away in the winding. However, the lower frequency range is more subjected to noise and
disturbances;
– measurement in the very high frequency range may acquire only a very small proportion of
the total PD energy, which results in sensitivity to signals originating only very close to the
sensor. However, this frequency range may be less susceptible to noise and disturbance.
For off-line PD testing to obtain appropriate sensitivity to PD from the whole winding it is ad-
visable to use wide band PD measuring systems. The lower cut-off frequency should be in the
range of several tens of kHz in accordance with IEC 60270.
TS 60034-27 © IEC:2006 – 27 –
It should be noted that depending on the winding design and the measurement arrangement
used, resonance phenomena that are in the frequency range of the PD measuring device may
occur and therefore may also influence PD results.
5.3 Effects of PD coupling units
For off-line PD measurements on stator windings and PD tests on winding components ca-
pacitive coupling units are often used. These consist of a high voltage capacitor and a low
voltage coupling device in series. When testing individual winding components, the coupling
device may also be connected in series to the test object (see Figure 5b). The low voltage
coupling device is connected to the transmission system.
The high voltage capacitor, the coupling device, the transmission system and the input im-
pedance of the measurement system represent a high-pass filter. Therefore, increased input
impedance or higher capacitance values lead to an increased sensitivity.
Figure 1 shows schematically the frequency response of an idealized PD pulse and the trans-
fer functions of different PD coupling units with a high voltage capacitor and a resistive meas-
uring impedance Z =R at the low voltage side. The marked overlap of the spectra of the PD
m
pulse and the coupling unit, shown in Figure 1, for an RC time constant of 5 ns, determines
the signal energy which can be measured. In practical cases, such systems show band pass
filter characteristics due to parasitic L and C components.
High voltage
1,0
C
k
0,9
RC time constant of
0,8
coupling unit in ns:
5 000 500 50 5
0,7
0,6
PD device
Z
m
0,5
0,4
PD pulse
spectrum
0,3
IEC 2278/06
0,2
0,1 PD energy
detected
time constant of PD coupling unit
1 3 5 7 9
10 10 10 10 10
for Z =R: τ = RC
m
Frequency Hz
IEC 2279/06
lower cut-off frequency:
f = 1/(2πRC)
lo
Components
Z measuring impedance
m
C coupling capacitor
k
Figure 1 – Frequency response of a PD pulse and coupling units
of various time constants
Frequency response
TS 60034-27 © IEC:2006 – 29 –
PD pulses are attenuated and dispersed especially at higher frequencies while propagating
through the winding. Therefore, measurement systems with lower cut-off frequency in the
lower frequency range usually provide an average good sensitivity to PD from the whole wind-
ing.
When taking measurements on individual winding components, the high voltage coupling ca-
pacitor is connected to the copper conductor. For PD measurements on complete windings,
the coupling unit is connected to the terminals of the machine or inside the frame directly to
the winding conductors.
The following low voltage coupling devices are typically combined with the high voltage ca-
pacitor:
– RLC filters or four-terminal networks (see IEC 60270) wherein an inductance serves to sup-
press the power frequency component;
– high-frequency current transformers (RF-CT) which may also serve to galvanically separate
the high voltage circuitry from the measuring device.
RF-CT connected with ground wires can also be used as a standalone coupling device. When
using fibre optical signal transmitters, the coupling devices can also be installed on the HV
side of the capacitor.
5.4 Wide-band and narrow band measuring systems
The principal difference between the various PD measuring systems is their bandwidth. The
PD pulses arriving at the terminals have a frequency spectrum characterized by the transmis-
sion function of the machine winding. The measured PD signal will be affected to a greater or
lesser degree depending on the bandwidth of the measuring system. Furthermore, micaceous
insulation systems are characterized by a high repetition rate of PD pulses. Figure 2 shows
typical pulse responses of different measuring systems. The upper trace of the oscillograms
represents the input pulse, and the lower trace the pulse response of the measuring system:
1) wide band system: (a) low pulse repetition rate, (b) increased rate, (c) high rate leading to
superposition of pulses,
2) narrow band system: (a) low pulse repetition rate, (b) increased rate, (c) high rate leading
to superposition of pulses.
Si l (V)
Si l (V)
TS 60034-27 © IEC:2006 – 31 –
1) Wide band system: Δf at –3 db: 210 kHz (2nd order-filters)
2,0
2,0
2,0
Calibrator pulses (offset = 1 V)
Calibrator pulses (offset = 1 V)
Calibrator pulses (offset = 1 V)
Filtered pulses (multiplier = 500) Filtered pulses (multiplier = 500)
Filtered pulses (multiplier = 500)
1,5
1,5
...
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