IEC 60034-27-1:2017

Rotating electrical machines - Part 27-1: Off-line partial discharge measurements on the winding insulation

IEC 60034-27-1:2017 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 winding insulation of rotating electrical machines.
This International Standard cancels and replaces IEC TS 60034-27 (2006). It constitutes a technical revision and reflects numerous improvements and corrections with respect to the previous publication.

Machines électriques tournantes - Partie 27-1: Mesurages à l’arrêt des décharges partielles effectués sur le système d'isolation des enroulements

L’IEC 60034-27-1:2017 présente des modalités communes concernant:
- les techniques et les appareils de mesure;
- la mise en place des circuits d'essai;
- les procédures de normalisation et d'essai;
- la réduction du bruit;
- la documentation des résultats des essais;
- l'interprétation des résultats des essais;
pour les besoins des mesurages à l’arrêt des décharges partielles dans les systèmes d'isolation des enroulements des machines électriques tournantes.
Cette Norme internationale annule et remplace l’IEC TS 60034-27, publiée en 2006. Elle constitue une révision technique avec de nombreuses améliorations et corrections par rapport à la publication précédente.

General Information

• Status: Published
• Publication Date: 12-Dec-2017
• ICS: 29.160.01 - Rotating machinery in general
• Technical Committee: TC 2 - Rotating machinery
• Drafting Committee: MT 10 - TC 2/MT 10
• Current Stage: PPUB - Publication issued
• Start Date: 13-Dec-2017
• Completion Date: 05-Jan-2018

Relations

• Replaces: IEC TS 60034-27:2006 - Rotating electrical machines - Part 27: Off-line partial discharge measurements on the stator winding insulation of rotating electrical machines
• Effective Date: 05-Sep-2023

Overview

IEC 60034-27-1:2017 is an international standard developed by the International Electrotechnical Commission (IEC) that focuses on off-line partial discharge (PD) measurements on the winding insulation of rotating electrical machines. This standard establishes a comprehensive framework including measurement techniques, test circuit arrangements, normalization and testing procedures, noise reduction strategies, and guidelines for documenting and interpreting partial discharge test results.

This standard replaces the earlier IEC TS 60034-27 (2006) and introduces significant technical revisions to improve accuracy and reliability in partial discharge testing. It is specifically designed for off-line testing to assess insulation condition and detect partial discharges that can lead to insulation deterioration and eventual machine failure.

Key Topics

• Partial Discharge Fundamentals: IEC 60034-27-1 explains the nature of partial discharges in rotating machines, covering different types such as internal discharges, slot discharges, end-winding gap discharges, and those caused by foreign conductive materials.

• Measurement Techniques and Instrumentation: The standard details frequency response considerations, effects of PD coupling units, and characteristics of measuring instruments. It addresses both wide-band and narrow-band measurement systems.

• Test Circuits and Procedures: Guidance is provided on standardized test circuits for individual winding components and complete windings, including specific configurations for accurate PD detection. It also outlines test voltages, conditioning steps, and safety requirements.

• Data Presentation and Visualization: IEC 60034-27-1 defines minimum data presentation requirements alongside advanced graphical methods, such as Phase-Resolved Partial Discharge (PRPD) patterns, to enhance understanding of PD activity.

• Noise Reduction and Disturbance Suppression: This includes multiple methodologies for mitigating electrical noise and external interference, such as frequency limiting, masking techniques, digital filtering, and signal processing to improve PD measurement sensitivity.

• Interpretation of Results: The standard provides instructions for interpreting PD magnitude, inception/extinction voltages, patterns recognition, and trending over time to assess insulation health and predict potential failures.

• Documentation: Clear guidance is given on test reporting, ensuring comprehensive recording of test conditions, results, and interpretations to support maintenance and diagnostics decisions.

Applications

IEC 60034-27-1:2017 is essential for engineers, maintenance personnel, and researchers involved with rotating electrical machines such as motors and generators across different industries, including power generation, manufacturing, and transportation. Its applications include:

• Condition Monitoring: Regular off-line PD testing helps detect insulation defects early, preventing catastrophic failures and extending machine service life.

• Quality Control in Manufacturing: Standardized PD measurements ensure winding insulation quality before machines enter service.

• Maintenance Planning: Data-driven insights enable effective predictive maintenance strategies to reduce downtime and optimize repair schedules.

• Research and Development: Provides a technical basis for developing improved insulation materials and test equipment.

• Certification and Compliance: Assists manufacturers and operators in meeting international safety and reliability requirements.

Related Standards

IEC 60034-27-1:2017 complements and references other key IEC standards and technical reports, including:

• IEC 60034 Series: Covers various aspects of rotating electrical machines design, testing, and performance.

• IEC 60270: Specifies electrical measurement techniques for partial discharges, forming a foundation for PD detection.

• IEC TC 2 and TC 37 Publications: Include terminology, test methods, and guidelines related to electrical insulation and electrotechnical components.

• IEC 60034-27 (2006): The predecessor technical specification that IEC 60034-27-1 supersedes, with updated methods reflecting improvements.

By adhering to IEC 60034-27-1, stakeholders ensure precise, reliable off-line partial discharge measurements, aiding in the early detection of insulation issues and enhancing operational reliability of rotating electrical machines worldwide.

Standards Content (Sample)

IEC 60034-27-1 ®
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Rotating electrical machines –
Part 27-1: Off-line partial discharge measurements on the winding insulation

Machines électriques tournantes –
Partie 27-1: Mesurages à l’arrêt des décharges partielles effectués sur le
système d'isolation des enroulements
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IEC 60034-27-1 ®
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Rotating electrical machines –

Part 27-1: Off-line partial discharge measurements on the winding insulation

Machines électriques tournantes –

Partie 27-1: Mesurages à l’arrêt des décharges partielles effectués sur le

système d'isolation des enroulements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.160.01 ISBN 978-2-8322-5104-1

– 2 – IEC 60034-27-1:2017 © IEC 2017

CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 Nature of PD in rotating machines . 12
4.1 Basics of PD . 12
4.2 Types of PD in rotating machines . 13
4.2.1 General . 13
4.2.2 Internal discharges . 13
4.2.3 Slot discharges . 13
4.2.4 End-winding gap and surface discharges . 14
4.2.5 Foreign conductive materials discharges . 14
4.3 Pulse propagation in windings . 14
5 Measuring technologies and instrumentation . 15
5.1 General . 15
5.2 Influence of frequency response of measuring system . 15
5.3 Effects of PD coupling units . 16
5.4 Effect of the measuring instrument . 17
6 Visualization of measurements . 17
6.1 General . 17
6.2 Minimum scope of PD data presentation . 17
6.3 Additional means of PD data representation. 18
6.3.1 General . 18
6.3.2 Partial discharge pattern . 19
7 Test circuits . 19
7.1 General . 19
7.2 Individual winding components. 20
7.3 Complete windings . 21
7.3.1 General . 21
7.3.2 Standard measurements (SX.X) . 22
7.3.3 Optional, extended measurements (EX.X) . 23
7.3.4 Using integrated test equipment (IX.X) . 24
8 Normalization of measurements . 25
8.1 General . 25
8.2 Individual winding components. 26
8.3 Complete windings . 26
9 Test procedures . 28
9.1 Acquiring PD measurements on windings and winding components . 28
9.1.1 General . 28
9.1.2 Test equipment and safety requirements . 28
9.1.3 Preparation of test objects . 28
9.1.4 Conditioning . 29
9.1.5 Test voltages . 29
9.1.6 PD test procedure . 30

9.2 Identifying and locating the source of partial discharges . 32
10 Interpretation of test results . 32
10.1 General . 32
10.2 Interpretation of PD magnitude, inception and extinction voltage. 33
10.2.1 Basic interpretation . 33
10.2.2 Trend in PD in a machine over time . 34
10.2.3 Comparisons between winding components or between windings . 34
10.3 PD pattern recognition . 35
10.3.1 General . 35
10.3.2 Basic interpretation . 35
11 Test report . 37
Annex A (informative) Influence parameters of test frequency to testing procedure . 39
Annex B (informative) Alternative methods to determine discharge magnitudes . 40
B.1 Q , according to definition 3.14 . 40
m
B.2 Cumulative repetitive PD magnitude Q . 41
r
Annex C (informative) Other off-line methods for PD detection and methods for
localization . 43
Annex D (informative) External noise, disturbance and sensitivity . 44
D.1 General . 44
D.2 Sensitivity . 44
D.3 Noise and signal-to-noise ratio . 46
D.4 Disturbances . 46
Annex E (informative) Methods of disturbance suppression . 47
E.1 Frequency range limiting . 47
E.2 Phase window masking . 47
E.3 Masking by noise signal triggering . 47
E.4 Noise signal detection by measuring the propagation time . 47
E.5 Two-channel signal difference method . 48
E.6 Suppression of constant wave (CW) signals by digital filtering . 49
E.7 Noise and disturbance rejection using signal processing techniques . 49
Annex F (informative) Interpretation of PD magnitude data and phase resolved PD
patterns . 52
F.1 Instructions for interpretation of PRPD patterns . 52
F.1.1 Example of PRPD patterns . 52
F.1.2 Relative severity of different PD mechanisms . 54
F.1.3 Interpretation of the PD measurements from the line side and from the
star point . 55
F.1.4 Inductive discharges / Vibration sparking . 55
Annex G (informative) Test circuits for complete windings . 57
G.1 General . 57
G.2 Schemes and illustrations (see Figure G.1) . 57
Annex H (informative) Wide-band and narrow-band measuring systems . 62
H.1 General . 62
H.2 Wide band systems . 63
H.3 Narrow band systems . 63
Bibliography . 64

– 4 – IEC 60034-27-1:2017 © IEC 2017
Figure 1 – Frequency response of a PD pulse and coupling units of various time
constants . 16
Figure 2 – PD magnitude as a function of the normalized test voltage Q=f(U/U ) . 18
max
Figure 3 – Example of a PRPD pattern. 19
Figure 4 – Basic test circuits in accordance with IEC 60270 . 21
Figure 5 – Test circuit for PD measurement (S1.1) on complete windings . 22
Figure 6 – Normalization of the test circuit for measurement S1.1 . 27
Figure 7 – Test voltage applied to the test object during PD measurement. 30
Figure 8 – Example for identification and localization of PD sources . 36
Figure B.1 – Example for the indication of polarity effect . 40
Figure B.2 – Effect of A/D conversion accuracy and the calculation of Q , Example . 42
r
Figure D.1 – Recharging of the test object by various current components . 45
Figure E.1 – Without window masking . 47
Figure E.2 – With window masking . 47
Figure E.3 – Pulse currents through the measuring circuit . 48
Figure E.4 – Example of noise rejection . 50
Figure E.5 – Example of cross-talk rejection . 51
Figure F.1 – Example of PRPD patterns . 53
Figure G.1 – Illustrated diagrams for Ү- and Δ-connections, according to 7.3 . 61
Figure H.1 – Typical pulse responses of wide band and narrow band PD systems . 62

Table 1 – Connection configuration S1 for open star point . 22
Table 2 – Connection configuration S2 for closed star point . 23
Table 3 – Connection configuration E1 for open star point . 23
Table 4 – Connection configuration E2 for closed star point . 24
Table 5 – Connection configuration I1 for integrated equipment and open star point,
measurement on high voltage side . 24
Table 6 – Connection configuration I2 for integrated equipment and open star point,
measurement on star point side . 25
Table 7 – Connection configuration I3 for integrated equipment and closed star point . 25
Table A.1 – Recommended minimum measurement time and maximum slew rates . 39
Table F.1 – Severity associated with the main PD sources in rotating machines . 54

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ROTATING ELECTRICAL MACHINES –

Part 27-1: Off-line partial discharge measurements
on the winding insulation
FOREWORD
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International Standard IEC 60034-27-1 has been prepared by IEC technical committee 2:
Rotating machinery.
This International Standard cancels and replaces IEC TS 60034-27 (2006). It constitutes a
technical revision.
The main technical changes with regard to IEC TS 60034-27 (2006) are as follows:
st
• In 1 version the scope was not well defined, and open to a too wide range of
measurement frequencies. That has been corrected.
st
• In 1 version pulse magnitude was defined in different ways. Now, 2 definitions are given,
one for each method.
st
• In 1 version the types of PD were erroneous. Especially the definition of the most critical
“slot discharges” has been improved.

– 6 – IEC 60034-27-1:2017 © IEC 2017
• Adding one more common test arrangement to Clause 7.
• Adding Annex A.
• Adding Annex B.
• Adding Annex G.
• Moving part of the original text (valid for old fashioned instruments) to new Annex H.
The text of this International Standard is based on the following documents:
FDIS Report on voting
2/1877/FDIS 2/1887/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60034 series, published under the general title Rotating electrical
machines, can be found on the IEC website.
NOTE A table of cross-references of all IEC TC 2 publications can be found in the IEC TC 2 dashboard on the
IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
INTRODUCTION
For many years, the measurement of partial discharges (PD) has been employed as a means
of assessing the quality of new insulation systems and the condition of aged insulation
systems. It is also considered as a means of detecting localized sources of PD in used
electrical winding insulation arising from operational stresses in service. Compared with other
dielectric tests (e.g. the measurement of dissipation factor or insulation resistance) the
differentiating character of partial discharge measurements allows PD sources within the
insulation system to be detected.
In connection with the servicing and overhaul of rotating machines, the measurement and
analysis of partial discharges can also provide information on:
– presence of ageing effects and potential defects in the insulating 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 different methods of measurement in
existence but also the criteria and methods of analysing and finally assessing the measured
data are often different and not comparable. Consequently, there is a 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 (IEC 60034-27-2).
Both of these approaches have advantages and disadvantages with respect to one another.
While acknowledging the extensive world-wide use of on-line methods and their proven value
to industry, this international standard is confined to off-line techniques. This approach is
considered necessary to render this standard sufficiently concise to be of use by non-
specialists in the field of PD testing.
Limitations:
When PD measurements are performed on stator windings, several external factors will
inevitably affect the result. Consequently, PD measurements are only comparable under
certain conditions.
In a factory or site environment, the PD measurement results will be influenced by noise,
unless provisions have been made to reduce the influence of noise. Different hardware and
software methods, affecting for example measurement frequency band or noise cancellation
algorithms, are used in different equipment systems to separate relevant PD signals from
noise. Recalculation of the measured PD signal to an equivalent charge is an additional step
that will be dependent on the measurement and the calibration equipment that has been used
for normalization, as well as the method used.
Measurement conditions including temperature and moisture as well as test object set-up will
further affect the PD result. In case of a stator winding, the attenuation and dispersion of the
PD pulse during propagation will be dependent on the actual winding design and the origin of
the pulse.
– 8 – IEC 60034-27-1:2017 © IEC 2017
Based on the above reasons, absolute PD magnitude limits for the windings of rotating
machines, for example as acceptance criteria for production or operation are difficult to
define.
In addition, the degree of deterioration, and hence the risk of insulation system failure,
depends on the specific type of PD source and its location within the stator winding insulation,
both of which can influence the test results significantly.
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
discharges (for example insulation failure mechanisms, which are not accompanied by pulse
signals due to conductive paths between different elements of the insulation). Pulse signals
may further remain undetected in practice due to the impact of electrical noise and
disturbance conditions, which limit the detection sensitivity.
For individual bars and coils, absolute limits for PD magnitude are also difficult to establish
due to disparities between different test equipment and test setups. Therefore, no absolute
limits are given in the current version of this document.

ROTATING ELECTRICAL MACHINES –

Part 27-1: Off-line partial discharge measurements
on the winding insulation
1 Scope
This part of IEC 60034 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 winding insulation of rotating
electrical machines.
The measurement methods described in this document are applicable to stator windings of
machines with or without conductive slot coating and to the stator windings of machines made
with form wound or random wound windings. In special cases like high voltage rotor field
windings, this document is applicable as well. The measurement methods are applicable when
testing with alternating sinusoidal voltages from 0,1 Hz up to 400 Hz.
Interpretation guidelines are given in this document and are applicable only if all the following
requirements are fulfilled:
– Measurements performed with power frequency of 50 Hz or 60 Hz, or when testing with
power supply within a frequency range of 45 Hz to 65 Hz.
– Form wound windings and winding components such as bars and coils.
– Winding with conductive slot coating. This is usually valid for machines with voltage rating
of 6 kV and higher.
For machines with random wound windings, form-wound windings without conductive slot
coating, and testing at frequencies differing from power frequencies, the interpretation
guidelines are not applicable. The testing procedures for off-line PD-measurements of this
document can be used for assessing the uniform quality of manufacturing or/and the trending
of these kind of windings as well as converter driven machine windings.
NOTE Testing of low voltage machines with so called Type I insulation systems is defined in reference [10] .
Testing procedures for qualification of converter driven high voltage machines with so called Type II insulation
systems are dealt with in IEC 60034-18-42 (in addition to the optional electric tests described therein).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements 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.
___________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC 60034-27-1:2017 © IEC 2017
IEC 60034-18-32, Rotating electrical machines – Part 18-32: Functional evaluation of
insulation systems – Test procedures for form-wound windings – Evaluation by electrical
endurance
IEC 60034-18-42, Rotating electrical machines – Part 18-42: Partial discharge resistant
electrical insulation systems (Type II) used in rotating electrical machines fed from voltage
converters – Qualification tests
IEC TS 60034-27-2, Rotating electrical machines – Part 27-2: On-line partial discharge
measurements on the stator winding insulation of rotating electrical machines
IEC 60034-27-4, Rotating electrical machines – Part 27-4: Measurement of insulation
resistance and polarization index of winding insulation of rotating electrical machines
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
IEC 60270:2000/AMD1:2015
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.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
partial discharge
PD
localized electrical discharge that only partially bridges the insulation between conductors and
which can or cannot occur adjacent to a conductor
3.2
off-line measurement
measurement taken with the rotating machine at standstill and disconnected from the power
system
Note 1 to entry: The necessary test voltage is applied to the winding from a separate voltage source.
3.3
on-line measurement
measurement taken with the rotating machine in operation and connected to the power
system
3.4
stress control coating
paint or tape on the surface of the groundwall insulation outside the slot section whose
purpose is to smoothen the potential differences on the surface of high voltage stator bars
and coils
Note 1 to entry: 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.
3.5
conductive slot coating
conductive paint or tape layer in intimate contact with the groundwall insulation in the slot
portion of the coil or bar side, often called ‘semiconductive’ coating
Note 1 to entry: This coating provides electrical contact to the stator core.
3.6
slot discharges
discharges that occur between the outer insulation surface of the slot portion of a coil or bar
and the grounded core laminations
3.7
internal discharges
discharges that occur within the groundwall insulation
3.8
surface discharges
discharges that occur on the surface of the insulation or on the surface of winding
components in the winding overhang or the active part of the machine winding
3.9
pulse magnitude distribution
number of pulses within a series of equally spaced windows of pulse magnitude during a
predefined measuring time
3.10
pulse phase distribution
number of pulses within a series of equally spaced windows of phase during a predefined
measuring time
3.11
partial discharge pattern
number of pulses for a matrix of PD magnitude vs. AC cycle phase position for visualization of
the PD behaviour during a predefined measuring time
Note 1 to entry: Another type of representation may be used for the interpretation and source separation, such as
frequency vs. time.
3.12
coupling device
usually an active or passive four-terminal network that converts the input currents to output
voltage signals
Note 1 to entry: These signals are transmitted to the measuring instrument by a transmission system. The
frequency response 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
high voltage coupling capacitor of low inductance design and a low voltage coupling device in
series
– 12 – IEC 60034-27-1:2017 © IEC 2017
3.14
largest repeatedly occurring PD magnitude
Q
m
largest magnitude associated with a PD pulse repetition rate of 10 pulses per second (pps),
which can be directly inferred from a pulse magnitude distribution
Note 1 to entry: Other repetition rates may be used for defining the Q , for example 50 or 60 pulses per second.
m
If other rates are used, this needs to be indicated, for example as Q or Q .
m50 m60
3.15
weighted occurring PD magnitude

Q
iec
weighted magnitude recorded by a measuring system which has the pulse train response in
accordance with IEC 60270
Note 1 to entry: In this document, the symbol Q will be used as a placeholder for both definitions of charge, Q
m
and Q .
iec
3.16
noise
signals that clearly are not pulses and are not generated by the stator winding
3.17
disturbance
pulsed signals that clearly are not partial discharges but may have PD like characteristics
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 breakdown of the insulation. PD in
general requires a gas volume to develop, for example in gas filled voids embedded in the
insulation, adjacent to conductors or at insulation interfaces.
A partial discharge can occur when the local electrical field strength at an inhomogeneity
exceeds its breakdown strength. This process may result in several PD pulses during one
cycle of the applied voltage. In rotating machines with micaceaous insulation the occurrence
of numerous imperfections like small voids at new insulation and delaminations at aged
windings is unavoidable. Therefore, a superposition of PD sources of different intensity will
always be measured.
The amount of charge transferred in the discharge is closely related to the specific properties
of the inhomogeneity such as the dimensions and the specific dielectric properties of the
materials involved, for example surface properties, kind of gas, gas pressure, etc.
Stator winding insulation systems, including type II machines as defined in IEC 60034-18-42
are expected to experience PD activity in service. The insulation systems are inherently
resistant to partial discharges due to their inorganic mica components. However, significant
PD in these machines is usually a symptom of insulation deficiencies, such as a
manufacturing problem or in-service deterioration, rather than a direct cause of failure.
Nevertheless, depending on PD source and magnitude of the specific conditions at this point,
it may turn into a significant ageing factor of a local insulation ageing process. The time to
failure may not correlate with PD levels, but depends significantly on many factors for
example but not limited to operating temperature, wedging conditions, degree of
contamination, etc.
The measurement and the analysis of the specific PD behaviour can be 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 failure.
4.2 Types of PD in rotating machines
4.2.1 General
Partial discharges shall be generally expected in insulation systems of HV rotating machines,
but their magnitudes, amount and positions depend on the design, materials, manufacturing
processes, quality as well as on environmental and ageing conditions. For a given machine
design, the nature of the materials used, manufacturing methods, operating conditions, etc.,
can profoundly affect the quantity, location, characteristics, evolution and the significance of
PD. For a given machine, the various PD sources may be identified and distinguished in many
cases by their characteristic PD behaviour. Additional diagnostic tests and visual inspections,
if applicable, may verify the PD source.
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. As PD are normal for high voltage rotating electrical machines
the mica in the insulation is intended to provide an acceptable life under the specified ageing
conditions. See also IEC 60034-18-32 for detailed information.
4.2.2.2 Internal delamination
Internal delamination within the main insulation can be caused by improper resin impregnation
or curing of the insulation system during manufacturing or by mechanical or thermal over-
stressing during operation. Delamination can also develop due to ageing of insulation.
Delamination due to ageing is normally a long-term process. Therefore, delamination in old
insulation is a clear sign of insulation ageing. Large voids may develop over a large area
resulting in discharges of relatively high energy, which may significantly deteriorate the
insulation. In particular, delamination will reduce the thermal conductivity of the insulation,
which might lead to accelerated ageing. Thus, delamination needs careful consideration when
PD activity is being assessed.
4.2.2.3 Debonding between insulated conductor and groundwall insulation
Debonding PD between conductors and insulation material are generated within air or gas
filled elongated pockets (in longitudinal direction) that are embedded between the main
insulation and the conductor stack.
They may result from overheating or from extreme mechanical forces that both lead to
separation of large areas between these layers.
4.2.3 Slot discharges
Slot discharges in high voltage rotating machines could develop when the conduc
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