IEC 61251:2015
(Main)Electrical insulating materials and systems - AC voltage endurance evaluation
Electrical insulating materials and systems - AC voltage endurance evaluation
IEC 61251:2015 describes many of the factors involved in voltage endurance tests on electrical insulating materials and systems. It describes the voltage endurance graph, lists test methods illustrating their limitations and gives guidance for evaluating the sinusoidal a.c. voltage endurance of insulating materials and systems from the results of the tests. This International Standard is applicable over the voltage frequency range 20 Hz to 1 000 Hz. The general principles can also be applicable to other voltage shapes, including impulse voltages. The terminology to be used in voltage endurance is defined and explained. This first edition of IEC 61251 cancels and replaces the second edition of IEC TS 61251, published in 2008. This edition constitutes a technical revision and includes the following significant technical changes with respect to the second edition of IEC TS 61251:
a) upgrade from Technical Specification to an International Standard;
b) clarification of issues raised since publication of IEC TS 61251.
Systèmes et matériaux isolants électriques - Évaluation de l'endurance a la tension alternative
L'IEC 61251:2015 décrit plusieurs des facteurs intervenant dans les essais d'endurance à la tension de systèmes et de matériaux isolants électriques. Elle décrit le graphe d'endurance à la tension, donne une liste des méthodes d'essai en indiquant leurs limites et donne des lignes directrices pour évaluer l'endurance à la tension alternative de systèmes et de matériaux isolants à partir des résultats des essais. La présente Norme internationale est applicable sur la plage de fréquences de la tension allant de 20 Hz à 1 000 Hz. Les principes généraux peuvent également être applicables à d'autres formes de tension, y compris les tensions de chocs. La terminologie à utiliser dans le cadre de l'endurance à la tension est définie et expliquée. Cette première édition de l'IEC 61251 annule et remplace la deuxième édition de l'IEC TS 61251 parue en 2008. Cette édition constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à la deuxième édition de l'IEC TS 61251:
a) transformation d'une Spécification technique en Norme internationale;
b) clarification des questions soulevées depuis la publication de l'IEC TS 61251.
General Information
Relations
Standards Content (Sample)
IEC 61251 ®
Edition 1.0 2015-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electrical insulating materials and systems – AC voltage endurance evaluation
Systèmes et matériaux isolants électriques – Évaluation de l'endurance à la
tension alternative
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.
Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.
IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé Fax: +41 22 919 03 00
CH-1211 Geneva 20 info@iec.ch
Switzerland www.iec.ch
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.
About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.
IEC Catalogue - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
The stand-alone application for consulting the entire The world's leading online dictionary of electronic and
bibliographical information on IEC International Standards, electrical terms containing more than 30 000 terms and
Technical Specifications, Technical Reports and other definitions in English and French, with equivalent terms in 15
documents. Available for PC, Mac OS, Android Tablets and additional languages. Also known as the International
iPad. Electrotechnical Vocabulary (IEV) online.
IEC publications search - www.iec.ch/searchpub IEC Glossary - std.iec.ch/glossary
The advanced search enables to find IEC publications by a More than 60 000 electrotechnical terminology entries in
variety of criteria (reference number, text, technical English and French extracted from the Terms and Definitions
committee,…). It also gives information on projects, replaced clause of IEC publications issued since 2002. Some entries
and withdrawn publications. have been collected from earlier publications of IEC TC 37,
77, 86 and CISPR.
IEC Just Published - webstore.iec.ch/justpublished
Stay up to date on all new IEC publications. Just Published IEC Customer Service Centre - webstore.iec.ch/csc
details all new publications released. Available online and If you wish to give us your feedback on this publication or
also once a month by email. need further assistance, please contact the Customer Service
Centre: csc@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.
A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.
Catalogue IEC - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
Application autonome pour consulter tous les renseignements
Le premier dictionnaire en ligne de termes électroniques et
bibliographiques sur les Normes internationales,
électriques. Il contient plus de 30 000 termes et définitions en
Spécifications techniques, Rapports techniques et autres
anglais et en français, ainsi que les termes équivalents dans
documents de l'IEC. Disponible pour PC, Mac OS, tablettes
15 langues additionnelles. Egalement appelé Vocabulaire
Android et iPad.
Electrotechnique International (IEV) en ligne.
Recherche de publications IEC - www.iec.ch/searchpub
Glossaire IEC - std.iec.ch/glossary
Plus de 60 000 entrées terminologiques électrotechniques, en
La recherche avancée permet de trouver des publications IEC
en utilisant différents critères (numéro de référence, texte, anglais et en français, extraites des articles Termes et
comité d’études,…). Elle donne aussi des informations sur les Définitions des publications IEC parues depuis 2002. Plus
projets et les publications remplacées ou retirées. certaines entrées antérieures extraites des publications des
CE 37, 77, 86 et CISPR de l'IEC.
IEC Just Published - webstore.iec.ch/justpublished
Service Clients - webstore.iec.ch/csc
Restez informé sur les nouvelles publications IEC. Just
Published détaille les nouvelles publications parues. Si vous désirez nous donner des commentaires sur cette
Disponible en ligne et aussi une fois par mois par email. publication ou si vous avez des questions contactez-nous:
csc@iec.ch.
IEC 61251 ®
Edition 1.0 2015-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electrical insulating materials and systems – AC voltage endurance evaluation
Systèmes et matériaux isolants électriques – Évaluation de l'endurance à la
tension alternative
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 17.220.99; 29.035.01 ISBN 978-2-8322-2990-3
– 2 – IEC 61251:2015 © IEC 2015
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and symbols. 6
3.1 Terms and definitions . 6
3.2 Symbols . 7
4 Voltage endurance . 7
4.1 Voltage endurance testing . 7
4.2 Electrical stress . 7
4.3 Voltage endurance (VE) graph . 8
4.4 Short-time electric strength . 8
4.5 Voltage endurance coefficient (VEC) . 9
4.6 Differential VEC (n ) . 9
d
4.7 Electrical threshold stress (E ) . 9
t
4.8 Voltage endurance relationship . 10
5 Test methods . 11
5.1 Introductory remarks . 11
5.2 Tests at constant stress . 11
5.2.1 Conventional VE test . 11
5.2.2 Diagnostic measurements . 12
5.2.3 Detection of an electrical threshold . 12
5.3 Tests at higher frequency. 12
5.4 Progressive stress tests . 13
5.5 Preliminary tests to determine the initial part of the VE line . 15
5.6 Recommended test procedure . 15
6 Evaluation of voltage endurance . 15
6.1 Significance of the VEC . 15
6.2 Significance of the electrical threshold stress . 16
6.3 Dispersion of data and precision requirements . 16
6.4 Presentation of the results . 16
Annex A (informative) The Weibull distribution . 18
A.1 Weibull distribution times to dielectric breakdown . 18
A.2 Weibull distribution dielectric breakdown stresses . 18
A.3 Generalized Weibull distribution of the dielectric breakdown stresses . 18
A.4 Inverse power model for the time to dielectric breakdown . 19
Bibliography . 20
Figure 1 – General voltage endurance line . 8
Figure 2 – Determination of the differential VEC n at a generic point P of the VE line . 9
d
Figure 3 – Plotting the VE line in a progressive stress test using different rates of
stress rise . 14
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL INSULATING MATERIALS AND SYSTEMS –
AC VOLTAGE ENDURANCE EVALUATION
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
international 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,
Technical 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 organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization 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
interested 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
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
International Standard IEC 61251 has been prepared by IEC technical committee 112:
Evaluation and qualification of electrical insulating materials and systems.
This first edition of IEC 61251 cancels and replaces the second edition of IEC TS 61251,
published in 2008. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the second
edition of IEC TS 61251:
a) upgrade from Technical Specification to an International Standard;
b) clarification of issues raised since publication of IEC TS 61251.
– 4 – IEC 61251:2015 © IEC 2015
The text of this standard is based on the following documents:
FDIS Report on voting
112/338/FDIS 112/347/RVD
Full information on the voting for the approval of this standard 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.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
This International Standard covers insulating materials and systems. Voltage endurance tests
are used to compare and evaluate insulating materials and systems. It is complex to
determine the capability of electrical insulating materials and systems to endure a.c. voltage
stress. The results of voltage endurance tests are influenced by many factors. Therefore this
International Standard can be considered as an attempt to present a unified view of voltage
endurance for simplified planning and analysis.
– 6 – IEC 61251:2015 © IEC 2015
ELECTRICAL INSULATING MATERIALS AND SYSTEMS –
AC VOLTAGE ENDURANCE EVALUATION
1 Scope
This International Standard describes many of the factors involved in voltage endurance tests
on electrical insulating materials and systems. It describes the voltage endurance graph, lists
test methods illustrating their limitations and gives guidance for evaluating the sinusoidal a.c.
voltage endurance of insulating materials and systems from the results of the tests. This
International Standard is applicable over the voltage frequency range 20 Hz to 1 000 Hz. The
general principles can also be applicable to other voltage shapes, including impulse voltages.
The terminology to be used in voltage endurance is defined and explained.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62539, Guide for the statistical analysis of electrical insulation dielectric breakdown data
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
voltage endurance
VE
measures of the capability of a solid insulating material to endure voltage
Note 1 to entry: In this International Standard, only a.c. voltage is considered.
Note 2 to entry: This note only applies to the French language.
3.1.2
life
time to dielectric breakdown
3.1.3
voltage endurance coefficient
VEC
numerical value of the reciprocal of the slope of a straight line log-log VE plot
Note 1 to entry: This note only applies to the French language.
3.1.4
specimen
representative test object for assessing the value of one or more physical properties
3.1.5
sample
group of nominally identical specimens extracted randomly from the same manufacturing
batch
3.2 Symbols
c, c′ constants in the inverse-power model
E electric stress
E short-time electric strength
o
E electric threshold stress
t
f frequency
h, k constants in the exponential model
L life
m scale parameter in the Weibull distribution (one variable)
M scale parameter in the generalized Weibull distribution (two variables)
n exponent of stress in the inverse-power model coinciding with the VEC
n differential VEC
d
R dimensional ratio
t time
t time to dielectric breakdown at constant stress
c
t time to dielectric breakdown at constant stress E
o o
t time to dielectric breakdown with progressive stress
p
tan δ dissipation factor
α scale parameter (63,2 percentile) in the Weibull distribution of times to dielectric
breakdown at constant stress
β shape parameter in the Weibull distribution of times to dielectric breakdown at
constant stress
γ shape parameter of the Weibull distribution of the dielectric breakdown stresses from a
progressive stress test
4 Voltage endurance
4.1 Voltage endurance testing
To evaluate the voltage endurance of insulating materials or systems, a number of specimens
are subjected to a.c. voltage and their times to dielectric breakdown are measured. In practice,
several samples of many specimens are tested at different voltages to reveal the effect of the
applied voltage on the time to dielectric breakdown. The arithmetic mean time to dielectric
breakdown of each sample is the average time to dielectric breakdown of all specimens tested
at that voltage. The time at which a certain percentage of specimens break down is the
estimated time to dielectric breakdown with a probability equal to this percentage.
The statistical treatment of the data (either by analytical or graphical methods) allows the
extraction of additional data such as other failure percentiles or confidence bounds and,
possibly, determination of the distribution (Gaussian, Weibull, lognormal, etc.).
4.2 Electrical stress
In general, reference to electrical stress (voltage per unit thickness) instead of voltage is
required. For a uniform field, electrical stress is given by the voltage (effective value) divided
by the thickness of specimens.
– 8 – IEC 61251:2015 © IEC 2015
If the electric field is not uniform, the maximum value shall be considered by the relevant
equipment committees.
4.3 Voltage endurance (VE) graph
The VE graph represents the time to dielectric breakdown (life) versus the corresponding
value of electrical stress. In the VE graph, the electrical stress is plotted as the ordinate with
either a linear or logarithmic scale. The times to dielectric breakdown are plotted on the
abscissa with a logarithmic scale. The voltage endurance line on this graph gives the final
result of the VE tests as it allows clear and complete evaluation of voltage endurance of the
specimens under the specified test conditions. For maximum significance, materials or
systems shall be compared at equal thickness and using the same type of electrodes,
temperature, humidity and ambient gas, or as agreed by the relevant equipment committees.
An accurate plotting of the line requires more than three tests at different voltages and one or
more tests are required at voltages which result in times to failure longer than 1 000 h. In any
case, a minimum number of three tests is required to draw the VE graph.
The voltage endurance line is straight or curved. In the latter case, its trend can often be
approximated by a few straight regions: sometimes a first part for short times with a low slope,
a middle region (which can extend to long times) with a steeper slope and finally a further
trend of the line showing a tendency to become horizontal (see Figure 1, where a general VE
line is shown). It is likely that the shape of the VE graph changes significantly from one
material or system to another. With a curve as shown in Figure 1, the VEC is not constant,
and the VEC will be different at different times (see n in Figure 2).
d
Log E
E
o
E
t
t Log time to breakdown
o
IEC
Figure 1 – General voltage endurance line
4.4 Short-time electric strength
The short-time electric strength is measured using a linearly increasing voltage. The duration
of such a test, as used in this International Standard, is of the order of one minute up to some
tens of minutes. The arithmetic mean value of the breakdown field for the tested sample is E .
o
The results of electric strength tests (or, in general, of tests with increasing voltage) are not
represented directly in the VE graph. Instead, a constant voltage test at the same stress as
the mean electric strength, E (or very close to it, 0,9E or as agreed), is made to determine
o o
the time to dielectric breakdown, t , with constant stress. The point (E , t ) is the origin of the
o o o
VE line. More details on this procedure are given in 5.5. However, when this procedure is
used, the following precautions shall be taken.
a) The electric strength tests shall be carried out under the same conditions (humidity,
temperature, etc.), in the same test cell and with the same procedures as for the voltage
endurance tests.
b) The test specimens, the breakdown path and the conditions of the specimen after
dielectric breakdown shall be examined and recorded for future use in the analysis of the
results. The latter is to ensure that the mode of failure at high stress is the same as that of
the other specimens tested later at lower stress.
4.5 Voltage endurance coefficient (VEC)
The slope of the VE line, n, is an indicator of the response of a material or system to electrical
stress. The parameter n is dimensionless. With a small slope of the VE line (i.e. a large value
of VEC), even a small reduction of stress produces a great increase in life. The reciprocal of
the slope is taken to be consistent with the numerical value of the exponent n in Formula (1).
A large value of the VEC does not correspond necessarily to high electric strength. It can
happen that the material with lower VEC has a longer time to dielectric breakdown at a given
stress if its short-time electric strength is so high that its poorer endurance is compensated for.
The value of n shall be associated with a high mean electric strength before attributing a high
endurance to the material. What is most significant is the retention of usable electric
strength for long periods of time.
4.6 Differential VEC (n )
d
If the VE line is curved in log-log coordinates, its slope is measured by means of the tangent
at any point. For any electrical stress, and thus for any point on the line, the differential
, can be defined as the absolute value of the reciprocal of
voltage endurance coefficient, n
d
the slope of the curve at that point (Figure 2) according to the life model described in
Clause 5.
E
Log
VE line
E
o
Line for determining n
d
1,0
Log time to breakdown
t
o
IEC
Figure 2 – Determination of the differential VEC n at a generic point P of the VE line
d
4.7 Electrical threshold stress (E )
t
If the VE line tends to become horizontal with decreasing stress within the test stress-times,
this indicates the presence of a limiting stress, E , below which electrical ageing becomes
t
negligible. This limit is called the electrical threshold stress. The tendency of the line to
become horizontal is detected by means of tests of suitable duration. However, the tests do
not always succeed in revealing such a trend in a reasonable time. Some insulating materials
or systems do not show any electrical threshold stress even for very long test times.
– 10 – IEC 61251:2015 © IEC 2015
4.8 Voltage endurance relationship
The VE relationship is the mathematical model of life under electrical stress or voltage, i.e.
the formula relating electrical stress and time to dielectric breakdown, whose graphical
representation is given by the VE line. If this line is straight on log-log graph paper, the
formula is of the type:
−n
L = c E (1)
where
L is the time to dielectric breakdown or time to failure or life;
E is the electrical stress;
c and n are constants dependent on temperature and other environmental parameters.
Formula (1) constitutes the so-called inverse-power model, which is the voltage-life model
often encountered with voltage endurance data on solid electrical insulation. In this case the
VEC is n, and it is constant. When data are available for time to dielectric breakdown at two
constant-voltage stresses, this model shall be used to get a rough estimate of the value of n
by using Formula (2):
−n
L E
1 1
= (2)
L E
2 2
If the VE test data do not form a straight line on log-log paper, the use of the inverse-power
, other models have
model is incorrect. If the line approaches an electrical threshold stress, E
t
been proposed, among them
n
L = c′ (E – E ) , (3)
t
which becomes the inverse-power model if E tends to 0 and is preferably used when the data
t
for short and medium times fit a straight line on log-log coordinates. Alternatively, another
model is
k exp (− h E)
L = , (4)
E − E
t
which derives from the exponential model, corresponding to an approximately straight line in
semilog coordinates for E > E but gives infinite time to dielectric breakdown when E tends to
t
E . In Formulas (3) and (4), constants c′, n, k, h and E depend on temperature and other
t t
environmental conditions.
Formulas (3) and (4) generate two new formulas which define the trend of the VE line
, E ) and (L , E ). The following formulas are obtained:
between any two points, (L
1 1 2 2
−n
E − E
L
1 1 t
= , (5)
L E − E
2 2 t
L exp {− h (E − E )}
1 1 2
= . (6)
L (E − E )/ (E − E )
2 1 t 2 t
The formulas of the VE line for a straight line or a straight-line segment on log-log plot are
Formulas (1) and (2). When there is a tendency toward a threshold after an approximately
linear trend on log-log or semilog graph paper, Formulas (3), (4), (5) and (6) apply.
By taking the logarithms, the inverse-power model, Formula (1), becomes
ln (L) = ln (c) − n ln (E) . (7)
This is the formula of the straight VE line in log-log coordinates. Its slope is −1/n. As the
numerical value of the reciprocal of the slope is equal to n, the VEC can also be defined as
the exponent n in the inverse-power model.
5 Test methods
5.1 Introductory remarks
Different methods of carrying out the VE test can be used. The differences concern the way of
applying voltage (constant or increasing with time), the frequency (service or higher) and the
time at which the test is interrupted (the time to dielectric breakdown for all sample specimens
(complete life tests) or a shorter time for some of the specimens of the sample (censored life
tests).
In general to enable comparisons to be made, the type of ageing cell or test object shall be
the same, whatever the choice of the parameters above. However, with respect to the choice
of the frequency of the applied voltage, the amount of heating from either dielectric loss or
from partial discharges shall be such that the temperature rise from these causes is less than
3 K.
When testing materials, the ageing cell or test object should result in a uniform electric field.
This can be achieved by electrodes having a flat surface rounded at the edges. To avoid
partial discharges and flashover along the specimen surface, the specimen shall extend a
suitable distance beyond the edges of the electrodes. If preliminary tests indicate that this
extension beyond the electrodes is not enough to avoid partial discharges and flashover, the
electrodes shall be immersed or embedded in an appropriate dielectric having the same or
higher permittivity than that of the material under test.
The form and processing of the specimen will depend on the purpose of the test. For research
purposes, internal degradation studies as a function of cavity size and shape have been
performed. However, this lies outside the scope of this International Standard. Evaluation and
comparison of materials from the point of view of degradation by external discharge are dealt
with in IEC 60343.
For insulation systems, the test objects shall represent adequately the form taken in service
and be determined by the relevant IEC equipment committee.
5.2 Tests at constant stress
5.2.1 Conventional VE test
In the constant stress test, the magnitude of the voltage applied to each specimen is kept
constant during the test. This magnitude is usually selected in such a way that the arithmetic
mean time to dielectric breakdown of the sample is between a few tens and a few thousands
of hours. The time to dielectric breakdown of some specimens, especially at the lower
stresses, can be so long that it is impracticable to wait for dielectric breakdown of all
specimens of the sample. In this case, the interruption of the test after dielectric breakdown of
some of the specimens requires the use of statistical procedures for censored data (see
IEC 62539).
– 12 – IEC 61251:2015 © IEC 2015
Usually, three or four different levels of voltage or electric field are used, thereby providing
three or four points for the VE line. Four points are often not enough to demonstrate curvature
of the line. On the other hand, the amount of data required for tests at more than four
voltages is expensive to obtain.
The fit of the data to a straight line shall be established through regression analysis as
specified in IEC 62539. If the quality of fit is good, that is the correlation coefficient R is 0,90
or higher, the VE line can be fitted to a straight line, with the negative reciprocal of the slope
of the line being the VEC. If R is below 0,90, the VE line is curved and a straight line model
is not appropriate.
For any test voltage, the times to dielectric breakdown of the specimens of a sample can be
tested for their fit to various breakdown time probability functions. If the data fit the Weibull
distribution, the experimental data give rise to a straight line (on Weibull paper) whose slope
is the shape parameter, β, of the distribution (see Annex A). Proceeding in the same way for
every test at different voltages, the variance of β can be checked.
5.2.2 Diagnostic measurements
In some cases there is no need to measure diagnostics. In those cases where the
measurement of diagnostics is necessary, diagnostic quantities such as tan δ or partial
discharge shall be monitored during the test. Where tan δ or partial discharge versus time
curves obtained at different voltages are compared, similar patterns can be observed. This
provides a contribution to understanding ageing behavior and prediction of the behavior of the
VE line for other samples.
Short-time electric strength measurements can also be carried out on specimens that have
not failed after a fixed ageing time, in order to evaluate their state of ageing. Thus the short-
time electric strength is a diagnostic quantity to determine the degree of ageing caused by
electrical stress.
To investigate the ageing process thoroughly, it is useful to employ chemical and microscopic
analyses. The results are often related to the variation of macroscopic properties: short-time
electric strength, conductivity, tan δ, etc.
5.2.3 Detection of an electrical threshold
The experimental points sometimes show a tendency of the VE line to become horizontal after
long voltage exposure times. Moreover, many reports of VE investigations include points
indicating much longer times to failure at the lower levels of stress than expected from
extrapolation of the trend at higher voltages. These results can indicate the existence of an
electrical threshold. It is desirable to test the data for the presence of such a threshold (E ).
t
A check for the threshold voltage can be made by a test at elevated frequency, as illustrated
in 5.3. Another method which permits evaluation of the trend of the VE line at low stresses is
given in 5.6. The threshold stress is influenced by temperature, usually decreasing as
temperature rises. For temperatures higher than room temperature, the VE line is usually
displaced towards the left of the graph and the times to dielectric breakdown are shorter for
the same electric stress. The VE test is often carried out at room temperature but tests at
higher temperatures provide information on the type of ageing processes, on the shape of the
VE line and, in particular, on the existence of a threshold and its dependence on temperature.
5.3 Tests at higher frequency
In order to reduce the test times, the frequency of the applied voltage may be increased. The
time to dielectric breakdown, L , at power frequency f is often derived from the time to
f
dielectric breakdown, L , at the test frequency, f , by means of the following relationship:
h h
f
h
L = L (8)
f h
f
However, the validity of this relationship is not proved, especially for organic materials when
the test frequency is more than 10 times f. Sometimes, acceleration is found to be
proportional to the frequency ratio raised to a power different from unity. This exponent
depends also on temperature, environmental conditions and type of prevailing ageing
mechanism. Because permittivity and tan δ depend on frequency and temperature, dielectric
heating, which is proportional to the product of the frequency, permittivity and tan δ, affects
the time to dielectric breakdown. Also, partial discharges in micro-voids or defects inside the
material and/or on the specimen surface have a different influence at a different frequency.
Therefore, it is important that the interpretation of frequency-accelerated experiments is done
with caution.
High-frequency tests at low stresses can be performed to infer the existence and, possibly,
estimate the value of the electrical threshold. If the results of power-frequency tests seem to
indicate the possible presence of a threshold, a high-frequency test shall be made at a
voltage close to the voltage of the suspected threshold. If the time to dielectric breakdown
at that voltage is considerably longer than would be expected according to the trend of the
VE line at higher voltages combined with Formulas (3) to (6), the presence of the threshold
is almost certainly confirmed and its estimation can be performed through fitting of the life
curve to such formulas
5.4 Progressive stress tests
In the progressive stress test, the magnitude of the stress applied to each specimen in a
sample increases with time until failure. The rate of the stress rise shall be the same for all
specimens in a sample. However, to create a VE line, different rates of stress rise shall be
used on each sample (i.e. collection of specimens). See Figure 3.
In this test, all specimens fail. Statistical treatment of the data is particularly useful due to the
large quantity of information obtained. If the data relevant to each sample fit to the Weibull
distribution, the corresponding points fit a straight line in Weibull paper. The slope of the line
is the shape parameter γ of the distribution (see Clauses A.2 and A.3). Note that if γ is the
same at different rates of voltage rise, the VEC can be derived from the ratio of γ to β
(see Clause A.4). For this reason, in the VE test on materials and systems for which
constancy of the VEC is expected in the test voltage range, a good practice is to carry out a
progressive stress test in order to determine γ before starting with the constant stress tests.
The VEC can then be derived theoretically. This permits a check of the value of the VEC to be
made and thus the likely duration of the test program.
– 14 – IEC 61251:2015 © IEC 2015
Voltage
V
V
V
V
t t t t Time
1 2 3 4
IEC
Figure 3 – Plotting the VE line in a progressive
stress test using different rates of stress rise
Knowledge of the value of γ is of great importance when the results have to be reported for
specimens of different size, i.e. area or volume. The dielectric breakdown probability at the
same voltage stress is an increasing function of the dimensions of specimens. In order to
transform the data – for instance the dielectric breakdown stress with a given probability –
from the specimens for which these data have actually been obtained to specimens of
different dimensions, it is necessary to know the relationship between probability, stress and
dimensions. If the Weibull distribution is valid, the ratio between two stresses, E and E ,
1 2
corresponding to the same dielectric breakdown probability for two elements, 1 and 2, of
different area is given by
E
1 1 γ
= R , (9)
E
where R is the dimensional ratio, i.e. the ratio of the dimensions (area) of element 2 to those
of element 1. See Formula (A.2).
The progressive stress test data are usually less scattered than those from constant stress
tests. If the VE line is straight on a log-log plot, its slope is also the same for progressive
stress. The progressive stress data are related to those at constant stress by the following
formula:
t = t (n + 1) , (10)
p c
where t and t are the times to dielectric breakdown at progressive and constant stress,
p c
respectively, for the same value of stress and n is the VEC.
Since n is usually in the range 8 to 15, t is shorter than t . The times to dielectric
c p
breakdown with progressive stress are significantly shorter than the failure times from
constant stress tests. Therefore, the progressive stress test is useful only for evaluation of
the VEC in the short-times range. If the VEC is not constant, it is not possible to predict
time to dielectric breakdown at constant stress starting from progressive stress data. In any
case, no information on the long-time behavior of the test material, let alone on the
threshold, is obtainable by progressive stress testing.
NOTE n is typically between 9 and 12 for mica-epoxy materials.
5.5 Preliminary tests to determine the initial part of the VE line
Preliminary tests are useful to determine the initial high-voltage part of the VE line, as well as
an initial estimate for the value of n. These tests provide data for planning the future lower
voltage tests. They include the following:
a) A progressive stress test or a step voltage test similar to a short-time electric strength test.
The arithmetic mean dielectric breakdown voltage from this test is E . The aim is to
o
puncture the specimen rather than cause flashover of the specimen. The failure shall not
be a flashover and shall resemble the dielectric breakdowns obtained at lower voltages
and longer times, thus involving the same ageing mechanism. The time to dielectric
breakdown in this test is often longer than the value suggested in IEC 60243-1.
b) A constant stress test at or near E . The voltage shall be raised to the value of E without
o o
overshoots, and time t is calculated as the average of the breakdown times of the sample
o
specimens. A zero crossing switch can be used to initiate the test to avoid overshoots and
a counter to count the number of a.c. cycles to dielectric breakdown.
c) Constant stress tests at stresses slightly lower than E , for example 0,9 E , 0,8 E .
o o o
According to Formula (10), the theoretical ratio of the arithmetic mean time to dielectric
breakdown with progressive stress, t , to the arithmetic mean time to dielectric breakdown
p
with the constant stress, t , is n + 1. From this an estimate of the value of n at the initial part
c
of the VE line can be calculated. Note that the point (E , t ) is on the VE line.
o o
5.6 Recommended test procedure
In order to characterize insulating materials or systems comprehensively from the point of
view of electrical endurance, the following procedure is recommended.
a) Perform preliminary tests at high stress, as described in 5.4.
b) Perform constant stress tests at lower stresses. A sufficient number of tests at different
stresses shall be performed to plot the VE graph and to obtain a reliable prediction of the
long-time behaviour of the material under test. In any case, at least three test voltages are
required. Other diagnostic measurements are also useful.
When the graph shows a tendency towards a threshold stress, the following procedure is
often a useful check for the existence of a threshold. Perform a test at a stress about 5 %
below the expected threshold stress with increased frequency. After a few thousand hours,
remove some of the specimens and perform chemical-physical analysis and short-time
electric strength measurements. No statistically significant variation of properties with respect
to unaged specimens, e.g. decrease of electric strength, shall be found if the voltage applied
is below the threshold.
6 Evaluation of voltage endurance
6.1 Significance of the VEC
Considering a VE line, the larger the value of the VEC, the longer the time to dielectric
breakdown for t
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.
Loading comments...