Close and Return
---------~----------------------
JICABLE '99
(,10.1
Type testing of cables and accessories
BERUJN S.M., GRUNTJES R.J.B., ROELOfS G.P.T., Kema Nederland, Arnhem, The Netherlands
Résumé
Abstract
1
Parce que les câbles et leurs. accessoires
représentent une partie importante des réseaux
électriques, ils sont soumis à de nombreux essais,
dont les essais de type. Pour ces essais plusieurs
normes sont applicables. Les essais de type ne
sont pas superflus car il apparaît que 26% des
matériels essayés sont refusés suite à un défaut
de conception. Le taux de refus pour les
accessoires (33%) est supérieur à celui des câbles
(17%). Pour les accessoires, le VDE 278 est une
norme sévère; 50% des matériels sont refusés en
essai. Il ne se dégage pas de tendance particulière
sur le taux de refus, cependant le taux de
défaillance en essai des accessoires tend à
augmenter.
Because cables and accessories are an important
part of the electric infrastructure they are subjected
ta several tests. One of these tests are type tests.
For type tests several standards are available. Type
testing of cables and accessories is not a
superfluous luxury, while it appears that around 26%
of the abjects do not pass the type tests because of
design failures. The failure rate for accessories
(33%) is higher than for cables (17%). For
accessories, VDE 278 ls a severe standard, 50% of
the objects does not pass type tests according ta this
standard. No c1ear trend in failure rate can be seen
over the years, although the failure rate of
accessories has the tendency ta increase.
Introduction
Standards used for type testing of cables and
accessories
Cables and accessories are an important part of the
eiectric infrastructure. The necessity of a high quality
of manufacturing ls basically expected [1]. Because
of this, cab les and accessories are subjected ta
several tests in several manufacturing stages. One
can distinguish between for instance pre-qualification
tests, type tests, routine tests, acceptance tests and
after laying tests.
These tests can be performed according ta different
standards, depending on the rated voltage of the
cable or accessory and the requirements of the
purchaser, for instance VDE 278 [2], IEC 60502 [3],
IEC 60840 [4], etc.
To prove the correct design of the cable and
accessories, type tests are often required. The
manufacturer and the utilities have to make choices
between several standards and they have ta decide
if only the cable or accessories will be tested or that
the combination that will be used 'will be tested.
ln this paper results will be presented of type tests
performed on cables and accessories during the last
six years (1993-1998) at KEMA High-Voltage
Laboratory. Figures are presented about the total
number of cables and accessories tested, according
ta which standard, the number of failures and causes
of the failures. At the end of the paper some
conclusions are drawn and sorne recommendations
are given.
At this moment there are several national and
international standards for testing of MV and HV
cables and accessories. The most applied
international standards can be divided into four
categories, according ta the nominal voltage:
5
•
150 - 500 kV, IEC 60267 in preparation
• 6 - 30 kV, e.g. IEC 60502-2 and -4 and VDE 278
• 30 -150 kV, e.g. IEC 60840
• 1 and 3 kV, e.g. IEC 60502-1
From the 166 type tests performed in six years time,
most tests are performed according ta IEC 60840,
IEC 60502 and VDE 278, as is shown in Figure 1.
60
.
50
tlS 40
CIl
c.
~
'0
30
~
CIl
.Q
E 20
::>
Z
10
IEC 840
IEC502
VDE 278
Other
Figure 1 Number of type tests performed as.a
function of the standard
Close and Return
The category other standards contains for instance
Canadian or Dutch standards, however the number
of tests performed per standard is not enough to
make any statistical analysis. The three standards
used most for type testing and a new standard are
explained in the following four paragraphs.
IEC 60267
Until now no official IEC standard is available for
tests for power cables with extruded insulation and
their accessories above 150 kV (U m=170 kV). At this
moment the International Electrotechnical
Commission is preparing a new standard for rated
voltages above 150 kV up to 500 kV. The prequalification test will be one of the tests, described in
this standard. The publication of the final draft is to
be expected in the beginning 2001.
The electrical and non-electrical type tests as
described in part 2 are in general the same as the
tests described in IEC 60840. Regarding the
electrical type tests two important differences can be
mentioned. In the first place, IEC 60502 requires the
heating cycle test to be performed without applying
voltage and with a heating cycle of at least 8 hours
duration. IEC 60840 requires a heating cycle of at
least 24 hours duration and continuously voltage
should be applied to the sample. The number of
cycles amounts to 20 for both standards.
Secondly, the maximum allowed dielectric loss factor
for XLPE insulation in accordance with IEC 60840 is
much lower than the requirement in IEC 60502-2.
The non-electrical type tests in both standards are
comparable and have the same purpose: checking
the properties of the cable materials.
IEC 60840
VDE 278
ln February 1999 the flrst edition (from 1988) has
been replaced by the second edition. The first edition
of IEC 60840 only describes test methods and
requirements for power cables with extruded
insulation for rated voltages above 30 kV up to
150 kV.
VDE 278 is a German standard for power cable
accessories with extruded insulation for rated
voltages up to 30 kV. The latest edition was
published in November 1997 and consists of
different parts, which are based on the
Harmonisation Documents. VDE 278 is describing
tests for the.following type of accessories: joints, inand outdoor terminations, stop-ends and cable
connectors. The VDE 278 was a more severe
standard for accessories than the IEC 60502 from
1993, the latest edition is in general equal to, the
now more severe, part 4 of IEC 60502.
Type tests consist of the following electrical tests:
bending test, partial discharge test, tan
measurement, heating cycle voltage test, partial
discharge measurement, impulse withstand test and
a power frequency voltage test. Furthermore a
number of non-electrical type tests are required in
order to test the properties of the materials.
ô
This standard is often used for performing type tests
on HV cables as can be seen in Figure 1. Although
the first edition of tEC 60840 does not describe tests
for accessories of power cables for rated voltages
above 30 kV up to 150 kV, some manufacturers
have performed type tests according to IEC 60840
on a cable with joints and terminations as is shown in
Figure 1. Apparently there was a need for a
standard for accessories, therefor, a new edition has
been published which replaces the first edition and
describes tests for both power cab les with extruded
insulation and their accessories for the same voltage
rating. Regarding thé test programme no major
changes have occurred.
Ali figures presented in this paper are related to tests
performed in accordance with the first edition.
IEC60S02
Since April 1997, this standard consists of three
parts (1, 2 and 4) under the general title: Power
cables with extruded insulation and their accessories
for rated voltages from 1 kV up to 30 kV, which
replace the former tEC 60502 (1993). Part 2 and
part 4 of IEC 60502 concern rated voltages from 6
kV up to30 kV for tests on respectively cables and
accessories.·
Results of type tests performed
Data used for analysis
From 1993 until 1998, 166 Type test are performed
on cables and accessories for around 50 different
customers. From these 166 type tests, 75 were
performed on cables and 91 on accessories. In
Figure 2 it is shown how this ls divided over different
components.
Transition joints
'5%
Crossbonding
joints
7%
Straight-through
joints
19%
Terminations
23%
Figure 2 Data used for analysis
Cables
46%
Close and Return
Number of type tests not passed
Of the 166 Type tests performed, 123 were passed
and 43 were not passed. In this number the number
of tests not passed because of an assembling or
manufacturing fai/ure is not included. Thus, 26% of
the type tests are not passed because of design
failures. The number of type tests not passed,
ineluding assembling and manufaeturing faults is
much larger.
.Remarkable, but logie, is that the percentage of tests
not passed for aeeessories in general is considerable
higher (33%) than for cables (17%). As is shown in
Figure 3, the fai/ure rate is the highest for crossbonding joints (50%).
60
T········-·······································-····.......................................................
50
+---------------
~
~ 40
,..._
45% , - - - - - - - - - - - - - - - - -
~
40%
+-----------;:;;-------
35%
+----A~---f-~.___-___/'_:____--
30%
+----j---\---f---~=7''_ir___-­
e
i 25%-I-~~'
I.'!
e 20% +----j'-\-----'I----i-----+---.2
:f
15%
+---J-~r__+--'wI'---'<.----_+----
10%
+-----------"""'----,If------
5%
+----.:..----------r-;~ïiiiei;l
+---.---,---.-----,--,-=:=:;::::=:::,
0%
1992
-""1
1993
1994
1995
1996
1997
1998
1999
year
Figure 5 Trend in failure rate
+-----------
" 30 + - - - - - - - - - - Œ
Therefor, it can be concluded that the general fai/ure
rate for accessories will increase with the
introduction of the new IEC 60502-4.
e:0
~
seen that the fai/ure rate per year is depending more
on the standard according which the type tests have
been performed than on the years.
20+-----
LI-
10
The trend is not shown for each type of accessory
separately sincethe numbers are too few.
o
Cables
Transition
joints
Terminations
Straighttrough joints
Crossbonding
joints
20
Figure 3 Failure rate for different objects
Dolher
18
!lJIEC80840
16
14
Failure rate as a function of the standard
:l
BIEC60502
,; 12
From Figure 4 it can be seen that for aeeessories for
ratings 6-30 kV the failure rate is signifieant higher
for tests performed according to VDE 278 (around
50%) than for tests performed according to
IEC 60502 (around 10%). For cables the fai/ure rate
for tests aecording to IEC 60840 is higher than for
tests aecording to IEC 60502.
J
DlVDE 278
~ 10
QI
.Cl
~ 8
z
6
4
2
1993
1994
1995
1996
1997
1998
Year
60% -p======;;
,
E:lCables
Figure 6 Number of tests per standard per year
for cables
50%
~ 40% .j======'--~
.5
.!!
e 30% + - - - - - - -
Oother
i!!
.2
30 + - - - - - - - - = = ;
'iii
LI- 20% + - - - - - - -
25 + - - - - - - - - - ,
i'I
10%
.!! 20
'ô
+----1< :<1-------1
~
0%
IEC50502
VDE278
tEC 60840
Other
Standard
~ 15 + - - - - I.... I----~
:0
z
10+----
Figure 4 Failure rate for cables and accessories
for different standards
5
o
1993
Trend
At first sight, no elear trend can be seen over the
years in Figure 5. When taking into account the
figures presented in Figure 6 and Figure 7 it can be
1994
1996
1995
1997
1996
Year
Figure 7 Number of tests per standard per year
for accessories
Close and Return
Failure cause
From the 43 faults, 13 appeared in cables and 30 in
accessories. The fauits can be divided into 4
categories as can be seen in Table 1. In some types
of cables and accessories multiple faults occurred.
This can happen, because for type tests more than
one loop is used. It can therefor occur that a cable or
accessory does not pass the short-circuit test, does
not pass the non-eleetrical tests and fails during the
heating cycle.
Table 1 Number of faults for different moments in
the type tests
Moment in
type tests
Cables
Accessories
non-electrical
tests
7
2
before heating
cycle
4
10
during heating
cycle
3
9
after heating
cycle
2
13
From these figures it can be seen that for cables
around 70% of the faults occur in the first two stages,
while for accessories this is only around 35%. Most
faults in the before the heating cycle stage are the
presence of partial discharges for both cables and
accessories. In some of the cables a breakdown
occurred when the voltage was increased towards
the tests level. From the 9 faults of accessories
during the heating cycle 5 of them were because of
the difference of shrinking properties of the cable
and the accessories [6].
Conclusions and recommendations
Since cables and accessories are an important part
of the electric infrastructure and because 26% of the
type tests are not passed because of design failures,
type tests are nowadays necessary. From the figures
about the trend it can not be concluded that the
fallure rate will go down or go up, so also in the
future type tests will be important.
Several standards for type tests are available,
however the differences in requirements are
nowadays smaller than they were before. For type
tests of accessories according to IEC 60502, this
implies that the type tests will be more severe. This
will cause that the failure rate for accessories will go
up in the future.
From the 166 type tests performed the percentage of
tests not passed for accessories considerable higher
(33%) than for the percentage not passed cables
(17%). The failure rate is the highest for crossbonding joints.
Remarkable is that before the heating cycle test
starts, around 70% of the cables has already not
passed the type tests, while this percentage for
accessories is much smaller.
Since a number of faults during the heating cycle test
is caused because of the interaction between cable
and accessories and because sometimes the object
that ls not under test fails, it is important to test
comblnations. As is weil known the strength of a
chain is the strength of its weakest Iink.
References
[1] General reports, Group 21, Electra; February
1999, No 182, page 53
[2] VDE 278, Starkstromkabel-Garnituren für
Nennspannung U bis 30 kV, Anforderungen und
Prüfverfahren
[3] IEC 60502, : Power cables with extruded
insulation and their accessories for rated voltages
from 1 kV up to 30 kV
[4]IEC 60840, Power cables with extruded insulation
and their accessories for rated voltages from 30 kV
up to 150 kV,
[5] IEC60267, Power cables with extruded insulation
and their accessories for rated voltages from 150 kV
upto 500 kV.
[6] S.M. Benda-Berljn and G.P.T. Roelofs, 'Type
testing of cables and accessories, a way to improve
th
the quality of these products?', lO ISH '97,
Montreal, volume 4.