Evolution of the French MV cable system (includes summary in

EVOLUTION OF THE FRENCH MV CABLE SYSTEM
F Bouanat, F Charles, J H Fondeur, E Siméon, C Boisseau, J M David
Draka Paricable, Sagem, Pirelli Câbles et Systèmes SA, Nexans, EDF (France)
ABSTRACT
The 20 kV synthetic cable on the French
distribution network has an excellent experience
feedback ; The very innovative design at the
introduction of this cable on the network, has
barely changed during 20 years of operation.
Meanwhile, significant improvements of the
synthetic insulation, on the material point of view
as well as on the industrial process, have led to an
extension of this technology to the highest voltage
levels and to the development of more efficient
accessories.
Due to these technical achievements, EDF and
the manufacturers of this product have considered
that its optimisation was possible.
The chosen way to directly draw benefit from
progress of the synthetic insulation was to reduce
the insulation thickness of the cables from 5.5 mm
to 4.5 mm. These optimised cables were
developed, and the corresponding connection
accessories were also adapted.
Taking into account the reductions of the cable
diameters, and the increase in the electric field,
the compatibility of these new cables with the
accessories
has
been
carefully studied.
Accessories were validated according to the
European document HD 629.1, the French
document C 33-001, and additional breakdown
tests. Field experimentation field has been done
and showed that some tools needed to be
adapted. This project required also to remind to
jointers the good practice of the cable preparation.
Since March 2000, Electricité de France,
provisions exclusively this new cable and this has
allowed a sensible reduction of the cost of 20 kV
underground cables.
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EVOLUTION OF THE FRENCH MV CABLE SYSTEM
F Bouanat, F Charles, J H Fondeur, E Siméon, C Boisseau, J M David
Draka Paricable, Sagem, Pirelli Câbles et Systèmes SA, Nexans, EDF (France)
RESUME
Le câble synthétique 20 kV utilisé en France pour
le réseau de distribution souterrain présente un
excellent retour d’expérience ; la spécification du
câble, très innovante lors de l’introduction de ce
produit en réseau, a évolué à la marge pendant
les vingt années d’utilisation en réseau. Dans
l’intervalle, des avancées importantes ont été
réalisées en matière d’isolation synthétique, tant
du point de vue des matériaux que de leur mise
en œuvre industrielle, qui ont permis d’étendre
cette technologie jusqu’aux tensions les plus
élevées et de développer des matériels de
raccordement toujours plus performants.
Tirant parti de ces avancées techniques, EDF et
les constructeurs de ce produit ont considéré que
son optimisation était possible.
La piste retenue pour tirer directement bénéfice
des progrès en matière d’isolation synthétique a
été de réduire l’épaisseur de l’isolant des câbles
de 5,5 mm à 4,5 mm. Ces câbles optimisés ont
été développés, les matériels de raccordement
associés ont été adaptés.
Compte tenu des réductions de diamètre des
câbles, et de l’augmentation du champ électrique,
la compatibilité de ces nouveaux câbles avec les
systèmes de raccordement a fait l’objet d’une
attention
particulière.
Les
matériels
de
raccordement ont été validés à partir des essais
des documents européens HD 629.1, et français
C 33-001 ainsi que des essais additionnels de
montée au claquage. Une expérimentation en
réseau a mis en évidence la nécessité d’adapter
certains outillages de préparation des câbles. Ce
projet a nécessité un rappel sur les bonnes
pratiques de préparation de têtes de câble.
L’utilisation en réseau de ces câbles a été
généralisé à EDF en mars 2000 et a permis de
réaliser une réduction sensible du coûts des
câbles 20 kV pour les réseaux souterrains.
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EVOLUTION OF THE FRENCH MV CABLE
SYSTEM
F Bouanat, F Charles, J H Fondeur, E Siméon, C
Boisseau, J M David
Draka Paricable, Sagem, Pirelli
Systèmes SA, Nexans, EDF (France)
Câbles
et
RESUME
Le câble synthétique 20 kV utilisé en France pour
le réseau de distribution souterrain présente un
excellent retour d’expérience ; la spécification du
câble, très innovante lors de l’introduction de ce
produit en réseau, a évolué à la marge pendant les
vingt années d’utilisation en réseau. Dans
l’intervalle, des avancées importantes ont été
réalisées en matière d’isolation synthétique, tant du
point de vue des matériaux que de leur mise en
œuvre industrielle, qui ont permis d’étendre cette
technologie jusqu’aux tensions les plus élevées et
de développer des matériels de raccordement
toujours plus performants.
Tirant parti de ces avancées techniques, EDF et
les constructeurs de ce produit ont considéré que
son optimisation était possible.
La piste retenue pour tirer directement bénéfice
des progrès en matière d’isolation synthétique a
été de réduire l’épaisseur de l’isolant des câbles de
5,5 mm à 4,5 mm. Ces câbles optimisés ont été
développés, les matériels de raccordement
associés ont été adaptés.
Compte tenu des réductions de diamètre des
câbles, et de l’augmentation du champ électrique,
la compatibilité de ces nouveaux câbles avec les
systèmes de raccordement a fait l’objet d’une
attention
particulière.
Les
matériels
de
raccordement ont été validés à partir des essais
des documents européens HD 629.1, et français C
33-001 ainsi que des essais additionnels de
montée au claquage. Une expérimentation en
réseau a mis en évidence la nécessité d’adapter
certains outillages de préparation des câbles. Ce
projet a nécessité un rappel sur les bonnes
pratiques de préparation de têtes de câble.
L’utilisation en réseau de ces câbles a été
généralisé à EDF en mars 2000 et a permis une
réduction sensible du coûts des câbles 20 kV pour
les réseaux souterrains.
ABSTRACT
The 20 kV synthetic cable on the French
distribution network has an excellent experience
feedback ; The very innovative design at the
introduction of this cable on the network, has barely
changed during 20 years of operation. Meanwhile,
significant improvements of the synthetic insulation,
on the material point of view as well as on the
industrial process, have led to an extension of this
technology to the highest voltage levels and to the
development of more efficient accessories.
Due to these technical achievements, EDF and the
manufacturers of this product have considered that
its optimisation was possible.
The chosen way to directly draw benefit from
progress of the synthetic insulation was to reduce
the insulation thickness of the cables from 5.5 mm
to 4.5 mm. These optimised cables were
developed, and the corresponding connection
accessories were also adapted.
Taking into account the reductions of the cable
diameters, and the increase in the electric field, the
compatibility of these new cables with the
accessories
has
been
carefully
studied.
Accessories were validated according to the
European document HD 629.1 the French
document C 33-001, and additional breakdown
tests. Field experimentation has been done and
showed that some tools needed to be adapted.
This project required also to remind to jointers the
good practice of the cable preparation.
Since March 2000, Electricité de France, provisions
exclusively this new cable and this has allowed a
sensible reduction of the cost of 20 kV
underground cables.
CABLE DESIGN BEFORE YEAR 2000
EDF changed over to polymeric cable in 1979 and
only one design is used which is covered by the
French standard NF C 33-223 (the MV cable used
in France was originally the subject of an Electricité
de France specification, HN 33-S-23, later
converted to a national standard NF C 33-223).
The cable was originally designed on the basis of a
number of requirements (short-circuit current,
easiness of erection of accessories, option of
mechanical laying, overhead variant etc.).
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This cable has an oversheath
longitudinal watertight design.
radial
and
improved by the control of the cleanness and the
improvement of smoothness of the interfaces
between the layers.
Some relevant points have to be pointed :
- the insulation screen is longitudinally grooved,
facilitating the fixing of the hygroscopic powder
providing longitudinal watertightness of the cable.
Moreover, these grooves ensure a thermo mechanical bedding
At the beginning of the manufacturing of synthetic
MV cables, the lines utilised by the cable makers
were the existing lines at this time, i.e. lines for low
voltage rubber insulation. The cross-linking
process was the steam process and the extrusion
process was the system «1+2».
- the aluminium screen is bonded to a thick sheath
in high-strength PVC. The combination of
hygroscopic powder and aluminium screen bonded
to the sheath constitutes an extremely effective
barrier to water penetration into the cable.
Cables manufacturers as well as machinery
manufacturers developed dedicated and optimised
insulation lines for the MV cables in order to
improve the quality and performance of the MV
cables
The cable has three individual cores laid up twisted
together, each of them consisting of :
The main evolution is :
- a stranded aluminium conductor
- a 5.5 mm XLPE insulation
- a strippable insulation screen with grooves filled
with an hygroscopic powder
- a 0.2 mm longitudinal aluminium tape bonded to a
thick PVC jacket (2.6 to 3 mm depending on the
conductor cross section).
FEEDBACK AND IMPROVEMENTS
- a general optimisation of the catenary
vulcanisation lines to insulate the MV cables;
- the evolution of the curing processes improved
the control of level and stability of cross-linking for
the three layer insulation and also a drastic
reduction of moisture content in the insulation part;
- the evolution of extrusion process by the
simultaneous triple extrusion resulting in a better
quality of the interfaces between the semiconductor layers and the insulation layer.
Behaviour of the cable to water penetration
Evolution of accessories
From 1979, the behaviour and the reliability of this
cable is excellent. Some cases of water
penetration where identified with some of the very
first cable produced in the beginning of 80’s. Then
jacket materials and screen where improved, and a
radial water tightness test has been introduced in
1983 in the specification.
The radial watertightness test introduced consists
in an 3000 hours ageing in a water bath at the
temperature of 80°C. The water is added with NaCl
and Na2SO4 so as to reach the pH 8,5. At the end
of this test, the aluminium foil must not be
corroded, and no water penetration must occur
between insulation screen and aluminium foil.
This test is relatively difficult to pass with a PVC
sheath and a 0,2 mm thick aluminium foil. Since
this test was introduced, no cases of water
penetration or anticipated ageing of this cable were
found in the field.
Improvements in insulation manufacturing
Since the late 70’s, insulation materials improved.
The insulation materials that can be found on the
market are more pure than they were 20 years ago.
Especially, the quality of compounds has been
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Since the mid 90’s, cold shrink technology had a
great development. The manufacturers propose
now a relatively large range of accessories in this
technology. At present, EDF uses mainly cold
shrink technology for termination and joints. Taped
technology is now dedicated to a very low numbers
of cases. Deadbreak premoulded separable
connector are also widely used. These
technologies are easier to handle for the jointers
and the risk consequently of jointing errors are
reduced. The level of performance of these
accessories and the field experience are good.
Despite this fact, the jointing errors are still one of
the major cause of breakdown. It has been noticed
that the breakdown of accessories due to a jointing
errors occurs most of the time in the first years of
service.
This last past ten years, a strong effort was done
for improving jointing instruction. Especially, all
jointing instructions are now tested in field so as to
make sure the document is comprehensive and
understandable enough.
REDUCTION OF INSULATION THICKNESS
Reducing the insulation to 4.5 mm in year 2000
would take time to be solved without generating
comparable saves.
Cables made in the year 80’s and later have a very
good behaviour. The rate of failure is extremely
low. Accessories were significantly improved. That
means that the cable system made in the late 90’s
had higher performance than the older one. In
order to reduce the cost of the cable, it appears
possible to reduce the insulation thickness. French
cable manufacturers and EDF started to carry out
investigation in this way.
Cold shrink accessories used in France cover
several cross section. As an example, middle size
joints cover cross section from 95mm² to 240 mm².
In this case, the reduction of thickness of cable
insulation on 95mm² cable could have required to
adapt some joint in size. This was time consuming
in terms of manufacturing and qualification. So the
reduction of thickness only for the 150 and 240
mm ² cables seemed to be the best compromise.
The aim of this program was to obtain a reduction
of the cost of the cable relatively quickly and in the
meantime, to maintain the level of reliability of the
cable system.
In term of stress increase, the small and large
cross sections would cause technical difficulties.
As it is shown on the figure 1, when reducing the
insulation, the saves tend to get less and the stress
tends to get higher, especially on the external part
of the insulation.
As it is showed on figure 2, the internal stress on
150 mm² and 240 mm² cables with 4.5mm
insulation is comparable to the internal stress on
50 mm² with 5,5 mm insulation.
3,60
3,40
50%
Internal stress (kV/mm)
45%
40%
35%
30%
25%
3,20
3,00
2,80
2,60
2,40
20%
15%
2,20
50
10%
95
150
240
630
Cross section (m m ²)
5%
Insulation 4,5 mm
0%
5,5
5,3
5,1
4,9
4,7
4,5
4,3
4,1
3,9
Insulation 5,5 mm
Insulation (mm)
Save on insulation material
Increase of the internal gradient
Figure 2, Internal stress and cross section
Increase of the external gradient
Figure 1, Saves and increase of gradient
Small and wide cross sections where not
included in the program
It appears quickly that the most significant saves
could be done by reducing the insulation on the
cross section 150 mm² and 240 mm², as these
cross section are the most widely used on the
network.
Reducing the insulation thickness to larger and
smaller size could involve technical difficulties that
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As showed in figure 3, external stress for a 4,5 mm
insulation cables is approximately 15 % higher than
in a 5,5 mm insulation cable (630 mm²).
2,40
External stress (kV/mm)
2,20
2,00
1,80
1,60
1,40
1,20
1,00
50
95
150
240
630
Cross section (m m ²)
Feedback of the new system
Insulation 4,5 mm
Insulation 5,5 mm
Figure 3, External stress and cross section
Accessories
qualification.
adaptations
and
tests
The range of each accessories, was carefully
studied, to check the compatibility between the old
and the new cable. The increase in the electric field
and the new diameters showed the importance of
the quality of interface for slipped-on or shrunk
accessories.
The cold shrink joint technology, didn’t need any
modification, because of his own design and his
good intrinsic level of electric strength. Most of the
cold shrink termination already used with the 5.5
mm cable could succeed to the qualification tests
on a 4.5 mm insulated cable. Deadbreack
premoulded separable connector were modified
(with the use of more flexible material for the
reducer) to be able to accept largest range of
insulation diameter and external sheath diameter
for the cross section 150 mm ² and 240 mm ².
Taped technology were re-designed by an
additional stress control tapes, to control the
geometrical discontinuities on the prepared cable.
Nevertheless, the quality of the cable jointing
preparation remains very critical for this whole
design, especially for the new cable.
Accessories were validated according to the
European document HD 629.1, the French
document C 33-001, and additional breakdown
tests.
Experimentation on site
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A field experimentation was carried out with
prototype of the new cable. This experimentation
consisted in doing approximately 15 real
installations on the network. A large range of
cables and accessories where experimented.
Traditional laying has been tested as well as
mechanical laying with a trenching machine.
Remarks of the jointers where collected. This
experimentation hasn’t shown particular problem
for the installation of this cable. It revealed the
need to adapt existing tools (cone insulation
sharpener for example) and to stop using some
type of old tools that were not suitable anymore.
The experimentation also showed that it was
necessary to remind to jointers the good practice of
the cable preparation.
Since March 2000, Electricité de France, provisions
exclusively the new cable with an insulation
thickness of 4.5 mm (section 150mm² and 240
mm² only) without any particular problem. The
change to the new cable allowed a sensible
reduction of the cost of 20 kV underground cable
system.
This evolution is a new technical step that is
favourable to the installation of medium voltage
underground link.