High Voltage Energy Cables Go Underground – How
to Improve Installation Efficiency
Gerard Plumettaz, Jouni Heinonen
Plumettaz Holding SA, Bex, Switzerland
+41-244630630 · [email protected]
relatively short cable sections, at high installation cost. To
increase installation distances additional pushing and continuous
improvements on lubrication have enhanced the installation
performances over the last years. However cable pulling with a
rope requires manpower at both ends, i.e. on winch side and on
drum side of the duct. Further said process is less performing
when compared to installation lengths achieved with the new
process described hereafter at lower cost.
Abstract
A new method to install high voltage cables up to 225 kV using
water under pressure has already been successfully tested and
applied in France in recent years. The method enables the
installation of continuous sections of power cables in one step
into a duct up to 3.5 km long. Pressurized water and a pushing
device are used. Unlike pulling the installation process takes
place from one end of the duct only, therefore reducing
manpower and avoiding coordination hazards. Installation time
is halved when compared to pulling, hence cost saving. When
using this new method the pulling forces required for reaching
distances beyond those achieved with traditional installation
methods, like winching, are far lower than pulling forces
generally met with said traditional methods. This offers
increased safety for both personnel and cable.
2. High voltage cable installation
methods
2.1 Direct burying method
Generally, direct burying of power cables has been the preferred
technology. The method is nothing else than opening a trench
between two points manually or mechanically and installing the
cable in the trench. Also, in many countries the cable is placed
in pre-laid channels or culverts, which are subsequently filled
with sand and covered with a lid, providing additional protection
to the cable. Direct burying has been used due to its low direct
cost and lack of alternative technologies available. The method
is time consuming, creating a lot of disturbance to the
neighbourhood and requires a lot of manpower, which is costly
in many countries. Due to these drawbacks the method is
gradually being replaced by alternative methods described
hereafter.
This method, named “Watucab” (WAter TUbe CAble), provides
the means to remarkable overall project cost savings and
increased cable reliability
.
Keywords: Watucab, high voltage cables; duct; push-pull;
pulling; jetting; floating;
1. Introduction
Mainly for urban areas, or to cope with unfavorable climate
conditions, there is a growing trend to get high voltage energy
cables underground. The main reason behind this new
installation method is of social and economical nature. Indeed
traditional direct burial of power cable, requiring the opening of
long trenches, 500 m to 1 km long, along busy streets imposes
long lasting disturbance of several weeks or even months to
urban traffic and to resident businesses and inhabitants,
hereinafter named “the neighborhood” during construction
work. Cost is an other reason. Compared with the cable direct
burial method, the adoption of ducts offers the benefit of a
drastically reduced duration of the disturbance to the
neighborhood like difficult or impossible access. This, because
the duct burial procedure can be implemented step by step i.e. in
short trenches of approx. 100 m. Once the duct is in place the
trench is closed, the surface rehabilitated for traffic. Such
procedure is not conceivable for direct buried cable as too many
splices would be required. Further, connecting a duct is easy,
splicing a power cable is a delicate, time-consuming and costly
operation. Furthermore, a duct provides for an additional
protection to the power cable. It also enables an upgrading of the
cable connection, as ducted cables can be removed and replaced
by one of larger power capacity without need to reopen a trench.
2.2 Pulling method
Installing cables in pre-laid ducts is getting more and more
popular. Even though material cost might in some cases be
higher than those for direct burying, cable installation in ducts
offers long term benefits, as the cable can be replaced at the
later stage without disturbing the neighbourhood. The duct
offers also an additional protection to the cable again improving
its reliability.
The ducts used for cable installation are typically made of either
HDPE or PVC. Also PE corrugated dual wall ducts, offering
good radial rigidity once installed underground and good
flexibility combined with low weight thus facilitating their
installation are mainly adopted for medium voltage distribution
cables (see figure 1).
The main issue is how do we get the heavy high voltage cable
into the duct efficiently? Pulling the cable with a rope is an
established method, but the installation length is limited to
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Figure 1. PE dual wall duct
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sector. Worldwide adopted is the jetting method, especially for
the installation of optical cable, where air is used as propelling
fluid. In some countries, like Hungary, Denmark, Sweden and
France, water is used as the propelling fluid, this method is
commonly known as cable floating. One could wonder why the
jetting method was never used for energy cable installation?
This is due to the fact that the amount of air needed to install
large cables in large ducts exceeds the capacity (volume flow)
of field compressors available on the market. Thanks to the
much higher viscosity of water compared to that of air the
volume flow for floating is much less. Furthermore, the
Archimedes uplift of the water acting on the cable makes the
lengths reached by floating generally longer compared to jetting
by air.
For all ducted systems preparation is the same: the ducts
segments are laid in a trench and joined together before the
cable installation. The trench is typically opened and closed in
sections, thus reducing additional or exhaustive disturbances to
the neighbourhood and traffic. In the same time the jointing
chambers are built. After completion of the duct route between
each jointing chamber, including filling the trench, rehabilitating
for traffic the actual cable placement can start. A thin steel wire
or P-line is blown trough the duct. Typically a stronger rope or
winch-line is then pulled trough the duct. The winch-line is then
connected to the cable end and the cable is pulled with winch.
Quite often also lubricant is used to reduce the friction between
the cable and the duct. The method requires operators on both
ends of the duct to operate the winch and the cable drum. The
installation lengths achieved by this method are limited by
friction and the number of curves and the undulation. The
method is typically used for distances up to 1 km. The method is
presented in figure 2.
The system to float the cable is shown in figure 4. The cable is
fed into the pre-installed duct with a cable pusher, typically
driven by caterpillars or belts. Immediately after the cable
pusher a water inlet chamber attached to the duct. The role of
the water is, next to providing uplift to the cable, to reduce and
stabilise by cooling the friction between cable and duct, thus
enabling to reach greater lengths. Quite often a small amount of
environment friendly additive is introduced in the water to
reduce the friction. Another benefit provided by this system
when compared to the cable pull or the cable push-pull methods
is that the floating method is a one step process, avoiding steps
like P-line and winch-line installation. Also control of the whole
process is done from one side only, i.e. the drum side. This
remarkably reduces the required manpower and it also makes it
easier to control the cable feeding process. For installations in
ducts having an outer diameter of up to 60 mm the floating
method has been used successfully for the insertion of single
phase power cables 240 mm2 of up to 20 kV over 2 km in a
50/42 mm HDPE duct in one step (see figure 5). This method is
not applicable for floating in ducts of over 60 mm outer
diameter due to excessive water consumption, causing water
logistics problems.
Figure 2. Pulling method
2.3 Push-pull method
To increase the installation length achieved with pulling, in
particular in the projects where the cable has to pass several
curves, the idea to push the cable simultaneously with pulling
was introduced years ago. At first sight the method is equivalent
to the pulling method. Indeed preparation and equipment are the
same, except for a pusher placed between cable drum and duct
inlet. Typically the pusher is a two or three belt caterpillar
synchronised with the cable winch located at the other end of
the duct. Also in this case lubricant is generally used to reduce
the friction between the cable and the duct. The system is shown
in figure 3.
Figure 4. Floating method
Figure 3. Push-pull method
When compared to cable pulling the push-pull method has the
advantage of achieving longer lengths. For underground high
voltage cable installation, the distance between splices must be
maximised in order to minimise the cost. Therefore any
improvement in cable installation length performance is very
welcome.
2.4 Floating method
Cable installation by using fluids that propel the cable has been
widely used for more than 20 years in the telecommunications
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Figure 5. Floating of a single phase cable
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2.5 Watucab method
3.1 Reference cables
The patented Watucab method [1] is combining the benefits of
the push-pull and the floating methods. According to this
method a cable is fed into the pre-installed duct via a water inlet
chamber attached to said duct. A watertight pig [2] is attached to
the end of the cable. Pressurized water is fed into the duct via
the water inlet chamber, the pressure difference over the
watertight pig exerting a mechanical thrust, resulting in a pulling
force at the cable foremost end. In addition to this pulling force
a pushing force is exerted on the cable by means of a
mechanical pusher located between cable drum and water inlet
chamber. Similar to floating the cable is subjected to
Archimedes uplift and friction can be reduced by environmental
friendly additives in the water. The method is shown in the
figure 6.
Two reference cables are considered. Their respective
characteristics are shown on Table 1. These cables are currently
available and have been successfully installed with the new
Watucab method [3]. The associated ducts are commonly used
and fulfill conditions according to (1) and (2)
Cable #
Voltage
[kV]
Conductor
[mm2]
Screen
1
2
225
63
1200 Al
630 Al
Al
Al
Cable #
Outer dia Dc
[mm]
Lin. weight
[N/ m]
Stiffness
[N/ m2]
1
2
109
66
111
43
6000
1800
Cable #
Max pull
allowed [N]
Duct ID Diduct
[mm]
Ampl/ Period
[mm] / [m]
1
2
48000
25200
192
102
200 / 28
125 / 17.5
Table 1. Reference cables
Figure 6. Watucab method
3.2 Reference duct route
Due to the presence of a watertight pig, the water flow is limited
to the amount that travels with the cable, facilitating the water
logistics, also for ducts of over 60 mm outer diameter. The
Watucab method offers several important benefits when
compared to other high voltage cable installation methods:
1) longer installation lengths in one step hence fewer splices,
even
along
heavily
tortuous
duct
routes
2) one step process, no need for P-line or winch-line
3) control of the whole process from drum side only reducing
need for manpower and hazards inherent to communication or
coordination
4) cable integrity better insured by drastically lower tensile and
radial forces. As a result the Watucab method is much faster,
less costly and safer for cable, duct and environment. The
method has been tested and used several times in France with
good performance and the cable lengths of 3 to 4 km have been
reached in one step.
The reference duct route is intended to simulate typical urban
route conditions. It follows a horizontal plane and has 90° bends
with 10 m radius are evenly spread along the RDR i.e. every
150 m, the undulation defined by Amplitude A and Period T is
determined according to (2). (see figure 7)
3. Maximum distances by method
In this chapter the maximum distances achieved, using the
traditional pulling and push-pull method and the new watucab
method are compared using two reference cables and ducts over
reference duct route (RDR). The choice of duct which is specific
for each reference cable fulfills the generally applied rule
regarding the minimum ratio between cable outer diameter Dc
and the duct inner diameter Diduct [1] . Also, as sections of buried
ducts between bends are never perfectly straight it is assumed
that they are continuously undulating. The chosen undulation
period (T) and amplitude (A) match with field observation, T
being a multiple of the duct outer diameter Doduct and A being
equal to Doduct[2]
Diduct / Dc ≥ 1.5
T = 140 x Doduct [m]
A = Doduct [mm]
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Figure 7. Reference duct route (RDR)
3.3 Performances analysis
The installation performances achieved over the RDR are
obtained from calculations based on the theory of cable
installation in ducts [4]. The results obtained from said
calculations are closely matching with field experience
worldwide and can therefore be considered as highly reliable.
The performances achieved with each above mentioned
installation method are calculated with the same RDR
parameters as described in para. 3.2. Four different values for
the coefficient of friction m between reference duct inner wall
and cable jacket are considered: Mu = 0.1, 0.125; 0.15 & 0.2.
Performances achieved per method are described on Table 2.
(1)
(2)
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3.3.1 General observations
3.3.2 Forces exerted on the cable
3.3.2.1 Pulling force:
From table 2 and 3 the performances achieved using the Pulling
, Push-pull and Watucab method, one will note the significant
performance increase of 35 to 60% brought by the push-pull
method, respectively 87 to 95% by the Watucab method when
compared to conventional pulling . The coefficient of friction
Mu has a similar impact on all above mentioned installation
methods. This means that lubrication remains a key element
Mu = 0.2
Push-pull
40.00%
60.00%
Watucab
93.87%
100.00%
For the above described performance comparison, the pushing
forces Fs [N] is identical for the Push-pull and Watucab method.
our case i.e.10000 [N] for cable 1 and 2. see table 5
From table 4 below one notes the considerably lower pulling
forces exerted on cable 1 and 2 when compared to the pulling
force applied with the pulling or push-pull method. This
contributes to enhance the service reliability of the cable
Table 2. Relative length increase per method
having a direct impact on the installation length, from Table 2
one can note that, for 0.1 < Mu < 0.2, the installation length is
more or less inversely proportional to the coefficient of friction
Mu.
Cable #
1
2
Pulling
1500
1500
Mu = 0.1
Push-pull
2100
2400
Watucab
2850
3000
Cable #
1
2
Pulling
1200
1200
Mu = 0.125
Push-pull
1650
1950
Watucab
2250
2400
Cable #
1
2
Pulling
1001
1007
Mu = 0.15
Push-pull
1350
1650
Watucab
1950
1950
Pulling
750
750
Mu = 0.2
Push-pull
1050
1200
Watucab
1454
1500
Cable #
1
2
cable#
1
2
F / Fadm
Pulling
0.00%
0.00%
The pushing force Fs [N] applied in our case is 10000 [N]. This
corresponds to the push exerted by available pushing
mechanism which is common for both methods, i.e. Push-pull
and Watucab. This value satisfies the cable manufacturer
requirements.
48000
25200
48000
25200
17370
6120
36.19%
24.29%
Table 4. Pulling forces comparison
cable#
1
2
0
0
10000
10000
10000
10000
Fs Watucab/
Fs Push-pull
Cable #
1
2
3.3.2.2 Pushing forces:
[N]
Watucab
94.81%
93.64%
Where P is the water pressure [N/mm2] and Sduct is the duct
inner cross area [mm2]
Watucab
(F)
Mu = 0.15
Push-pull
34.87%
63.85%
F = P x Sduct [N]
Fs Watucab
Pulling
0.00%
0.00%
With Watucab the pulling forces F correspond to the force
exerted by the water tight pig. They are equal to
[N]
Cable #
1
2
Scond = 1200 resp 630 [mm2]
Push-pull
(Fadm)
Watucab
87.50%
100.00%
σadm = 40 [N/mm2]
Fs Push-pull
Pulling
0.00%
0.00%
Watucab
90.00%
100.00%
[N]
Cable #
1
2
Mu = 0.125
Push-pull
37.50%
62.50%
Where: σadm is the max admissible tensile load on the conductor
given by the cable manufacturer and S is the cable conductor
metallic cross section [mm2]. For reference cables 1 and 2
Pulling
(Fadm)
Mu = 0.1
Push-pull
40.00%
60.00%
F = Fadm = σadm x Scond [N]
Fs Pulling
Pulling
0.00%
0.00%
Cable #
1
2
The pulling force F [N] needed for achieving the installation
lengths shown on figure 2 are identical for the pull and pushpull methods and correspond to the maximum allowable pulling
forces Fadm applicable for cable 1 & 2. These forces F are equal
to:
100%
100%
Table 5. Pushing forces comparison
Table 3. Performance per method, installation length
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This buried duct route, made of 160/152 mm PVC includes a
total of fourteen 90° bends and 3 siphons evenly spread over
976 m. This represents a cumulated angular deflection of 1620°.
This is more severe than most urban duct routes.
4. Cost comparison of methods
Although it is very difficult to compare the methods in specific
countries in real monetary terms we have made an analysis of
the cost factors based on man-hours and number of splice
chambers required for the construction of a typical 21 km long
underground power line cases. Manpower needed for duct
placement and cable drum logistic is not taken into account here
as it remains more or less equivalent with all installation
methods analyzed here . The results are shown in table 5.
For validation purpose 1 km of 90 kV, 1000 mm² Al, with ext.
diameter 82 mm, 68 N/m has been successfully placed in the
above described test circuit. Furthermore said cable has been
placed and removed several times (for tests and demonstration
purpose) To remove the cable the Watucab process is reversed
i.e. water is injected from the opposite duct end after having
pivoted the pig in order to make it act on the cable extremity as
a pusher instead of a puller. The pusher is also reversed and
becomes a puller. In this way the cable is removed from the duct
and recoiled on the drum at a very low load. This demonstrates
that Watucab is a reversible process allowing for an efficient
upgrading of power-lines as the removed cable can be replaced
by an other with larger capacity.
Installation forces exerted on the cable : 6000 N for pulling
respectively 7000 N for pushing.
5.2
Projects / field experience
5.2.1 Floating method
Numerous customers’ projects have been accomplished and
successfully achieved for low and medium voltage applications
i.e. up to 25 kV networks. For instance:. lighting of highway
interchanges, power distribution in dense urban areas, and
special industrial application such power transmission in tunnels
and water shafts.
Table 5. Cost factor in cable installation methods
In the Neuchâtel region, Switzerland (see Figure 9), floating in
one step over a 2.7 km route, 3 x 12/20 kV, 125 mm2 Cu, linear
weight 14 N/m, ext. diameter 34.5 mm, linear weight 23 N/m.
Duct used: 3 x HDPE diameter 48/42 mm , wall mounted (water
shaft).
The numbers shown on table 5, based on recent experience,
clearly indicate the importance of the savings obtained by the
reduction of the quantity of joint chambers i.e., 60% when
compared with pulling and 37% when compared with push-pull.
The cost savings for labor is also significant i.e. 54% of the
amount of man-days needed
5. Examples of tests and achievements
using Floating and Watucab
5.1 Validation
A dedicated test circuit has been built in France as described on
figure 8.
Figure 9 : Neuchâtel
5.2.2 Watucab method
Over the last 4 years seven high voltage projects have been
successfully carried out in France. The aim of the projects was
to replace overhead power line sections heavily exposed to
storms or, in urban areas, to meet new requirements for urban
planning. To this day more than 100 km of single phase cable of
63 to 225 kV have been installed using the Watucab method.
Section lengths range from 1.6 to 3.3 km. Two typical projects
are:
Figure 8 : trial circuit
In Normandy, France (see Figure 10), installation over a 6.1 km
long route, 3 x 90 kV, 630 mm2 Al single phase cable, ext.
diameter 72 mm, linear weight 49 N/m. Duct used: 3 x HDPE
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5) Faster and less costly installation
6) reversible process allowing network upgrade
diameter 160/132 mm. Average section length 2.03 km, longest
section: 2.3 km
The high voltage cables are installed in increasing amounts under
ground for several reasons. The main drivers are better protection
of the cable against weather hazards and public acceptance than
for aerial cables. The main limiting factor for underground cables
has been the higher installation cost than for aerial cable
networks. Watucab offers an efficient solution to install high
voltage cable under ground at lower cost compared to any other
underground cable installation method. The system has proved to
be viable and offering important benefits to the cable companies,
installers and the end customers
7. Acknowledgements
Special thanks to Willem Griffioen for his precious support and
advice during the preparation of this paper.
8. References
[1] EP 1456923. “Method for Installing a High or Medium
Voltage Power Cable in the Ground.” Plumettaz SA
[2] EP 1518307 « Pig for installing a cable in a conduit »
Plumettaz SA
[3] M. Le STUM & Al., “Report on the Use of Extruded Cables
on the French Grid”.Chap. 4 B1-204, CIGRE 2006,
www.cigre.org
[4] W. Griffioen, «Installation of Cables in Ducts”, Plumettaz
SA Bex (CH) (1993) ISBN 9072125 37 1
Figure 10 : Normandy
In Brittany, France (see Figure 11), installation over a 19 km
long route, 3 x 63 kV, 800 mm2 Al single phase cable, ext
diameter 68 mm, linear weight 51 N/m. Duct used: 3 x HDPE
125/102 mm The average section length is 3.17 km, longest
section: 3.31 km..
The Authors
Gerard Plumettaz received a MS degree
in mechanical engineering at the Swiss
Federal Institute of Technology, Zürich,
in 1970 with an emphasis on machine
tool techniques. Joined his family
business, Plumettaz SA, Bex, Switzerland, in 1971 and became instrumental in
product design, development and
marketing. Initial task was to design
and develop winching concepts for
military tank retrieval. Here specialized
winching techniques led to the design of underground placement
methods. Until 2009, CEO of Plumettaz SA. Today Chairman of
Plumettaz Holding SA, he is continuing to be active in the pursuit
of advanced methods in underground placement technology.
Jouni Heinonen holds a MS degree in
mechanical engineering from the
Tampere University of Technology, in
Finland. He began his career in 1986 as
Product manager at Falcon Chemicals
in Finland and then joined Nokia Cable
Machinery in Finland as Product
Development Engineer. In 1988
he joined Nokia-Maillefer Oy as
Product Development Manager and
moved to Switzerland to join NokiaMaillefer SA, initially as Project leader
to finally become Managing Director in 1996. From 1998 he held
the role of Executive VP of Business Group Plastics of Nextrom
and became CEO of Nextrom Holding SA from 1999. From 2005
to 2008, he held the role of CEO of Gurit Holding AG. Today, he is
acting as CEO of Plumettaz Holding SA in Switzerland, a leading
manufacturer of cable laying equipment.
Figure 11 : Brittany
6. Conclusions
A new method for installation of high voltage energy cables in
pre-installed ducts has been developed and successfully applied.
The method is applicable for ducts with an outer diameter superior
to 60 mm. The main benefits gained from this innovation are
1) longer installation lengths in one step saving expensive joints
2) one step process
3) process control from drum side only enabling easy visual
communication between active parties thus avoiding hazards
4) Increased protection of cable integrity
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