Information will be given on the method of installation of cables in ducts and tunnels, which methods get presently increased attention in urban areas. In particular attention will be paid to the conductor material of cables, copper or aluminium, and if there is a preferred choice to recommend based on the typical material properties and related experience.

1. DNV GL © 2013 SAFER, SMARTER, GREENERDNV GL © 2013
Cables in ducts and tunnels
28-3-2014
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What kind of conductor material is preferred,
aluminium or copper?
Power Cable Academy, a Copper Alliance initiative,
WIM BOONE, DNV GL, 25-03-2015

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Table of contents
 INTRODUCTION
 APPROACH
 DIFFERENT OPTIONS FOR INSTALLING CABLE
 INTERNATIONAL PRACTICE
 MULTI- PURPOSE TUNNEL
 COMPARISON COPPER/ALUMINIUM IN DUCTS
 COMPARISON COPPER/ALUMINIUM IN TUNNELS
 EVALUATION
 CONCLUSIONS
 REFERENCES
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Introduction
 Different cable installation methods:
- Direct buried
- In ducts
- In tunnels
 What are the pros and cons of ducts and tunnels as installation methods
 What is the impact of these installation methods on the selection of conductor
material?
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Approach
 Investigation of international situation
 List of pros and cons
 Impact on selection of conductor material
 Future aspects
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Different installation methods
Direct buried Duct Pipe Tunnel
Economics 3 2 2 1
Inconvenience 2 3 3 3
Current rating 3 2 2 4
Maintenance 1 2 2 3
Protection 2 3 3 4
Major advantage 4
Advantage 3
Disadvantage 2
Major disadvantage 1
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International Practice Ducts
 In urban areas
 Large number of crossing
services
 Restricted use of mechanical
diggers
 Civil activities and cable
installation are separate
 Excellent mechanical protection
 One of the safest types of
installation regarding safety
in case of short circuit
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Source: CIGRE TB 338

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International practice Tunnels
 If space in underground is scare and
no ducts are possible
 Tunnels are used for a long time for:
– Traffic
– Waterways
– Power cables
– Other services (telecommunication,
sewage, gas pipes, city heating etc.)
 Tunnels are designed to enable easy maintenance, repair and replacement of
services in the tunnel
 Ventilation
 Nowadays multi purpose tunnels (electricity, oil, gas, water, etc.) are becoming
more and more common because of cost sharing.
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Source: CIGRE TB 338

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Multi purpose tunnel
 Multi purpose tunnel=car pooling
 Pros:
- Sharing investment costs
- Organizing and maintaining underground infrastructure in a structured way
 Cons:
- Complicated management structure
- Mutual impact of different infrastructures
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Essential services in Multi-Purpose Tunnel
 Lighting
 Ventilation
 Drainage of seepage
 Emergency procedures
 Communications
 Fire& smoke detection
 Fire extinguishing measures
 Pipe leak detection
 Protection of personnel
 Vermin barriers
 EMC control
 Temperature control
 Emergency power supplies
 Management of the multi-purpose tunnel
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Cooling
 Cooling inside duct and tunnel is
convection and radiation
 The heat transfer from the wall to
the surrounded soil is conduction
Duct Tunnel10

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Current rating in duct
0
500
1000
1500
2000
2500
150 250 350 450 550 650
CurrentRating[A]
External diameter duct [mm]
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Comparison
0
500
1000
1500
2000
2500
0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5
CurrentRating[A]
Thermal resistivity soil [Km/W]
3000 Al 2000 Cu 2000 Al 1200 Cu 1000 Al 630 Cu
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 Comparison of the current rating of cables with Copper conductors with cables
with Aluminium conductors
 A Copper cable has a smaller diameter with the same current rating

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Method to calculate current rating in tunnel
 Thermal Model for one cable installed inside a duct in a tunnel, with the heat
sources and thermal resistance
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Tg Tm Tt To
qg qa qti qto qo
qtl
Wt
Ttc1 Ttc2
Ttr
WaWdWg
Ws
Tt
qdo
qdl
Tdc1 Tdc2
Tdr
qdi
Heating sources
Cable Duct Duct wall Tunnel Tunnel wall Ambient
Heat transfer to the
air in the duct
Heat transfer
to ambient

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Temperature versus ventilation speed in tunnel
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 Higher ventilation speed will increase the
cooling capacity
 Higher ventilation speed will increase the
current rating of the cable
-Maximum conductor temperature
-Maximum jacket temperature
-Maximum air temperature in duct
-Average air temperature in tunnel
Ventilation speed [m/s]
Temperature[°C]

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Pros and cons of ducts
If the cable will be installed in a duct there are the following advantages:
 The cable has a mechanical protection against 3rd party damage
 After the installation of the ducts it is not needed to open the streets for
maintenance of repair of the cables
 It is a proven technology
The disadvantage of installing the cable in ducts are:
 High installation cost
 After installing diameter restriction for new cables
 Reduced current rating compared to direct buried
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Pros and cons tunnels
If the cable will be installed in a tunnel there are the following advantages:
 The cables are very well protected against mechanical damage
 After the installation of the tunnel it is not needed to open the streets for
maintenance of repair of the cables
 Higher current ratting compared with direct buried, in particular when forced air
cooling is applied
 It is a proven technology
 The underground infrastructure, in case of MP tunnel, is well ordered
 It is easy to perform maintenance to the cable
The disadvantages of installing the cable in tunnels are:
 Very high investment costs, however this could be reduced by using multi-
purpose tunnels, where the costs are shared by different parties
 Complicated organizational structure and mutual risk between cable and other
infra structures in MP tunnel
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Comparison Copper/Aluminium - ducts
 In ducts:
– The power cables are pulled through the ducts from manhole to manhole
– The distance between the manholes is depending on the maximum pulling
cables length
– The maximum cable pulling length depends on the tensile strength and the
specific weight
– Copper has higher tensile strength but also higher specific weight than
aluminium, finally calculations showed a lower maximum pulling length than for
aluminium
– Copper has a lower coefficient of expansion than aluminium, causing less
thermal-mechanical problems at the joints
– Copper has a lower specific electrical resistivity than aluminium therefore
requiring smaller copper conductor cross-section than for aluminium for the
same current rating, resulting in less space and more flexibility.
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Comparison Copper/Aluminium - tunnels
 In tunnels:
– Copper conductor cables have the advantage to be used for longer lengths
(stored on a drum for longer length because of reduced radial size) and higher
tensile strength
– Because of smaller radial size, copper conductor cables have a smaller size, are
more flexible, but have smaller cooling surface
– Because of the lower coefficient of expansion of copper, the sag of cables
between the clamps due to thermal effects is less. Consequently copper
conductor cables require less space
– The power cables are secured with clamps to hold the power cables in place in
case of a short circuit current and to allow controlled expansion during normal
operation. The weight of copper cables is twice the weight of aluminium,
therefore requiring special attention.
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Future trends
 More tunnels and ducts will be applied for installing cables
 The future tunnel will be multi purpose tunnel and will probably therefore become
more attractive than the duct
 Legislative pressure will increase to use tunnel or ducts
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Evaluation of main pros and cons
 For ducts copper conductor has following pros and cons
 Pros:
- Limited radial size
- Lower coefficient of expansion
 Cons:
- Higher specific weight
 For tunnels copper conductor has following pros and cons,
 Pros:
- Lower coefficient of expansion
- Longer installed length
- High mechanical strength
 Cons:
- Higher specific weight
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Conclusions
 Copper has a strong position in existing ducts, because of the radial size
 For new ducts and for tunnels there are pros and cons, although copper has good
possibilities due to low coefficient of expansion and particularly in case of tunnel
application due to longer allowable cable lengths
 Legislative pressure will increase to use tunnel or ducts for installing cables.
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References
 CIGRE Technical Brochure 194, Construction, laying and installation techniques for
extruded and self contained fluid filled cable systems, oktober 2011
 CIGRE Technical Brochure 403, Cable systems in multi purpose or shared
structures, February 2010
 CIGRE B1_109_2012, EHV Cable Installation Technolgy for Tunnels and Ducts in
Japan
 CIGRE B1-108_2004, 1600 MVA Electrical power transmission with an EHV XLPE
cable system, in the underground of London
 CIGRE 21-109_1998, New 400 kV XLPE long distance cable systems, their first
application for the power supply of Berlin
 Question No 5 from Tatsuo Kurata, Japan, registration number 925, EHV Cable
Installation Technology for effective utilization of existing infrastructure
 Jicable paper 2007, HV Power cables installed in multi purpose tunnels. a
challengeable option, by Frank de Wild and Wim Boone

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