Choices and Considerations for the Selection of Power Cables in
Tunnel Installations
Brian Gregory
Chief Engineer
BlCCGeneral UK Cables
Supertension & Subsea Systems
Tel: 020 8 225 2223
Email: brian.gregory@supertension.com
1.O
Introduction
This paper considers cables in the voltage range of 33kV to 550kV
2.0
Cable types:
The following types are available:
Fluid filled (FF) self contained cable (up to 550kV), these cables have extruded
metallic sheaths and operate at a comparatively low internal pressure of 5.3-8
bar, although in deep shaft installations they have been used at pressures in
excess of 20 bar. One notable installation being the 525kV 1000mm2 cable in
1972 in the Grand Coulee Dam Hydro Electric scheme in the USA. A second
notable installation being the 400kV 2580mm2 cable in 1973 in the cable tunnel
underneath the River Severn in the UK. The 400kV cable was installed inside
reinforced PVC pipes such that the cables could be watercooled, permitting the
rating of 2600MVA to be achieved on a single circuit. These cables are still
operating satisfactorily.
FF cables can be insulated with either paper tapes or polypropylene paper (PPL)
tapes. Both types are impregnated with DDB, a synthetic hydrocarbon insulating
fluid. Paper taped cable has exhibited the best service performance with respect
to electrical reliability. Early types of cable experienced problems with oil leaks
resulting from the use of non-alloy lead sheaths and non-polymeric oversheath
corrosion protection. 275 and 500kV cables were installed in the UK after 1965
and these employed extruded lead alloy or aluminium sheaths and PVC or
polyethylene oversheaths. A number of successful measures were implemented
by 1980 to eliminate fluid leaks, these being aluminium sheaths, polyethylene
oversheaths, reinforced plumbs and synthetic rubber ‘0’ring seals etc. Generally
extruded aluminium sheathed cables are preferred for tunnel installations as
these have the best cyclic fatigue life and the best fire performance.
PPL tapes give a combination of lower dielectric heat loss, higher electrical
breakdown strength and lower capacitive reactance than a paper cable, for these
reasons FF PPL cable has become the preferred type at 275kV, 400kV and
500kV. The low loss characteristics of PPL insulation give less benefit when
installed in air inside a tunnel than when laid direct in the ground. This is
because a) the thermal resistance between the conductor and ambient is much
8/ 1
lower for an in-air installation and b) it can be shown mathematically that the
dielectric heat loss only has to flow through 50% of the thermal resistance of the
insulation. Prototype PPL cables were manufactured at 132kV in 1973 and at
275kV in 1980. The first service installations in the UK occurred in 1992 at
275kV and 400kV. Significant installations had occurred in earlier years at 345kV
in the USA and at 500kV in Japan. The service experience of the cable and
accessories has been excellent and equals that of paper cable, which it closely
emulates.
0
Fluid filled high pressure pipe type (up to 345kV)
This cable evolved in the USA. Non metallic sheathed cores are pulled into preinstalled steel pipes. The pipe is then evacuated and filled with a hydrocarbon
insulating fluid. The cable is pressurised to typically 10 bar. Such cables have
been successfully used in horizontal tunnels, however the cores tend to slide
within the pipe necessitating the use of special cable restrictors to prevent
damage at joint positions. This design of cable is becoming obsolete because of
adverse corrosion and high volume leak experience with old cables in laid direct
installations.
Self contained gas filled cable (up to 132kV)
Several types of gas filled cables have been employed, the only surviving type is
the pre-impregnated gas filled cable. The insulation is comprised of paper tapes
which have been impregnated with a viscous hydrocarbon compound. The tapes
are then lapped onto the conductor. The insulation is sealed within a smooth
aluminium sheath. The cable is filled with dry nitrogen gas at a pressure of
typically 12-14 bar. The gas permeates the butt gaps between the paper tapes
and is an important part of the electrical insulation. This type of cable historically
was the preferred type for installation in tunnels where a fire risk existed, for
example in road tunnels. Fire tests have confirmed that the gas cable has a
superior fire performance compared to extruded XLPE types and FF paper types.
Following the melting of the metallic sheath the nitrogen gas helps to extinguish
flames and the outer layers of paper tapes form a carbon layer inhibiting further
combustion. Although still manufactured, the use of this type of cable has
declined for laid direct installations in favour of extruded XLPE cables, because
of adverse experience with gas leaks in old cables.
Gas filled high pressure pipe type
This type of cable is similar to the FF high pressure pipe type cable with the
exception that the steel pipe is pressurised with nitrogen gas and not a
hydrocarbon fluid. This design of cable has superior fire and environmental
acceptability, but has the same disadvantage of tendency for the cores to slip in
inclined shafts. These cables are still manufactured for laid direct installations.
0
Extruded Crosslinked Polyethylene Cables (XLPE)
This has become the preferred type for use at 33-132kV because of the reduction
in maintenance and the elimination of gas or oil leaks. Extruded XLPE cables
812
have the added advantage in a tunnel installation that the risk of fire propagation
is significantly reduced (because of the absence of hydrocarbon insulating fluid).
However XLPE is a hydrocarbon material and will sustain combustion, albeit at a
much reduced rate, until the fire is extinguished. Extruded XLPE cables have
recently been installed at 500kV in custom designed tunnels in Japan. A major
consideration with an EHV XLPE circuit is the lack of reliability of the accessories,
particularly the joints. The absence of an insulating liquid or gas in the cable
results in the risk of partial discharge (sparking) at the interface between the
cable insulation and joint insulation. The quality of the cable manufacture,
accessory manufacture and on-site assembly has to be much higher than for a
fluid or gas filled cable. The straight joints employed in Japan use an on-site
extrusion moulding technique, however this requires a) the provision of space
within the tunnel at joint bay positions and b) the use of highly sophisticated
materials treatment and extrusion plant. It takes approximately five weeks to
make three joints and although this has been acceptable for a new installation,
this is considered to be too long for repair purposes in returning a failed circuit
back into service. In other Countries there has been a preference for the
development of premoulded accessories. An earlier paper describes the
installation of 380kV XLPE cables together with premoulded accessories in two
tunnels underneath Berlin. An advantage of a tunnel installation is that the joints
are more accessible for the measurement of partial discharges, both during
commissioning and in service, such that a defective joint can be dismantled
before disruptive failure and major damage occurs. However partial discharge or
failure from a fluid filled joint in a paper or PPL cable is virtually unheard of. Thus
XLPE cable systems have to be regarded as electrically less reliable than FF or
GF types and provision needs to be made for the rapid repair and replacement of
cable and accessories following an in-service failure in the tunnel.
.
XLPE cables are at risk to failure due to the permeation of water into the
insulation, resulting in the phenomenon of ‘water treeing’. The rate of water
treeing is very slow if the core screen design stress of the cable is less than
2kV/mm and if the moisture content of the insulation is less than 80%. For this
reason it is possible to make economies by using non metal sheathed cables at
33kV and lower voltages. Such cables can employ a water tree retardant grade
or type of insulation and screens to further reduce the risk. At higher voltages it
is necessary to design the cable to operate at much higher stresses and so a
metallic sheath is applied to the cable. The risk exists that should the cable be
damaged either during installation or in service, then water can travel along
complete section lengths (500-1OOOm), thereby necessitating the removal of the
affected cable. It is good practice to specify that the conductor and sheath of
these cables be waterblocked such that water cannot penetrate longitudinally.
Although cables installed inside a tunnel are well protected from third party
damage in service, they are more vulnerable to damage than buried cables
during installation, because all materials, tools and vehicles have to pass through
the tunnel and close to the cables. It is unusual for a cable tunnel to be
completely dry along it’s length. Thus XLPE cables in tunnels need to be
carefully protected against mechanical damage and water penetration.
Cables with thin (0.2-1")
metal foil sheaths to provide a moisture barrier have
been installed in tunnels. These cables usually require additional stranded earth
wire conductors to be applied to the extruded core to carry short circuit fault
currents along the cable. The use of metal foil barriers is comparatively new for
large diameter, high power, high
voltage cables and concem exists that
the foils may be unable to withstand
either damage during installation or
thermomechanical fatigue in service.
A recent paper in JICABLE 1999
described the development of a Triple
energy
absorbing
foil
sheath
construction for 132kV XLPE cables
which were installed in a cable tunnel
under London. These cables were laid
direct in cement bound sand in the floor
of the tunnel and were not exposed to
thermomechanical movement. There
has been no published work on the
thermomechanical Performance of
either flexible or rigidly cleated large
diameter, large conductor foil sheathed
cables. Engineering practice has been
to employ extruded metal sheathed
cables using the thermomechanical
design/cleating
formulae
and
experimental fatigue results from fluid
filled pressure cables.
Three circuits of rigidly constrained 132kV
XLPE cable being installed in CBS filled
troughs in a tunnel under London
Extruded Ethylene Propylene Rubber (EPR) Cables
These cables have been predominantly employed in Italy at voltages up to
15OkV. They are more flexible than XLPE cables and have a better short term
overload performance. EPR insulation has a superior water tree performance to
XLPE cable. Cables without metal sheaths have been commonly installed at
33kV and prototypes are now available for installation at 150kV in Italy.
Gas insulated lines (GIL)
Gas insulated busbar has long been used in association with SF6 insulated
switchgear. In some substations the lengths of busbar have been of the order of
500m long. This has raised the prospect of installing long lengths either in laid
direct or tunnel applications. Gas insulated lines are much larger in diameter
than flexible cables (eg prototype 400kV designs are approximately 600"
OD
compared with 150"
OD for a flexible cable). The advantage of GIL is that it is
possible to carry a much higher current rating on one cable per phase. There are
several major engineering problems to be perfected. The conductor and metallic
enclosure are rigid and are usually provided with expansion joints. The
expansion joints have a limited fatigue life and care must be taken in their
selection and maintenance. It is usual to segregate pneumatic sections of the
GIL, to prevent major gas loss in a catastrophic event in service. This is
achieved by an insulating barrier. A prototype service installation in Japan at
275kV employed an anchor construction to transmit the pneumatic forces at the
insulating barrier to the tunnel walls at periodic intervals and to the tunnel shafts.
The presence of the larger diameter GIL and the additional forces on the tunnel
wall required a more expensive tunnel construction than conventional cable. Gas
insulated busbar has been recorded to be less reliable than flexible cable due to
a) the presence and generation of particulate contamination and b) the
occurrence of partial discharge and tracking on the support insulators resulting
from space charge, The latter problems are of great significance in a long length
transmission line, as the GIL is assembled on-site in short 14m lengths, thereby
increasing the number of support insulators and opportunities for the presence of
particulate contamination.
Single Core and Three Core Constructions
Three core cables within one flexible metal sheath are available up to 15OkV in a
fluid filled cable. For higher transmission voltages, single core cables are
preferred to permit longer despatch lengths and fewer accessories. XLPE and
EPR cables are not capable of operating at the high design stresses of fluid filled
cable and so it has been practice to prefer single core cables at 33kV and above.
Single core cables are equally acceptable in a tunnel installation as space
restrictions are usually less severe than laid direct installations.
3.0
Design of Cable System for Tunnel Installation
There are two methods of fixing the cable within a tunnel, the first being a flexible
system and the second being a rigid system.
Flexible System
The cable is partially unconstrained such that it can expand and contract during load
cycling. This reduces compressive and tensile forces within the cable and
accessories, however it exposes the cable components to the risk of damage from
fatigue failure. The lowest cost method is installation in a vertically sagged system,
often referred to as a Holttum system in which the cable is fixed to the tunnel wall or
roof via cleats, these being usually padded to prevent damage to the oversheath.
The distance between cleats and the vertical sag is calculated to limit the bending
moment strain in the cable sheath adjacent to the cleat, to less than the fatigue limit
for a 40 year life. It is possible to use a lead alloy sheath, however a corrugated
aluminium sheath is both lighter in weight and has a superior fatigue performance.
Significant electromechanical forces occur between the cables during a system
through fault and it is usual practice to prevent the cables from flying apart at mid
span by the application of a shorting strap between the three phase cables.
The behaviour of a taped cable is significantly different from an extruded cable. The
insulation in a taped cable has no inherent stiffness because the tapes are designed
to slide over each other during expansion and contraction. Extruded insulation has
an inherent stiffness, however the calculation of the second moment of area is
extremely complex. XLPE and EPR insulation has non-linear stress strain
characteristics and non linear temperature characteristics. Thus the effective
stiffness of the cable reduces significantly when heated under full load and
progressively increasing deformation adjacent to the cleats can be expected. This
deformation can become permanent due to the 'compression set' characteristic of
the polymer/elastomer. The practice in some countries has been to employ long
spans for extruded cables, however this is usually for lower voltage, lighter weight
cables with smaller conductors. Also some countries design at lower conductor
temperatures, for example 7OoC for PE cables in France and 85OC for XLPE cables
in Japan. As mentioned previously, engineering practice especially for large
conductor EHV cables has been to design the span and sag based on fluid filled
cable practice.
The cables can alternatively be horizontally snaked on cable trays, this technique
being used for 275kV and 500kV XLPE cables in Japan. The cable weight is
uniformly supported and the sheath strain is better controlled. This technique is
frequently employed in the vertical shaft leading to the tunnel.
Rigid System
This is the preferred method for the higher voltage, larger conductor cables. The
cable is constrained such that it cannot move during load cycling. The cable can be
laid in troughs filled with CBS or close cleated. The distance between cleats is
calculated to prevent buckling of the metallic sheath. It is most important that the
accessories are designed to withstand the compressive thrust and retraction from
the conductor and sheath during load cycling. This can amount to 2kN/mm2 of
copper cross section (ie 6 tonnes for a 2000mm2 conductor)
Combined Flexible and Rigid Systems
It is not always possible to design for a single system, for example there maybe a
need for a rigidly constrained cable to pass over an expansion joint in a tunnel or to
be connected to switchgear or transformers that are allowed to move during load
cycling. Similarly if a flexible sag system changes direction it is possible for the
conductor or insulated core to expand asymmetrically into the adjacent span of cable
resulting in local distortion. It is usual to lock the cable at such positions by using
close cleated locking waves or by installing an anchor straight joint in which the
conductor is mechanically bolted to rigid insulation (epoxy resin casting), which in
turn is bolted to a cleat.
4.0
Design of Tunnel
A tunnel with vertical walls provides more space for parallel circuits (it is usual in
Japan to install three circuits on one wall), this being a better investment for the
future. The economics for deep tunnels dictates that these be circular in cross
section. It would normally be expected to install a minimum of one circuit on each
side of the tunnel. Conventional flexible cables with ratings up to 2000MVA occupy
less space than GIL, however above that rating it will be necessary to install two
cables per phase compared to one GIL per phase.
Modem prefabricated joints on XLPE cable and straight joints on fluid filled cable can
be assembled in the centre of small diameter tunnels and then moved to their
permanent position on the wall. Significantly more space is required in the assembly
of GIL.
The diameter of the vertical shafts maybe determined by the size of the tunnelling
machine and in this case it may be sufficiently large to lower either a drum of flexible
cable or a 14m section of GIL to the horizontal tunnel. In some tunnel routes space
does not permit a large diameter shaft. A flexible cable has the advantage that it can
be lowered either a) under it’s own weight for short depths, b) through a vertical pipe
for medium depths or c) attached to a steel bond wire for great depths. Within the
tunnel it is possible to transfer long lengths of flexible cable (eg Ikm) by the use of
rollers or overhead hoist.
The tunnel should be designed with fixing points to suit the particular cable size and
weight, rather than the cable be designed to suit the dimensions of cable segments.
The tunnel needs to be designed to withstand the thermomechanical loads imposed
by the cable system. In a flexible system, either rigidly or flexibly cleated these loads
are of minimal magnitude and are distributed. In the case of GIL these loads can be
of high magnitude, particularly if pneumatic loads are also anchored, and will be
concentrated at discrete positions along the tunnel and shafts, which will need to be
strengthened. For both fluid filled and GIL facilities will need to be provided for
evacuation of the cablelaccessories and for filling with either hydrocarbon fluid or
SFGlnitrogen gas.
Environmental Considerations
Gas filled paper insulated cables are filled with nitrogen gas and have no significant
environmental concerns. Fluid filled cables contain the hydrocarbon fluid, DDB,
which meets the requirements for biodegradability. Early types of GIL in tunnels
contained pure SF6 gas in very large volumes, which, should a leak occur, posed a
major risk to the Earth’s upper atmosphere, for this reason prototype GIL has been
proposed with the SF6 gas diluted with nitrogen in a 1 : l O ratio, but with the pressure
increased from typically 4 to 6 bar. It has been necessary to develop new types of
gas handling equipment both to mix the gas and to separate it for
reclamationldisposal.
Safety Considerations
Magnetic Fields
The advantage of a tunnel is that the magnetic field at ground surface is effectively
zero. The use of solidly bonded cable and GIL sheaths permits a current to flow in
approximate antiphase to the conductor current, thereby reducing the external
magnetic field in the tunnel walkway. The rating of high power cables is
considerably increased by the use of specially bonded sheaths, such that circulating
sheath currents are eliminated. The external magnetic field can be reduced by
installing the cables in close trefoil formation. GIL has the advantage that thick
aluminium alloy enclosures are required to contain the gas pressure and these
permit the flow of very high antiphase currents which virtually eliminate the extemal
magnetic field. Most cable tunnel installations are designed to not require access for
maintenance when the circuits are alive.
Fire Precautions
It is unusual for cable tunnels to contain additional combustible material. It is good
practice to employ high speed circuit breakers, without auto-reclose, such that the
minimum of energy is fed into an electrical fault, thereby reducing the risk of ignition
of cable materials. This is particularly important for fluid filled cables. It is usual to
install temperature monitoring and firekmoke detection systems such that the source
of a failure or fire can be immediately located and remedial measures taken. It is
also good practice to install fire extinguishing equipment. Cables can be provided
with cable oversheaths with different fire performances. PVC is extensively used in
the EHV cable tunnels in Japan, but is not recommended for use in the UK because
of the liberation of toxic fumes during a fire. Polyethylene sheaths liberate far less
fumes but can propagate fire. RPS grades of PVC are available for use in tunnels
and buildings and non halogen containing sheaths are available with low smoke and
fume properties (LSF).
5.0
Cable Commissioning Tests
Fluid Filled Cables
These are subjected to a DC HV withstand test. The test equipment is mobile and is
transported on normal road transport. Access is gained to the cable system via
outdoor terminations or test ports in gas insulated busbar at the ends of the circuit.
Cable circuits of long length can be tested.
Extruded XLPE Cables
Initial experience showed that failures, particularly of accessories, could occur after a
short period in service, irrespective of whether a DC test had been performed or not.
Additionally there is concern that DC testing can generate a permanent space
charge within the cable. In extreme circumstances long periods of DC energisation
have been shown to be capable of increasing the electrical stress adjacent to the
screens by up to 800%, although this is unlikely during a 15 minute test, there is still
concern. CIGRE has therefore recommended that DC testing be discontinued and
that circuits be AC tested. The preferred method is to use mobile AC test sets at
near 50Hz frequency. Sufficient mobile AC test sets are now available to cover
installations throughout Europe at transmission voltages of up to 132kV. Two test
sets exist that can test in combination a 400kV circuit, at a test voltage to earth of
400kV. However, all of these test sets have a limited output in both current and
tuneable inductive reactance, such that it may not be possible to test circuits above
10-20km in length at 400kV. In these circumstances soak testing for 1-7 days at Uo,
whilst connected to the grid system, is recommended. It is now becoming feasible to
fit partial discharge transducers to each joint and termination, such that partial
discharge testing can be performed during commissioning.
6.0
Conclusion
A wide choice of cable types exist to suit different tunnel situations.
At voltages up to and including 220kV, extruded XLPE cables offer a combination
of adequate electrical performance, low maintenance and good fire performance.
At voltages of 275kV and above the most reliable cable system remains the fluid
filled paper/PPL cable, both in terms of the cable dielectric and the accessory
dielectric. The risk of fire spread is greater with this type of cable and
precautions are required to be taken.
XLPE cables and in particular their joints are becoming available at 275, 400 and
500kV, initially with small conductors. However these are electrically less reliable
and care is required to be taken in the specification of type approval tests,
prequalification test, manufacturing routine tests, manufacturing sample tests and
in jointer training.
Prototype GIL systems are being proposed and although offering the advantage
of high ratings, pose significant engineering problems with respect to electrical
reliability and service life.
Wherever possible the cable tunnel construction should be designed to suit the
thermomechanical requirements of the cable system to achieve maximum circuit
reliability. Flexible cables apply minimal loads to the tunnel wall. GIL cables can
impose discrete loads at certain positions and this needs to be taken into account
in the tunnel design.
Brian Gregory