U.S. patent application number 16/506643 was filed with the patent office on 2021-01-14 for power cable system with cooling capability.
The applicant listed for this patent is NKT HV Cables AB. Invention is credited to Robert Wayne Hobson.
Application Number | 20210012927 16/506643 |
Document ID | / |
Family ID | 1000005300471 |
Filed Date | 2021-01-14 |
United States Patent
Application |
20210012927 |
Kind Code |
A1 |
Hobson; Robert Wayne |
January 14, 2021 |
Power Cable System With Cooling Capability
Abstract
A power cable system including a power cable, and an evaporator
pipe assembly extending along the power cable, wherein the
evaporator pipe assembly having an inner liquid pipe including a
pressurised liquid refrigerant, and an outer gas pipe arranged
outside of and coaxially with the inner liquid pipe, wherein the
inner liquid pipe is provided with a plurality of openings
distributed along its length, and wherein the openings provide
fluid communication between the inner liquid pipe and the outer gas
pipe, allowing part of the pressurised liquid refrigerant to escape
from the inner liquid pipe to the outer gas pipe and evaporate in
the outer gas pipe, thereby cooling the power cable.
Inventors: |
Hobson; Robert Wayne; (Holly
Springs, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NKT HV Cables AB |
Lyckeby |
|
SE |
|
|
Family ID: |
1000005300471 |
Appl. No.: |
16/506643 |
Filed: |
July 9, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02G 3/03 20130101; H01B
7/423 20130101; F28D 7/00 20130101 |
International
Class: |
H01B 7/42 20060101
H01B007/42; F28D 7/00 20060101 F28D007/00; H02G 3/03 20060101
H02G003/03 |
Claims
1. A power cable system comprising: a power cable, and an
evaporator pipe assembly arranged external to the power cable and
extending along the power cable, wherein the evaporator pipe
assembly comprises an inner liquid pipe including a pressurised
liquid refrigerant, and an outer gas pipe arranged outside of and
coaxially with the inner liquid pipe, wherein the inner liquid pipe
is provided with a plurality of openings distributed along its
length, and wherein the openings provide fluid communication
between the inner liquid pipe and the outer gas pipe, allowing part
of the pressurised liquid refrigerant to escape from the inner
liquid pipe to the outer gas pipe and evaporate in the outer gas
pipe, thereby cooling the power cable.
2. The power cable system as claimed in claim 1, wherein the liquid
refrigerant is arranged to flow along the length of the evaporator
pipe assembly.
3. The power cable system as claimed in claim 1, wherein the
evaporator pipe assembly extends along the entire or essentially
the entire length of the power cable.
4. The power cable system as claimed in claim 1, further
comprising: a compressor unit having a suction side and a discharge
side, wherein the outer gas pipe is connected to the suction side
and the inner liquid pipe is connected to the discharge side.
5. The power cable system as claimed in claim 4, wherein the
compressor unit is configured to compress the evaporated
refrigerant to liquid state as the evaporated refrigerant passes
through the compressor unit and discharge it on the discharge side
to the inner liquid pipe.
6. The power cable system as claimed in claim 4, wherein the
compressor unit is configured to be controlled to change the
suction in the outer gas pipe to provide the evaporator pipe
assembly with a controllable cooling functionality.
7. The power cable system as claimed in claim 1, wherein the inner
liquid pipe and the outer gas pipe are made of metal.
8. The power cable system as claimed in claim 1, comprising an
outer pipe, wherein the power cable and the evaporator pipe
assembly are arranged in the outer pipe.
9. The power cable system as claimed in claim 1, wherein the liquid
refrigerant is carbon dioxide.
10. The power cable system as claimed in claim 1, wherein the outer
pipe is filled with a fluid.
11. The power cable system as claimed in claim 10, wherein the
fluid is a liquid.
12. The power cable system as claimed in claim 11, wherein the
liquid is water.
13. The power cable system as claimed in claim 11, comprising a
first liquid tank, and a liquid movement system configured to move
the liquid from the first liquid tank and back to the first liquid
tank through the outer pipe.
14. The power cable system as claimed in claim 13, wherein the
liquid movement system is configured to move the liquid with
turbulent flow through the outer pipe.
15. The power cable system as claimed in claim 13, wherein the
liquid movement system comprises at least one pump.
16. The power cable system as claimed in claim 2, wherein the
evaporator pipe assembly extends along the entire or essentially
the entire length of the power cable.
17. The power cable system as claimed in claim 2, further
comprising: a compressor unit having a suction side and a discharge
side, wherein the outer gas pipe is connected to the suction side
and the inner liquid pipe is connected to the discharge side.
18. The power cable system as claimed in claim 5, wherein the
compressor unit is configured to be controlled to change the
suction in the outer gas pipe to provide the evaporator pipe
assembly with a controllable cooling functionality.
19. The power cable system as claimed in claim 2, wherein the inner
liquid pipe and the outer gas pipe are made of metal.
20. The power cable system as claimed in claim 12, comprising a
first liquid tank, and a liquid movement system configured to move
the liquid from the first liquid tank and back to the first liquid
tank through the outer pipe.
21. The power cable system as claimed in claim 1, wherein the power
cable is spaced apart from the evaporator pipe assembly.
Description
TECHNICAL FIELD
[0001] The present disclosure generally relates to power cables,
and in particular to power cable cooling.
BACKGROUND
[0002] A type of power cable system present in some regions of the
world are often referred to as high pressure fluid filled (HPFF)
cable systems, also known as pipe cables. HPFF cable systems
typically include an outer pipe and one or more power cables
arranged in the outer pipe. Typically, a high-pressure liquid such
as oil is arranged in the outer pipe for squeezing gas bubbles in
the cable insulation to a very small size and prevent the formation
of partial discharge locations. The liquid also transfers heat from
the power cable(s) to the outer pipe.
[0003] It would be desirable to increase the ampacity of power
cables such as those of an HPFF cable system. The ampacity is
related to the heat developed in the cable. The ampacity is limited
by how fast the heat from the losses in the cable system is
dissipated to the surrounding medium. By removing the heat from the
pipe cable and not relying on the heat to dissipate through the
surrounding medium, it would be possible to increase the ampacity.
One way to cool a power cable is by circulating the fluid,
typically oil or water, through the outer pipe. The fluid is
circulated through air or water-cooled heat exchangers which are
uniformly spaced apart along the transmission line. This method
requires recirculation of the fluid. Often, existing underground
cables do not have facilities to provide recirculation of
fluid.
[0004] U.S. Pat. No. 3,955,042 discloses the cooling of power
cables by a closed-cycle evaporation-condensation process. Cables
are arranged in a pipe which is high pressure oil-filled. The pipe
is enclosed by an outer pipe adapted to extend along a portion of a
length of the underground cables and partially filled with volatile
liquid. An enclosed heat chamber is connected to the outer pipe.
The connection and the enclosed heat exchanger are located above a
level of the volatile liquid within the outer pipe. The volatile
liquid is evaporated within the outer pipe by heat generated within
the power cables. The evaporated liquid is transferred by virtue of
pressure differential from the outer pipe to the enclosed heat
exchanger, wherein the evaporated volatile liquid is condensed and
from which it flows by gravity back to the outer pipe.
[0005] A drawback with the solution disclosed in U.S. Pat. No.
3,955,042 is that the evaporated volatile liquid travels along the
power cable and is thereby heated as it flows in the axial
direction of the power cable. The cooling effect is hence reduced
along the power cable.
SUMMARY
[0006] In view of the above, an object of the present disclosure is
to provide a power cable system which solves or at least mitigates
existing problems of the state of the art.
[0007] There is hence provided a power cable system comprising: a
power cable, and an evaporator pipe assembly extending along the
power cable, wherein the evaporator pipe assembly comprises an
inner liquid pipe including a pressurised liquid refrigerant, and
an outer gas pipe arranged outside of and coaxially with the inner
liquid pipe, wherein the inner liquid pipe is provided with a
plurality of openings distributed along its length, and wherein the
openings provide fluid communication between the inner liquid pipe
and the outer gas pipe, allowing part of the pressurised liquid
refrigerant to escape from the inner liquid pipe to the outer gas
pipe and evaporate in the outer gas pipe, thereby cooling the power
cable.
[0008] The liquid refrigerant can evaporate in the outer gas pipe
due to a lower pressure in the outer gas pipe compared to the
pressure in the inner liquid pipe. The boiling point of the liquid
refrigerant is hence lowered in the outer gas pipe.
[0009] The evaporator pipe assembly forms a long evaporator running
parallel with the power cable. The openings operate in choked flow.
The liquid refrigerant exits the openings at a lower pressure and
temperature and at a fixed mass flow rate regardless of the
operating pressure inside the inner liquid pipe into the outer gas
pipe. The heat of evaporation absorbs and removes the heat. A more
uniform cooling along the length of the power cable may thereby be
achieved without recirculation. The cooling capability of the outer
gas pipe is refreshed in a region of each opening. The axial
distance between the openings may be adapted based on the cooling
needs of the power cable.
[0010] The power cable may be a low voltage, medium voltage or
power cable.
[0011] The cooling may further be controlled by controlling the
pressure inside the outer gas pipe, because this pressure together
with the characteristics of the refrigerant determines the boiling
temperature of the refrigerant. Depending on the pressure in the
outer gas pipe and the refrigerant selected, the boiling
temperature could be as low as below zero degrees Celsius. The
outer surface of the outer cooling pipe may hence be cooled to
below zero degrees Celsius. Due to the more uniform cooling, the
power cable system enables a higher ampacity of the power cable. In
particular, a power cable of different type than HPFF cables will
be able to operate with the same or similar ampacity as the
existing HPFF cable systems. The power cable may for example have
an insulation system comprising cross-linked polyethylene (XLPE),
polypropylene or cellulose-based material such as paper.
[0012] The outer gas pipe may for example have an outer diameter in
the range of 2-10 cm, such as 2-8 cm or 3-7 cm or 3-6 cm. Smaller
dimensioned pipes may better withstand high internal pressures.
[0013] The power cable system may comprise a plurality of power
cables, for example three power cables, extending parallel with
each other.
[0014] According to one example, the evaporator pipe assembly is
arranged in the power cable. The evaporator pipe assembly may be
located in the centre of the cable, radially inside of the
conductor. The conductor may hence have a hollow centre. The inner
liquid pipe and the outer gas pipe could be connected to the
outside world through a termination or cable joint.
[0015] According to one embodiment the liquid refrigerant is
arranged to flow along the length of the evaporator pipe
assembly.
[0016] According to one embodiment the evaporator pipe assembly
extends along the entire or essentially the entire length of the
power cable.
[0017] One embodiment comprises a compressor unit having a suction
side and a discharge side, wherein the outer gas pipe is connected
to the suction side and the inner liquid pipe is connected to the
discharge side.
[0018] The compressor unit may comprise a condenser provided on the
discharge side and connected to the inner liquid pipe.
[0019] According to one embodiment the power cable system comprises
a plurality of compressor units, and the evaporator pipe assembly
comprises a first Tee connection configured to connect the outer
gas pipe to the suction side of several of the compressor units and
a second Tee connection configured to connect the inner liquid pipe
to the discharge side of several of the compressor units. Multiple
compressor units are thereby able to draw the evaporated
refrigerant from and feed liquid refrigerant into the evaporator
pipe assembly. This provides mechanical redundancy.
[0020] The number of compressor units required may depend on the
amount of heat that must be removed from the power cables and the
amount of heat that each compressor unit can remove.
[0021] According to one embodiment the compressor unit is
configured to compress the evaporated refrigerant to liquid state
as the evaporated refrigerant passes through the compressor unit
and discharge it on the discharge side to the inner liquid
pipe.
[0022] According to one embodiment the compressor unit is
configured to be controlled to change the suction in the outer gas
pipe to provide the evaporator pipe assembly with a controllable
cooling functionality. Thus, based on the suction provided by the
compressor unit, the cooling effect on the power cable may be
controlled.
[0023] According to one embodiment the inner liquid pipe and the
outer gas pipe are made of metal. The metal may for example
comprise steel, such as stainless steel, copper, aluminium, an
alloy based on steel, copper or aluminium, or any other suitable
metal.
[0024] One embodiment comprises an outer pipe, wherein the power
cable and the evaporator pipe assembly are arranged in the outer
pipe.
[0025] According to one embodiment the liquid refrigerant is carbon
dioxide. Carbon dioxide, which has refrigerant designation number
R744, has proved to be especially useful for certain high voltage
applications. Other refrigerants or fluids may however
alternatively also be used.
[0026] According to one embodiment the outer pipe is filled with a
fluid. The heat transfer between the power cable and the evaporator
pipe assembly may thereby be facilitated.
[0027] According to one embodiment the fluid is a liquid. Liquid
provides a more efficient heat transfer than gas. The liquid does
not remove the heat from the power cable but transports the heat
from the outer surface of the power cable to the outer surface of
the outer gas pipe.
[0028] According to one embodiment the liquid is water. The liquid
could alternatively be any other suitable liquid, such as an
oil.
[0029] One embodiment comprises a first liquid tank, and a liquid
movement system configured to move the liquid from the first liquid
tank and back to the first liquid tank through the outer pipe.
[0030] According to one example the power cable system comprises a
second liquid tank, wherein the first liquid tank is arranged at a
first end region of the power cable and the second liquid tank is
arranged at a second end region of the power cable, wherein the
liquid movement system is configured to alternatingly move the
liquid between the first liquid tank and the second liquid tank
through the outer pipe.
[0031] According to one embodiment the liquid movement system is
configured to move the liquid with turbulent flow through the outer
pipe. Due to the turbulent flow, the heat exchange between the
power cable and the evaporator pipe assembly is made more
efficient. The turbulent flow makes the liquid temperature uniform
in the cross section of the outer pipe. It is therefore not
necessary with much liquid storage, for example only around an
amount corresponding to 30-50 meters of outer pipe length.
[0032] According to one embodiment the liquid movement system
comprises at least one pump.
[0033] The liquid movement system may according to one example
comprise a bladder tank configured to act as liquid storage. The at
least one pump may be configured to pump liquid from the bladder
tank through the outer pipe and to be set in reverse to send the
liquid back towards the bladder tank.
[0034] Generally, all terms used in the claims are to be
interpreted according to their ordinary meaning in the technical
field, unless explicitly defined otherwise herein. All references
to "a/an/the element, apparatus, component, means, etc. are to be
interpreted openly as referring to at least one instance of the
element, apparatus, component, means, etc.", unless explicitly
stated otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The specific embodiments of the inventive concept will now
be described, by way of example, with reference to the accompanying
drawings, in which:
[0036] FIG. 1 is a perspective view of a cross section of a section
of an example of a power cable system;
[0037] FIG. 2 schematically shows a side view of an example of a
power cable system; and
[0038] FIG. 3 schematically shows a side view of an example of a
power cable system.
DETAILED DESCRIPTION
[0039] The inventive concept will now be described more fully
hereinafter with reference to the accompanying drawings, in which
exemplifying embodiments are shown. The inventive concept may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided by way of example so that this
disclosure will be thorough and complete, and will fully convey the
scope of the inventive concept to those skilled in the art. Like
numbers refer to like elements throughout the description.
[0040] FIG. 1 shows an example of a power cable system 1. The power
cable system 1 comprises an outer pipe 3 and an evaporator pipe
assembly 5. The power cable system 1 furthermore comprises a
plurality of power cables 7a-7c. The power cable system 1 could
alternatively comprise only a single power cable. Further, it is to
be noted that according to some examples the power cable system
does not comprise any outer pipe 3.
[0041] The outer pipe 3 may for example be made of metal. The metal
may for example be steel, such as stainless steel, aluminium,
copper, or any other metal material suitable for pipe construction.
The outer pipe 3 could alternatively be made of another material
than metal, for example a polymeric material.
[0042] Each power cables 7a-7c comprises a respective conductor 9a,
9b, 9e and a respective insulation system 10a, 10b, 10c surrounding
the conductor 9a, 9b, 9c. The insulation system boa, 10b, 10c may
for example comprise XLPE, polypropylene or cellulose material such
as paper. In the case of only one power cable, the power cable
comprises a conductor and an insulation system surrounding the
conductor. The insulation system may in this case for example
comprise XLPE, polypropylene or cellulose material such as
paper.
[0043] In the present example, the power cables 7a-7c are arranged
in the outer pipe 3. The power cables 7a-7c extend along the outer
pipe 3 inside the outer pipe 3.
[0044] The evaporator pipe assembly 5 is arranged in the outer pipe
3. The evaporator pipe assembly 5 extends along the outer pipe 3
inside the outer pipe 3. The evaporator pipe assembly 5 extends
along the power cables 7a-7c. The evaporator pipe assembly 5 may
extend along a majority of the length of the power cables 7a-7c,
for example along their entire length or essentially along their
entire length.
[0045] In examples which do not comprise an outer pipe, the
evaporator pipe assembly extends along the one or more power cables
but without protection from an outer pipe.
[0046] The evaporator pipe assembly 5 comprises an inner liquid
pipe 5a and an outer gas pipe 5b. The outer gas pipe 5b is arranged
radially outside of and coaxially with the inner liquid pipe 5a.
The inner gas pipe 5a may for example be made of metal. The metal
may for example be steel such as stainless steel, copper or
aluminium. The outer gas pipe 5b may for example be made of metal.
The metal may for example be steel such as stainless steel, copper
or aluminium.
[0047] The inner liquid pipe 5a comprises a plurality of openings
or orifices 5c. The openings 5c are through-openings extending
radially through the inner liquid pipe 5a. The openings 5c are
distributed along the length of the inner liquid pipe 5a. The
openings 5c provide fluid communication between the inner liquid
pipe 5a and the outer gas pipe 5b.
[0048] The inner liquid pipe 5a contains a pressurised liquid
refrigerant. The liquid refrigerant may for example be carbon
dioxide, CO.sub.2. In case of carbon dioxide, the carbon dioxide
could be pressurised to at least 25 bar, such as at least 30 bar
inside the inner liquid pipe 5a. Other liquid refrigerants are also
envisaged, in particular any refrigerant that by way of a pressure
drop provides reduced temperature at the lower pressure and can
remove heat from the walls of the outer gas pipe 5b.
[0049] The liquid refrigerant is set to flow through the inner
liquid pipe 5a. When the liquid refrigerant reaches an opening 5c,
part of the liquid refrigerant escapes from the inner liquid pipe
5a through the opening 5c to the outer gas pipe 5b. The liquid
refrigerant escaping through the opening 5c will then be subjected
to an adiabatic expansion, in which it still in liquid form but
colder than in the inner liquid pipe 5a. The pressure of the liquid
refrigerant in the outer gas pipe 5b is typically several times
lower than inside the inner liquid pipe 5a. The refrigerant is
selected such that at the pressure in the outer gas pipe 5b, the
boiling point of the liquid refrigerant is very low. For carbon
dioxide, the boiling point could for example be below zero degrees
Celsius. As the liquid refrigerant in the outer gas pipe 5b
evaporates, the phase change consumes large amounts of heat emitted
by the power cables 7a-7c. The outer surface of the outer gas pipe
5b obtains the temperature of the boiling point of the liquid
refrigerant contained in the outer gas pipe 5b. The power cables
7a-7c are hence cooled.
[0050] According to one example, the power cable system 1 may
comprise a fluid such as a liquid arranged inside the outer pipe 3
but outside of the evaporator pipe assembly 5. The liquid may fill
or essentially fill the available space inside the outer pipe 3.
The liquid may for example be water but could alternatively be an
oil or any other liquid with relatively low viscosity. The liquid
may be set to move with a turbulent flow inside the outer pipe 3
along the outer pipe 3.
[0051] FIG. 2 schematically shows a side view of the power cable
system 1 provided with further system components. The exemplified
power cable system 1 comprises a plurality of compressor units
C1-C3. Each compressor unit C1-C3 has a suction side and a
discharge side. Each compressor unit C1-C3 may comprise a
compressor, a condenser, and an accumulator tank for the liquid
refrigerant.
[0052] For reasons of simplicity, the outer pipe 3 and the power
cables 7a-7c are not shown in FIG. 2. The evaporator pipe assembly
5 is shown connected to the compressor units C1-C3.
[0053] A first section 5-1 of the evaporator pipe assembly 5 is
connected to a first compressor unit C1 at one end and to a second
compressor unit C2 at the other end. The inner liquid pipe 5a has
one end connected to the discharge side of the first compressor
unit C1 and the other end connected to the discharge side of the
second compressor unit C2. The outer gas pipe 5c is connected to
the suction side of the first compressor unit C1 and to the suction
side of the second compressor unit C2.
[0054] Although both sides of the outer gas pipe 5b of the first
section 5-1 of the evaporator pipe assembly 5 are connected to the
suction side and both sides of the inner liquid pipe 5a are
connected to the discharge side, the pressure drop over the first
section 5-1 should be sufficient to drive the liquid refrigerant
and evaporated refrigerant in the evaporator pipe assembly 5. The
liquid refrigerant is thereby able to move back and forth between
adjacent compressor units inside the evaporator pipe unit 5.
[0055] According to one example, the first compressor unit C1 and
the second compressor unit C2 may be configured to be controlled
such that a pressure difference is obtained between their suction
sides and between their discharge sides to enable the evaporated
refrigerant and the liquid refrigerant to flow between the first
compressor unit C1 and the second compressor unit C2.
[0056] A second section 5-2 of the evaporator pipe assembly 5 is
connected to the second compressor unit C2 at one end and to a
third compressor unit C3 at the other end. The inner liquid pipe 5a
has one end connected to the discharge side of the second
compressor unit C2 and the other end connected to the discharge
side of the third compressor unit C3. The outer gas pipe 5c is
connected to the suction side of the second compressor unit C2 and
to the suction side of the third compressor unit C3.
[0057] Although both sides of the outer gas pipe 5b of the second
section 5-2 of the evaporator pipe assembly 5 are connected to the
suction side and both sides of the inner liquid pipe 5a are
connected to the discharge side, the pressure drop over the second
section 5-2 should be sufficient to drive the liquid refrigerant
and evaporated refrigerant in the evaporator pipe assembly 5.
[0058] According to one example, the second compressor unit C2 and
the third compressor unit C3 may be configured to be controlled
such that a pressure difference is obtained between their suction
sides and between their discharge sides to enable the evaporated
refrigerant and the liquid refrigerant to flow between the second
compressor unit 2 and the third compressor unit C3.
[0059] In view of the above, the power cable system may comprise a
control system. The control system may be configured to control the
suction of the compressor units C1-C3 to change the pressure in the
outer gas pipe 5b. The pressure in the outer gas pipe 5b may
thereby be controlled. Hence, the boiling point of the liquid
refrigerant may be controlled.
[0060] The distance between two adjacent compressor units such as
the first compressor unit C1 and the second compressor unit C2 may
for example be a few hundred metres such as at least 300 meters,
for example at least 500 metres, or at least 700 metres. Typically,
the distance between two adjacent compressor units may be less than
2000 metres, for example less than 1500 metres. The compressor
units C1-C3 may be located at splice pits accessible by
manholes.
[0061] The accumulation tanks of the compressor units C1-C3 may
comprising liquid refrigerant. The liquid refrigerant may be stored
in the accumulation tank under high pressure. The pressure may for
example be several times higher than the pressure of the liquid
refrigerant in the inner liquid pipe 5a. The compressor units C1-C3
could thereby be able to cycle and store the liquid refrigerant.
The power cable system 1 may comprise a control system configured
to control one or more valves to control the amount of liquid
refrigerant in the inner liquid pipe 5a provided from the liquid
tank. The control system could be configured to control the one or
more valves based on the temperature of the power cable(s). The
power cable system 1 may for example comprise a plurality of
temperature sensors for example in the form of a distributed
temperature sensing system configured to measure the temperature of
the power cable(s), fed to the control system.
[0062] FIG. 3 schematically shows a side view of an example of the
power cable system 1. The power cable system 1 depicted in FIG. 3
may comprise one or more compressor units as shown in FIG. 2 but
are for reasons of simplicity not included in FIG. 3. The power
cable system 1 comprises a first liquid tank T1 and a second liquid
tank T2. The power cable system 1 comprises a liquid movement
system 13. The liquid movement system 13 may comprise one or more
pumps. The liquid movement system 13 is configured to move the
liquid in the outer pipe 3 between the first liquid tank T1 and the
second liquid tank T2. The liquid movement system 13 may be
configured to move the liquid with a turbulent flow through the
outer pipe 3 between the first liquid tank T1 and the second liquid
tank T2. The first liquid tank T1 may for example be arranged at a
first end of the outer pipe 3. The second liquid tank T2 may for
example be arranged at a second end of the outer pipe 3.
[0063] As an alternative to the above, the power cable system 1
could comprise only a single liquid tank, e.g. the first liquid
tank T1. In this case first liquid tank T1 should be placed
appropriately along the length of the outer pipe 3. The liquid
movement system 13 would in this case be configured to pump liquid
from the first liquid tank T1 through the outer pipe 3 and back to
the first liquid tank T1.
[0064] The inventive concept has mainly been described above with
reference to a few examples. However, as is readily appreciated by
a person skilled in the art, other embodiments than the ones
disclosed above are equally possible within the scope of the
inventive concept, as defined by the appended claims.
* * * * *