U.S. patent number 10,855,063 [Application Number 15/870,249] was granted by the patent office on 2020-12-01 for method in the manufacturing of an insulated electric high voltage dc termination or joint.
This patent grant is currently assigned to NKT HV CABLES AB. The grantee listed for this patent is NKT HV CABLES AB. Invention is credited to Anders Lindgren, Carl-Olof Olsson, Markus Saltzer.
United States Patent |
10,855,063 |
Olsson , et al. |
December 1, 2020 |
Method in the manufacturing of an insulated electric high voltage
DC termination or joint
Abstract
A method in the manufacturing of an insulated electric high
voltage DC termination or joint includes providing an insulated
electric high voltage DC cable including a high voltage DC
conductor, a polymer based insulation system surrounding the high
voltage DC conductor, the polymer based insulation system including
an insulation layer and a semiconducting layer surrounding the
insulation layer, and a grounding layer surrounding the
semiconducting layer; removing the grounding layer and the
semiconducting layer in at least one end portion of the high
voltage DC cable, mounting a field grading adapter or joint body in
the at least one end portion of the high voltage DC cable; and
subjecting the insulation layer of the polymer based insulation
system in the at least one end portion of the high voltage DC cable
for a heat treatment procedure, while being covered by the mounted
field grading adapter or joint body.
Inventors: |
Olsson; Carl-Olof (Vasteras,
SE), Saltzer; Markus (Fislisbach, CH),
Lindgren; Anders (Karlskrona, SE) |
Applicant: |
Name |
City |
State |
Country |
Type |
NKT HV CABLES AB |
Lyckeby |
N/A |
SE |
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Assignee: |
NKT HV CABLES AB (Lyckeby,
SE)
|
Family
ID: |
1000005217333 |
Appl.
No.: |
15/870,249 |
Filed: |
January 12, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180138674 A1 |
May 17, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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14649131 |
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9991687 |
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PCT/EP2013/051890 |
Jan 31, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02G
1/14 (20130101); H01B 13/0016 (20130101); H01B
13/0165 (20130101); H01B 13/0026 (20130101); H01B
13/0162 (20130101); H02G 1/12 (20130101); H02G
15/18 (20130101); H02G 15/003 (20130101); H02G
1/16 (20130101); H02G 15/188 (20130101); H02G
15/068 (20130101); Y10T 29/49123 (20150115); Y10T
29/49119 (20150115) |
Current International
Class: |
H02G
1/14 (20060101); H01B 13/016 (20060101); H02G
15/00 (20060101); H01B 13/00 (20060101); H02G
15/18 (20060101); H02G 1/12 (20060101); H02G
1/16 (20060101); H02G 15/068 (20060101); H02G
15/188 (20060101) |
References Cited
[Referenced By]
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0 170 440 |
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EP |
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EP |
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2 093 774 |
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EP |
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2 498 262 |
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EP |
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2001169428 |
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JP |
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2011-515791 |
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JP |
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WO 92/03865 |
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WO |
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WO 2004/038735 |
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May 2004 |
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WO |
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WO |
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Other References
Hansen et al., "Effect of Atomic Oxygen on Polymers," Journal of
Polymer Science: Part A, vol. 3, No. 6, Jun. 1, 1965, pp.
2205-2214, XP055184481. cited by applicant .
Canadian Office Action and Search Report, dated Jun. 20, 2017, for
Canadian 2,898,837. cited by applicant .
Chinese Office Action and Search Report, dated Jun. 14, 2017, for
Chinese 201380072055.7, with an English translation. cited by
applicant .
Korean Office Action and English translation, dated May 1, 2017 for
Korean Application No. 10-2015-7022615. cited by applicant .
European Office Action, dated Feb. 7, 2018, for European
Application No. 13704032.5. cited by applicant.
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Primary Examiner: Tugbang; A. Dexter
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Divisional of U.S. patent application Ser.
No. 14/649,131 filed on Jun. 2, 2015, which was filed as the
National Phase of PCT International Application No.
PCT/EP2013/051890 filed on Jan. 31, 2013, all of which are hereby
expressly incorporated by reference into the present application.
Claims
The invention claimed is:
1. A method in the manufacturing of an insulated electric high
voltage DC termination or joint comprising the steps of: providing
an insulated electric high voltage DC cable comprising a high
voltage DC conductor, a polymer based insulation system surrounding
the high voltage DC conductor, the polymer based insulation system
comprising an insulation layer and a semiconducting layer
surrounding the insulation layer, and a grounding layer surrounding
the semiconducting layer; removing the grounding layer and the
semiconducting layer in at least one end portion of the high
voltage DC cable, thereby exposing the insulation layer of the
polymer based insulation system in the at least one end portion of
the high voltage DC cable; mounting a field grading adapter or
joint body in the at least one end portion of the high voltage DC
cable, thereby covering the insulation layer of the polymer based
insulation system in the at least one end portion of the high
voltage DC cable, the field grading adapter or joint body being
part of the high voltage DC termination or joint, wherein at least
one substance is present in the insulation layer of the polymer
based insulation system in the at least one end portion of the high
voltage DC cable in a non-homogenous distribution; and subjecting
the insulation layer of the polymer based insulation system in the
at least one end portion of the high voltage DC cable for a heat
treatment procedure, while being covered by the mounted field
grading adapter or joint body, thereby equalizing a concentration
of the at least one substance in the insulation layer of the
polymer based insulation system in the at least one end portion of
the high voltage DC cable.
2. The method of claim 1, further comprising the steps of:
temporarily covering the field grading adapter or joint body by a
cover impermeable to the at least one substance present in the
insulation layer of the polymer based insulation system in the at
least one end portion of the high voltage DC cable in a
non-homogenous distribution before performing the step of
subjecting; and removing the cover after the step of
subjecting.
3. The method of claim 2, wherein a second insulated electric high
voltage DC cable is provided, the second insulated electric high
voltage DC cable comprising a second high voltage DC conductor; a
polymer based insulation system surrounding the second high voltage
DC conductor, the polymer based insulation system comprising an
insulation layer and a semiconducting layer surrounding the
insulation layer; and a grounding layer surrounding the
semiconducting layer; the grounding layer and the semiconducting
layer of the second insulated electric high voltage DC cable are
removed in at least one end portion of the second insulated
electric high voltage DC cable, thereby exposing the insulation
layer of the polymer based insulation system of the second
insulated electric high voltage DC cable in the at least one end
portion of the second insulated electric high voltage DC cable; and
a field grading adapter or joint body of the second insulated
electric high voltage DC cable is mounted in the at least one end
portion of the second insulated electric high voltage DC cable,
thereby covering the insulation layer of the polymer based
insulation system of the second insulated electric high voltage DC
cable in the at least one end portion of the second insulated
electric high voltage DC cable, wherein the at least one substance
is present in the insulation layer of the polymer based insulation
system in the at least one end portion of the second insulated
electric high voltage DC cable in a non-homogenous distribution,
and wherein the step of subjecting further comprises the step of:
subjecting the insulation layer of the polymer based insulation
system of the second insulated electric high voltage DC cable in
the at least one end portion of the second insulated electric high
voltage DC cable for the heat treatment procedure, while being
covered by the mounted joint body of the second insulated electric
high voltage DC cable, thereby equalizing the concentration of the
at least one substance in the insulation layer of the polymer based
insulation system of the second insulated electric high voltage DC
cable in the at least one end portion of the second insulated
electric high voltage DC cable.
4. The method of claim 1, wherein a second insulated electric high
voltage DC cable is provided, the second insulated electric high
voltage DC cable comprising a second high voltage DC conductor; a
polymer based insulation system surrounding the second high voltage
DC conductor, the polymer based insulation system comprising an
insulation layer and a semiconducting layer surrounding the
insulation layer; and a grounding layer surrounding the
semiconducting layer; the grounding layer and the semiconducting
layer of the second insulated electric high voltage DC cable are
removed in at least one end portion of the second insulated
electric high voltage DC cable, thereby exposing the insulation
layer of the polymer based insulation system of the second
insulated electric high voltage DC cable in the at least one end
portion of the second insulated electric high voltage DC cable; and
a field grading adapter or joint body of the second insulated
electric high voltage DC cable is mounted in the at least one end
portion of the second insulated electric high voltage DC cable,
thereby covering the insulation layer of the polymer based
insulation system of the second insulated electric high voltage DC
cable in the at least one end portion of the second insulated
electric high voltage DC cable, wherein the at least one substance
is present in the insulation layer of the polymer based insulation
system in the at least one end portion of the second insulated
electric high voltage DC cable in a non-homogenous distribution,
and wherein the step of subjecting further comprises the step of:
subjecting the insulation layer of the polymer based insulation
system of the second insulated electric high voltage DC cable in
the at least one end portion of the second insulated electric high
voltage DC cable for the heat treatment procedure, while being
covered by the mounted joint body of the second insulated electric
high voltage DC cable, thereby equalizing the concentration of the
at least one substance in the insulation layer of the polymer based
insulation system of the second insulated electric high voltage DC
cable in the at least one end portion of the second insulated
electric high voltage DC cable.
5. The method of claim 1, wherein the field grading adapter or
joint body comprises rubber as a field grading material.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to high voltage electrical
power equipment. More particularly, the invention relates to a
method in the manufacturing of an insulated electric high voltage
DC termination or joint.
DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION
Extruded solid insulation based on a polyethylene, or a crosslinked
polyethylene (XLPE), has for almost 40 years been used for AC
transmission and distribution cable insulation. Therefore the
possibility of the use of XLPE for DC cable insulation has been
under investigation for many years. Cables with such insulations
for DC transmission have no restrictions on circuit length and they
also have a potential for being operated at higher temperatures,
thus offering a possibility to increase the transmission load.
It is, however, well known that this type of XLPE composition
exhibits a strong tendency to form space charges under DC electric
fields, thus making it unsuitable in insulation systems for DC
cables. However, it is also known that extended degassing, i.e.,
subjecting the crosslinked cable insulation to high temperatures
for long periods of time, will result in a decreased tendency to
space charge accumulation under DC voltage stress. It is generally
believed that the heat treatment removes the peroxide decomposition
products, such as acetophenone and cumyl alcohol, from the
insulation whereby the space charge accumulation is reduced.
Degassing is, however, a time-consuming batch-process comparable
with impregnation of paper insulations and thus as costly.
Therefore it is advantageous if the need for degassing is
removed.
EP 2093774 A1 provides a method for producing an insulated electric
high voltage DC cable or a high voltage DC termination or joint
with a polymer based, preferably polyethylene, electrical
insulation system suitable for use in DC transmission and
distribution networks carried out in a manner such that there is no
need for any lengthy time consuming batch-treatment (e.g. heat
treatment) of the cable to ensure stable and consistent dielectric
properties and a high and consistent electric strength of the cable
insulation. The resulting cable insulation further exhibits a low
tendency to space charge accumulation, a high DC breakdown
strength, and a high impulse strength and high insulation
resistance.
The method comprises subjecting the polymer based insulation system
for a heat treatment procedure while the outer surface of the
polymer based insulation system is covered by a cover impermeable
to at least one substance present in the polymer based insulation
system in a non-homogenous distribution, thereby equalizing the
concentration of the at least one substance in the polymer based
insulation system. The method may be performed for a DC cable
subsequent to subjecting the DC cable for a heat treatment while
the outer surface of the extruded polymer based insulation system
is not covered by any impermeable cover to thereby remove another
substance, e.g. methane, present in the polymer based insulation
system after crosslinking. The at least one substance may include a
rest or byproduct from crosslinking, preferably a peroxide
decomposition product, and/or an additive, e.g. an antioxidant.
The heat treatment procedure is performed at a temperature of
between 50 and 120.degree. C., and most preferably between 70 and
90.degree. C., and is relatively fast.
SUMMARY
While the above approach provides high quality high voltage DC
cable, the present inventor has noted that it is in many situations
insufficient for high voltage DC terminations and joints. During
the manufacturing thereof, the outer layers of a high voltage DC
cable end portion are stripped, i.e., removed, uncovering the
polymer based insulation system before a rubber based field grading
device is mounted. The stripping of the outer layers including a
grounding layer is performed in air and may take several hours for
a thick cable. During this operation, the distribution of one or
more substances in the exposed portion of the polymer based
insulation system, especially close to an outer surface thereof, is
altered due to diffusion of the one or more substances to the
exposed surface of the polymer based insulation system, followed by
evaporation from the surface to the surrounding air. This problem
has been shown to occur even if the concentrations of the one or
more substances in the polymer based insulation system in the high
voltage DC cable were perfectly equalized prior to stripping.
An object is therefore to remedy this problem and to offer a method
in the manufacturing of an insulated electric high voltage DC
termination or joint that provides for a high voltage DC
termination or joint, wherein the distribution of at least one
substance in the polymer based insulation system in a cable end is
improved, thereby achieving a reasonably good radial distribution
of the conductivity within the polymer based insulation system.
In one aspect a method in the manufacturing of an insulated
electric high voltage DC termination or joint comprises (i)
providing an insulated electric high voltage DC cable comprising a
high voltage DC conductor; a polymer based insulation system
surrounding the high voltage DC conductor and comprising an
insulation layer and a semiconducting layer surrounding the
insulation layer; and a grounding layer surrounding the
semiconducting layer; (ii) removing the grounding layer and the
semiconducting layer in at least one end portion of the high
voltage DC cable, thereby exposing the insulation layer of the
polymer based insulation system in the at least one end portion of
the high voltage DC cable; (iii) temporarily covering the
insulation layer of the polymer based insulation system in the at
least one end portion of the high voltage DC cable by a cover
impermeable to at least one substance present in the insulation
layer of the polymer based insulation system in the at least one
end portion of the high voltage DC cable in a non-homogenous
distribution; (iv) subjecting the insulation layer of the polymer
based insulation system in the at least one end portion of the high
voltage DC cable for a heat treatment procedure, while being
covered by the cover, thereby equalizing the concentration of the
at least one substance in the insulation layer of the polymer based
insulation system in the at least one end portion of the high
voltage DC cable; and (v) removing the cover.
The method is preferably performed in an initial phase of the
manufacturing of the insulated electric high voltage DC termination
or joint, before any termination or joint adapters or bodies are
mounted.
Preferably, the cover is also covering any exposed portions of the
semiconducting layer of the polymer based insulation system in the
at least one end portion of the high voltage DC cable to prevent
diffusion from those portions during the heat treatment.
In one embodiment, a field grading adapter or joint body is
pre-equalized. The field grading adapter or joint body is mounted
in an end portion of a piece of cable having an exposed insulation
layer with a similar composition as the insulation layer of the
polymer based insulation system of the high voltage DC cable (to
which the field grading adapter or joint body later is to be
mounted), thereby covering the exposed insulation layer of the
piece of cable. The field grading adapter or joint body is
subjected to a heat treatment procedure such that one or more
substances from the insulation layer of the piece of cable diffuse
into the field grading layer. The field grading adapter or joint
body is removed from the piece of cable and the inner surface of
the field grading adapter or joint body is temporarily covered with
a cover, which is impermeable to the one or more substances
diffused into the field grading layer, until the field grading
adapter or joint body is mounted in the at least one end portion of
the high voltage DC cable. In the above process, the field grading
adapter or joint body is prepared to obtain approximately the same
distribution of substances as would be obtained after equalization
with the field grading adapter or joint body mounted to the high
voltage DC cable, but can be made in advance and at a more
convenient location.
In a second aspect a method in the manufacturing of an insulated
electric high voltage DC termination or joint comprises the step of
(i) providing an insulated electric high voltage DC cable
comprising a high voltage DC conductor, a polymer based insulation
system surrounding the high voltage DC conductor and comprising an
insulation layer and a semiconducting layer surrounding the
insulation layer and a grounding layer surrounding the
semiconducting layer; (ii) removing the grounding layer and the
semiconducting layer in at least one end portion of the high
voltage DC cable, thereby exposing the insulation layer of the
polymer based insulation system in the at least one end portion of
the high voltage DC cable; (iii) mounting a field grading adapter
or joint body, preferably comprising rubber as a field grading
material, in the at least one end portion of the high voltage DC
cable, thereby covering the insulation layer of the polymer based
insulation system in the at least one end portion of the high
voltage DC cable, the field grading adapter or joint body being
part of the high voltage DC termination or joint and at least one
substance being present in the insulation layer of the polymer
based insulation system in the at least one end portion of the high
voltage DC cable in a non-homogenous distribution; and (iv)
subjecting the insulation layer of the polymer based insulation
system in the at least one end portion of the high voltage DC cable
for a heat treatment procedure, while being covered by the mounted
field grading adapter or joint body, thereby equalizing the
concentration of the at least one substance in the insulation layer
of the polymer based insulation system in the at least one end
portion of the high voltage DC cable.
When a high voltage DC cable end is mounted with a rubber based
field grading adapter or joint body, there is probably not
immediately an equilibrium distribution of chemicals between the
cable insulation and the adapter or body. The substances present in
the insulation layer may diffuse into the adapter or body and
substances present in the adapter or body may diffuse into the
insulation layer. In order to obtain a robust insulation system,
the distribution of substances needs to be sufficiently uniform,
and this is obtained by the above heat treatment procedure.
In one embodiment, the field grading adapter or joint body is
temporarily covered by a cover impermeable to the at least one
substance present in the insulation layer of the polymer based
insulation system in the at least one end portion of the high
voltage DC cable in a non-homogenous distribution before performing
the heat treatment and the cover is removed after the heat
treatment. This is generally not required since the layer, e.g.
rubber layer, of the field grading adapter or joint body is
typically so thick that even if diffusion to the surrounding takes
place at the outer surface of the field grading adapter or joint
body, this will not affect the concentration distribution in the
polymer based insulation system.
During the heat treatment any exposed portions of the
semiconducting layer of the polymer based insulation system in the
at least one end portion of the high voltage DC cable may be
covered by insulating or semiconducting material to prevent or
reduce diffusion from those portions during the heat treatment.
In a further embodiment, a second insulated electric high voltage
DC cable is provided, the second high voltage DC cable comprising a
high voltage DC conductor, a polymer based insulation system
surrounding the high voltage DC conductor and comprising an
insulation layer and a semiconducting layer surrounding the
insulation layer, and a grounding layer surrounding the
semiconducting layer; the grounding layer and the semiconducting
layer of the second high voltage DC cable are removed in at least
one end portion of the second high voltage DC cable, thereby
exposing the insulation layer of the polymer based insulation
system of the second high voltage DC cable in the at least one end
portion of the second high voltage DC cable; and the field grading
adapter or joint body, which here is a joint body, is mounted in
the at least one end portion of the second high voltage DC cable,
thereby covering the insulation layer of the polymer based
insulation system of the second high voltage DC cable in the at
least one end portion of the second high voltage DC cable, wherein
the at least one substance is present in the insulation layer of
the polymer based insulation system in the at least one end portion
of the second high voltage DC cable in a non-homogenous
distribution, and wherein the step of subjecting comprises
subjecting the insulation layer of the polymer based insulation
system of the second high voltage DC cable in the at least one end
portion of the second high voltage DC cable for the heat treatment
procedure, while being covered by the mounted field grading adapter
or joint body, thereby equalizing the concentration of the at least
one substance in the insulation layer of the polymer based
insulation system of the second high voltage DC cable in the at
least one end portion of the second high voltage DC cable. Also in
this embodiment, while the entire joint could be temporarily or
permanently covered as above during the heat treatment procedure,
this is not generally required.
During the heat treatment any exposed portions of the
semiconducting layer of the polymer based insulation system in the
at least one end portion of the second high voltage DC cable may
also be covered by insulating or semiconducting material to prevent
or reduce diffusion from those portions during the heat
treatment.
In particular, the concentration of the at least one substance may
be equalized adjacent to the outer surface of the insulation layer
of the polymer based insulation.
The polymer based insulation system may comprise a compounded or
crosslinked polyethylene (XLPE) composition. The at least one
substance may in such case include rest or byproducts from
crosslinking reactions, preferably a peroxide decomposition
product.
Alternatively, or additionally, the polymer based insulation system
may comprise a thermoplastics and the at least one substance may
include one or more additive, such as an antioxidant, low molecule
weight fractions of the polymer used in the polymer based
insulation system, moisture, and/or process chemicals.
The rubber of the field grading adapter or joint body may comprise
rest or byproducts from the crosslinking reaction, one or more
antioxidant, one or more plasticizers, process chemicals, and/or
moisture.
The heat treatment procedure may be performed for example by
applying thermal bands or heating bands on the parts to be heated,
by generating heat in the conductor which will increase the heat of
the insulation layer, in a heating chamber, such as an oven, or in
or by any other suitable heating device known in the art.
The heat treatment procedure may be performed at a temperature of
between 50 and 120.degree. C., and most preferably between 70 and
90.degree. C. and for a time that is dependent on the temperature
at which the heat treatment procedure is performed. Generally, the
temperature and time are selected to provide a sufficiently uniform
distribution of substances and conductivity. This will depend on
the substances, and especially the substances dominating the
conductivity characteristics. In a system with generally very low
concentrations of substances, e.g. antioxidants and moisture, there
may still be large influences on the conductivity due the presence
of them. Nevertheless, the heat treatment times are less or much
less than the times required for degassing of the substances as
disclosed in the prior art section.
The step of providing an insulated electric high voltage DC cable
comprising a high voltage DC conductor, a polymer based insulation
system surrounding the high voltage DC conductor, and a grounding
layer surrounding the polymer based insulation system may include
the following steps. The polymer based insulation system is covered
by a cover impermeable to the at least one substance present in the
polymer based insulation system in a non-homogenous distribution,
and the polymer based insulation system is subjected for a heat
treatment procedure while the outer surface of the polymer based
insulation system is kept covered by the cover, thereby equalizing
the concentration of the at least one substance in the polymer
based insulation system. The cover may be part of the high voltage
DC cable and comprises the grounding layer or may be a temporary
cover, which is removed after the heat treatment procedure of the
polymer based insulation system of the high voltage DC cable.
Shortly speaking, the high voltage DC cable used in the present
invention has been treated in accordance with the principles
disclosed in EP 2093774 A1, the contents of which being hereby
incorporated by reference.
The invention can be applied in the manufacturing and/or
installation of high voltage DC cable systems for HVDC systems.
Hereby, cable systems for higher voltage levels can be achieved at
lower total cost.
For cable systems that are on the edge of meeting the electrical
requirements, the use of the invention will reduce the risks of
breakdowns during testing and operation.
Applying the invention, the conductivity characteristics of the
insulation system will already at installation be close to the
conductivity characteristics that are obtained for a system that
has been used during long time. Hereby, the risks associated with a
new installation will be reduced and the user could expect a more
reliable operation.
Further characteristics of the invention and advantages thereof
will be evident from the following detailed description of
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic flow scheme of a method for providing an
insulated electric high voltage DC termination or joint according
to an embodiment.
FIG. 2 illustrates schematically in a perspective view an end of a
high voltage DC cable during the method illustrated in FIG. 1.
FIG. 3 is a schematic flow scheme of a method for providing an
insulated electric high voltage DC termination or joint according
to a further embodiment embodiment.
FIG. 4 illustrates schematically in a perspective view an end of a
high voltage DC cable during the method illustrated in FIG. 3.
DETAILED DESCRIPTION OF EMBODIMENTS
A method in the manufacturing of an insulated electric high voltage
DC termination or joint according to one embodiment will now be
described with reference to FIGS. 1 and 2. The latter Figure shows
an end portion of an insulated electric high voltage DC cable 20
during the method, the high voltage DC cable 20 comprising from
inside and out a high voltage DC conductor 21; a polymer based
insulation system 22-24 comprising a semiconducting layer 22, and
an insulation layer 23, and a further semiconducting layer 24; a
grounding layer 25; and an outer covering or sheath 26. The
grounding layer 25 may comprise an extruded layer of lead or a
combination of copper wires and a thin diffusion tight layer.
The polymer based insulation system 22-24 may be extruded, molded,
or manufactured in any convenient manner. The insulation layer 23
may be a crosslinked polyethylene layer, a thermoplastic layer, or
a layer of other suitable material. The high voltage DC cable 20
may be manufactured as disclosed in EP 2 093774 A1 to obtain an
equalized high voltage DC cable 20.
The method in the manufacturing of an insulated electric high
voltage DC termination or joint, which is performed in an initial
phase of the manufacturing of the insulated electric high voltage
DC termination or joint, begins, in a step 12, by providing the
high voltage DC cable. Next, the outer covering or sheath 26, the
grounding layer 25 and the semiconducting layer 24 are, in a step
13, removed in one end portion 27 of the high voltage DC cable 20,
thereby exposing the insulation layer 23 in the at least one end
portion 27 of the high voltage DC cable 20. Thereafter, the
insulation layer 23 is, in a step 14, temporarily covered in the
end portion 27 of the high voltage DC cable 20 by a cover
impermeable to at least one substance present in the insulation
layer 23 in the end portion 27 of the high voltage DC cable 20 in a
non-homogenous distribution. The insulation layer 23 is, in a step
15, in the end portion 27 of the high voltage DC cable 20,
subjected for a heat treatment procedure, while being covered by
the cover, thereby equalizing the concentration of the substance in
the insulation layer 23 of the polymer based insulation system
22-24 in the end portion 27 of the high voltage DC cable 20.
Finally, the cover is, in a step 16, removed, and the method is
ended. FIG. 2 shows the resulting end portion 27 of the high
voltage DC cable 20.
Preferably, the cover is also covering any exposed portions of the
semiconducting layer 24 of the polymer based insulation system
22-24 in the at least one end portion 27 of the high voltage DC
cable 20 to prevent diffusion from those portions during the heat
treatment.
A high voltage DC cable joint is manufactured by repeating the
above method for a second high voltage DC cable, and by mounting a
joint body to the high voltage DC cable ends.
A high voltage DC cable termination is manufactured by mounting a
field grading adapter to the end portion 27 of the high voltage DC
cable 20.
It shall be appreciated that when the cover has been removed in
step 16, the polymer based insulation system 22-24 has to be
covered as soon as possible, e.g. by mounting the field grading
adapter or joint body or by assembling a final diffusion
barrier.
The field grading adapter or joint body may be pre-equalized by
means of mounting the field grading adapter or joint body in an end
portion of a piece of cable having an exposed insulation layer with
a similar composition as the insulation layer 23 of the polymer
based insulation system 22-24 of the high voltage DC cable 20,
thereby covering the exposed insulation layer of the piece of
cable, subjecting the field grading adapter or joint body to a heat
treatment procedure such that one or more substances from the
insulation layer of the piece of cable diffuse into the field
grading layer, and removing the field grading adapter or joint body
from the piece of cable and covering the inner surface of the field
grading adapter or joint body with a cover, which is impermeable to
the one or more substances diffused into the field grading layer,
until the field grading adapter or joint body is mounted in the end
portion 27 of the high voltage DC cable 20.
With reference next to FIGS. 3 and 4, a method in the manufacturing
of an insulated electric high voltage DC termination or joint
according to another embodiment will be described. The method
begins with providing, in a step 32, an insulated electric high
voltage DC cable 20, e.g. the high voltage DC cable 20 as disclosed
with reference to FIGS. 1 and 2.
The outer covering or sheath 26 and the grounding layer 25 and the
semiconducting layer 24 are, in a step 33, removed in at least one
end portion 27 of the high voltage DC cable 20, thereby exposing
the insulation layer 23 of the polymer based insulation system
22-24 in the end portion 27 of the high voltage DC cable 20. A
field grading adapter or joint body 41 (FIG. 4) is, in a step 34,
mounted in the end portion 27 of the high voltage DC cable 20,
thereby covering the insulation layer 23 of the polymer based
insulation system 22-24 in the end portion 27 of the high voltage
DC cable 20, the field grading adapter or joint body 41 being part
of the high voltage DC termination or joint and comprising a rubber
based material. Finally, the insulation layer 23 of the polymer
based insulation system 22-24 is, in a step 35, in the end portion
of the high voltage DC cable, subjected for a heat treatment
procedure, while being covered by the mounted field grading adapter
or joint body 41, thereby equalizing the concentration of the
substance in the insulation layer 23 of the polymer based
insulation system 22-24 in the end portion 27 of the high voltage
DC cable 20. FIG. 4 shows the resulting end portion 27 of the high
voltage DC cable 20 after equalization with the field grading
adapter or joint body 41 mounted.
Alternatively, the field grading adapter or joint body 41 is
temporarily covered by a cover impermeable to the substance present
in the insulation layer 23 of the polymer based insulation system
22-24 in the end portion 27 of the high voltage DC cable 20 in a
non-homogenous distribution before performing the heat treatment
procedure, wherein the cover is removed after the heat treatment
procedure.
During the heat treatment any exposed portions of the
semiconducting layer 24 of the polymer based insulation system
22-24 in the at least one end portion 27 of the high voltage DC
cable 20 may be covered by insulating or semiconducting material to
prevent or reduce diffusion from those portions during the heat
treatment.
In one embodiment, a second insulated electric high voltage DC
cable is provided, wherein the second high voltage DC cable may be
identical to the first high voltage DC cable 20. The outer covering
or sheath, the grounding layer, and the semiconducting layer of the
second high voltage DC cable is removed in at least one end portion
of the second high voltage DC cable, thereby exposing the
insulation layer of the second high voltage DC cable in the end
portion of the second high voltage DC cable. The joint body 41 is
mounted also to the end portion of the second high voltage DC
cable, thereby covering the insulation layer of the second high
voltage DC cable in the end portion of the second high voltage DC
cable. The above disclosed heat treatment procedure comprises
subjecting also the insulation layer of the second high voltage DC
cable in the end portion of the second high voltage DC cable for
the heat treatment, while being covered by the mounted joint body
41, thereby also equalizing the concentration of the substance in
the insulation layer of the second high voltage DC cable in the end
portion of the second high voltage DC cable.
If the polymer based insulation system 22-24 comprises a compounded
or crosslinked polyethylene (XLPE) composition, the substance may
be a rest or byproduct from crosslinking reactions, preferably a
peroxide decomposition product.
If the polymer based insulation system 22-24 comprises a
thermoplastics, the substance may be an additive, such as an
antioxidant, low molecule weight fractions of the polymer used in
the polymer based insulation system, moisture, or a process
chemical.
If the field grading adapter or joint body 41 comprises a rubber
materiel, it may contain rest or byproducts from the crosslinking
reaction, one or more antioxidant, one or more plasticizers,
process chemicals, and/or moisture, which may diffuse into the
polymer based insulation system 22-24 after the mounting of the
field grading adapter or joint body 41.
The heat treatment procedure may be performed at a temperature of
between 50 and 120.degree. C., and most preferably between 70 and
90.degree. C. and for a time that is dependent on the temperature
at which the heat treatment procedure is performed and on the
substance concentrations, materials, and geometry of the end
portion 27 of the high voltage DC cable 27. Generally, the
temperature and time are selected to provide a sufficiently uniform
distribution of substances and conductivity for each cable and
application.
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