U.S. patent application number 09/966270 was filed with the patent office on 2002-04-04 for electric dc-cable with an insulation system.
Invention is credited to Bostrom, Jan-Ove, Campus, Alfred, Carstensen, Peter, Ericsson, Anders, Farkas, Andreas, Gustafsson, Anders, Gustafsson, Bill, Nilsson, Ulf.
Application Number | 20020039654 09/966270 |
Document ID | / |
Family ID | 26663167 |
Filed Date | 2002-04-04 |
United States Patent
Application |
20020039654 |
Kind Code |
A1 |
Gustafsson, Bill ; et
al. |
April 4, 2002 |
Electric DC-cable with an insulation system
Abstract
An insulated DC-cable and method for production of an insulated
DC-cable with an insulating system comprising an extruded
cross-linked polyethylene based insulation disposed around the
conductor. The extruded polyethylene based compound comprises
additives such as cross-linking agent, scorch retarding agent and
antioxidant. The scorch retarding agent comprises a compound (D),
2,4-diphenyl-4-methyl-pentene-1 and the antioxidant comprises a
compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionicacid and
thiodiglycol. The compounded polyethylene based resin composition
is extruded and cross-linked at a temperature and for a period of
time sufficient enough to cross link the insulation. The
temperature and the period of time upon extrusion and cross-linking
are limited so as to substantially suppress or essentially avoid
undesired polar by-products being formed in the cross-linked
composition.
Inventors: |
Gustafsson, Bill;
(Stenungsund, SE) ; Nilsson, Ulf; (Stora Hoga,
SE) ; Campus, Alfred; (Eysins, CH) ;
Carstensen, Peter; (Huddinge, SE) ; Gustafsson,
Anders; (Alingsas, SE) ; Farkas, Andreas;
(Stenungsund, SE) ; Ericsson, Anders; (Karlskrona,
SE) ; Bostrom, Jan-Ove; (Odsmal, SE) |
Correspondence
Address: |
DYKEMA GOSSETT PLLC
FRANKLIN SQUARE, THIRD FLOOR WEST
1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Family ID: |
26663167 |
Appl. No.: |
09/966270 |
Filed: |
October 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09966270 |
Oct 1, 2001 |
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09582092 |
Sep 29, 2000 |
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09582092 |
Sep 29, 2000 |
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PCT/SE98/02432 |
Dec 22, 1998 |
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Current U.S.
Class: |
428/375 ;
264/104 |
Current CPC
Class: |
C08K 5/375 20130101;
Y10T 428/2933 20150115; C08K 5/01 20130101; H01B 3/441 20130101;
C08L 23/04 20130101; C08L 23/04 20130101; C08L 23/04 20130101; C08K
5/375 20130101; C08K 5/1345 20130101; C08K 5/1345 20130101; C08K
5/01 20130101 |
Class at
Publication: |
428/375 ;
264/104 |
International
Class: |
D02G 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 1997 |
SE |
9704825-0 |
Claims
1. An insulated direct current electric cable, a DC-cable,
comprising a conductor and an extruded, cross-linked conductor
insulation disposed around the conductor, wherein the extruded
insulation comprises a polyethylene based compound with additives
including a cross-linking agent, a scorch retarding agent comprises
a compound (D), 2,4-diphenyl-4-methyl-pentene-1, and that the
antioxidant comprises a compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol.
2. A DC-cable according to claim 1, characterized in that the
extruded insulation has a thickness of less than 10 mm and that the
antioxidant system is a combined two component antioxidant system
consisting of compound (B), a di-alkyl-thio-dipropionate, and
compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol.
3. A DC-cable according to claim 2, characterized in that the
combined antioxidant system is added at a level of from 0.1 to 0.8%
by weight.
4. A DC-cable according to claim 3, characterized in that the
combined antioxidant system is added at a level, preferably of from
0.2 to 0.5% by weight.
5. A DC-cable according to claim 1, characterized in that the
antioxidant system consists of compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydr- oxyphenyl) propionicacid and
thiodiglycol, only
6. A DC-cable according to claim 5, characterized in that compound
(C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol, is added at a level of from 0.1 to
0.5% weight.
7. A DC-cable according to claim 6, characterized in that compound
(C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol, is added at a level of from 0.2 to
0.4% by weight.
8. A DC-cable according to any of the preceding claims,
characterized in that the scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pent- ene-1, is added at a level of from 0.1
to 1.0% by weight.
9. A DC-cable according to claim 8, characterized in that the
scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pentene-1, is added at a level of from 0.2 to
0.5% by weight.
10. A DC-cable according to any of the preceding claims,
characterized in that the peroxide cross-linking agent is added at
a level of from 1.0 to 2.4% by weight.
11. A DC-cable according to claim 11, characterized in that the
peroxide cross-linking agent is added at a level of from 1.2 to
1.8% by weight.
12. A DC-cable according to any of the preceding claims,
characterized in that the peroxide cross-linking agent is dicumyl
peroxide.
13. A DC-cable according to any of the preceding claims,
characterized in that the polyethylene resin is a low density
polyethylene with a melt flow rate, MFR, within a range from 0.5 to
10 g/10 min.
14. A method for production of an insulated DC-cable comprising a
conductor and an extruded cross-linked polyethylene based conductor
insulation according to any of the preceding claims, comprising the
following steps; providing a conductor; compounding a polyethylene
based resin composition, wherein a peroxide cross-linking agent, a
scorch retarding agent and an antioxidant system is added;
extruding the compounded polyethylene based resin composition to
form a conductor insulation disposed around the conductor in the
DC-cable; cross-linking the extruded insulation, characterized in
that a scorch retarding agent comprising compound (D),
2,4-diphenyl-4-methyl-pentene-1, and an antioxidant system
comprising compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol is added to the polyethylene resin upon compounding;
and that process variables such as the composition of polyethylene
based resin, the extrusion and cross-linking temperatures and the
time periods for extrusion and cross-linking are controlled such
that any formation of water in the extruded, cross-linked
insulation is suppressed.
15. The method according to claim 14, characterized in that the
additions of the peroxide cross-linking agent and the scorch
retarding agent (D), 2,4-diphenyl-4-methyl-pentene-1, is balanced
and that the composition is extruded and cross-linked at a
temperature at which the cross-linking is completed to a desired
level, that the amount of excessive peroxide cross-linking agent
present in the cross-linked insulation is minimized.
16. The method according to claim 14 or 15, characterized in that
upon compounding a two component antioxidant system comprising
compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol, and compound (B), a
di-alkyl-thio-dipropionate, is added to the polyethylene resin.
17. The method according to claim 16, characterized in that the
combined antioxidant is added at a level of from 0.1 to 0.8% by
weight.
18. The method according to claim 16 or 17, characterized in that
the compounded resin composition is extruded and cross-linked at a
product temperature below 230.degree. C.
19. The method according to claim 18, characterized in that the
compounded resin composition is extruded and cross-linked at a
product temperature within a range of from 200.degree. C. to
225.degree. C.
20. The method according to claim 14 or 15, characterized in that
upon compounding an antioxidant consisting of only of compound (C),
a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid
and thiodiglycol, is added to the polyethylene resin.
21. The method according to claim 20, characterized in that
compound (C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol, is added at a level of from 0.1 to
0.5% by weight.
22. The method according to claim 20 or 21, characterized in that
the compounded resin composition is extruded and cross-linked at a
product temperature above 230.degree. C.
23. The method according to claim 22, characterized in that the
compounded resin composition is extruded and cross-linked at a
product temperature with a range of from 240.degree. C. to
350.degree. C.
24. The method according to any of the preceding claims,
characterized in that the scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pent- ene-1, is added at a level of from 0.1
to 1.0% by weight.
25 The method according to any of the preceding claims,
characterized in that the peroxide cross-linking agent is dicumyl
oxide and added at a level of from 1.0 to 2.4% by weight.
26. The method according to any of the preceding claims,
characterized in that the polyethylene resin is a low density
polyethylene with a melt flow rate, MFR, within a range from 0.5 to
10 g/10 min.
Description
TECHNICAL FIELD
[0001] The present invention relates to an insulated electric
direct current cable, a DC-cable, with a current- or
voltage-carrying body, i.e. a conductor and an insulation system
disposed around thc conductor, wherein the insulation system
comprises an extruded and cross-linked polyethylene
composition.
[0002] The present invention relates in particular to an insulated
electric DC cable for transmission and distribution of electric
power. The extruded insulation system comprises a plurality of
layers, such as an inner semi-conductive shield, an insulation and
an outer semi-conductive shield. At least the extruded insulation
comprises a cross-linked polyethylene based electrically insulating
composition with a system of additives such as cross-linking agent,
scorch retarding agent and anti-oxidant.
BACKGROUND ART
[0003] Although many of the first electrical supply systems for
transmission and distribution of electrical power were based on DC
technology, these DC systems were rapidly superseded by systems
using alternating current, AC. The AC systems had the desirable
feature of easy transformation between generation, transmission and
distribution voltages. The development of modern electrical supply
systems in the first half of this century was exclusively based on
AC transmission systems. However, by the 1950s there was a growing
demand for long transmission schemes and it became clear that in
certain circumstances there could be benefits by adopting a DC
based system. The foreseen advantages include a reduction of
problems typically encountered in association with the stability of
the AC-systems, a more effective use of equipment as the power
factor of the system is always unity and an ability to use a given
insulation thickness or clearance at a higher operating voltage.
Against these very significant advantages has to be weighed the
high cost of the terminal equipment for conversion of the AC to DC
and for inversion of the DC back again to AC. However, for a given
transmission power, the terminal costs are constant and therefore,
DC transmission systems were rendered economical for the schemes
involving long distances. Thus DC technology becomes economical for
systems intended for transmission over long distances as for when
the transmission distance typically exceed the length for which the
savings in the transmission equipment exceeds the cost of the
terminal plant.
[0004] An important benefit of DC operation is the virtual
elimination of dielectric losses, thereby offering a considerable
gain in efficiency and savings in equipment. The DC leakage current
is of such small magnitude that it can be ignored in current rating
calculations, whereas in AC cables dielectric losses cause a
significant reduction in current rating. This is of considerable
importance for higher system voltage. Similarly, high capacitance
is not a penalty in DC cables. A typical DC-transmission cable
include a conductor and an insulation system comprises a plurality
of layers, such as an inner semi-conductive shield, an insulation
base body and an outer semi-conductive shield. The cable is also
complimented with casing, reinforcement etc to withstand water
penetration and any mechanical wear or forces during, production
installation and use.
[0005] Almost all the DC cable systems supplied so far have been
for submarine crossings or the land cable associated with them. For
long crossings the mass-impregnated solid paper insulated type
cable is chosen because there are no restrictions on length due to
pressurizing requirements. It has been supplied for operating
voltages of 450 kV. To date an essentially all paper insulation
body impregnated with a electric insulation oil has been used but
application of laminated material such as a polypropylene paper
laminate is being persuaded for use at voltages up to 500 kV to
gain advantage of the increased impulse strength and reduced
diameter.
[0006] As in the case of AC transmission cables, transient voltages
is a factor that has to be taken into account when determining the
insulation thickness of DC cables. It has been found that the most
onerous condition occurs when a transient voltage of opposite
polarity to the operating voltage is imposed on the system when the
cable is carrying full load. If the cable is connected to an
overhead line system, such a condition usually occurs as a result
of lightning transients.
[0007] Extruded solid insulation based on a polyethylene, PE, or a
cross linked polyethylene, XLPE, has for almost 40 years been used
for AC transmission and distribution cable insulation. Therefore
the possibility of the use XLPE and PE for DC cable insulation has
been under investigation for many years Cables with such
insulations have the same advantage as the mass impregnated cable
in that for DC transmission there are no restrictions non circuit
length and they also have a potential for being operated at a
higher temperatures. In the case of XLPE, 90.degree. C. instead of
50.degree. C. for conventional DC-cables. Thus offering a
possibility to increase the transmission load. However, it has not
been possible to obtain the full potential of these materials for
full size cables. It is believed that one of the main reasons being
the development of space charge in the dielectric when subjected to
a DC-field. Such space charges distort the stress distribution and
persist for long periods because of the high resistivity of the
polymers. Space charges in an insulation body do when subjected to
the forces of an electric DC-field accumulate in a way that a
polarized pattern similar to a capacitor is formed. There are two
basic types of space charge accumulation patterns, differing in the
polarity of the space charge accumulation in relation to the
polarity. The space charge accumulation results in a focal increase
at certain points of the actual electric field in relation to the
field, which would be contemplated when considering the geometrical
dimensions and dielectric characteristics of an insulation. The
increase noted in the actual field might be 5 or even 10 times the
contemplated field. Thus the design for a cable insulation must
include a safety factor taking account for this considerably higher
field resulting in the use of thicker and/or more expensive
materials in the cable insulation. The build up of the space charge
accumulation is a slow process, therefore this problem is
accentuated when the polarity of the cable after being operated for
a long period of time at same polarity is reversed. As a result of
the reversal a capacity field is superimposed on the field
resulting from the space charge accumulation and the point of
maximal field stress is moved from the interface and into the
insulation. Attempts have been made to improve the situation by the
use of additives to reduce the insulation resistance without
seriously affecting the other properties. To date it has not been
possible to match the electrical performance achieved with the
impregnated paper insulated cables and no commercial polymeric
insulated DC cables have been installed. However, successful
laboratory tests have been reported on a 250 kV cable with a
maximum stress of 20 kV/mm using XLPE insulation with mineral
filler (Y.Maekawa et al, Research and Development of DC XLPE
Cables, JiCable`91, pp. 562-569) This stress value compares with 32
kV/mm used as a typical value for mass-impregnated paper
cables.
[0008] An extruded resin composition for AC cable insulation
typically comprises a polyethylene resin as the base polymer
complemented with various additives such as a peroxide
cross-linking agent, a scorch retarding agent and an anti-oxidant
or a system of antioxidants. In the case of an extruded insulation
the semi-conductive shields are also typically extruded and
comprise a resin composition that in addition to the base polymer
and an electrically conductive or semi-conductive filler comprises
essentially the same type of additives. The various extruded layers
in an insulated cable in general are often based on a polyethylene
resin. Polyethylene resin means generally and in this application a
resin based on polyethylene or a copolymer of ethylene, wherein the
ethylene monomer constitutes a major part of the mass. thus
polyethylene resin may be composed of ethylene and one or more
monomers which are co-polymerisable with ethylene. LDPE, low
density polyethylene, is today the predominant insulating base
material for AC-cables. To improve the physical properties of the
extruded insulation and its capability to withstand degradation and
decomposition under the influence of the conditions prevailing
under production, shipment, laying, and use of such a cable the
polyethylene based composition typically comprises additives such
as;
[0009] stabilizing additives, e.g., antioxidants, electron
scavengers to counteract decomposition due to oxidation; radiation
etc;
[0010] lubricating additives, e.g. stearic acid, to increase
processability;
[0011] additives for increased capability to withstand electrical
stress, e.g. an increased water free resistance, e.g. polyethylene
glycol, silicones etc; and
[0012] cross-linking agents such as peroxides, which decompose upon
heating into free radicals and initiate cross-linking of the
polyethylene resin, sometimes used in combination with unsaturated
compounds having the ability to enhance the cross-linking
density.
[0013] The number of various additives is large and the possible
combinations thereof is essentially unlimited. When selecting an
additive or a combination or group of additives the aim is that one
or more properties shall be improved while others shall be
maintained or if possible also improved. However, in reality it is
always next to impossible to forecast all possible side effects of
a change in the system of additives. In other cases the
improvements sought for are of such dignity that some minor
negative have to be accepted, although there is always an aim to
minimize such negative effects.
[0014] A typical polyethylene based resin composition to be used as
an extruded, cross-linked insulation in an AC-cable comprises:
[0015] 97,1-98,9% by weight of low density polyethylene (922
kg/m.sup.3) of melt flow rate 0,4-2,5 g/10 min.
[0016] 0,1-0,5% by weight of an antioxidant SANTONOX R.RTM.
(Flexsys Co) with the chemical designation
4,4'-thio-bis(6-tert-butyl-m-cresol,) here referred to as compound
(A) 1,0-2,4% by weight of a cross linking agent, DICUP R.RTM.
(Hercules Chem) with the chemical designation dicumyl peroxide.
Although some disadvantages with the use of Santonox R.RTM. as an
antioxidant have been known for a long time its advantages (e.g.
its ability to prevent scorch i.e. premature cross linking) have
outweighed these drawbacks. Furthermore it is well known that this
cross linked composition exhibits a strong tendency to form space
charges under DC electric fields, thus making it unusable in
insulation systems for DC cables. However, it is also known that
extended degassing, i.e. exposing the cross linked cable at high
temperatures to a high vacuum for long periods of time, will result
in a somewhat decreased tendency to space charge accumulation under
DC voltage stress. It is generally believed that the vacuum
treatment removes the peroxide decomposition products, such as
"acetophenone" and "cumyl alcohol", from the insulation whereby the
space charge accumulation is reduced. Degassing is 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. Most known cross-linked polyethylene
compositions used as extruded insulation in AC-cable exhibit a
tendency for space charge accumulation which renders them
unsuitable for use in insulation systems for DC-cables.
OBJECT OF THE INVENTION
[0017] It is an object of the present invention to provide an
insulated DC-cable with an electrical insulation system suitable
for use as a transmission and distribution cable in networks and
installations for DC transmission and distribution of electric
power. The cable shall comprise a solid extruded conductor
insulation that can be applied and processed without the need for
any lengthy time consuming batch-treatment such as impregnation or
degassing, i.e. vacuum treatment of the cable. Thereby reducing the
production time and thus the production costs for the cable and
thereby offering the possibility for an essentially continuous or
at least semi-continuous production of the cable insulation system.
Further, the reliability, low maintenance requirements and long
working life of conventional DC-cables comprising a mass
impregnated paper-based insulation shall be maintained or improved.
That is, the cable according to the present invention shall have
stable and consistent dielectric properties and a high and
consistent electric strength. The cable insulation shall exhibit a
low tendency to space charge accumulation, a high DC breakdown
strength, a high impulse strength and high insulation resistance.
The replacement of the impregnated paper or cellulose based tapes
with an extruded polymeric insulation shall as an extra advantage
open for an increase in the electrical strength and thus allow an
increase in operation voltages, improve handleability and
robustness of the cable.
[0018] It is also an object to provide a cable comprising an
extruded, cross linked insulation based on polyethylene which has
low or no space charge accumulation in the insulation during DC
electric stresses, thereby eliminating or at least substantially
reducing any problem associated with space charge accumulation. It
shall also provide a capacity to reduce safety factors in design
values used for dimensioning the cable insulation
[0019] It is further the object to provide a method for
manufacturing the insulation of such an insulated DC cable
according to the present invention. The process according to this
aspect of the present invention for application and processing of
the conductor insulation shall be essentially free from operating
steps requiring a lengthy batch treatment of complete cable lengths
or long lengths of cable core. The process shall also exhibit a
potential for being used in a continuous or semi-continuous way for
production of long lengths of DC-cable.
SUMMARY OF THE INVENTION
[0020] A DC electric power cable comprising a conductor and an
extruded, cross linked sold conductor insulation disposed around
the conductor, wherein the extruded insulation comprises a
polyethylene based compound to which additives including a cross
linking agent, a scorch retarding agent and an antioxidant system
have been made during compounding. According to the present
invention in it s general concept the scorch retarding agent
comprises compound (D), 2,4-diphenyl-4-methyl-pentene-1, and the
antioxidant system comprises compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol.
[0021] The compounded polyethylene based insulation is typically
extruded and heated to an elevated temperature and for a period of
time long enough to cross link the insulation. The temperature and
the period of time is controlled so as to substantially suppress or
essentially avoid any undesired by-products being formed in the
cross-linked composition.
[0022] The cable insulation can be applied with an essentially
continuous process without the need for lengthy batch treatments as
e.g. vacuum treatment. The low tendency for space charge
accumulation and increased DC breakdown strength of conventional
DC-cables comprising an impregnated paper insulation is maintained
or improved. The insulating properties of the DC-cable according to
the present invention exhibit a general long term stability such
that the working life of the cable is maintained or increased. This
achieved by the balance addition of the scorch retarding agent, the
peroxide cross-linking agent and the antioxidant system in
combination with the controlled process temperatures and processing
times according to the invention as exemplified in the coming.
DETAILED SPECIFICATION OF THE INVENTION
[0023] One embodiment of the present invention, especially suited
for DC-cables comprises a combined two component antioxidant
system, where a primary phenolic antioxidant compound (C), a
diester of 3,-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol is complemented with a so called secondary antioxidant
(or thio-synergist) compound (B), a di-alkyl-thio-dipropionate,
such as, DSTDP, di-stearyl-thio-dipropianate or alternately, DLTDP,
di-lauroyl-thio-dipropionate. Preferably the alkyl is a linear or
branched saturated alkyl group with 8 to 20 carbon atoms.
[0024] A DC-cable according to this embodiment comprises the
combined two component antioxidant consisting of compound (C) and
compound (B) added to the polyethylene at a level of from 0.1 to
0.8% by weight, preferably of from 0.2 to 0.5% by weight. Compound
(C), a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol is well known and commercially
available under the trade name IRGANOX 1035.RTM. from Ciba-Geigy
and known for use as a component in antioxidant systems for
extruded, cross linked insulation's in AC cables, where it
typically is added in amount of about 0.2 weight. To increase the
antioxidant efficiency a hydroperoxide decomposer compound (B) is
added. Compound (B) is, in the form of, DSTDP,
di-stearyl-thio-dipropianate commercially available under the trade
name IRGANOX PS802, from Ciba-Geigy plc or HOSTANOX SE2 from
Hoechst AG alternatively is DLTDP, di-lauroyl-thio-dipropianate
commercially available under the trade name IRGANOX 800, from
Ciba-Geigy plc and HOSTANOX SEI, Hoechst AG. However, compound (C)
or the combination of compound (C) and (B) does not have the scorch
retarding properties of compound (A). Therefore compound (D) is
added as a scorch retarding agent, whereby any undesired premature
cross linking during the extrusion process is essentially avoided.
The scorch retarding agent (D), 2,4-diphenyl-4-methyl-pentene-1, is
commercially available from Nippon Oil and Fats under the tradename
Nofmer MSD.RTM.. The use of this modified combined system that
incorporates the scorch retarding agent (D) as a complement to the
antioxidant system comprising compound (C) has proven successful to
suppress any tendency for scorch and as an extra advantage it has
increased the final cross linking ratio in the polyethylene
composition.
[0025] Thus a DC-cable according to this embodiment comprises a
conductor and an extruded, cross-linked polyethylene based
insulation disposed around the conductor where the polyethylene
compound comprises a peroxide cross-linking agent, a scorch
retarding agent consisting of compound (D) and a combined
antioxidant system consisting of compounds (B) and (C). The DC
cable according to this embodiment must since compound (b) under
certain conditions catalyzes water-formation from cumyl alcohol be
processed under conditions where this water formation is
suppressed. Therefore the cable insulation in a DC-cable according
tho this embodiment is processed at temperatures sufficiently low
to suppress this formation of water. The product temperature during
extrusion and cross-linking is kept below 230.degree. C. this shall
be compared with a typical product temperature of about 300.degree.
C. for processing of conventional XLPB systems. Preferably the
product temperature is kept within a range of from about
200.degree. C. The term "product temperature" means in this
application the maximum temperature at any point within the
product, i.e. the extruded insulation. The processing time at this
temperature is kept below 10 minutes, preferably within a range
from 2 minutes to 5 minutes. These restrictions in product
temperature and times at this temperature restrict the thickness of
the insulation which can be extruded and cross-linked to a
thickness of approximately 10 mm or less. the DC-cable according to
the embodiment also exhibits a minimized content of excess peroxide
cross-linking agent in the cross-linked polyethylene insulation.
This is obtained by the limited and controlled addition of the
cross-linking agent in combination with the promotion of grafted
bridges between polymer chains provided by scorch retarding agent,
compound (D). The insulation system is typically a three layered
system comprising a first inner semi-conductive shield, an
insulation, with a thickness deemed to be suitable in view of the
electrical or mechanical forces acting on the cable and the thermal
situation upon use, and a second outer semi-conductive shield. This
insulation system is typically applied by a true triple extrusion
process, but can of course be applied by other suitable extrusion
processes.
[0026] The scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pen- tene-1, is according to the present
invention added at a level of from 0.1 to 1.0% by weight.
Preferably is it at a level of from 0.2 to 0.5% by weight.
[0027] The peroxide cross-linking agent is added at a level of from
1.0 to 2.4% by weight, preferably at a level of from 1.2 to 1.8% by
weight. Preferably the peroxide cross-linking agent is dicumyl
peroxide.
[0028] the polyethylene resin is a low density polyethylene with a
melt flow rate MFR, within a range from 0.5 to 2. g/10 min. MFR is
determined at 190.degree. C./2.16 kg according to ISO 1133 cond.
4.
[0029] According to an alternative embodiment of the invention the
DC cable comprises an extruded, cross-linked polyethylene based
insulation disposed around the conductor where the polyethylene
compound comprises a peroxide cross-linking agent, a scorch
retarding agent consisting of compound (D),
2,4-diphenyl-4-methyl-pentene-1, and as antioxidant compound (C), a
diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol, only. By this strictly specified combination of
additives consisting of a specified antioxidant component, a
likewise specified scorch retarding agent and a peroxide
cross-linking agent a low content of polar by-products in the
cross-linked composition is assured as any tendency to formation of
such compounds upon processing at elevated temperatures is
suppressed. In addition to the substantial freedom from such
by-product formation in the resin composition the combination of
additives according to this embodiment of the present invention as
in the general concept of the present invention offers the
advantage that the added amount of the peroxide cross-linking agent
can be reduced and controlled within narrow margins and still
ensure the desired ratio of cross-linking. The polyethylene
compound used in this embodiment of the DC-cable can be
cross-linked without any upper limits in temperature and time. A
DC-cable according to this embodiment comprises an antioxidant
consisting of compound (C) only. Compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol, is added to the polyethylene resin at a level of from
0.1 to 0.5% by weight, preferably of from 0.2 to 0.4% by
weight.
[0030] The DC cable according to this embodiment is provided with
an insulation which is applied using conventionally used processing
conditions. thus the compounded resin composition is extruded and
cross-linked at a product temperature above 230.degree. C., but
below 400.degree. C. where thermal degradation occurs. there is not
any restriction as to the time at the elevated extrusion and
cross-linking temperatures and therefore DC-cables with a larger
thickness of insulation can be produced. A cable according to this
embodiment is typically extruded and cross-linked at a temperature
of 230.degree. C. or above for up to 120 minutes or more.
Preferably the cable insulation is processed at a temperature
within a range of from 240.degree. C. to 350.degree. C. and
typically at 270.degree. C. for a time period within a range of
from 20 minutes to 120 minutes. DC-cables according to this
embodiment may exhibit an insulation system of any thickness. Apart
from the increased product temperature and processing time the
extrusion process for application of the insulation according to
this embodiment of the invention is essentially the same as
described in the foregoing. the scorch retarding agent, compound
(D), is according to this embodiment of the present invention also
added at a level of from 0.1 to 1.0% by weight. Preferably is it at
a level of from 0.2 to 0.5% by weight. The peroxide cross-linking
agent is likewise added at a level of from 1.0 to 2.4% by weight,
preferably at a level of from 1.2 to 1.8% by weight. Preferably the
peroxide cross-linking agent is dicumyl peroxide. The polyethylene
resin is likewise typically a low density polyethylene with a melt
flow rate MFR, within a range from 0.5 to 10 g/10 min. MFR is
determined at 190.degree. C./2.16 k.g, according to ISO 1133 cond.
4.
[0031] A DC-cable according to the present invention typically
comprises from the center and outwards;
[0032] a conductor of any desired shape and constitution, such as a
stranded multi-wire conductor, a solid conductor or a segmental
conductor;
[0033] a first extruded semi-conducting shield disposed around and
outside the conductor and inside the conductor insulation;
[0034] a metallic screen; and
[0035] a sheath arranged outside the metallic screen. the cable can
when deemed appropriate be complemented with further features such
as reinforcing wires outside the outer extruded shield, sealing
compound or a water swelling powder for filling any interstices in
and around the conductor, other metal/polymer interfaces may be
sealed in order to prevent water from spreading along such
interfaces. The three layers of the insulation system is typically
applied around the conductor using a true triple extrusion
process.
[0036] The present invention also provides a method for the
production of a DC-cable as described in the foregoing. In its most
general form the process for production of an insulated DC-cable
comprising a conductor and an extruded cross-linked polyethylene
based conductor insulation includes the following steps;
[0037] laying or otherwise forming a conductor of any desired shape
and constitution;
[0038] compounding a polyethylene based resin composition
comprising additions of a peroxide cross-linking agent, a scorch
retarding agent and an antioxidant system;
[0039] extruding the compounded polyethylene based resin
composition to form a conductor insulation disposed around the
conductor in the DC-cable preferably the three layered insulation
system comprising the insulation layer complemented with the two
semi-conducting shields is applied using a true triple extrusion
process;
[0040] cross-linking the extruded insulation
[0041] wherein according to the present invention a scorch
retarding agent comprising compound (D),
2,4-diphenyl-4-methyl-pentene-1, is added to the polyethylene resin
upon compounding;
[0042] an antioxidant comprising compound (C), a diester of
3,-(3,5-di-tert-butyl-4-hydroxy-phenyl) propionicacid and
thiodiglycol, is added to the polyethylene resin upon compounding;
and
[0043] that the compounded polyethylene based resin composition is
extruded and cross-linked at an elevated temperature and for a
period of time long enough to cross link the insulation. The
temperature is controlled and the period of time is limited so as
to substantially suppress or essentially avoid undesired polar
by-products being formed in the cross-linked composition.
Preferable the additions upon compounding are balanced such that
the amount of excessive peroxide cross-linking agent present in the
cross-linked insulation can be minimized.
[0044] According to one embodiment a scorch retarding agent
comprising compound (D), 2,4-diphenyl-4-methyl-pentene-1 and a
combined two component antioxidant system comprising compounds (b),
a di-alkyl-thio-dipropionate, and (C), a diester of
3-(3,5-di-tert-butyl-4-- hydroxyphenyl) propionic acid and
thiodiglycol, is added to the polyethylene resin upon compounding.
The compounded polyethylene based resin composition is thereafter
extruded and cross-linked at a suitable product temperature and for
a suitable period of time, typically below of 230.degree. C. or
above for 10 minutes or less. Preferably the cable insulation is
processed at a temperature within a range of from 200.degree. C.
for a temperature within a range of from 2 minutes to 5 minutes. As
compound (b) catalyzes water-formation from cumyl alcohol the cable
insulation is processed under the conditions given in the foregoing
sentence whereby this water formation is suppressed. A DC-cable
according to this embodiment is most suited in installations where
a reduced insulation thickness is sought for and suitable, that is
a insulation thickness of approximately 10 mm or less. This
DC-cable also comprise a minimized content of excess peroxide
cross-linking agent obtained by a limited and controlled addition
of the cross-linking agent in combination with the promotion of
grafted bridges between polymer chains provided by scorch retarding
agent, compound (D), 2,4-diphenyl-4-methyl-pentene-1. According to
this embodiment a combined antioxidant consisting of compound (C),
a diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionic acid
and thiodiglycol, and compound (B), a di-alkyl-thio-dipropionate,
is added to the polyethylene at a level of from 0.1 to 0.8% by
weight, preferably of from 0.2 to 0.5% by weight.
[0045] According to one alternative embodiment a scorch retarding
agent comprising compound (D) and an antioxidant consisting of
compound (C) only, is added to the polyethylene resin upon
compounding. The compounded polyethylene based resin composition is
thereafter extruded and cross-linked at a suitable temperature and
for a suitable period of time, typically above of 230.degree. C. or
above for up to 120 minutes or more. Preferably the cable
insulation is processed at a product temperature within a range of
from 240.degree. C. to 350.degree. C. for a temperature within a
range of from 20 minutes to 120 minutes. Formation of undesired
by-products in the extruded, cross-linked insulation is essentially
eliminated or at least substantially suppressed by the strictly
controlled selection of additives. the balanced addition of the
peroxide cross-linking agent and the scorch retarding agent results
in a substantial reduction of excess peroxide in the polyethylene
composition after cross-linking. Thus any tendency for space charge
accumulation upon use of the DC-cable is substantially reduced and
the DC-breakdown strength is substantially increased. DC-cables
according to this embodiment is typically produced with any
thickness of the insulation that is deemed to be required by the
electrical or mechanical forces acting on the cable or the thermal
situation upon use. According to this alternative embodiment an
antioxidant consisting of compound (C) only, a diester of
3-(3,5_di-tert-butyl-4-hydroxyphenyl) propionic acid and
thiodiglycol, is added to the polyethylene resin at a level of from
0.1 to 0.5% by weight, preferably of from 0.2 to 0.4% by
weight.
[0046] A DC cable according to the present invention with an
insulation that comprises an extruded, cross linked polyethylene
compound with the specific combination of additives and processing
mentioned above exhibiting considerable advantages such as;
[0047] A low tendency for space charge accumulation,
[0048] An increased DC breakdown strength.
[0049] Further advantages with DC cable according to the present
invention are among others;
[0050] An essential elimination of the scorch problem, thus
ensuring a homogeneous cable insulation,
[0051] A high and controlled degree of cross linking which provides
the desired high temperature mechanical properties to the cable
insulation; and
[0052] Improved heat aging properties, which ensures the desired
long working life of the cable also when used at high loads.
[0053] The specific combinations of chosen systems of additives and
set-ups of process parameters, including a suitable compounding and
processing during extrusion and cross-linking acts to suppress any
formation of space charge accumulating by-products upon
cross-linking. The scorch retarding agent (D),
2,4-diphenyl-4-methyl-pentene-1, that promotes the formation of
grafted bridges between the polymer chains in the polyethylene,
offers the possibility to ensure that a desired ratio of
cross-linking is safely achieved even with a reduced and strictly
controlled addition of peroxide cross-linking agent. thus an
essential elimination or a substantial reduction of excess peroxide
remnants in the insulation of the DC-cable can be achieved. Apart
from the technical advantages obtained as a result of this, as
discussed earlier, this is also advantageous considering the cost
of the peroxide cross-linking agent. Consequently a DC-cable
according to the present invention with an insulation comprising an
extruded, cross-linked polyethylene compound with this specific
combination of additives and process in the described way exhibit
considerable advantages such as a low tendency for space charge
accumulation, improved and stable electrical properties. A DC-cable
according to the present invention comprising an extruded,
cross-linked conductor insulation as defined in the foregoing
ensures long term stable and consistent dielectric properties and a
high and consistent electric strength as good or better than for a
conventional wound and impregnated cable. This is especially
important due to the long life such installations are designed for,
and the limited access for maintenance of such installations in
remote locations or even sub-sea. Further an insulated DC-cable as
described in the foregoing offers the possibility of an essentially
continuous or semi-continuous process for the application of the
insulation without any need for batch treatments, such as degassing
or impregnation to ensure the performance and stability of the
cable installation. Thereby the possibilities are offered for a
reduced production time by the adoption of an essentially
continuous process free from operating steps requiring batch
treatment of complete cable lengths or part lengths and lead to
cost advantages compared with maintained or even improved
properties in relation to conventional cables.
[0054] Further the essential elimination of the tendency for
accumulation of space charges combined with the increase in the
electrical strength allow an increase in operation load, especially
in operation voltages. While the possibility of using the extruded
cross-linked insulation at increased conductor temperatures opens
for an increase in current densities.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] The present invention shall be described more in detail
under reference to the drawings and examples.
[0056] FIG. 1 show a cross-section of a typical DC-cable according
to the present invention for transmission of electric power
comprising an extruded, cross-linked insulation.
[0057] FIG. 2a to 2d show space charge recordings for comparative
tests on plates with compositions as used in prior insulated
AC-cables and for compositions according to the present
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES
[0058] the DC-cable according to the embodiment of the present
invention shown in FIG. 1 comprises from the center and
outwards;
[0059] a stranded multi-wire conductor 10;
[0060] a first extruded semi-conducting shield 11 disposed around
and outside the conductor 10 and inside a conductor insulation
12;
[0061] an extruded conductor insulation 12 with an extruded,
cross-linked composition as described in the foregoing;
[0062] a second extruded semi-conducting shield 13 disposed outside
the conductor insulation 12;
[0063] a metallic screen 14; and
[0064] an outer covering or sheath 15 arranged outside the metallic
screen 14.
[0065] the cable can if deemed appropriate be further complemented
in various ways with functional layers or other features. It can
for example be complemented with a reinforcement in form of
metallic wires outside the outer extruded shield 13, a sealing
compound or a water swelling powder introduced in metal-polymer
interfaces or a system of radial achieved by e.g. a corrosion
resistant metal polyethylene laminate and longitudinal water
sealing achieved by water swelling material, e.g. tape or powder
beneath the sheath 15.
EXAMPLE 1
[0066] Comparative Tests
[0067] Test plates with compositions as used in prior art insulated
AC-cables and in accordance with the present invention for use in
insulated DC-cables were produced, processed and subjected to an
evaluation of the tendency for space charge accumulation by
recording space charge profiles using the Pulsed ElectroAccoustic
(PEA) technique. The PEA technique is well known within the art and
described by Takada et al. in IEEE Trans. Electr. Insul. Vol.
EI-22(No.4). pp 497-501(1987).
[0068] a,A 2 mm thick test plate of a polyethylene composition
comprising:
[0069] about 98% by weight of low density polyethylene (922
kg/m.sup.3) of melt flow rate 0.8 g/10 min.
[0070] about 0,4% by weight of an antioxidant SANTONOX R.RTM.
(Flexsys Co) with the chemical designation
4,4'-thio-bis-(6-tert-butyl-m-cresol), and about 1,6% by weight of
a cross linking agent, DICUP R.RTM. (Hercules Chem) with the
chemical designation dicumyl peroxide, was molded at 130.degree.
C.
[0071] Two semi-conductive electrodes were molded on the test plate
and the assembly was cross-linked in an electric press at
180.degree. C. for 15 minutes.
[0072] the 2 mm thick cross-linked test plate was subsequently
tested at 50.degree. C. in a device for PEA analysis were the plate
was inserted between two flat electrodes and subjected to a 40 kV
direct voltage electric field. that is one electrode was grounded
and the other electrode was held at a voltage potential of +40 kV.
The space charge profile as shown in FIG. 2a was recorded for the
test plate. Were arbitrary units for space charge/volume is
presented as a function of the test plate thickness, i.e. 0 is at
the grounded electrode and x indicated the distance from the
grounded electrode in the direction towards the +40 kV
electrode.
[0073] b, A 2 mm thick test plate of a the same polyethylene
composition comprising as in comparative example a was molded, also
at 130.degree. C. Two semi-conductive electrodes were molded on
this test plate and the assembly was cross-linked in an electric
press at 180.degree. C. for 15 minutes. Subsequently the test plate
was degassed in a vacuum oven for 72 hours at 80.degree. C. and a
pressure of <10.sup.-3 torr.
[0074] The 2 mm thick cross-linked test plate was subsequently
tested at 50.degree. C. in a device for PEA analysis were the plate
was inserted between two flat electrodes and subjected to a 40 kV
direct voltage electric field. That is one electrode was grounded
and the other electrode was held at a voltage potential of +40 kV.
The space charge profile as shown in FIG. 2b was recorded for the
test plate. Were arbitrary units for space charge/volume is
presented as a function of the test plate thickness, i.e. 0 is at
the grounded electrode and x indicates the distance from the
grounded electrode in the direction towards the +40 kV
electrode.
[0075] c, A 2 mm thick test plate of a polyethylene composition
comprising:
[0076] about 97.7% by weight of a low density polyethylene with a
MFR of 1.2 g/10 min,
[0077] about 1.7% by weight of a peroxide cross-linking agent in
the form of dicumyl peroxide,
[0078] about 0.3% by weight of compound (D),
2,4-diphenyl-4-methyl-pentene- -1, as a scorch retarding agent,
and
[0079] about 0.3% by weight of an antioxidant system consisting
of
[0080] compound (B) in the form of DSTDP,
di-stearyl-thio-dipropianate and
[0081] compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol, the ratio of compound B to compound C was 1:3, was
molded at 130.degree. C.
[0082] Two semi-conductive electrodes were molded on the test plate
and the assembly was cross-linked in an electric press at
180.degree. C. for 15 minutes.
[0083] The 2 mm thick cross-linked test plate was subsequently
tested at 50.degree. C. in a device for PEA analysis were the plate
was inserted between two flat electrodes and subjected to a 40 kV
direct voltage electric field. That is one electrode was grounded
and the other electrode was held at a voltage potential of +40 kV.
The space charge profile as shown in FIG. 2c was recorded for the
test plate. Were arbitrary units for space charge/volume is
presented as a function of the test plate thickness, i.e. 0 is at
the grounded electrode and x indicates the distance from the
grounded electrode in the direction towards the +40 kV
electrode.
[0084] d, A 2 mm thick test plate of a polyethylene composition
comprising:
[0085] about 97.7 by weight of a low density polyethylene with a
MFR of 1 g/10,
[0086] about 1.4% by weight of a peroxide cross-linking agent in
the form of dicumyl peroxide,
[0087] about 0.4% by weight of a scorch retarding agent in the form
of compound (D), 2,4-diphenyl-4-methyl-pentene-1, and
[0088] about 0.3% by weight of an antioxidant system consisting of
compound (C), a diester of 3,-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionicacid and thiodiglycol, was molded at 130.degree. C.
[0089] Two semi-conductive electrodes were molded on the test plate
and the assembly was cross-linked in an electric press at
180.degree. C. for 15 minutes.
[0090] the 2 mm thick cross-linked test plate was subsequently
tested at 50.degree. C. in a device for PEA analysis were the plate
was inserted between two flat electrodes and subjected to a 40 kV
direct voltage electric field. That is one electrode was grounded
and the other electrode was held at voltage potential of +40 kV.
the space charge profile as shown in FIG. 2d was recorded for the
test plate. Were arbitrary units for space charge/volume is
presented as a function of the test plate thickness, i.e. 0 is at
the grounded electrode and x indicated the distance from the
grounded electrode in the direction towards the +40 kV
electrode.
CONCLUSIONS OF COMPARATIVE TESTS
[0091] The space charge of profiles of the samples in example 1a,
1b, 1c, and recorded 3 hours after the application of the DC
voltage are shown in FIG. 2a, 2b, 2c, and 2d, respectively. It is
clearly seen that the space charge accumulation in the insulation
material traditionally used in AC XLPE cables (see FIG. 2a) is
high. after degassing the space charge accumulation decreases but
still have a significant magnitude (see FIG. 2b). However, the
tendency for space charge accumulation is substantially reduced for
the two compositions according to the present invention represented
with comparative examples FIGS. c and d. The reduction space charge
accumulation is as good as or better than the reduction achieved
after the long time degassing process, examples in FIGS. 2a and
b.
EXAMPLE 2
[0092] a polyethylene based resin composition was compounded as
described in the following. to a low density polyethylene resin
with a MFR of 1.2 g/10 min following additions were made,
[0093] a peroxide cross-linking agent in the form of dicumyl
peroxide was added in an amount of 1.7% by weight,
[0094] a scorch retarding agent in the form of compound (D),
2,4-diphenyl-4-methyl-pentene-1, was added in an amount of 0.3% by
weight, and an antioxidant system consisting of
[0095] compound (B) in the form of DSTDP,
di-stearyl-thio-dipropianate and
[0096] compound (C), a diester of
3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol, was added to the polyethylene resin in a total amount
of 0.3% by weight. The ratio of compound B to compound C was 1:3.
The compounded resin composition was extruded to a 8 mm insulation
on a shielded, stranded multi-wire conductor and cross-linked at a
product temperature of 225.degree. C. and for a limited processing
time of 5 minutes.
[0097] No significant level of water was detected in the extruded,
cross-linked insulation. As water formation typically occurs at
high temperature in a system comprising compound (B) that normally
catalyzes water-formation from cumyl alcohol, the low water content
was taken as proof that the water formation was successfully
suppressed by the reduced product temperature and processing
temperature.
[0098] The tendency for space charge accumulation was, when tested,
low and substantially reduced in relation to the space charge
accumulation normally detected in conventional extruded
cross-linked polyethylene based compositions as used for Ac-cables.
thus the cable was considered suitable for use as DC-cable.
[0099] Further benefits were found in the low content of excess
peroxide cross-linking agent in the cable insulation after
cross-linking and the precise control of the cross-linking degree.
These advantages were obtained by the limited and controlled
addition of the cross-linking agent in combination with the
promotion of grafted bridges between polymer chains provided by
scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pentene-1.
EXAMPLE 3
[0100] A polyethylene based resin composition was compounded as
described in the following. To a low density polyethylene resin
with a MFR of 1 g/10 min the following additions were made,
[0101] a peroxide cross-linking agent in the form of dicumyl
peroxide was added in an amount of 1.4% by weight,
[0102] a scorch retarding agent in the form of compound (D),
2,4-diphenyl-4-methyl-pentene-1, was added in an amount of 0.4% by
weight, and an antioxidant system consisting of compound (C), a
diester of 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionicacid and
thiodiglycol, only was added to the polyethylene resin in an amount
of 0.3% by weight. The compounded resin composition was extruded
and cross-linked at a temperature of 300.degree. C.
[0103] The tendency for space charge accumulation was, when tested,
low and substantially reduced in relation to the space charge
accumulation normally detected in conventional extruded
cross-linked polyethylene based compositions as used for AC-cables.
Thus the cable was considered suitable for use as a DC-cable.
[0104] Further benefits were found in the low content of excess
peroxide cross-linking agent in the cable insulation after
cross-linking and the precise control of the cross-linking degree.
These advantages were obtained by the limited and controlled
addition of the cross-linking agent in combination with the
promotion of grafted bridges between polymer chains provided by
scorch retarding agent, compound (D),
2,4-diphenyl-4-methyl-pentene-1.
[0105] A cable produced according to this example was found to be
suitable for any thickness of the insulation that is deemed to be
required by the electrical or mechanical forces acting on the cable
and by the thermal situation upon use.
* * * * *