Note: Descriptions are shown in the official language in which they were submitted.
CA 02339541 2001-02-05
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AN ELECTRIC DC-CABLE WITH AN INSULATION SYSTEM COMPRISING AN
EXTRUDED POLYETHYLENE COMPOSITION AND A METHOD FOR
MANUFACTURING SUCH CABLE
TECHNICAL FIELD
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 the conductor, wherein the insulation system comprises an
extruded and
cross-linked polyethylene composition.
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
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rapidly superseded by systems using alternating current, AC.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
I950s 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.
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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 voltages.
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 complemented with casing, reinforcement etc. to withstand water
penetration and any
mechanical wear or forces during, production installation and use.
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.
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.
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 of XLPE and PE for DC
cable insulation
has been under investigation for many years. Cables with such insulation have
the same
advantage as the mass impregnated cable in that for DC transmission there are
no restrictions
on circuit length and they also have a potential for being operated at higher
temperatures. In
the case of XLPE, 90 °C instead of 50 °C for conventional mass-
impregnated 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
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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 local 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 field 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.
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 resins 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;
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- stabilizing additives, e.g. antioxidants, electron scavengers to counteract
decomposition due
to oxidation; radiation etc.;
- lubricating additives, e.g. stearic acid, to increase processability;
- additives for increased capability to withstand electrical stress, e.g. an
increased water tree
resistance , e.g. polyethylene glycol, silicones etc.; and
- 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;
- scorch retarders to avoid premature cross-linking.
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.
A typical polyethylene based resin composition to be used as an extruded,
cross-linked insulation in an AC-cable comprises:
100 parts by weight of low density polyethylene (922 kg/m3) with melt flow
rate (MFR,) of
0,4 - 2,5 g/10 min.
0,1 - 0,5 phr (parts per $undred Iesin) of an antioxidant, e.g. SANTONOX R~
(Flexsys Co)
with the chemical designation 4,4'-thio-bis(6-tert-butyl-m-cresol), or other
antioxidants or
combination of antioxidants
1,0 - 2,5 phr of a cross linking agent, DICUP R~ (Hercules Chem) with the
chemical
designation dicumyl peroxide.
However, it is well known that all cross linked polyethylene compositions used
as extruded
insulation in AC-cable systems exhibit strong tendency to accumulate space
charge under
DC-electric stress, thus making them unsuitable for use in insulation systems
for DC-cables.
It is also known that extended degassing, i.e. exposing the cross linked cable
at high tempera-
tures 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.
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OBJECTS OF THE IIWENTION
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,
make the cable handy and improve robustness.
It is also an object to provide a cable comprising an extruded, cross linked
insulation based on poiyethylene 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
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.
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SL>TvIMARY OF THE INVENTION
6
It has now surprisingly been found that excellent results with regard to
accumulation of space charge under the influence of a DC-electric field can be
achieved by
incorporating in XLPE compositions for electric cables a specific glycerol
fatty acid ester
additive, optionally in combination with further additives.
The present invention thus provides a DC-electric power cable comprising a
conductor and an extruded, cross linked solid insulation system comprising at
least three
layers disposed around the conductor, characterized in that the extruded
insulation system
comprises a polyethylene based compound to which additives including a cross
linking
agent, a scorch retarding agent, an antioxidant and an additive comprising a
glycerol fatty
acid ester of the general formula ( I )
R1O(C3Hs(OR'-)O)nR3 ( I )
where
n >_ 1, preferably, because of commercial availability, n = 1-20, and more
preferably
n = 3-8,
R~, R-', and R3, which are the same or different, designate hydrogen or the
residue of a
carboxylic acid with 8-24 carbon atoms,
with the proviso that there are at least two free OH groups and at least one
residue of a
carboxylic acid with 8-24 carbon atoms in the molecule. In case R2 and R3 both
represent
hydrogen (H) atoms and R~ = R the carboxylic residues the formula will take
the simple form
of(II)
RO(CH2CH(OH)CH20)nH ( II )
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 optimize the
cross linking
process.
The cable insulation system can be applied on the conductor 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
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7
exhibit a general long term stability such that the working life of the cable
is maintained or
increased.
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 an extruded cross-linked polyethylene based
conductor
insulation includes the following steps:
laying or otherwise forming a conductor of any desired shape and constitution;
compounding a polyethylene based resin composition comprising additions of a
cross-linking agent, a scorch retarding agent, antioxidant and a spare charge
reducing
additive
extruding the compounded polyethylene based resin composition to forth 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)
cross-linking the extruded insulation
wherein according to the present invention a space charge reducing additive
comprising a glycerol fatty ester of the general formula ( I ), is added to
the
polyethylene resin upon compounding;
R~O(C3Hs(OR2)O)nR3 ( I )
where
n >_ 1, R', R2, and R3, which are the same or different, designate hydrogen or
the
residue of a carboxylic acid with 8-24 carbon atoms, with the proviso that
there are at
least two &ee OH groups and at least one residue of a carboxylic acid with 8-
24
carbon atoms in the molecule.
and wherein the compounded polyethylene based resin composition is extruded
and
cross-linked at an elevated temperature and applied pressure and for a period
of time
long enough to cross link the insulation.
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8
Other distinguishing features and advantages of the present invention will
appear
from the following specification and appended claims.
DETAILED DESCRIPTION OF THE INVENTION
In order to use extruded polyethylene or cross linked polyethylene (XLPE) as
an insulation for DC-cables several factors have to be taken into account. The
most important
issue is the space charge accumulation under DC-voltage stress. The present
invention
accomplish such significant decrease in the space charge accumulation
typically occurring in
an operating DC-cable by incorporating a low amount of an additive of the
general structure
( I ) into the polyethylene or the cross linkable polyethylene compound. The
compound of
the general structure ( I ) is a mono- or polyglycerol ether where at least
one OH group forms
an ester with a carboxylic acid with 8-24 carbon atoms. Preferably, the
compound of
structure ( I ) is a monoester, i.e. it contains one carboxylic acid residue
with 8-24 carbon
atoms per molecule. Further, the ester forming carboxylic acid preferably
forms ester with a
primary hydroxylic group of the glycerol compound. The compound of formula ( I
) may
include 1-20, preferably 1-15, most preferably 3-8 glycerol units, i.e. n in
the formula ( I ) is
1-20, preferably 1-15, and most preferably 3-8.
When R~, RZ, and R3 in formula ( I ) do not designate hydrogen they designate
the residue of a carboxylic acid with 8-24 carbon atoms. These carboxylic
acids may be
saturated or unsaturated and branched or unbranched. Illustrative, non-
limiting examples of
such carboxylic acids are lauric acid, myristic acid, palmitic acid, stearic
acid, oleic acid,
linoleic acid, linolenic acid, and behenic acid. When the carboxylic residue
is unsaturated the
unsaturation may be utilized for binding the compound of structure ( I ) to
the ethylene
polymer of the composition and thus effectively prevent migration of the
compound of
structure ( I ) from the composition.
In formula ( I ) R1, R2 and R3 may designate the same carboxylic acid residue,
such as stearoyl, or different carboxylic residues, such as stearoyl and
oleyl.
To prevent migration and exudation, the compound of structure ( I ) should be
compatible with the composition in which it is incorporated, and more
particularly with the
ethylene base resin of the composition.
The compounds of structure ( I ) are known chemical compounds or may be
produced by known methods. Thus, a compound of formula ( I ) where n = 3 is
commercialized as Atmer~184 (or 185) by ICI, Great Britain, and one where n in
average is
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8, having one fatty acid residue per molecule, can be obtained from ICI under
the
denomination SCS 2064~. Other known commercial compounds which can be
described by
formula ( II ) are TST 22I ~ (n = 6 and R = linoleic acid residue (unsaturated
C 18 acid)) TST
21 S~ {n = 6 and R = steatic acid (saturated C 18 acid)), and TST 216~ (n = 6
and R =
behenic acid (unsaturated C22 acid)) all supplied by Danisco, Denmark.
The compound of formula ( I ) is incorporated in the composition of the
invention in an amount effective for inhibiting space charge accumulation
under DC-stress.
Generally~this means that the compound of formula ( i ) is incorporated in an
amount of
about 0,05-2 % by weight, preferably 0,1-1 % by weight of the composition.
In addition to the compound of formula ( I ) the composition of the compounds
for the DC-cables of the present invention may include conventional additives,
such as
antioxidants to counteract decomposition due to oxidation, radiation, etc.;
lubricating
additives, such as stearic acid; cross linking additives, such as peroxides
which decompose
upon heating and initiate cross linking; and other additives such as scorch
retardant agents
arid compatibilizers. The overall amount of additives, including the compound
of formula
( I ) in the composition of the present invention should not exceed about 10 %
by weight of
the composition.
Besides the compound of formula ( I ) and other conventional and optional
additives mentioned above the composition of the invention predominantly
comprises an
ethylene polymer as indicated earlier. The choice and composition of the
ethylene polymer
varies depending on whether the composition is intended as an insulating layer
of an electric
cable or as an inner or outer semi conductive layer of an electric cable.
A composition for an insulating layer of an electric cable according to the
invention may for example comprise about 0,05 % to about 2 % by weight of the
compound
of formula ( I ) together with other conventional and optional additives; 0 to
about 4 % by
weight of a peroxide cross linking agent; the remainder of the composition
substantially
consisting of an ethylene polymer. Such ethylene polymer preferably is an
LDPE, i.e. an
ethylene homopolymer or a copolymer of ethylene and one or more alpha-olefins
with ~-8
carbon atoms, such asl-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. The
amount of
alpha-olefin comonomer(s) may be in the range from about 1 % to about 40 % by
weight of
the ethylene monomer. A copolymer of ethylene together with minor amounts,
i.e. up to ~
by weight of one or more polar comonomer(s), eg. vinyl acetate,
methylacrylate,
ethylacrylate, butylacrylate or dimethylamino-propylmethacrylamide (DMAPMA)
can also
be used.
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Similarly, a composition for a semiconductive layer of an electric cable may
comprise about 0,05 % to about 2 % by weight of the compound of formula ( I )
together
with other conventional and optional additives; about 30-80 % by weight of an
ethylene
polymer; carbon black in an amount at.least sufficient to make the composition
semiconductive, preferably about 15-45 % by weight of carbon black; 0 to about
30 % by
weight of an acrylonitrile-butadiene copolymer; and 0 to about 4 % by weight
of a peroxide
cross linking agent. In this connection the ethylene polymer is an ethylene
copolymer of the
composition as described for the insulating layer or an ethylene copolymer,
such as EVA
(ethylene-vinylacetate), EMA (ethylene-methylacrylate), EEA (ethylene-
ethylacrylate), or
EBA (ethylene-butylacrylate).
A DC-cable according to the present invention with an extruded, cross linked
insulation system comprising a cross-linked polyethylene composition, XLPE,
and an
additive of structure ( I ) exhibit considerable advantages such as;
- A substantially reduced tendency for space charge accumulation and
accordingly an
increased DC breakdown strength.
The cable according to the following examples the present invention also
offers good
performance and stability of the extruded cable insulation system even when
high
temperatures have been employed during extrusion, cross linking or other high
temperature
conditioning..
The DC-cable according to the present invention offers the capability of being
produced by an essentially continuous process without any time consuming batch
step such
as impregnation or degassing, thereby opening for substantial reduction in
production time
and thus the production costs without risking the technical performance of the
cable.
In order to further facilitate the understanding of the invention some
illustrating,
non-limiting examples will be given below. In the examples all compositions
are given as
part per hundred parts of resin by weight, unless othenvise stated.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention shall be described more in detail while referring to the
drawings and examples. Figure 1 shows a section-view of a cable for high-
voltage direct
current transmission of electric power according to one embodiment of the
present invention.
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Figure 2 shows the configuration of the test plates. Figures 3 to 14 show
space charge
recordings for measurements on plates with XLPE compositions as used in prior
insulated
AC-cables and for compositions according to the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS, EXAMPLES.
The DC-cable according to the embodiment of the present invention shown in
figure 1 comprises from the center and outwards;
- a stranded mufti-wire conductor 10;
- a first extruded semi-conductive shield 11 disposed around and outside the
conductor 10
and inside a conductor insulation 12;
- an extruded conductor insulation 12 with an extruded, cross-linked
composition;
- a second extruded semi-conductive shield 13 disposed outside the conductor
insulation 12;
- a metallic screen 14; and
- an outer covering or sheath 15 arranged outside the metallic screen 14.
The DC-cable can when deemed appropriate be further complemented in
various ways with various 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 moisture barriers 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. The conductor need not be stranded
but can be of
any desired shape and constitution, such as a stranded mufti-wire conductor, a
solid
conductor or a segmental conductor.
The test plate 20 used for measurement of the space charge distribution shown
in figure 2, comprises two semi-conductive electrodes 21 made of a carbon
black filled
ethylene copolymer and the insulation body 22 with the composition given in
Table 1.
Figure 3, 5, 7, 9, 1 l, and 13 show the distribution of space charge in
arbitrary
units in the "voltage-on" mode as a function of distance from the grounded
electrode.
Similarly figure 4, 6, 8, 10, 12, and 14 show the distribution of space charge
in arbitrary units
in the "voltage-off' mode as a function of distance from the grounded
electrode (note the
scales in "voltage-on" mode and "voltage-off' mode are different).
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In order to facilitate the understanding of the invention some illustrating,
non-
limiting examples will be given below. In the following examples test plates
with various
compositions were manufactured and subjected to measurements of space charge
accumulation by recording the space charge profiles. The profiles were
recorded using the
Pulsed Electro Acoustic (PEA) technique. The PEA technique is well known
within the art
and described by Takada et al. in IEEE Trans. Elec. Insul. Vol. EI-22 (No 4),
pp. 497-SO1
(1987). The space charge profiles shown in the following examples are either
"voltage-on"
i.e. the recorded space charge profiles under electrical stress after 3 hours
DC-voltage
application, or "voltage-off', i.e. the recorded space charge profiles
immediately after
grounding of the electrodes (prior to grounding a DC-voltage was applied for 3
hours).
The compositions shown in Table 1 were all made in a conventional manner by
compounding the components in an extruder. The test plates were manufactured
in a two-
step process. In the first step the insulation was press molded from an
extruded tape at 130 °
C for 10 minutes into circular plates with a diameter of 210 mm and a
thickness of 2 mm. In
the second operation two semiconductive electrodes were mounted in the center
on each side
of the circular insulation plates and the assembly was heated to 180 °C
for 1S minutes in an
electric press unless otherwise stated. The high temperature cycle was made in
order to
complete the cross linking. The test plates were hereafter cooled to ambient
temperatures
under pressure. Mylar~ films were used as backing during the press molding.
The
semiconductive electrodes were made of a commercial product, LE OS00~ from
Borealis,
Sweden. This compound comprises ethylene-butylacrylate copolymer and acetylene
black.
The dimensions of these electrodes were 1 mm in thickness and SO mm in
diameter. Figure 2
show the configuration and the dimensions of the test plates.
The space charge profiles of the test plates were recorded by a device for PEA
analysis at SO °C. One electrode was grounded and the other was held at
a voltage of +40 kV,
i.e. the electric field strength in the plate was 20 kV/mm. In the space
charge profiles figure
3-14 the electric charge per unit volume is presented as a function of the
test plate thickness,
i.e. zero is the position of the grounded electrode and x indicates the
distance from the
grounded electrode in the direction towards the high voltage (+40 kV)
electrode. In the
"voltage on" mode the space charge profile was recorded after 3 hours of
voltage application.
In the "voltage-off' mode the space charge profile was recorded immediately
after grounding
of the high voltage electrode (i.e. after 3 hours at +40 kV). The space charge
profiles are
given in arbitrary units of charge per volume insulation. The amplification
used during
"voltage-off' is higher than during "voltage-on". However, the scales used for
all samples in
either mode are comparable.
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Example 1, 2, and 3 are comparative examples. The composition of the
insulation material in these examples correspond to the invention disclosed in
the Swedish
patent application No. 9704825-0 (1997-12-22).
EXAMPLE 1
A 2 mm thick test plate of polyethylene of composition A (see Table 1 )
equipped with two semiconductive electrodes and cross linked at 180 °C
for 15 minutes
was tested at 50 °C in a device for PEA analysis. 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 + 40kV.
The space charge
profile as shown in figure 3 was recorded, in the so called "voltage-on" mode
after 3 hours of
exposure to the DC-voltage stress. The charge per unit volume is presented in
arbitrary units
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.
Figure 4 shows the space charge profile immediately after grounding of the
high voltage electrode at the end of the 3 hours high voltage electrification
in the so called
"voltage-off' mode. The charge per unit volume is presented in arbitrary units
(different
from that used in the "voltage-on" mode) 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 original high voltage electrode.
EXAMPLE 2
In order to test the effect of removing all volatile from the insulation
system a
test plate of the same kind as in example 1 and cross linked at 180 °C
for 15 minutes was
treated in a high vacuum at 80 °C for 72 hours. After this treatment
the space charge profiles
were recorded. Figure 5 shows the "voltage-on" mode and figure 6 the "voltage-
off' mode.
EXAMPLE 3
In order to test the effect of cross linking conditions a test plate of the
same
kind as in example 1 was cross linked at 250 °C for 30 minutes. The
test plate was tested in a
device for PEA analysis. Figure 7 shows the "voltage-on" mode and figure 8 the
"voltage-
on" mode.
EXAMPLE 4
A 2 mm thick test plate of polyethylene of composition B (see Table 1)
equipped with two semiconductive electrodes and cross linked at 180 °C
for 15 minutes
was tested at 50 °C in a device for PEA analysis. The plate was
inserted between two flat
electrodes and subjected to a 40 kV direct voltage electric field. That is one
electrode was
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grounded and the other electrode was held at a voltage potential of + 40kV.
The space charge
profile as shown in figure 9 was recorded, in the so called "voltage-on" mode
after 3 hours of
exposure to the DC-voltage stress. The charge per unit volume is presented in
arbitrary units
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.
Figure 10 shows the space charge profile immediately after grounding of the
high voltage electrode at the end of the 3 hours high voltage electrification
in the so called
"voltage-off' mode. The charge per unit volume is presented in arbitrary units
(different
from that used in the "voltage-on" mode) 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 original high voltage electrode.
EXAMPLE 5
In order to test the effect of removing all volatile from the insulation
system a
test plate of the same kind as in example 4 and cross linked at 180 °C
for 15 minutes was
treated in a high vacuum at 80 °C for 72 hours. After this treatment
the space charge profiles
were recorded. Figure 11 shows the "voltage-on" mode and figure 12 the
"voltage-off'
mode.
EXAMPLE 6
In order to test the effect of cross linking conditions a test plate of the
same
kind as in example 4 was cross linked at 250 °C for 30 minutes. The
test plate was tested in a
device for PEA analysis. Figure 13 shows the "voltage-on" mode and figure 14
the "voltage-
on" mode.
When comparing the space charge profiles in example 1, 2, and 3 with the
space charge profiles in example 4, 5, and 6 it is evident that the compound
of formula ( I ) is
an extremely effective space charge reducing agent. It is clearly seen from
table ? that the
space charge accumulated under similar conditions is more than 50 % lower when
a
compound of formula ( I ) is added to the insulation composition.
In order to show the robustness of the space charge accumulation suppressing
effect of the compound of formula ( I ) the following experiments, presented
in example 7, 8,
and 9, were perforated.
EXAMPLE 7
In order to check eventual concentration dependence of compound of formula
I ) a 2 mm thick test plate of polyethylene of composition C (see Table 1 )
equipped with 1<vo
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semiconductive electrodes and cross linked at 180 °C for 15 minutes was
tested at 50 °C in a
device for PEA analysis. The space charge profiles in "voltage-on" mode and
"voltage-off'
mode were identical to figure 9 and 10, respectively.
EXAMPLE 8
In order to check the influence of the antioxidant system on the space charge
reducing power of the compound of formula ( I ) three test plates of
composition D, E, and F
(see Table 1), respectively, was manufactured and tested as described in
example 1. All three
test plates showed space charge profiles in both "voltage-on" mode and
"voltage-off' mode
which were identical to figure 9 and figure 10, respectively.
EXAMPLE 9
In order to investigate the influence of other additives on the space charge
reducing power of a compound of formula ( I ), three different compositions G,
H, and I (see
Table 1), respectively, was manufactured and tested as described in example 1.
All three test
plates showed space charge profiles in both "voltage-on" mode and "voltage-
off' mode
which were identical to figure 9 and figure 10, respectively.
It is evident from the results of example 7, 8, and 9 that the addition of a
compound of formula ( I ) is an effective space charge reducing agent in a
very broad range
of cross linked polyethylene compositions.
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TABLE 1
Composition of XLPE insulation compounds
Compound No. A B C
LDPE*, MFR2= 0,8 100 100 100
LDPE*, MFR2= 2 -
Irganox 1035** 0,2 0,2 0,2
Irganox PS 802*** 0 4 0 4 0,4
> >
Antioxidant 3 - - -
Antioxidant 4 _ _
Compound of formula ( I ): - 0,6 0,9
polyglyceryl mono-fatty acid ester
(SCS 2064)****
N-methylpyrrolidone _ -
Compatibilizer 1 - -
Compatibilizer 2 _ - -
Dicumylperoxide 1,8 1,8 1,8
Scorch retarding agent***** 0,4 0,4 0,4
Total 102,8 103,4 103,7
* LDPE, low density polyethylene, i.e. polyethylene prepared by radical
polymerization at
high pressure (density = 0,922 g/cm3).
** Irganox 1035~, diester of 3-(3,S-di-tert-butyl-4-
hydroxyphenyl)propionicacid and
thiodiglycol, Ciba-Geigy.
*** Irganox PS 802~, di-stearyl-thio-dipropionate, Ciba-Geigy.
**** ICI, Great Britain
***** 2,4-diphenyl-4-methyl-pentene-1, Nofmer MSD~, Nippon Oil and Fats.
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TABLE 1 (cont.)
Composition of XLPE insulation compounds
Compound No.,. D E F
LDPE*, MFR.,= 0,8 100 100 100
LDPE*, MFR = 2 _ _ -
Irganox 1035** 0 15 0 2 0 2
,
Irganox PS 802*** _ _ -
Antioxidant 3 0,08 - -
Antioxidant 4 - 0,2 0,2
Compound of formula ( I ): 0,6 0,6 0,35
polyglyceryl mono-fatty acid ester
(SCS 2064)****
N-methylpyrrolidone _ - _
Compatibilizer 1 _ _
Compatibilizer 2 _ _
Dicumylperoxide 1 8 1 8 1 8
Scorch retarding agent***** 0 4 0 4 0 4
> > >
Total 103,3 103,2 102,95
* LDPE, low density polyethylene, i.e. polyethylene prepared by radical
polymerization at
high pressure (density = 0,922 g/cm3).
** Irganox 1035~, diester of 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propionicacid and
thiodiglycol, Ciba-Geigy.
*** Irganox PS 802~, di-stearyl-thin-dipropionate, Ciba-Geigy.
**** ICI, Great Britain
***** 2,4-diphenyl-4-methyl-pentene-1, Nofmer MSD~, Nippon Oil and Fats.
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TABLE 1 (cont.)
Composition of XLPE insulation compounds
Compound No. . G H I
LDPE*, MFR2= 0,8 100 - 100
LDPE*, MFR2= 2 - 100 -
Irganox 1035** 0,2 0,2 0,2
Irganox PS 802*** 0,4 0,4 0,4
Antioxidant 3 - -
Antioxidant 4 _ _ -
Compound of formula ( I ): 0,35 0,35 0,7
polyglyceryl mono-fatty acid ester
(SCS 2064)****
N-methylpyrrolidone 0,07 0,05 0,07
Compatibilizer 1 - 0 35 -
Compatibilizer 2 0,25 - -
Dicumylperoxide 1 8 1 8 1 8
> > >
Scorch retarding agent***** 0,4 0 4 0 4
Total 103,47 103,55 103,57
* LDPE, low density polyethylene, i.e. polyethylene prepared by radical
polymerization at
high pressure (density = 0,922 g/cm3).
** Irganox 1035~, diester of 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propionicacid and
thiodiglycol, Ciba-Geigy.
*** Irganox PS 802~, di-stearyl-thio-dipropionate, Ciba-Geigy.
**** ICI, Great Britain
***** 2,4-diphenyl-4-methyl-pentene-1, Nofmer MSD~, Nippon Oil and Fats.
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TABLE 2
Relative magnitudes of the accumulated space charge in "voltage-off" mode
. (after 3 hours of DC electrification at 20 kV/mm).
EXAMPLE 1 2 3 4 5 6
Composition according A A A B B B
to table 1
Cross linking temperature,180 180 250 180 180 250
C
After processing (80C/72- + _ _
hours/vacuum)
Relative magnitude of 100 70 160 50 35 60
space
charge in "voltage-off'
mode
Figure No. ~ 4 ~ 6 8 I 10 12 14
~ I
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