Note: Descriptions are shown in the official language in which they were submitted.
~220S33
ELECTRIC POWER CABLE
1 BACKGROUND OF THE INVENTION
This invention relates to improvements in and
concerning an electrically insulated cable impregnated with
insulating oil.
For an oil-impregnated insulating layer (or
dielectric layer) in an oil-immersion electric cable,
electrically insulating paper has heretofore most often been
used. Recently, however, propylene film has been used
instead as the oil-immersion insulating layer. In addition
to far exceeding electrically insulating paper in terms of
dielectric breakdown voltage, this film has several
advantages such as a low dielectric loss tangent and a
dielectric constant approximating the dielectric constant of
the insulating oil.
lS The conventional polypropylene insulated cable f
however, has a disadvantage that swelling with the
insulating oil occurs to an exceptionally great extent.
When the cable is used in any application involving oil
immersion, it has entailed various restrictions. When a
cable is wrapped in polypropylene film and the resultant
polypropylene insulated cable is immersed in insulating oil,
for example, the film swells so as to tighten its pressure
1220533
1 on the cable and consequently deprive the cable of its
flexibility and impair the fluidity of the insulating oil
between the insulating film turns. As a measure of avoiding
this trouble, the film may be more loosely wound around the
conductor. When the film is loosely wound, however, there
is a possibility of the film slipping out of place or being
wrinkled.
Another disadvantage suffered by the conventional
polypropylene insulated cable is that, where edges of the
insulating film overlap, the fluidity of the insulating oil
between the adjacent turns is liable to be low, possibly to
the extent of inducing dielectric breakdown.
When polypropylene film is impregnated with any of
the alkylbenzene type oils which are preponderantly used as
insulating oils in OF cables of the EHV class, the film
swells as the temperature of the ambient air increases so
that the film increases in thickness, possibly to the extent
of notably increasing the interface pressure between the
overlapping plies of film, causing the film to sustain
rupture such as due to thermal expansion or contraction of
the cable, and thus Eorcing upper plies of the film to Eall
into gaps formed between adjacent lower plies of the film
and consequently causing damage. The result is generally
degraded electrical characteristics.
-3~
1 SUMMARY OF THE: IN~ENTION
Accord;ngly, a primary object of the invention is
the provision of an oil-immersion electrically insulated
cable suffering only nominal loss and enjoying excellent
insulation by having an insulation layer formed on a con-
ductor by winding at least a sheet of improved polypropylene
film on the conductor, thereby eliminating the above-no-ted
defects, including e~cessive swelling. Another object of
the invention is to provide an oil-immersion electrically
insulated cable of the type above, which has an insulation
layer formed on a conductor by alternately winding at
least a sheet of polypropylene film and at least a sheet
of kraft paper, thereby, in addition to the elimination of
swelling problem, eliminating insufficient fluidity of the
insulating oil in the insulating layer.
This invention accomplishes the object noted
above by providing an electric power cable, comprising a
conductor; insulation surrounding said conductor, said
insulation having at least portions thereof formed by wind-
ing on said conductor polypropylene film, characterized inthat said conductor polypropylene film has a density in a
range of 0.905 to 0.915 g/cm3, a birefringence in a range
of 0.020 to 0.035, a ratio of a lengthwise tensile strength
to a widthwise tensile strength in a range of 5 to 15, an~
a thickness in a range of 70 to 300 microns; and insulating
oil with which said insulation is impregnated.
Other objects and characteristics of this
1220533
1 inven~ion will become apparent from the further disclosure
of this invention to be made in the following detailed
description of preferred embodiments, with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF T~E DRAWINGS
Figs. lA and lB are sectional views of typical
insulation structures according to the present invention;
Fig. 2 shows a cross section of an oil-immersion
insulated electric power cable;
1~ Fig. 3 is a perspective view of an oil-immersion
insulating layer in the oil-immersion insulated electric
power cable of Fig. 2;
Fig. 4 is a cross section of a typical insulation
structure contemplated by the invention, illustrating in an
enlarged view a portion of the oil-immersion insulating
layer involving a change of layer structure; and
Fig. 5 is a cross section of another embodiment of
the invention.
DESCRIPTION OF T~E PREFERRED EMBODIMENTS
The term "polypropylene (hereinafter referred to
as "PP" for short) as used herein means polypropylene of a
grade having an isotacticity of at least 90%, preferably at
least 95~, and more preferably at least 97%, and a melt
index in the range of 0.5 to 40 9/10 minutes, preferably 1
1220533
1 to 20 g/10 minutes. An isotacticlty below the lower limit
mentioned above is undesirable because such increases the
degree of swelling with the insulating oil. If the melt
index is below the lower limit mentioned above, the amount
of swelling with the insulating oil also increases. On the
other hand, if the melt index is above the upper limit
mentioned above, the amount of the polymer dissolved in the
insulating oil is increased, and consequently the viscosity
of the insulating oil rises. In the PP species of the grade
described above, those materials which prove suitable for
the manufacture of the cable of this invention fulfill the
requirement that the temperature of melt crystallization
(TmC) be in the range of 105 to 120C, preferably 108 to
118C. A PP species having a TmC below the lower limit
mentioned above suffers from a great increase of the degree
of swelling with the insulating oil. A PP species having a
TmC exceeding the upper limit mentioned above exhibits an
inferior film-forming property and produces a homogeneous
film with difficulty and, consequently, aggravates
dielectric faults.
The PP film to be used for the cable of this
invention is required to have a density in the range of
0.905 to 0.915 g/cm3, preferably 0.907 to 0.912 g/cm3. If
the density is below the lower limit mentioned above, the
1220S33
1 degree of swelling with the insulating oil is increased.
Conversely, if the density is above the upper limit
mentioned above, the PP film becomes brittle and the
mechanical strength of the insulating layer of the cable is
insufficient. The birefringence of the PP film to be used
for the cable of this invention is required to fall in the
range of 0.020 to 0.035, preferably 0.025 to 0.032. If the
birefringence is less than the lower limit mentioned above,
the swelling of the PP film with the insulating oil
increases beyond the tolerable extent. If it exceeds the
upper limit, the PP film is liable to sustain cracks which
could cause dielectric breakdown. Such a PP film,
therefore, does not meet the objects of this invention. The
ratio of strengths in the two axial directions of the PP
film to be used for the cable of this invention~ namely, the
quotient of the tensile strength of the film in the
longitudinal direction divided by the tensile strength
thereof in the lateral direction, is required to be in the
range of 5 to 15, preferably 7 to 12. If this ratio is less
than the lower limit mentioned above, the swelling of the PP
film with the insulating oil increases beyond a tolerable
extent. Conversely, if this ratio exceeds the upper limit,
the differences of properties exhibited by the PP film in
axes of varying directions increase beyond a tolerable
~220533
-- 7 --
1 extent, and consequently the workability of the PP film
while being wound on the cable to form an insulator is
notably degraded (for example, by stretching, wrinkling, or
rupturing).
Now, a typical method for producing the film to be
used for the cable of this invention will be described by
way of illustration. The PP recin is melted, extruded in
the form of a sheet through an extrusion die, wound on a
cooling drum, and left to cool and solidify The PP sheet
thus obtained is passed between a set of reducing rolls and
rolled with a rolling ratio (the quotient of the thickness
of sheet after rolling divided by the thickness of sheet
before rolling) in a range of 5 to 12, preferably 7 to 10.
The pressure of rolling is desirably in a range of
10 to 3000 kg/cm, preferably 100 to lOOO kg/cm, and the
temperature of the reducing rolls is desirably in a range of
60 to 160C. The PP sheet can be easily rolled uniformly
at a high rolling ratio by wetting the surface of the PP
sheet with a suitable liquid (such as water, an aqueous
solution of surface active agent, alkylene glycol,
polyalkylene glycol, glycerin, or an electrically insulating
oil) while the PP sheet is entering the reducing rolls.
The PP film obtained by the foregoing rolling
treatment (generally in a thickness in a range of 10 to 300
~ZZ05~3
-- 8 --
1 microns) is again heated to 100 to 150C and subjected to
heat treatment at this temperature for a period of 1 to 20
seconds until it slackens by 0.5 to 10% of the original size
in the longitudinal direction.
The present invention is characterized by
possessing the features described above. The oil-immersion
electrically insulated cable of the present invention can be
obtained in a more desirable form by limiting the ratio of
thermal shrinkage of the PP film in the longitudinal
direction to the range of 0.1 to 5%~ preferably 0.5 to 3%.
If the ratio of thermal shrinkage exceeds the upper limit
mentioned above, a disadvantage results in that the
insulating layer is liable to tighten and consequently
wrinkle. If this ratio is less than the lower limit, the
film in the insulating oil is liable to stretch in the
longitudinal direction and the insulating layer wound on the
cable slacken. A typical method for limiting the ratio of
thermal shrinkage in the longitudinal direction resides in
heating the PP film produced by the method described above
to a temperature in a range of 80 to 140C, preferably, 90
to 130C, and retaining the film at this temperature for a
period of 0.5 to 50 hours, preferably, 1 to 20 hours, with
the film held in a tense state or allowed to slacken by 0.1
to 5% of the original size in the longitudinal direction.
12Z0533
1 By this aging heat treatment, the ratio of thermal shrinkage
in the longitudinal direction can be confined within the
range of 0.1 to 5%, preferably, 0.5 to 3~.
The thickness of the PP film sheet is limited to
the range of 70 to 300 microns for the following reason. If
the thickness is smaller than 70 microns, there is a fair
possibility that the PP sheet will sustain fracture and
consequently fail to provide a mechanical strength required
for permitting the wrapping the PP tape around a conductor
to produce an insulating layer and, in the finished OF
cable, fail to retain a strength necessary for enabling the
cable to resist flexing, and cause the resultant insulating
layer to sustain abnormalities such as wrinkles, dents, and
collapses which adversely affect the electrical properties
of the cable.
Geneeally, the required thickness of the
insulating layer is obtained by adjusting the number of
plies of film tape wrapped on the conductor. If the
thickness of the tape is small, the number of plies of the
tape is proportionally increased, with the result that the
size of the e~uipment needed increases and the amount of
work involved in mounting, replacing, and splicing film
tapes increases. If the thickness of the PP film exceeds
300 microns, the PP tape has an excessively high stiffness
12Z05~3
- 10 -
l such that the tape, when being wound on the conductor to
produce an insulating layer thereon, offers resistance in
conforming to the contour of the cylindrical shape of the
conductor and, in the produced OF cable, gives rise to
abnormalities such as separation in the insulating layer and
irregular distribution of gaps left between edges of
adjacent turns of the tape, which tends to adversely affect
the electrical properties of the cable as a whole.
Generally, for the formation of an insulating
layer in a cable, the film tape is wound on the conductor in
such a manner as to permit the occurrence of gaps between
edges of adjacent turns of film tape. Thus, the thickness
of oil layers formed in such gaps increases with the
thickness of the tape. In an OF cable, the electrical
strength of the oil layer is lower than the insulating
strength of the tape portion. The fact that the oil layers
notably increase in size, therefore, does not prove very
favorable.
In the light of the various conditions described
above, OF cable is produced by preparing PP films of varying
sheet thicknesses in the range of 70 to 300 microns, cutting
these PP films into PP tapes of a suitable width, winding on
the inner side of the insulating layer (the side bordering
on the conductor and, therefore, experiencing severe
~220s313
1 electrical stress) PP tapes of smaller thickness, which are
rather inferior mechanically but quite superior
electrically, and, on the outer side of the insulating layer
(the side where the electrical stress is less toward the
outside and the effects of flexing exerted thereon increase
toward the outside), PP tapes of greater thickness, which
are rather inferior electrically but quite superior
mechanically.
The term "kraft paper" as used herein means
ordinary insulating paper which has been conventionally used
in OF cables of the EHV class. The thickness of the kraft
paper to be used in the cable of this invention is limited
to the range of 70 to 300 microns for the same reasons given
above with respect to the thickness of the PP film.
Regarding the insulating oil for use in the cable
of the invention, the inventors have found that
alkylbenzenes containing an aromatic ring, particularly DDB
(dodecyl benzene), which is commonly used in cables, best
suits the purpose of the invention. Generally, the
following conditions are adopted as criteria for selecting
the insulating oil:
(A) The oil should be readily available at low
cost.
(B) The oil should possess excellent and stable
~220533
- 12 -
1 electrical properties.
(C) The oil should be highly compatible with the
component materials of the insulating layer of the cable.
Specifically, in the case of the invention, the oil should
be amply compatible with the PP film.
Regarding conditions (A) and (B), DDB proves to be
an ideal insulating oil. With respect to condition ~C),
however, DDB is not ideal in that it causes swelling of the
PP film.
Generally, the degree of compatibility between a
film and insulating oil is determined by their respective SP
values ~index of solubility~; the similarity between the
film and the insulating oil in a particular combination
increases and the ability of the insulating oil to swell the
film also increases as the SP values of the film and the
insulating oil approach each other. Both PP and DDB have SP
values approximating 8. Thus, their combination has been
heretofore held to be less desirable than the other
combinations, such as PP and polybutene oil or PP and
silicone oil, because it has high mutual compatibility and
entails a high degree of swelling.
After much study in this respect, the inventors
have found that, since the swelling of the PP film by the
insulating oil is caused by this oil penetrating the
lZ20533
l amorphous phase of the film, the fortification of the
amorphous phase, which constitutes one electrically weak
point of the PP film, renders the lubricating oil in the
combination susceptible to heavy swelling of the PP film
more desirable from the electrical point of view. Moreover,
DDB proves all the more desirable electrically in the sense
that it possesses a benzene ring which causes it to excel in
gas absorbing properties and resistance to corona discharge.
According to the inventors' studies, the impulse breakdown
value of one PP film layer, with the value in th~
combination with DDB taken as unity (l), is about 0.8 in the
combination with .polybutene, and 0.6 to 0.7 in the
combination with silicone oil. This trend applies to the AC
breakdown strength of the PP film. For the outstanding
electrical properties of DDB in combination with the PP film
to be retained intact without any sacrifice of the
compatibility of the DDB with the PP film, the following
solution has been devised:
Since, as described above, thorough impregnation
of the PP film with DDB is advantageous from the standpoint
of electrical properties, the cable is left standing at the
maximum expected actual working temperature (generally in a
range of 85 to 95C) for 24 to 48 hours to allow the PP
film to swell to saturation prior to the shipment of the
12;~0S33
1 cable. This conditioning is effective to ensure the cable
possesses good electrical properties from the outset of its
servlce.
Through a study of the PP film, it has been
ascertained that the amount of swelling of the PP film with
DDB can be restrained by optimizing the film's density,
birefringence, and ratio of strength in two axial directions
within the ranges mentioned abo~e. To be specific, actual
measurements indicate that the ratio of increase in the
thickness of the film by swelling in the present combination
of PP film and DD8 is one-half that in the combination of
homo-casting PP film and DDB.
To remedy the insufficiency, the surfaces of
either or both the kraft paper and PP film are embossed to
produce bosses of a size sufficient for absorbing an
increase of the thickness of the PP film due to swelling,
preventing the pressure within the layer of the insulating
tape from abnormally rising, and maintaining the fluidity of
the lubricating oil.
When one or both surfaces of the PP film and the
kraft paper to be used in the cable of the invention are
coarsened by embossing, the surface roughness R thus
produced is required to be in a range of 1 to 50 microns,
preferably 2 to 40 microns. If the surface roughness is
122053.~
- 15 -
1 less than the lower limit mentioned above, the amount of
swelling of the PP film to be absorbed by the bosses is
insufficient and the fluidity of the insulating oil in the
layer is deficient, thereby inducing dielectric breakdown.
Conversely, if the surface roughness exceeds the upper limit
mentioned above, the PP film may be impaired by the
embossing treatment and the bosses formed occupy too much
volume and therefore continue their existence even after the
swelling of the film, with a possible result that oil
passages may occur between overlapping film tapes, which
effect degrades the electrical strength of the layer.
The coarsening of the surface of the PP film may
be effected by an embossing treatment, for example. To be
specific, the PP film of the invention is passed between
embossing rolls held at 90 to 140C to coarsen either or
both of the opposite surfaces of the PP film, with the
surface roughness RmaX falling in the range of 1 to 50
microns, preferably 2 to 40 microns.
For the production of the film for use in the
cable of the present invention, the combination of rolling
and embossing treatments proves to be most desirable.
Optionally, other methods may be used. For example, the
rolliny treatment of the aforementioned combination may be
replaced by a combination of rolling and stretching
~2Z0533
- 16 -
1 treatments or by a stretching treatment using closely spaced
rolls. Also, the embossing treatment of the aforementioned
combination may be replaced by a sand blasting process or
etching process to effect the desirea surface coarsening.
For the purpose of coarsening the surface of the
kraft paper, an embossing treatment with embossing rolls is
most desirable~ Otherwise, a process of spraying water
drops may be utilized.
As the number of sheets of PP film superposed on
the insulating layer is increased, ~here are obtained
improvements in the dielectric loss tangent (tan ~) and
dielectric constant (), in addition to such advantages as
lowered swelling of the film with the insulating oil,
improved fluidity of the insulating oil within the
insulating layer, improved mechanical properties of the
insulating layer and enhanced workability of the PP film
when the film is wound on the conductor, and fewer
occurrences of excessive tightening or loosening of the
winding of the layer. Consequently, the cable which is
obtained is suitable for use as EHV through UHV classes of
275 to 1000 KV, particularly the UHV class.
The alternate winding of kraft paper and PP film
is essential for the manufacture of the cable of this
invention for the following two reasons: First, this
1220533
1 winding provides the produced cable with improved mechanical
strength which a cable insulated exclusively with PP film
does not easily attain. Secondly, the interposition of
kraft paper containing a polar group between the opposed
surfaces of PP film and the distribution thereof throughout
the entire insulating layer serves to improve the electrical
strengths, particularly impulse strength, especially,
positive impulse strength.
The thermal expansion coefficient of kraft paper
is extremely small, in fact, about two orders of magnitude
lower than the thermal expansion coefficient of PP-film.
The Young's modulus of kraft paper is small compared with
that of PP film. When a plurality of sheets of kraft paper
are superposed and exposed to changes of temperature due to
load variation, the kraft paper exhibits an extremely high
flexibility. When the kraft paper is cut into tapes, the
edge faces of the tapes are very smooth and do not form
rigid cutting edges as observed in cut edge faces of PP
film. When a sheet of kraft paper is combined with a sheet
of PP film in such a manner that it has at least one surface
thereof bordering on the sheet of PP film, there are derived
numerous advantages, including the fact that the cushioning
effect of kraft paper greatly facilitates the control of the
inner pressure between adjacent tapes, permits the
1220S33
- 18 -
1 conditions of-cable production to be selected in very wide
ranges, and renders the production easy such that the
adjacent tapes in the produced cable are allowed to slide
smoothly over each other and do not suffer mutual
displacement because the kraft paper absorbs flexion exerted
thereon during handling prior to actual installation of the
cable. Further, the kraft paper very smoothly absorbs any
increase of the thickness of the PP tape due to swelling and
thermal expansion and permits the inner pressure between the
adjacent tapes to be easily maintained at the optimum level-
and discourages formation of gaps between the adjacent
tapes.
Where two cable ends are joined, it is usual that
the winding of the insulation layer and the subsequent
winding of tapes across the joint of the two cable ends are
both carried out manually. In this case, if the entire
insulating layer is formed exclusively of PP film tapes, it
is rather difficult for the inner layer of PP tapes to be
tightly wound manually. When PP film and kraft paper are
alternately wound as contemplated by this invention, the
inner layer can be very easily tightened with the kraft
paper and the tightly wound condition of the inner layer can
be easily retained intact. Thus, the alternate winding
facilitates the work of cable production and stabilizes and
1220533
-- 19 --
1 enhances the quality of the produced cable. All these
factors enhance the mechanical and electric properties of
the insulating layer.
Purely from the electrical point of view, plastic
film, which lacks a polar group and has carbon and hydrogen
atoms arranged very neatly and orderly, is slightly inferior
in resistance to corona discharge to kraft paper, which
contains a polar group and has carbon and hydrogen atoms
distributed randomly. The reason for this difference
remains yet to be clarified. This trend is conspicuous
particularly with respect to the impulse strength,
especially, the positive impulse strength. As a result of
much study, the inventors have found that the combination of
kraft paper and the PP film of this invention manifests
outstanding properties because the kraft paper gives rise to
uniformly distributed barrier interfaces in the insulating
layer. This discovery has led to the provision of a cable
of excellent electrical strength.
From the standpoint of material costs, even a P~
film, one of rather inexpensive plastic materials, is still
more than twice as expensive as kraft paper. Thus, the
economy of the cable improves as the proportion of kraft
paper in the combination of kraft paper and PP film is
increased. Also for thorough coordination between the
1220533
- 20 -
1 performance (~-tan ~ ) and the economy of the cable, the
cable of this invention, having alternate windings of kraft
paper and PP film, manifests its outstanding effects to the
fullest e~tent.
Now, working examples of the invention will be
described with reference to the accompanying drawings. Fig.
2 is a cross section of an oil~immersion electrically
insulated cable. In this diagram, reference numeral
denotes a path for oil, 2 a conductGr, 3 an oil-immersion
insulating layer wound on the conductor, 4 a metallic sheath
of aluminum or lead enclosing the oil-immersion insulating
layer, and 5 a corrosion-proofing layer superposed on the
sheath 4.
One version of the alternate winding is
illustrated in Fig. lA. Fig. lA is a cross section
illustrating a typical insulation structure according to the
present invention. It depicts the portion Z of the cross
section of Fig. 2 in the form of an enlarged model.
Specifically, Fig. lA represents one version of the
insulation structure wherein sets each composed of one sheet
of PP film 3a and one sheet of kraft paper 3b are repeated
throughout the entire insulating layer. Since the ratio of
PP film and kraft paper in this structure is roughly 1 : 1,
the value of ~ tan ~ of the completed cable is intermediate
~2Z0533
- 21 -
l the respective values of ~-tan ~ of the two materials.
Since this value for kraft paper is 3.4 x 0.2 % and that of
PP film is about 2.2 x 0.02 %, the overall value of the
complete cable is equivalent to 2.8 x 0.1 %. With this
structure, the dielectric loss tangent is reduced (to the
order of (2.8 x 0.1 %)/(3.4 x 0.2 %) = 0.41) compared with
that of the conventional cable using kraft paper exclusively
in the insulating layer. Thus, the cable of the present
invention proves highly useful for the EHV class of 275 to
S00 kV.
Since this structure has sheets of kraft paper
providing an excellent cushioning effect, each interposed
between adjacent sheets of PP film, it can cope easily with
enlargement of the PP film due to swelling. This structure
is produced most easily because it offers ample allowance
for surface coarsening of the kraft paper and provides good
control of the winding tension of the tapes. Even for the
sake of flexibility and other mechanical properties of the
completed cable, the fact that the kraft paper manifests an
excellent cushioning effect is a highly desirable merit.
Further, since sheets of kraft paper are interposed between
adjacent sheets of PP film and thus are distributed
throughout the entire thickness of the insulating layer,
providing the effect of a barrier, virtually no loss occurs
~2Z0533
- 22 -
1 in the electric breakdown strengths, specifically, the
positive impulse breakdown strength measured with the
conductor side with the higher stress being the positive
pole. The thickness effect of the plastic film, i.e., the
loss of breakdown strength which occurs when a layer formed
exclusively of sheets of PP film is given an increased
thickness is eliminated. Thus, the cable using this
insulation structure also excels electrically.
As described above, the cable of the insulation
structure of Fig. lA is stable and excellent both
mechanically and electrically. Thus, it is suitable for use
in EHV to UHV classes of 275 to 1000 kV.
For a further reduction in the dielectric loss
tangent, which is proportional to the square of the
transmission voltage and tan ~ , the cable is required to
possess a still lower value of tan ~ . To meet this
demand, the inventors have consequently developed the
i~sulation structure illustrated in Fig. lB. More
specifically, in this structure, sets, each consisting of
two sheets of PP film 3a and one sheet of kraft paper 3b
interposed therebetween, are repeated throughout the entire
insulating layer. In this insulation structure, the
cushioning effect of the kraft paper and the resistance
offered to corona discharge by the barriers of kraft paper
lZZ0533
- 23 -
1 are excellent. In the cable using this insulation
structure, the dielectric constant (E) iS ~2 x 2.2 + 3.4)/3
= 2.6 and the dielectric loss tangent (tan ~) is (2 x 0.02 %
~ 0.2 %)/3 = 0.087 %. Thus, the value of ~-tan ~ of this
cable is (2.6 x 0.087)/(3.4 x 0.2) = 0.33 as compared with
the cable using the insulating layer made exclusively of
kraft paper. Thus, the cable attains the desired reduction
of dielectric loss tangent at substantially no sacrifice of
other mechanical and electrical properties.
In this insulation structure, since t~e mixing
ratio of kraft paper as a cushioning component is decreased
both locally and overall to two-thirds that in the structure
of Fig. lA~ it becomes necessary to slightly increase the
aforementioned amount of surface coarsening of the PP ~ilm
and kraft paper and to increase slightly the tape winding
tension. Nevertheless, since all the sheets of the PP film
border on kraft paper and thus make the most of the
cushioning effect of the kraft paper, the produced cable
proves amply practicable from the standpoint of manufacture
and flexibility.
From the electrical point of view, although the
cable suffers a slight loss in its positive impulse
property, it retains the barrier effect of the kraft paper
and the resistance to corona discharge as expected.
1220533
- 24 -
1 The cushioning effect of the kraft paper i.s
derived more safely and desirably by using raw kraft paper
containing water in a ratio of 3 to 6% above the level of
the moisture in the air or moisture-adjusted kraft paper
having its thickness increased in advance by the addition of
water rather than dry kraft paper having its moisture
content lowered to 1% or lower in advance, as has been done
for insulation layers made solely of kraft paper. Unlike
the winding of dry kraft paper which entails an extra
process for drying, special storage designed to keep the
paper dry, and a taping machine specially designed to permit
the paper to be wound in a dry state, the winding
contemplated by this invention is easy to perform and quite
inexpensive.
The invention have further impxoved the breakdown
property of the cable as follows. Specifically, they have
found it highly desirable to use sheets of kraft paper
having a high dielectric constant and high resistance to
corona discharge in several, for instance, three to ten,
lowermost plies closest to the conductor and consequently
s~bjected the most electric stress. With this technique,
particularly the positive impulse strength of the cable can
be improved without suffering any discernible rise of the
value of ~ tan ~ of the cable as a whole.
12ZOS33~
- 25 -
1 Fig. 3 is a perspective view of the oil-immersion
insulating layer in the oil-immersion electrically insulated
cable. In the diagram, 6 denotes a lower (left-hand) oil-
immersion insulating layer, 7 an upper (right-hand) oil-
immersion insulating layer, and 8 a portion where a change
of layers (change of taping head) occurs in the gap winding
- oil-immersion insulating layer and where the depth of the
oil gap equals the thickness of two plies of tape. This
particular portion constitutes another weak point of the
cable.
To overcome this weak point, the present invention
- uses tapes of kraft paper, as shown in Fig. 4, in the plies
destined to be exposed to the portions 8a of the layer
change in the oil-immersion insulating layer (the portions
indicated by the arrow in Fig. 4) where the depth of the oil
! gap e~uals the thickness of two plies of tape, so that any
local breakdown of the oil gap 8a will be prevented from
readily developing into total breakdown of the cable by the
barrier effect of the kraft paper. In Fig. 4, plies of PP
film are used at the vicinity of layer changes. Optionally,
these two plies may be formed of kraft paper so that a total
o four plies of kraft paper are present, two above and two
below each point of change of layer. This structure has
been demonstrated to be quite effective. Arranging plies of
1;~20S33
- 26 -
l kraft paper at areas of layer change is more effective
closer to the conductor side where the electric stress is
prominent. Where the mixing ratio of kraft paper is desired
to be lowered to reduce the value of ~ tan ~ , it is
advantageous to adopt this approach at the areas of layer
change in only the lowermost five or so layers from the
boundary of the conductor.
Concerning particularly the reduction of the
positive impulse property, which demands due attention in
the application of the PP film to the cable, improvements
are attained notably by the arrangement of the kraft paper.
These improvements add all the more to the effectiveness of
the cable of the present invention.
The amount of surface coarsening of the PP film
and kraft paper contemplated by this invention varies widely
with the class of voltage, the size of the conductor, the
kind of the cable, and the insulating oil to be used. It is
particularly affected by the combination of the specific
combination of PP film and insulating oil.
In the cable of the present invention, the
combination of PP film with DDB has been demonstrated to
enable the cable to attain excellent electrical properties,
although other insulating oils can also provide excellent
results. For a POF cable, for example, polybutene-type
J ZZ~5;~3
- 27 -
1 insulating oils of high viscosity are most often used. When
such an insulating oil is used, since the amount of swelling
of the PP film is small, it suffices to form bosses of a
size of 2 to 10 microns, for example, only on the sheets of
kraft paper used in the insulation structure of Fig. lA Of
course, it otherwise suffices to form bosses of a size of
about 5 microns only on the sheets of PP film. In the case
of the insulation structure of Fig. lB involving the
combination of PP film with DDB, it suffices to form bosses
of a size of 6 to 20 microns on all sheets of the PP film,
to form bosses of a size of 20 to 40 microns on every other
sheet of PP film, or to form bosses of a size of S to 10
microns on all the sheets of PP film and bosses of a size of
1 to 5 microns on the sheets of kraft paper. With bosses of
this size, it is possible to optimize the inner pressure
between the adjacent tapes throughout the entire insulating
layer by adjusting the tape width and controlling the tape
winding tension.
The appropriateness of this inner pressure was
determined by holding a given cable at the highest working
temperature (85 to 95C, for example) for 24 hours, thereby
to amply swell the layer of PP film, then bending the cable
twice alternately in opposite directions into a loop of a
diameter about 20 times the outermost diameter of the
lZ2053~
1 insulating layer, and disassembling the cable and visually
examining the insulating tapes in the insulating layer for
possible sign of irregularities.
In any event, it is essential that the cable be
manufactured by setting the amount of surface coarsening in
the range of 1 to 50 microns, depending on the application
of the cable and the type of insulating oil, and that the
the taping conditions be coordinated with the selected
amount of surface coarsening. Once the cable is produced,
it is then desirable and necessary to have the PP film swell
at the highest working temperature of the cable.
The cable of the present invention provides the
following outstanding features by producing a cable for
which the values of density, birefringence, ratio of
strengths in two axial directions, and surface roughness of
the PP film used in the cable are within their respectively
ranges herein defined, superposing sheets of the PP film and
sheets of kraft paper in the described manner, and properly
coarsening either or both of the opposite surfaces of the
tapes:
(1) The swelling of the cable with the insulating
oil is minimal.
(2) The fluidity of the insulating oil between
the overlapping plies of the film is satisfactory.
12Z~)533
- 29 -
1 (3) The film exhibits outstanding mechanical
properties for an insulating layer and enjoys good
workability while being wound on the conductor.
(4) The insulating layer wound on the conductor
neither tightens nor slackens easily.
(5) The formation of an insulating layer across a
joint of two cable ends is easy and the insulating layer SQ
formed is reliable.
(6) The film excels in both dielectric constant
and dielectric loss tangent so that required properties are
provided at no sacrifice of economy.
(7) The cable excels in breakdown properties. It
particularly is excellent in the positive property.
The terms and methods of measurement as used in
the present invention will now be described below.
(1) Isotacticity: A given PP sample is extracted
from boiling N-heptane. The weight of the extracted residue
is divided by the original weight of the sample. The
quotient is multiplied by 100. The product, expressed as
percent, is used to represent the isotacticity.
(2) Melt index: This physical property is
measured under the conditions L of ASTM D-1238-73.
(3) Temperature of melt crystallization (TmC): A
sample 5 mg in weight is placed in a tester, for instance,
122~)~33
- 30 -
1 Model DSC-II, made by Perkin Elmer Corp., with the
atmosphere inside the tester displaced with nitrogen. Then,
the sample is heated to raise its temperature at a rate of
20C/minute to 200~C and then held at this level of 200C
for five minutes. Then, the hot sample is cooled at a rate
of 20~C/minute, causing in the meantime the tester to
describe a peak of the heat generated as a consequence of
the crystallization of the molten sample. The temperature
at the apex of the curve so described i5 reported as TmC.
(4) Density: This property is measured in
accordance with ASTM D-1505.
(5) Birefringence: By the use of an Abbe
refractometer, the refractive index in the longitudinal
direction (Ny) and that in the lateral direction (Nx) of a
given sample of film are measured. The difference obtained
by subtracting Nx from Ny is the birefringence. In this
measurement, a sodium D ray is used as the light source and
methyl salicylate as the mounting medium.
(6) Ratio of strengths in two axial directions:
The tensile strength in the longitudinal direction, ay
(kg/mm2), and the tensile strength in the lateral direction,
~x (kg/mm2) of a sample of film are measured by the method
of ASTM D-882-67. The quotient of ~y divided by ~ x
represents the ratio of strengths.
~220S33
- 31 -
1 (7) Surface roughness (RmaX) The roughness,
RmaX, of a sample of film is measured by the method
described in JIS B-0601-1976. The cutof value is fixed at
0.5 mm.
(8) Ratio of thermal shrinkage: A specimen 200 mm
in length and 10 mm in width is cut from a sample of film,
with the longitudinal direction of the specimen taken as the
direction of measurement of this ratio. The specimen is
held for 15 minutes in an oven in which hot air at 120C is
circulated. After this, the specimen is removed from the
oven and measured for length at room temperature. ~ith L
representing the length (in mm) found by the measurement,
the ratio of thermal shrinkage is defined by the following
equation:
Ratio of thermal shrinkage (%) =
100 x (200 - L)/200.
(9) Degree of swelling of cable insulating layer
with insulating oil:
A lamination of a desired number of specimens each
30 mm x 30 mm is subjected to a pressure of about 1 kg/cm2
by a spring. The thickness of the lamination in the wound
state is noted as tl. In this state, the lamination is
dried to a desired extent and then immersed in insulating
oil. It is heated to a temperature to be tested, for
12Z0~33
- 32 -
1 example, 85 to 95C, and kept as it is for 4 to 24 hours to
swell the PP film completely. The thickness of the
lamination swelled completely is noted as t2. Then, the
degree of swelling (~) is found in accordance with the
following equation:
tl - t2
Degree of swelling (%) = 100 x t
(10) Fluidity of insulating oil:
A sample of film is wrapped around a conductor to
form a cable. This cable is immersed in the insulating oil
and then impregnated with the oil under a vacuum.
Thereafter, the cable is disassembled and visually examined
to determine whether or not the insulating oil has been
dispersed in all gaps in all the plies of the film. A
rating is made on a three-point scale defined as follows.
Rank A Thorough uniform dispersion of the oil
throughout the gaps.
Rank B: ~here are points at which slight
insufficiency of oil dispersion occurs.
Rank C: There are planes in which total absence
of oil dispersion occurs.
For use as an oil-immersion insulating material,
the film is required to be rated as Rank A. In applications
to low voltage cables, a film rated as Rank B may be
1220~33
- 33 -
1 acceptable. A film rated as Rank C should be rejected as an
oil-immersion insulating materialO
(11) Electrically insulating oil: This is a
generic term applicable to all known electrically insulating
oils such as mineral oil, castor oil, cottonseed oil,
alkylbenzene, diallyl alkanes, polybutene oil, and silicone
oil.
The present invention will now be describea more
specifically below with reference to actual examples and
comparative examples.
A volume of P~ resin pellets having an isotactic
structure content of 97.6~, melt index of 6 g/10 minute, and
a TmC of 110.5C were supplied to an extruder and melt
extruded through a T-shaped die at 260C in the form of a
sheet. The molten sheet was wound on a cooling drum at 30C
and allowed to cool and solidify to produce a sheet about
1000 microns in thickness. This sheet was passed between a
set of reducing rolls (roll diameter 250 mm) and rolled to
about 9 times the original length. The rolling was carried
out with a rolling pressure of 500 kg/cm and roll
temperature of 140C. The sheet surface was wetted with
polyethylene glycol. The rolled film about 90 microns in
thickness was introduced into an atmosphere at 130C and
subjected to a 10-second heat treatment, causing it to
1220~,33
- 3~ -
1 slacken by 1% in the longitudinal direction. Then, the film
was passed between embossing rolls held at 130l~ to transfer
print a sand blast pattern about 100 mesh in surface
roughness on both surfaces of the film. The film, in a
tense state, was held for 10 hours in an atmosphere at 120C
to carry out an aging heat treatment and thereafter left to
cool gradually to room temperature. This film was cut into
tapes 22 mm in width.
The properties exhibited by the PP film were as
follows:
Density (g/cm3): 0.910
Birefringence: 0.030
Ratio of strengths in two axial directions: 10.2
Ratio of thermal shrinkage (%): 1.3
Surface roughness (Rmax~ (microtls): 12~5
Vegree of swelling (%): 3.1
Average thickness of PP film (microns): 100
For comparison, a commercially available
nonstretched PP film and a biaxially oriented PP film were
tested for the same properties. The results were as
follows:
Biaxially
Nonstretched oriente~
PP film PP film
Density 0.899 0.905
:12Z053~3
1 Birefringence 0.003 0.013
Ratio of strengths in
two axial directions 1~3 1.9
Ratio of thermal
shrinkage (%) 0 2.8
Surface roughness
(Rmax) (microns) 0.6 0.9
Degree of swelling (~)
of flat PP film at 80C 11.6 6.3
The properties of the kraft paper used in
combination of the PP film were as follows:
Density (g/cm3): 0.80
Airtightness (Gurley sec): About 3000
Thickness (microns~: 100
Tapes 22 mm in width of the kraft paper were wound
with one-third overlap on a stranded conductor 200 mm2 in
cross section in a varying structure as indicated in the
Table below, The cables consequently obtained were dried
then impregnated with DDB at room temperature and left to
stand at lOO~C for 48 hours for thorough swelling of the PP
film. The cables were allowed to cool to normal room
temperature, bent twice alternately in opposite directions
into a loop of a diameter about 20 times the outside
diameter of the insulating layer, and disassembled for
visual inspection of the condition of the insulating layers.
~Z205~3
-- 36 --
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a Q a ~ 8 0 ~ c~. O _ '
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_ ~Vw ~ r O ~ ~ ~ O O ~ _ ¢
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lZZOS33
1From these results, it is noted that the
insulating layer of this invention swells only slightly with
the insulating oil, shows good fluidity of the insulating
oil and good bending properties, possesses a high impulse
5strength, experiences only nominal loss of positive impulse
strength, permits attainment of the desired value of E-tan ~
and, thereforet proves highly advantageous for use in the
production of an oil-immersion electrically insulated cable.
As mentioned hereinbeforel the inventors have
10succeeded in realizing an oil-impregnated insulating power
cable which is of high quality and high practicability by
using a specific combination of PP film and kraft paper.
The loss (~-tan ~) is limited by the thickness
limitation of the kraft paper as mentioned previously. In
15order to reduce the loss, it is necessary to reduce the
thickness of the paper as a whole below this lower limit
value of about 70 microns, which is impossible for reasons
mentioned before.
According to the present invention, a thinner
20kraft paper can be used together with PP film. The PP film
to be used together with the thinner kraft paper should be a
low swelling PP film having roughened surfaces.
Two sheets of kraft paper sandwich the low
swelling PP film to form a multiple (in this case three)
~2Z0533
- 38 -
1 layer laminated structure, referred to as PP laminated
paper, to be used as a substitute for the kraft paper.
Table 2 shows examples of specifications of the PP laminated
paper.
Table 2
Sample lamination A B C
Total thickness (um) 125 155 200
PP thickness (7~m) 45 55 80
Paper thickness (7~m) 40.40 40.60 60.60
Density (g/cm3) 0.88 0.90 0.90
Deg. of Swelling at 100C (%) 4.0 3.8 3.3
f 2.75 2.79 2.81
tan ~ (%~ 0.07 0.08 0.09
~tan ~ l.9xO.1 2.2xO.1 2.5xO.l
Assuming a value of ~tan ~ of the PP laminated
paper of 2.8 x 0.1 %, the value of ~tan ~ of the cable
having an insulating layer prepared by alternately winding
the PP film and the PP laminated paper is:
~(2.2 + 2.8)/2j~[(0.02Q + 0.1%)/2~ = 2.5 x 0.06%.
The value of ~ tan ~ of a cable having an
insulating layer prepared by winding a combination tape of
one PP laminated paper and two PP films is:
[(2~2 x 2 + 2.8)/3]~L(0.02% x 2 ~ 0.1%~/3]
= 2.4 x 0.047%.
1220~33
- 39 -
1 The LatiOS of these values to those of a cable
having an insulating layer composed of only the kraft paper
are, respectively, as follows:
(2.5 x 0.06)/(3.4 x 0.2) = 0.22, and
~2.4 x 0.047)/(2.4 x 0.2~ = 0.17.
As is clear from these ratios, the use of the PP
laminated paper according to the present invention results
;n a remarkably reduced loss in the cable.
Table 3 shows comparative data of cables having
insulating layer composed of PP films and the PP laminated
papers. In Table 3, sample no. 1 is the same as sample no.
- 1 in Table 1.
Table 3
Sample No. 1 10 11
insulating kraft paper rolled PP film rolled PP film
tape and PP laminated and PP lami-
paper nated paper
sample(1) change of (1) ~1)
structure layer by } do. } do.
six plies (2) (2)
~2) three (3) alternate (3) alternate
plies combination combination
of a laminated of a lami-
PP paper and a nated PP
PP film paper and
two films
E 3.4 2.52 2.43
tan ~ (%) 0.23 0.055 0.049
~220~33
- 40 -
1 negative
im~ulse 1.0 1.9 1.8
positive
impulse 0.95 1.4 1.
fluidity of
insulating
oil A A A
Results of no
disassembly abnormality do. doO
As is clear from Table 3, the test results fo~
samples Nos. 10 and 11 show that these are usable i~
practice. In producing a cable on the basis of either
sample No. 10 or ~o. 11, it may be necessary to roughen the
surfaces of the PP film to the extent of 20 to 40 microns or
to roughen the surfaces of the PP film to the extent of 5 to
10 microns and those of the PP laminated paper to the extent
of 3 to 10 microns, while the tension of the tape when wound
is controlled to be sufficiently small.
According to another embodiment of the present
invention, which is shown in Fig. 5, the high stress
produced around the conductor 10 of an AC cable can be
minimized to further improve the dielectric breakdown
strength of the insulating layer thereof by employment of a
so-called "~-graded insulating layer" in which a portion of
the insulating layer 11 adiacent the conductor 10 has a
value of ~ which is reduced with the distance from the
conductor. According to this embodiment~ the thickness o~
lZ20S33
1 the insulating layer can be reduced correspondingly, and
thus the size of such a cable can be reduced. It is as
important as the reduction of ~-tan ~ to reduce the size of
the cable by reducing the thickness of the insulating layer,
This is particularly true for cable used in the EHV to U~V
ranges.
In Fig. 5, the insulating layer is composed of
five layers 11 to 15, in which the layer 11 is formed of
kraft paper, a layer 12 of an alternating combination of a
sheet of kraft paper and a sheet of the PP film, a layer, 3
of an alternating combination of a sheet of kraft paper and
two sheets of the PP film, a layer of an alternating
combination of a sheet of the PP laminated paper and a sheet
of the PP film and a layer of an alternating combination of
a sheet of the PP laminated paper and two sheet of the PP
films.
Table 4 shows data of a typical example of the e-
graded cable shown in Fig. 5.
~2Z0~33
- 42 -
1 Table 4
Insulating
layer Construction ~-tan ~ (%)
Conductor
side 11 kraft paper 3.4x0.2
12 alternate combination of 2.8x0.1
a kraft paper ana a
rolled PP f ilm
13 alternate combination of 2.6x0.087
a kraft paper and two
rolled PP films
14 alternate combination of 2.5x0.06
a PP laminated paper and
a rolled PP film
oute 15 alternate combination of 2.4x0.047
surface of a PP laminated paper and
insulating two rolled PP films
laye~
In Table 4, the dielectric loss t~ tan ~) of the
kraft paper, the PP laminated paper and the low swelling PP
film are about 3.4 x 0.2 (%)~ 2.8 x 0.1 (%) and 2.2 x 0.02
(%), respectively. One or more layers among the layers 1 to
5 in Table 4 may be omitted, if necessary, according to the
class of the cable.
The cable having the ~-graded insulating layer
exhibits an improvement of the dielectric breakdown voltage
by 3 to 10% relative to a cable having no ~-graded layer.
As described hereinbefore, the insulating oil
impregnated power cables according to the present invention
which utilize as at least portions of its insulating layer a
~ZZ0~33
- 43 -
1 PP film having a low swelling and good mechanical properties
and a kraft paper of a natural polar material or a PP
laminated paper, with at least portions thereof being
roughened, exhibits remarkably improvements in dielectric
loss, dielectric breakdown voltage, and reliability.
