Note: Descriptions are shown in the official language in which they were submitted.
1057043
This invention relates to a power cable wherein
the deterioration of plastic insulation caused by a phenomenon
called "water tree" is effectively prevented and to the acces-
sories of the cable. More particularly it relates to a method
of preventing such deterioration of plastic electrical insula-
tion used for power cables and accessories, the method being
suitable for application to power cables for relatively low
voltage of the class 3 to 22 KV, through to high voltage power
cable of the class 66 to 154 KV.
It is well known that power cables insulated with
plastic typified by cross-linked polyethylene, polyethylene,
ethylene-propylene rubber and butyl rubber possess many advan-
tages. On account of these advantages, plastic insulated
power cables are widely used. When they have been in service
for a long period of time, however, a phenomenon called "water
tree" occurs wherein exterior water enters the insulation and
diffuses and condenses therein. The insulation material of
the cable thus deteriorates and dielectric breakdown may
result. In order to prevent this, a metal sheath of lead,
aluminum or the like is generally provided over the insulation
layer.
"Water tree" can be prevented in a cable that is
provided with a metal sheath. However, the cable is then not
only very expensive but also is difficult to handle due to
the increased weight by the addition of the metal layer.
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1057043
These are the drawbacks of the conventional cables.
Generally, this invention provides a power cable
in which such drawbacks are alleviated and a method is pro-
vided for prevention of "water tree" in the insulation layer
of the cable.
The mechanism of the initiation and development
of the "water tree" phenomenon has not been previously known.
The present inventors, however, have successfully clarified
the mechanism and then arrived at the method of this inven-
tion preventing the occurrence of "water tree".
"Water tree" takes place where the electric fieldis strong and a mass of minute voids filled with water is
formed. The voids are formed when the chemical potential~w
of the water contained in the minute voids is decreased by an
electric field. This can be expressed in the following
formula:
~w = ~o ~ 2 o(7p)TEl _____--- (1)
In Formula (1) above,~O represents the chemical
potential of water in the minute voids when there is no
electric field; ~ the specific inductive capacity of water;
~O the dielectric constant of vacuum; p the density of water;
El the electric field in a mass of minute voids filled with
water; and T the temperature.
In Formula (1), (~ap)T ~ 0
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1057043
Therefore, ~w~ and the water where there is no
electric field, namely external water, enters and diffuses into
the minute voids. As a result of this, the minute voids
increase to form the "water tree".
Assuming that the minute voids are of spherical
shape and that the electric field in the insulation of the
cable is Eo, the value of El can be obtained from the following
formula:
E = 3~ Eo _______- (2)
~2 + 2~1
wherein 2 = C_ j-~/w~o (complex specific inductive
capacity) -------- (3)
a: conductivity of water
w: angular frequency
As apparent from Formula (1), the smaller the
value of El, the lower the growth rate of the "water tree"
will be. Accordingly, "water tree" can be prevented by
making the value of El smaller.
In accordance with this invention, an electrolyte
which dissolves in water and increases the electric conduc-
tivity of the water is added to the insulation material duringpreparation of the insulator to provide a small value of El
80 that "water tree" can be prevented. In other words, the
principle that El in Formula (2) becomes smaller as ~ in
Formula (3) becomes greater is utilized in the invention.
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The invention will become more apparent from the
following description:
Any inorganic or organic matter that dissolves
in water to increase the electric conductivity of the water
may be employed as the electrolyte. The preferred electro-
lytes for use accoxding to the invention include strong elec-
trolytes as for example sodium chlorlde, sodium sulfate,
potassium chloride, potassium sulfate, other alkali metal
salts, alkali earth metal salts, ammonium chloride and other
ammonium salts, cupric sulfate and other metal salts, sodium
acetate, other salts of other carboylic acid, salts of organic
sulfonic acids, etc., of which sodium sulfate is most effec-
tive in preventing the "water tree". These electrolytes
neither move in the insulation, nor enter into the insulation.
The quantity of the electrolyte to be added is
at least 10 7% by weight of the insulation material. Addi-
tion of the electrolyte in excessive quantities would produce
an adverse effect in terms of the insulation characteristics
and the infiltration of water due to osmotic pressure. To
avoid this, the amount added should be less than 1% by weight
of the insulation material.
To ensure an effective prevention of "water tree",
it is also preferable to disperse the electrolyte as evenly
as possible, in the form of micro-particles of less than
several~m in the plastic material which is used to form the
insulation. The best results can be obtained by the use of
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plastic insulation formed with plastic material prepared
with the micro-particles of the electrolyte evenly dispersed
therein.
The process for achieving even dispersion of the
minute particles of an electrolyte measuring less than
severalJ~m may be selected from the following:
(1) Mixing of micro-particles of electrolyte
and the insulation material in a mixer extruder called
"Brabender Plastograph" (Trade Mark) or by rolls or the
like;
(2) Dissolving the particles of electrolyte in,
for example, water or alcohol, and mixing the solution with
the insulation material by suitable means as for example
rolls or the mixer extruder called "Brabender Plastograph"
(Trade Mark);
~ 3) Immersing pelletized insulation material
into the aforementioned solution. Then, after the micro-
particles have been made to stick to the surfaces of the
pellets by evaporation of the solvent, molding the insulation
by a conventional process using an extruder or the like;
(4) Adding the aforementioned particles of
electrolyte to additives usually used with the insulation
materials. The mixture is then used in molding the insulation.
The electrolyte material can be added by any of
the foregoing processes. Furthermore, in the case of insu-
lations that are molded by a method other than extrusion
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molding, as for example mold joints, the electrolyte material
may be added beforeh~nd to the plastic tape which is a mold
material, or the micro-particles of the electrolyte may
be allowed to stick to the tape surface beforehand. With
this arrangement the "water tree" resulting from any flaws
in the adhesive surfaces can be effectively prevented.
The features and advantages of this invention
will more fully appear from the following detailed descrip-
tion taken in connection with the accompanying drawing which
is a cross-sectional view illustrating a plastic insulated
power cable.
In the drawing, the reference numeral 1 indicates
a core conductor and 3 a plastic insulation layer. Normally
a semiconductive layer 2 is provided between the core conduc-
tor 1 and the plastic insulation layer 3. Outwardly of the
insulation layer 3, there is provided a plastic sheath 6,
made for example of polyvinyl chloride, there being another
semiconductive layer 4 and a screening layer 5 of copper
tape therebetween.
When the present invention is applied to the
plastic insulated power cable illustrated in the drawing,
however, the screening layer 5 is no longer required and can
be omitted. Then, the structure of the cable can be simplified
with the electrolyte mixed in the plastic insulation layer 3
and with the insulation arranged round the conductor 1.
The particular advantages to the plastic insulated
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power cable prepared in accor~ance with this invention include:
(1) The use of a metal sheath for preventing
the infiltration of water is no longer required.
(2) "Water tree" can be prevented even if
there are foreign matter and voids in the insulation layer
or flaws on the surface of the insulation layer and the
semiconductive layer such as protrusionsor the like.
(3) Since the electrolyte to be added for the
purpose of preventing "water tree" is available at a low cost,
the use of it causes negligible increase in the cost of
material. Such increase in the cost of material is only
about 1% of, for example, the cost of a cros~-linked poly-
ethylene insulated power cable, while the metal sheath which
is employed conventionally to prevent "water tree" causes an
increase in cost of 100%.
(4) The addition of the electrolyte does not
cause any increase in weight and any difficulty in the work
with the cable.
(5) The electric characteristics of the power
cable are unaffected by the addition of the electrolyte.
The following description covers some of the
preferred embodiments of this invention by way of examples:
EXAMPLE 1
Micro-particles (grain size not exceeding l~ m)
of sodium chloride, sodium sulfate, ammonium chloride, copper
sulfate or sodium acetate were added in the proportions shown
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57043
in Table 1 and mixed with DCP (di-cumyl peroxide) which is
employed as a cross-linking agent for polyethylenP. Using
each mixture, a cross-linked polyethylene insulated power
cable of the class of 6 KV was prepared by a conventional
method. Each of the power cable samples prepared in this man-
ner was subjected to a test carried out by immersing it in
water and applying high voltage of 8 KV thereto for a
period of 180 days. After the test, the samples were checked
for "water tree". However, no samples showed occurrence of
'Iwater tree", while a sample of power cable which had been
prepared without addition of electrolyte showed the occur-
rence of "water tree" as shown in Table 1 as a comparison
example.
TABLE 1
Electrolyte added Amount added to DCP (wt %) "Watbr Tree"
NaCl 0.01 None
" O.1 "
" 1 1'
a2S4 0 05
" 0.1 "
NH4C1 0.1
CuSO4 0.2 ,.
Sodium acetate 0.5 "
Not added -- Occurred
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EXAMPLE 2
Micro-particles of sodium sulfate of grain size
not exceeding l~m were mixed with polyethylene in a propor-
tion of 0.01% using a "Brabender Plastograph". Using this
mixture, a cross-linked polyethylene cable of the class of
6 KV similar to those described in Example 1 was prepared.
The cable was subjected to water immersion and high voltage
of 8 KV for 180 days in the same manner as in Example 1.
However, no "water tree" occurred.
EXAMPLE 3
An aqueous solution of sodium sulfate was prepared
and polyethylene pellets immersed therein and then removed.
The aqueous solution adhering to the surfaces of the pellets
was quickly dried with hot air. The proportion of sodium
sulfate deposited and adhering to the pellet surfaces to
that of the polyethylene was 0.02~ by weight. A power cable
similar to those of Example 1 was prepared using the poly-
ethylene pellets. The power cable did not show any "water
tree" after it had undergone the same test as in Example 1.
EXAMPLE 4
An aqueous solution of sodium sulfate was prepared
and the solution added while polyethylene was subjected to a
roll mixing process. The solution was evaporated during the
process and the sodium sulfate mixed with the polyethylene.
The polyethylene composition thus obtained was used for the
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~05~043
preparation of a power cable, which was subjected to the test
in the same manner as in Example 1. However, no "water tree"
was formed.
EXAMPLE 5
Sodium chloride or sodium sulfate were adhered to
the surfaces of polyethylene pellets by the same method as
in Example 3. Then, using the pellets, cross-linkable poly-
ethylene tapes for mold joints were prepared. Following this,
cross-linked polyethylene mold joints of the class of 20 XV
were prepared from the tapes. Each joint sample thus obtained
was subjected to a test carried out for a period of 12 months
by placing the joint in water and applying high voltage of 8 KV
thereto. Table 2 shows the test results for those samples
in comparison with a sample which was prepared without such
additives. Table 2 also shows a sample which was prepared
by adhering sodium sulfate to the tape surface only.
TABLE 2
AdditivesRatio to polyethylene, wt % "Water tree"
NaCl 0.05 None
" 0.1 "
Na2S4 0 05
~ O.1 "
No additive --- Occurred
Na2SO4 only to 0.01 None
tape surface
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As shown in Examples 1 through 5, "water tree" of
the plastic insulations of power cables and accessories can be
effectively prevented in accordance with this invention. With
the conventionally employed water screening layer such as a
metal sheath thus no longer required for preventing "water
tree", the invented method gives a great advantage in terms
of costs.
E ~ ~LES 6 - 9 AND COMPARISON EXAMPLES 1 AND 2
Cross-linkable polyethylene tapes were prepared
from compositions which were obtained by blending 0.002 part
by weight (Example 6), 0.02 part by weight (Example 7),
0.2 part by weight (Example 8) and 0.5 part by weight
(Example 9) respectively of sodium sulfate with 100 parts
by weight of polyethylene. Using each of the non-bridged
polyethylene tapes, a mold joint part of 20 KV cross-linked
polyethylene insulated power cable was prepared.
In addition to those joint part samples, joint
parts were also prepared, for comparison, one from a cross-
linkable polyethylene tape (Comparison Example 1) and another
from another cross-linkable polyethylene tape made of poly-
ethylene containing a cross-linking agent with 0.5 part by
weight of talc blended in 100 parts of the polyethylene
(Comparison Example 2). These comparison samples of mold
joint parts of 20 KV cross-linked polyethylene insulated power
cables were prepared in the same manner as the other samples.
The six different mold joint parts of 20 RV
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1057043
cross-linked polyethylene insulated power cables prepared as
described were subjected to tests in water with high voltage
of 8 KV applied for a period of 18 months. After the test,
each mold joint sample was examined for the presence or
absence of "water tree" and also for dielectric strength.
The test results are shown in Table 3.
TA~3LE 3
Kinds of "Water tree" in Dielectric strength
mold_~ints insulator layer Before test Aftertest
Sample of Example 6 None More than AC180KV AC190 KV
Sample of Example 7 " " " " AC200 KV
Sample of Example 8 " " " " AC180 KV
Sample of Example 9 " " " " AC200 KV
Comparison Example 1 Occurred " " " AC 80 KV
Comparison Example 2 " " " " AC 70 KV
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