Since about 1970,
the cross-linked Polyethylene (XLPE) -insulated
power cables have been used in Germany. This
insulation possesses very good electrical, mechanical
and thermal characteristics in medium voltage
networks. This type of insulation has excellent
chemical resistance and is also resistant to
cold temperatures. Due to various advantages,
the XLPE-insulated type has vastly displaced
the traditional classic paper-insulated types
in many sectors.
In order to prevent
the penetration of moisture and also to extend
the duration of life, the XLPE insulated medium
voltage cables are designed with longitudinally
waterproof shielding including an additional
swell tape and PE outer jacket.
The manufacture of this jacket is based on high
density polyethylene (HDPE), in which an additive
organic peroxide is mixed. Due to the heating
and pressure the molecule chains are joined together,
assuring the transition from thermo plastic to
elastic condition.
In comparison to PVC and paper-insulated cables,
the advantage of XLPE-insulated medium voltage
power cables is that they possess a low
dielectric factor, such as it is 100 times smaller
than of PVC-insulated cables.
Moreover, a better dielectric constant value
has has an effect on the low mutual capacitance,
the short circuit to ground and the charging
current of XLPE-insulated cables.
The good properties of these cables remain constant
at a wide temperature range.
Characteristics
of XLPE
Permissible
operating temperature:
For permanent
(normal) operation:
+ 90°C/+194°F
In short circuit:
+250°C/+482°F
In overload operation
and damage by sea:
up to +130°C/266° F
Specific
heat resistance:
3.5 K x m/W
Dielectric constant:
2.4
Specific resistance
(20°C):
min. 1016 Ohm
x cm
Loss factor (tan δ)
(20°C):
max. 0.5 x 10-3
Density:
0.92 g/cm3
Breaking strength:
min. 200%
Tensile strength:
min. 12.5 N/mm2
Conductor & Inner
Semi-Conducting Layer
Conductor
Copper or Aluminum, round, multi-wire
stranded and compact, according to VDE
0295 and HD 383.
Inner Semi-Conducting Layer
Semi-conducting compound, cross-linked,
min. wall thickness 0.3 mm.
Insulation
Cross-linked Polyethylene
(XLPE), compound type 2XI1 according to DIN
VDE 0207 part 22 an HD 620.1
Insulation nominal wall thickness
6/10
kV
=
3.4
mm
12/20
kV
=
5.5
mm
18/30
kV
=
8.0
mm
Outer
Semi-Conducting Layer
Outer semi-conducting layer is
extruded together with the inner
semi-conducting layer and the insulation
in one working process and are spliced
with each other
Semi-conducting compound, cross-linked,
wall thickness 0.3 to 0.6 mm
Concentricity
of Conductor
The difference between the maximum and
minimum value of 0.5 mm should not be exceeded.
Semi-Conducting
Type
Over the outer semi-conducting layer, a
semi-conducting tape must be used.
Shielding
Shielding of copper wires must have a minimum
diameter of 0.5 mm and over that a copper
tape applied helically with a minimum thickness
of 0.1 mm.
Copper cross-section according to DIN VDE
0273 and 0276.
Separator
Over the shielding as well as under the
outer jacket a separating layer must be used
(e.g. tape).
Outer
Jacket
PE compound DMP2 according to HD 620.1
and 2YM3 to DIN VDE 0276 part 3, black, or
PVC compound DMV6 according to HD 620.1 and
YM5 to DIN VDE 0207 part 5, red
Wall thickness = 2.5 mm, for 1 x 500 mm2 /
30 kV = 2.6 mm
Cable
Installation
In order to avoid any damage,
the XLPE insulated medium voltage power cables
should be carefully laid and installed. It
must be ensured that the cables should not
be pulled over hard or sharp edges. The cable
ends must be sealed water-tight. After cutting
the length both ends must be sealed immediately.
An installation depth of 60 to 80 cm is recommended.
Single conductor cables are normally arranged
in a trefoil touching or triangular shape.
For installation in conduits, especially the
influence of thermal insulation of air space
between the cable and the inner wall of the conduit
should be considered. The inner diameter of the
conduit should be at least 1.5 times that of
the diameter of the cable.
Bending
Radius
During the installation
of XLPE cables, the bending radius should not
be below the following values:
Cable without
metal jacket
=
15
x cable Ø
Cable with
aluminum-laminated jacket
=
30 x
cable Ø
Temperature
Range
During the installation,
the temperature should not be below the following
values:
For XLPE
insulation + PVC jacket
=
- 5°C
For XLPE
insulation + PE jacket
=
-20°C
Max.
Permissible Tensile Strength
By pulling the conductors
with a pulling head (not for armored cables)
P = No. of conductors x conductor cross-section
x δ
δ = permissible pulling tension N/mm2
- for copper conductor:
50 N/mm2
- for aluminum conductor:
30 N/mm2
Current
Carrying Capacity
According to VDE 0276 part
620, -5C or HD 620 S1
Laying
Underground
Laying depth:
0.7 - 0.8 m
Ground temperature in
laying depth:
+20°C/+68°F
Specific heat resistance:
1.0 K m/W
Load factor:
0.7 (EVU-load)
Outdoor
Installation
Air temperature:
+30°C/86°F
Load factor (permanent
load):
1.0
Installation
in Conduits
Cables for conduit system
installation in ground, a reduction of the
current carrying capacity with a factor of
0.85 is recommended.
Test
Voltages
Kind of Voltage
Test
Voltage
Test in kV
U0/U
= 6/10 kV
U0/U
= 12/20 kV
U0/U
= 18/30 kV
Voltage Test a.c. in
kV
15
30
45
Voltage Test d.c. in
kV
48
96
144
Voltage Test a.c. (1000
h)
18
36
54
Voltage
Test of Cable System
During the operation or
after installing the medium voltage power cables,
the dielectric can be tested with alternating
or direct current. The test duration continues
30 minutes.
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