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XLPE
INSULATED POWER CABLES
CABLE (Al-)XLPE/SWA/PVC 6-35
kV
XLPE, Cross-linked polyethylene, has stable intermolecular bonds
between polyethylene particles created by thermochemical action, due to
the presence of organic peroxide. XLPE has the same electricl
properties as the conventional polyethylene and as result of stable
intermolecular bonds, has much better thermal and mechanical properties.
This provides XLPE insulated cables to be used at maximum continuous
conductor temperature of 90 �C while conventional polyethylene insulated
cables to be used at that of 75 �C. Maximum overload conductor operating
temperature is 130 �C ( operating at the emergency overload temperature
shall not exceed 100 hours per year). Maximum conductor short-circuit
temperature is 250 �C. This provides an important advantage in cable
ratings and is of special significance in countries or situations where
the ambient temperature is high and derating factors have to be
applied. The cross-linking tehnology is based on entirely "dry"
cross-linking and cooling process. This eliminates micro inclusions of
water from the insulation which, unless prevented, lead to so called
"water treeing" effect. A triple crosshead extrudes the screen over the
conductor and screen over insulation in a single technological operation.
All that gives the cable excellent electrical, thermal and mechanical
properties.
DESCRIPTION Three-core XLPE/SWA/PVC
cable is being made in accordance with IEC 60502/98. Copper or
aluminium compacted stranded circular conductor, conductor screen of
extruded layer of semi-conducting XLPE, XLPE insulation, insulation screen
of extruded layer of semi conducting XLPE, semi-conducting bedding tape,
metallic screen consisting of copper wires applied helically with a
counterspiral of copper tape. Three cores are being laid-up together with
suitable fillers, covered with PVC separation sheath, armoured with
galvanized steel wires applied helically and covered with PVC outersheath
over in black colour.
APPLICATION XLPE-insulated
cables can be used for nearly all installations : in the ground, in cable
ducts, indoors, in power stations, in industrial plants , in switching
stations. Thanks to their better thermal characteristics they are
particularly recommended where :
- high abmient temperatures exict
- cables are exposed to the sun
- cables are laid in ducts in a concrete
block
- cables are grouped
- high emergency loads are important
- high short-circuit currents are required
- high earth leakage currents have to be
transmitted by the screens
Minimum bending
radius:
|
cable during instalation |
cable placed in position
adjacent to joints or terminations |
NON-ARMOURED |
15D |
12D |
ARMOURED |
12D |
10D |
D - outer cable
diameter
Current carrying capacity has been
calculated according to IEC 287/82 (100 % load factor) and following
conditions:
- Depth of laying 0.70 m, and for 20/35 kV
cables 1.0 m.
- Soil thermal resistivity 1.0 K
m/W.
- Ground temperature 20oC,
ambient air temperature 30oC, maximum conductor operating
temperature 90oC.
Sheaths bonded at both ends of an
electrical section
RATING FACTORS
|
Ambient temperature
oC |
15 |
20 |
25 |
30 |
35 |
40 |
Factor |
Cable in
ground |
1.04 |
1.00 |
0.96 |
0.93 |
0.89 |
0.85 |
Cable in air |
1.14 |
1.09 |
1.05 |
1.00 |
0.95 |
0.90 |
|
Soil thermal
resistivity (Km/W) |
Factor |
0.7 |
1.0 |
1.2 |
1.5 |
2.0 |
2.5 |
3.0 |
1.12 |
1.00 |
0.94 |
0.86 |
0.77 |
0.71 |
0.66 |
COMPARISON OF CHARACTERISTIC VALUES OF
INSULATING COMPAUNDS
|
Type of insulating
compaunds |
UPET |
PET |
PVC |
Maximum continous conductor operating temperature
oC |
90 |
70 |
65 |
Maximum conductor short-circuit temperature
oC |
250 |
150 |
160 |
Dielectric power factor at 50Hz,
20oC |
4 x
10-4 |
2 x 10
-4 |
10-2 |
Relative permittivity at 50Hz,
20oC |
2.3 |
2.3 |
4-8 |
Volume resistivity at 20oC
ohm.cm |
1017 |
1017 |
1014 |
Thermal resistivity K.m/W |
3.5 |
3.5 |
6.0 |
Tensile strenth, minimum
N/cm2 |
200 |
150 |
250 |
Elongation-at-break, minimum % |
350 |
550 |
250 | |