6kV / 10kV Medium Voltage 2 Phase Cable Xlpe Insulated Wire Fire Resistant

6kV / 10kV Medium Voltage 2 Phase Cable Xlpe Insulated Wire Fire Resistant

Since about 1970 the cross-linked polyethylene (XLPE)-insulated power cables have been used in Germany. The XLPE-insulation possesses very good electrical, mechanical and thermal characteristics in

medium voltage networks. This type of insulation is very chemically resistant and also resistant to cold. Due to various advantages, the XLPE-insulated type has vastly displaced the traditional classical 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 water-proof screen including an additional swell tape and a PE outer sheath.
The manufacture of this sheath 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 to each other, assuring the transition from thermoplastic 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 an effect on the low mutual capacitance, the short-circuit to ground and the charging current of XLPE-insulated cables.
The good properties of XLPE-insulated cables remain constant through a wide temperature range.
 
Characteristics of XLPE

• permissible operating temperature

• For permanent (normal) operation + 90°C
• In short circuit +250°C
• In overload operation and

damage by sea up to +130°C
 

• Specific heat resistance 3,5 K · m/W
• Dielectric constant 2,4
• Specific resistance (20°C) min. 1016 Ohm · cm
• Loss factor (tan o) (20°C) max. 0,5 · 10–3
• Density 0,92 g/cm3
• Breaking strength min. 200%
• Tensile strength min. 12,5 N/mm2

Conductor

• Copper or aluminium, round, multiwire stranded and compact, according to VDE 0295.

Inner semi-conducting layer

• Semi-conducting compound, cross-linked, mini-mum wall-thickness 0,3 mm.

Insulation

• Cross-linked polyethylene (XLPE), compound type DIX8 according to HD 620 S2.
• Insulation nominal wall-thickness for 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 mini-mum 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.

Screen

• Screening 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 HD 620 S2.

Separator

• Over the screen as well as under outer sheath, a separating layer must be used (e. g. taping).

Outer sheath

• PE compound DMP2 according to HD 620 S2 black or
• PVC compound DMV6 according to HD 620 S2 red
• Wall-thickness = 2,5 mm,

for 1x 500 mm2 /30 kV = 2,6 mm

Laying of Cable

In order to avoid any damage, the XLPE-insulated medium voltage cables should carefully laid and installed. It must be ensured that the cables should not be pulled over the hard or sharp edges.

The cable ends must be water-tight-sealed. After cutting the length both ends must be sealed immediately.

A laying depth of 60 to 80 cm is recommended. Single conductor cables are normally arranged in a trefoil touching or triangular shape.

For laying in conduits, the influence of thermal insulation of air space between the cable and the inner wall of conduit should be specially 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 laying of XLPE cables, the bending radius should not be below of the following values:

• Cable without metal sheath = 15 x cable Ø
• Cable with Alu-laminated sheath = 30 x cable Ø

Temperature range

During the installation, the temperature should not be below the following values:

• for XLPE-insulation + PVC sheath = – 5°C
• for XLPE-insulation + PE sheath = –20°C

Max. permissible tensile strength

By pulling the conductors with a pulling head (not for armoured cables)
 

P = No. of cores x conductor cross-section x o

o = permissible pulling tension N/mm2

– For Cu-conductor: 50 N/mm2

– For Alu-conductor: 30 N/mm2
 

Current carrying capacity

according to VDE 0276 part 620 or HD 620 S2
 

Laying in earth (ground)

• Laying depth 0,7 – 0,8 m
• Earth temperature

in the laying depth 20°C

• Specific heat resistance 1,0 K · m/W
• Load factor 0,7 (EVU-Load)

Laying in air

• Air temperature 30°C
• Load factor (permanent load) 1,0

Laying in conduits

Cables for conduit systems laying in earth, a reduction of the current carrying capacity with a factor of

0,85 is recommended.
 

Test voltages
 

Voltage test to cable system

During the operation or after laying the medium voltage power cables, the dielectric can be tested with

alternating or direct current. The test duration continues 30 minutes.