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The basics of high voltage cables for underground transmission
The basics of high voltage cables for underground transmission (on photo: London Power Tunnel Highbury 132kV Cables; credit: murphygroup.co.uk)

Thermal Design of Cables

Underground cables are installed in trenches of rectangular cross-section. After excavation of the trench, a layer of sand is placed in it to serve as a bedding, as shown in Figure 1.

The length of cable is pulled in along the trench and covered with a further layer of sand. Sand free from flints and stone is employed to avoid damage to the cable serving during pulling and initial back filling. Above the cable and sand bedding are placed cover tiles to protect the cable from mechanical damage from subsequent excavation activities.

 Cross-section of Trench and buried cable
Figure 2 – Cross-section of Trench and buried cable

The excavated material is replaced in the trench and stamped to consolidate it. The minimum trench width that can be conveniently excavated is about 700 mm (27 inches), and for safety reasons, the minimum depth of burial in normal circumstances is 900 mm (36 inches).

An underground cable carrying current will have in addition to the conductor loss, dielectric loss and losses in the sheath. These produce heat which are conducted away from the cable to the surface, producing a temperature gradient.

When more than one single core cable is laid together (as is required for three phase systems exceeding 150 kV), the heat produced by one conductor affects the other and the heat factors need to be modified. When the spacing between the cables is increased, the heat produced by the circulating currents between the cables will increase whereas the eddy current losses decrease.

Thus there is an optimum spacing for cables and various alternatives may have to be evaluated before the economic arrangement is finally selected.


Current rating of Cables

In a cable, the factor which ultimately limits the current carrying capacity is the maximum operating temperature which may be sustained by the cable throughout its life without risk of damage or deterioration.

The heat generated in the cable is due to (a) ohmic loss in the conductor, (b) the dielectric loss in the insulating medium and (c) the sheath and intersheath losses.

The heat so generated is radiated to the surroundings. The current that can be carried depends on the conductivity of the surrounding medium as well, so that the same cable would have different ratings depending on whether the cable is buried or not.

Title:The basics of high voltage cables for underground energy transmission  – J. R. Lucas
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The basics of high voltage cables for underground energy transmission
The basics of high voltage cables for underground energy transmission

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