Present Score of Cables in A number of Backfills Utilizing the Finite Factor Technique
The thermal resistance of the exterior surroundings which surrounds energy cables has the most important influence on the temperature rise of the conductor and therefore the present ranking in contrast with different elements. That’s the reason loads of effort is taken by asset homeowners and cable techniques designers to scale back the exterior thermal resistance for cables to maximise their current-carrying capability.
It’s common follow to put in buried teams of low- or medium voltage cables inside duct banks which permits circuits to be well-organised and guarded in the identical trench (or a number of trenches). Concrete is commonly used to encompass the cables in a duct financial institution and has comparatively low thermal resistivity but in addition just isn’t topic to moisture migration (drying-out impact), which ensures greater cable present rankings.
Excessive-voltage cables are generally buried in trenches and surrounded with an accredited bedding materials (sometimes thermally stabilised sand) which has appropriate low thermal resistivity. Backfilling is handiest when the thermal resistivity of the native soil is excessive but in addition will be an efficient technique of avoiding moisture migration (resulting in excessive thermal resistivities) within the neighborhood of the cables. A number of transmission cable circuits could also be put in in separate backfills inside the identical transmission easement. Joint bays made from precast concrete sections or manufactured on-site and will be unfilled or crammed with stabilised sand.
As soon as cables are put in in a duct financial institution or a backfilled trench the remaining house left is crammed both with the native soil (typically sieved for rocks) or a special materials introduced in from off-site.
Essentially the most extensively used paperwork for cable present ranking calculations are the IEC 60287 [ref. 1] sequence of requirements (which adopts an equal thermal mannequin solved utilizing analytical strategies). Whereas most real-world circumstances together with these described above contain a number of zones with completely different thermal resistivity, the IEC normal gives a calculation technique to permit for the presence of a single a duct financial institution or backfill solely and the size are additionally restricted.
For correct present ranking calculations of cables put in in multi-layered or multi-zoned soils with various thermal resistivity the Finite Factor Technique (FEM) is confirmed to be extremely efficient.
The IEC 60287 and FEM calculations herein had been carried out utilizing ELEK Cable HV Software program™.