Cables in Troughs

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The thermal rating of cables installed in unfilled or in filled troughs is determined using the CYMCAP/UNF and the CYMCAP/MDB modules respectively.

Features

In these modules, a trough (or a trench) is defined to be a long shallow rectangular-shaped excavation, where the walls, bottom and cover are made of concrete. The cables can be installed on the floor, hanging from supports on the walls or racks. The trough can be filled with a material with good thermal properties or it can be left unfilled (air filled). The heat transfer mechanism is different for filled and unfilled troughs and therefore they are treated independently.

Unfilled Troughs

Initially, the only option to rate unfilled trough installations was to use the IEC standard. In this approach the cables ratings are calculated as for cables in free air, but the temperature inside the trough is computed according to the IEC Standard 60287-2-1©. The module has been improved significantly and includes three options in addition to the IEC standard to model a given trough installation: Slaninka Method 1, Slaninka Method 2 and Anders-Coates Method.

With the IEC standard, thermal resistivities of the soil and the trough’s cover are ignored. With the Slaninka Method 1, the thermal resistivity of the trough’s cover is considered. The Slaninka Method 2 considers both the thermal resistivities of the cover and the soil surrounding the trough. With the Anders-Coates method, in addition to the thermal resistivities of the soil and the cover, the wind velocity above the trough is taken into account. In all options the user can choose whether the trough is exposed to sun radiation or it is shaded. The approaches are all based on field research by independent parties and published in scientific journals.

Filled Troughs

Filled troughs are treated in the CYMCAP/MDB module as multiple backfills. Cables in filled troughs are rated in the CYMCAP software using:

  • Finite elements method to compute the external-to-the-cable thermal resistance T4
  • IEC Standards procedures to perform efficiently ampacity calculations

Besides, the module offers:

  • Computes the temperature and steady-state unequally loaded ampacity
  • Facilities to move the troughs down and model asymmetrical troughs
  • Ability to perform cyclic loading rating through the use of load factors

 

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