Ampacity of Vertical Cable Ducts Entering a Wind Turbine Foundation

Long cable routes often encounter varying installations throughout their run. This could be due to the cable traversing diverse terrains, structures, or environments. The different installation conditions of each cable section affect the current-carrying capacity of the cable. A specific installation of one section would probably derate the cable more than any other section along its run. Engineers would often have to assess the current rating of each section to determine the final cable rating. The section that gives the lowest current-carrying capacity should be considered the most conservative estimate of the cable rating.

ELEK Cable HV Software can be used to perform accurate cable ampacity calculations.

An interesting case to consider when addressing this concept is the installation of wind turbines. Wind turbines are often located far from their respective substations, leading to long cable runs that must traverse the terrain safely. These wind turbines may be offshore or onshore, with their transmission lines buried under the seafloor or ground. Regardless, these cable runs will have vertical sections to terminate the cables at the turbines.

Below is an example of a conduit entry to the termination point of a wind turbine. The drawing also describes an image of an actual installation on site. From the underground horizontal section, the cable will need to rise to the equipment above, leading to the necessity of vertical cable sections. From the drawing, it can be seen that the vertical cable section is still enclosed in the foundation's concrete.

How do we treat the vertical cable section in calculations?

Answers:

1. Treat the vertical cable section as a standard buried section or;
2. Ignore the vertical cable section entirely.

Here are the explanations for the two possible approaches.

Treat it like a standard buried section

If the vertical section is surrounded by soil or concrete, as described above, it can be treated like a standard horizontal buried section. In the ELEK Cable High Voltage software, there is no way of modelling the vertical cable section as anything other than a horizontal buried section.

Ignore the vertical cable section

The alternative approach is to ignore the vertical cable section entirely. Engineers sometimes prefer a rule-of-thumb for particularly short cable sections that are less than 5 % of the total run. These short cable runs are ignored with the assumption that they would have little to no effect on the rating of cables that have long routes. This may apply to wind turbine cables with long spans and a short vertical section at the termination point.

There are instances when the second approach is not recommended, especially if the installation condition of the vertical section is entirely different from that of the horizontal section. For example, if the vertical section is enclosed in the air instead of buried, a separate calculation may need to be performed.

Ultimately, it is up to the engineer to decide whether the short section should dictate the circuit's current rating.

References: 

ELEK Cable High Voltage V7.5