Validation of the ampacity of buried cables
An important estate company contacted Powersys for a cable warming study. Several underground power cables had to be installed for an important project. The client needed to validate the ampacity of several cables in different configurations, i.e. the maximum current the cables can carry. The main purpose was to give to him enough information to be able to validate the system.
Model
The first step was to model the cables. The core and sheath characteristics have a huge influence on the thermal power that is dissipated. Moreover, every layer conducts the thermal power and limits the cable ability to exhaust thermal power. Therefore, layers thermal insulation determines the steady-state temperature.
To be able to reproduce very accurately the heat emission produced by the Joule effect, we focused on modelling the cable’s geometry and material. We also strive to represent as accurately as possible the way the cables are installed in their duct.
Some data were lacking. We compensated this lack by using our experience to estimate realistic values for the missing characteristics. Also, an uncertainty existed in the thermal resistivity of the materials around the cables. To manage it, we performed a parametric analysis with the different possible values.
Fig1. Cable modelling for ampacity study
Study
To ensure the results of our study be very representative of reality, we based it on IEC standards 60287 and 60853.
From the model we had built in the first part, we computed to find the maximum current that can be carried by the cable without exceeding the maximum cable temperature.
The client needed to get the cables ampacity for a wide range of fill conductivity. To answer this requirement, we established ampacity curves (temperature of the cable regarding the thermal resistivity of the environment). Such curves were carefully established for each configuration in which the client wanted to know the cables ampacity. Eventually, the client was able to anticipate the maximum current that can flow through the cables in any situation and adapt the project to take into account those new data.
