What is the FEM Module?
The FEM calculation module in Cable HV Software performs 2D finite element analysis on buried cable systems to obtain current ratings. The FEM module is very accurate and extends the capabilities of the software beyond the limitations of the IEC standard calculations.
Advantages of FEM calculations for cable current ratings
The advantages of FEM-based cable rating calculations include:
1. Any HV cable installation and configuration can be modelled.
Your exact installation conditions can be modelled which leads to better accuracy. For example, with FEM you can model multiple zones of varying soil thermal resistivity.
2. The heat transfer between the ground and air is modelled.
The IEC standard assumes that no heat is transferred between the ground surface and the air above, known as the isothermal assumption. FEM on the other hand allows the convection of heat generated by the buried cables which flows into the ambient air to be considered.
For shallow buried cables the difference in current ratings between IEC standard and FEM is often significant. A recent study by National Grid using FEM calculations concluded the IEC method overrates cables buried at 1 m or less.
3. Accurate ratings for groups of cables.
The mutual heating effects for groups of cables are more accurate using FEM. The IEC standard approach is to calculate the mutual heating for each cable separately and combine heat rise as an algebraic sum using superposition. The difference with FEM is that the mutual heat flux influence and disturbance are considered simultaneously.
Steps to performing the calculations
The high-level steps to an FEM cable rating calculation are as follows:
1. Add the cable models into the study.
2. Set the native soil and ground surface conditions.
3. Add and position the cable circuits, ducts, backfills or troughs.
The FEM user interface is fully interactive. You can select and move circuits or individual cables in custom arrangements. You can resize and position backfills, ducts or troughs using the mouse. You can also enter exact locations or dimensions into the corresponding controls.
You can customise the names of circuits and colours of backfills. The software automatically detects and provides warnings if there are overlapping objects, which are not allowed.
