Ampacity of Underground Power Cables
Asset managers and network operation managers need to know the transport capacity of power cables in normal and in emergency situations without jeopardizing the reliability of the cable system. The ampacity or current rating calculation gives the answer
Calculations are made for static, cyclic or dynamic situations. The way the cable is installed and the soil/ambient properties mainly determine the ampacity. The current loading is improved by load monitoring and use of measurement of actual temperature of the cable sheath, by hot spot investigation and by improvement of the thermal soil properties. Additionally, the emission of magnetic fields at the soil surface may influence the ampacity of underground power cables.
Static current rating
This calculation is based on IEC 60287, and is applied to medium voltage and high voltage cables. The results of the calculations are slightly conservative, and although local conditions are not taken into account, the calculation of the static current rating is important and is widely applied.
Cyclic and emergency current rating
These calculations are based on IEC 60853. The effects of cyclic loading and of the thermal capacitances of the cable materials and soil are taken into account. The cyclic model is used for higher current medium voltage and high voltage cables and produces results that are slightly more conservative than the calculations for the static current rating.
Dynamic current rating optimization with and without temperature measurements
Dynamic current rating optimization can be realized by considering the actual temperature values of the cable sheath and soil, and the dynamic thermal behavior of the cable and the environment.
Short-circuit current operating limits
Short-circuit calculations can be performed to determine the (in)adequacy of the power cable and installation for the application. The methods of IEC 60949 and IEC 61443 are used for this purpose.
Thermal dynamic modeling
The power of thermal dynamic modeling is that the actual operating and installation conditions are taken into account. Investments to upgrade the system can be postponed since the cable can be loaded to its actual full capacity limit.
Hot spot investigations
Using this technique, the position of the critical thermal points of the cable circuits are identified so that the current rating can be increased without jeopardizing the reliability of the system.
Thermal soil properties
Determination of the actual thermal resistivity of the soil is very important since the thermal characteristics of the surrounding soil and the laying depth of the cable circuit can alter the circuit ampacity by more than 50%.
Magnetic field calculations
Local authorities may set limits to the intensity of magnetic fields at the soil surface. While reduction of the magnetic field strength emitted is possible, it will result in a reduced circuit ampacity. For this reason a balanced approach must be followed during the design stage of the project.