|
|
|
Ampacity of underground power cables
Load monitoring, Optical fiber technology, Thermal dynamic modeling, Hotspot investigations, Soil thermal properties, Magnetic field calculations, Thermal capabilities
|
|
|
Asset managers and managers network operation need to know the amount of energy they are allowed to transport in normal and in emergency operation 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, but since reductions in magnetic field strength result in a reduction in circuit ampacity, a balanced approach must be applied.
|
|
|
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 made for most applications.
|
|
|
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 circuits 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
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.
|
|
|
Current rating optimization
|
|
|
Short-circuit current operating limits
Short-circuit calculations can be performed to determine the
adequacy of the 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.
|
|
|
|
|
|
|
|
