In recent years a lot of research has been done in order to increase the power flow of underground cables and to develop the equipment to effectively monitor the weather and thermal state of the cable by creating accurate thermal models. As a result of applying dynamic thermal rating technologies, the capacity is usually increased by 5 to 15%. Few factors determine the thermal rating of the underground, among them the soil temperature and thermal resistivity of the earth, and they change very slowly, don’t get affected much by weather and current loading.
The main challenge with underground installations is to accurately measure the maximum current value that can flow through the circuit breakers.
The current capacity carried in specific cable circuit breakers depends on certain aspects, such as cable construction, the soil, the temperature, and the sheath-bonding method. Only the soil properties are variable, and the others are constant.
The soil is affected by the weather change in different seasons and the cable heating, hence the current carrying capacity changes drastically. Dynamic current rating of a cable circuit is a crucial factor in order to utilize it in the full capacity all year round; because it is always the biggest challenge for power operators to choose the right power load for the underground cable.
That is why monitoring thermal conditions of the buried cable circuits and installed distributed temperature sensing (DTS) systems is crucial.
A real-time operating system is created to capture different load current parameters, cable surface, and soil temperatures to provide input to the real-time operating system. The data about current and temperature is passed to a computer through the fiber optic connection. The computer gathers load and temperature data and provides an updated ampacity rating.
Once the dynamic current rating data is received, it has to be analyzed within a set period of time. The results are usually used to develop risk management strategies. There are certain challenges when calculating line ampacity, such as conductor properties and atmospheric conditions, which have to be considered. Each of these factors increases the level of uncertainty when determining ampacity.