How temperature affects cable performance: Limits and Best Practices
Modern medium and high voltage cables are complex high-tech products with a wide variety of design elements. However, if we try to identify the main elements of the cables, there are probably only 4 of them:
These and other elements of the cable structure are constantly exposed to temperature in normal operation, during overloads, and in various emergency modes (for example, during short circuits). In these modes, currents flow through the cores and screens of the cables and heat them, sometimes to temperatures of hundreds of degrees. If such temperatures are not dangerous for the metallic core and screen, then they pose a danger to the polymer insulation of the cable.
It is the insulation that is the element of the cable that most "painfully" perceives an increase in temperature since insulation is influenced by two factors at the same time:
🔥 temperatures of the hot core and screen;
⚡ the electric field of high strength existing between the core and the grounded screen
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Combination of these factors lead to the aging (degradation) of insulation, and the task is to make these processes go as slowly as possible. To do this, when designing the cable line, the selection of the core and screen, should be based on the following rules:
✔️ In normal operation, the XLPE temperature should not exceed 90℃ (for cables with PP insulation 110℃).
✔️ In case of overloads, the temperature should not exceed 105°C (for PP 130°C), for a certain time within the year (depending on the region and the regulations: it may be, for example, no more than 100 hours per year or no more than 3*72=216 hours per year).
✔️ In case of short circuits, the temperature of the core and the screen should not exceed 250°C, however, for some designs of the screen and its adjacent elements, it is allowed to have a screen temperature of up to 350°C.
It can be seen that the calculation of the insulation temperature is an important part of the cable line design and allows to reasonably select the cross-section of its core and screen. In case of a risk of temperature rise, it is necessary to increase the cross-section of the core and / or screen.
If you need help or advice in checking the temperature conditions of the cable and selecting the cross-sections of the core and screen, then you can contact Cabelte.