Electric power industry consulting
The project usually justifies a specific screen cross-section. However, in practice, there are reasons why the actual screen cross-section may differ from the expected:1️⃣ The cable was bought from a stock (there was no time to wait since it was a line repair that was required), and as a result, a cable arrived at the
If the cable insulation is damaged, the current in the cable core (Ic) and the current in the cable screen (Is) can reach the value of the network short-circuit current. One of the important steps in selecting a cable core and screen is to check their resistance to short-circuit (fault) currents, taking into account the
Three-core cables of medium voltage 6-35 kV, as a rule, are used only for small core cross-sections – up to 240 mm2 (although there are 300, 400 and more). Each of the three phases A, B, and C on top of XLPE insulation has its own individual screen, most often made of copper tape (foil)
The screen (and core) must withstand the short-circuit (fault) current (If), taking into account its time (tf). For XLPE- and EPR-insulated cables, the temperature during a short-circuit should not exceed 250°C. In the lack of information, this temperature is applicable for both the core and the screen. However, according to IEC 61443, if the screen
Modern MV-HV cable has conductive screen. Such screen, after grounding, make it possible to equalize the electric field in the cable insulation and eliminate this field outside the cable. In practice, the question arises – how to choose the screen cross-section? Cable factories have their own opinion. Let’s assume that a screen is made out
The method of symmetrical components is used to calculate the normal and emergency modes of the electric network. The method represents an any mode (even asymmetric) as a composition of three separate symmetrical ones:✅ Positive sequence “1”.✅ Negative sequence “2”.✅ Zero sequence “0”. To use the method, it is important that the longitudinal impedances Z1,
High-voltage cables must have a conductive metal screen (sheath). The selection of the screen cross-section is very important. Without selecting the screen cross-section (and bonding/grounding scheme), it is impossible:✅ To ensure the resistance of the cable screen to fault (short-circuit) currents.✅ To perform a thermal calculation of the cable line and select the core cross-section.