To design the cable line, we need to pass the following stages:
✅ Evaluate the electrical strength of the cable outer sheath (jacket) and agree on what is the permissible voltage on the screen.
✅ Select the grounding/bonding scheme of the screens (number of sections, cycles, etc.)
✅ Perform the thermal calculation of the cable line and select the cross section of the core.

It can be seen that, even if we have the most accurate methods (IEC, CIGRE, software) of thermal calculation (step 3), we cannot use them without information on the sheath strength (step 1).

In order to reduce power losses in the screens, many 6-35 kV cable lines and almost all 110-500 kV cable lines have either single-end grounding of the screens or their cross-bonding. At the same time, the current in the cable cores, due to its magnetic field, induces a voltage of Us on the screens relative to the ground. This voltage appears:
1️⃣ In normal mode.
2️⃣ In case of short circuits external to the cable.
3️⃣ For lightning and switching current impulses in the core.

In cases 1 and 2, the limitation of Us is performed by the optimisation of the number of single-end grounded sections or number of cross-bonging cycles. In case 3, the limitation of Us is performed due to surge arresters installed in the link boxes (Sheath Voltage Limiters, SVLs).

Today let’s talk only about case 2. We can calculate the Us voltage on screens during external short circuits (for example, according to a book), but what can we compare it with? What voltage Us should be considered permissible? What do we even know about sheath strength?

The outer sheath of 6-500 kV cables has a thickness of 4-6 mm. According to IEC 60229, such a sheath is tested:
➡️ DC 25 kV (1 min), at the factory;
➡️ DC 10 kV (1 min), after installation.

There is no information in IEC on how to convert a DC 10 kV (1 min) test voltage to the AC 50 Hz voltage that we can consider as a permissible for outer sheath for the duration of a short circuit (usually less than 1 sec). Let us decide on it.

In terms of its amplitude, a DC 10 kV (1 min) voltage corresponds to an AC 7 kV (50 Hz, 1 min). If the sheath is ready to withstand DC 10 kV for 1 minute, then it will definitely withstand AC 7 kV for 1 minute (since AC voltage reaches a maximum of 10 kV only at some moments in time, and at other moments the voltage is less than 10 kV).

Thus, the sheath tested according to IEC 60229, is guaranteed to withstand AC 7 kV (50 Hz, 1 min). This means that it is guaranteed to withstand AC of 7 kV (50 Hz, 1 sec), since, obviously, if the sheath withstands the voltage for 1 min, it will definitely withstand the same voltage but for a shorter time (1 sec). So, for all 6-500 kV cables, based on IEC 60229, during external short circuits, it can be argued that the sheath withstands an AC 7 kV (50 Hz, 1 sec). It is this voltage that is recommended to be used to select the screen bonding/grounding.

IEC 60229 focuses on DC. To fill in the IEC gaps, a number of studies were done in laboratories. The tests I know of, conducted in the Netherlands (220 kV) and the former USSR (330 kV), have shown that the new sheath is ready to hold AC 50 Hz at 30-50 kV for minutes! Thus, in fact, the sheath of the new 6-500 kV cables seems to be very durable. Of course, while laying, the sheath may be scratched or pushed through. But even if we assume that the sheath has lost half of its thickness, then it will withstand at least half of 30-50 kV, that is, 15-25 kV. However, it’s too early to celebrate. The fact is that if we want the SVLs to withstand the AC 50 Hz voltage applied to them of 15-25 kV, then they must have the maximum continuous operating voltage of 10-20 kV (Umcov). These SVLs have such residual voltages and voltage drops on the connecting wires (bonding cables, bc) that they are not able to protect the sheath from impulse processes (lightning etc).

Thus, as it was shown in the post, it is not recommended to design the line so that, in case of external short circuit, AC 50 Hz voltage in the screen is more than 7 kV. This allows the use of standard SVL with Umcov=6 kV in screens, and they protect the sheath well from impulses. But the selection of SVL is a separate topic, which will be discussed in one of the new posts.