For lines with single-core cables, there is a problem of selecting the optimal screens’ bonding type. It is especially relevant for 110-500 kV lines, where grounding of screens on both sides is almost prohibited, and 99% of lines have either grounding of screens on one side or cross-bonding of screens. In both cases, it is necessary to select surge arresters installed between the screens and the ground and designed to limit voltage impulses on the cable outer sheath. Such arresters are also known as Sheath Voltage Limiters (SVL). The question arises about how to select such an SVL, and today I would like to make the first post of several.

1️⃣ The outer sheath of 110-500 kV single-core cables is made of the same standard materials (PE, polymer composition, etc.) and has the same standard thickness of about 4-6 mm. Therefore, we have every reason to believe that the electrical strength of the sheath varies little from project to project and remains at the same level. This raises a reasonable question – why should we use there SVLs of a different voltage class?

2️⃣ The length and type of so-called “bonding cables” connecting the SVL (installed in the link box) and cable joints (or terminations) is more or less standardized for the industry. Considering points 1 and 2, it is clear that the residual SVL voltage can be assumed to be approximately the same, for all 110-500 kV lines.

3️⃣ The cable sheath specified in point 1 can be considered, as a first approximation, as “insulation of class 6 kV”. To protect such a sheath, SVL of 6 kV class (the maximum continuous operating voltage Umcov) can be used as the base. This is exactly a standard solution – a standard SVL installed at a standard distance from a standard joint (or termination) will provide a standard level of sheath protection.

4️⃣ It is important to understand that although the SVL is installed to protect against switching (and sometimes lightning impulses), it is also exposed to an AC 50 Hz voltage, which is induced on the cable screens in normal mode (tens of volts) or during a short circuit (several kilovolts). Since the SVL should not be damaged by such a voltage, the screens’ bonding type should be selected so that, with a short circuit (external to the line), the voltage on the screen does not exceed, say, 7 kV. If the voltage exceeds, then it is necessary to change the screens’ bonding:
✅ increase the number of sections, each of which is grounded once;
✅ install an equipotential bus along the line (it does not always help);
✅ increase the number of cross-bonding cycles.

So, there is no such a problem as “SVL selection”. There is a problem of “selection of the screens’ bonding type.”

You might say, “it’s clear with Umcov, but what about the selection of SVL energy absorption capability”? Here I will answer – have you ever seen an SVL in operation with an energy of more than 2 kJ/kV?

All this is described in the book on cables.

There are also some things in previous posts:
➡️ About SVL with Umcov = 3-5 kV, please read there.
➡️ About SVL with Umcov = 8 kV, please read there.