It is not enough to have money to buy a good cable. It is also important to find the time and knowledge to make the necessary calculations and select the best way to lay a cable line, taking into account the screen bonding type. Today, let’s look at a simple calculation example of a double-circuit 35 kV line with single-core cables, each with a 300 mm2 Cu core and a 50 mm2 Cu screen. We will perform the thermal calculations using ELEK Software – Electrical Power Systems Design, for the following typical arrangements of phases A,B,C:
1️⃣ trefoil;
2️⃣ in one pipe with an outer diameter of 160 mm;
3️⃣ in three pipes with an outer diameter of 110 mm.

We will not consider laying three phases in a row today, since, firstly, we are limited in the size of the post. And secondly, laying three phases in a row is not recommended for medium-voltage 6-35 kV networks for a number of reasons (we will discuss it in the future).

So, today we have laying options 1,2,3 only. For each of them, we will consider two screen bonding types:
✅ no losses in the screens (can be achieved by single-end grounding of the screens or their cross-bonding);
✅ with losses in the screens (solid grounding of the screens).

In total, we get 6 main options: 1(a,b), 2(a,b), 3(a,b). Let the soil have a temperature of 20 °C and a resistivity of 1.2 Km/W. The results of calculations of permissible currents (capacities) are shown in the Table and divided into three groups:
➡️ green (maximum currents, conventionally assumed to be 1.0);
➡️ yellow (average currents, above 0.9);
➡️ red (minimum currents, below 0.9).

Worst-case scenarios:
✔️ Option 2b: in one pipe, with losses in the screens (422 A);
✔️ Option 3b: in three pipes, with losses in the screens (399 A).

Options 2b and 3b are bad not only because they provide the minimum current, but also because losses in the screens require payment. In the book on HV cables it was showed that the cost of losses in the screens of a line with a length of only a few kilometres can reach 1 million euros over 30 years of operation.

We see that the maximum current can be achieved in two equal ways:
✔️ Option 1a: without pipes, without loss in screens (gives 473 A);
✔️ Option 3a: in three pipes, without losses in the screens (gives 472 A).

By understanding the advantages and disadvantages of the six options discussed in the table, we can make an informed decision about how best to lay the cables and which bonding type to choose. The capacity current for the six variants varies from 399 A (0.84) to 473 A (1.0), i.e. by 16%.

We see that the permissible current is not determined only by the core cross-section, but depends on a significant number of factors. Such factors will be different for each line, and therefore it is difficult to advise in advance which of the six options will be better. You should make such calculations yourself, knowing the features of the project.

You can discuss the post in our public LinkedIn group “High Voltage Cable Lines“.