Single-core cables of 6-35 kV usually have a two-side grounding of the screens (“solid bonding”). This leads to the fact that in normal operation, the magnetic field of the core induces 50 Hz currents in the screens. At a given current in the core, the induced currents in the screens depend on:
✅the cross-section of the screen and its material;
✅the distance between the phases ABC.

It seems to us that if there are positive sequence currents (load currents) in the cores, then there are positive sequence currents induced in the screens. Unfortunately, this is not the case, and in fact, the screens have currents (Is):
➡️of positive sequence (Is1);
➡️of ZERO sequence (Is0).

The zero sequence current in the screens is noticeable primarily when ABC phases are arranged in a row. This current is closed in a loop formed by ABC screens and the ground. Since there are resistances of two grounding systems (at the beginning and at the end of the cable line) in the path of this current, the value of this current (Is0) is usually no more than a dozen Amperes. However, the situation dramatically changes when we are dealing not with a single-circuit, but with a double-circuit line.

The zero sequence currents in the screens of the double-circuit line are no longer closed through the ground, but through the screens of the neighbouring circuit. Thus, there are no grounding systems in the loop for these currents (Is0), which significantly increases the magnitude of zero sequence induced currents in the screens. As can be seen from the example (the left part of the oscillogram), the core current of Ic1=500 A gives up to Is0=100 A of zero sequence current in the loop formed by the screens ABC1 and ABC2. There will also be positive sequence currents in the screens (Is1), but let’s focus on zero (Is0).

The situation becomes even more complicated if the cores of the two circuits are switched on in parallel (by circuit breakers of sections of busbars). In this case, the currents in the loop formed by the screens ABC1-ABC2, by their magnetic field induce currents in the loop formed by the cores ABC1-ABC2 – this is a kind of reverse transformation from screens to the cores. Thus, we have a zero sequence current in the cores. As can be seen from the example (the right part of the oscillogram), a zero sequence current in the core (Ic0) is up to 50-100 A.

In a network with an isolated neutral, a 50-100 A zero sequence current passing through the cores of two circuits (directions of Ic0 in two circuits are opposite) can cause a false operation of ground fault protection. And such cases are known from practice where, in absolutely normal operation, cable line was disconnected by ground fault. To eliminate the situation:
✅arrange phases ABC of each circuit into triangle (trefoil formation);
✅if you really want to have in row formation, then let it be of opposite type.

Please read Chapter 7.2 of the book (hardcopy is available).