We have to design the cable length that the customer wants. However, let’s discuss which line length would be particularly inconvenient for us and why. The answer to this question depends on the voltage class and the method of neutral grounding. Today, let’s only discuss 110-500 kV networks with a grounded neutral. For such networks, there are several factors that we need to think about:
✅ The charging current of the line and its reactive power.
✅ Risks of damage to power transformers.
✅ The ability to implement the necessary type of screen grounding.

The first two points have already been discussed in the group, and we won’t talk about them. Also, let’s not talk about the fact that factories don’t agree to produce short lengths. Today we will only talk about the third one – the selection of the optimal bonding/grounding of the screens.

So, there are three basic cases:
1️⃣ Grounding on both sides;
2️⃣ Grounding on one side;
3️⃣ Cross-bonding (1 complete cycle is shown, although there may be 2-3 or more).

Case (1) is dangerous with induced currents and losses in the screens. The larger the screen cross-section and the distance between the phases, the more noticeable the screen currents and the losses are. For 110-500 kV lines, these factors are such that both-sides grounding is very rare to meet.

Case (2) has no currents or losses, but has an induced voltage. This voltage is different in the symmetrical mode (normal mode or three-phase short circuit, external to the cable) and in the unbalanced mode (single-phase short circuit, external to the cable). With a single-phase short circuit, the voltage reaches 600-700 V for every 1 km of length and 1 kA of current. Therefore, with a short-circuit current in the cores of, say, 20 kA, we have 12-14 kV/km on the screens, which is dangerous for the cable sheath and the SVL installed in the link-boxes. Therefore, for such (2), it is better not to have a length of more than 0.3-0.6 km.

Case (3) has no currents and losses, but has an induced voltage. It reaches its maximum at the cross-bonding nodes. The voltage there is formed differently than in (2), and the case of a single-phase short circuit is no longer calculated. Usually, the induced voltages are such that the normal mode is the most important. Depending on the current and the distance between the phases, it is necessary to have one complete cycle for every 3-6 km of the line route.

To have (3), we need to divide the line into three sections. Since the typical drum length of a 110-500 kV cable (in order not to have problems with delivery) is 400-600 m, it is inconvenient to have (3) for lines less than 3*0.4=1.2 km long.

Thus, for lines between 0.3-0.6 km and 1.2 km, the length it is already bad to do single-end grounding, but not enough for cross-bonding. It turns out that the length of 0.6-1.2 km is inconvenient. In this range, single-end grounding can be used, but if the so-called equipotential conductor is laid along the route, it’s a headache, and we’ll talk about it in a future post.