We often think that 6-35 kV medium-voltage (MV) networks are simpler than 110-500 kV high-voltage (HV) networks. Of course, this is not the case, and it is in MV networks that many serious problems are concentrated – among them, for example, the selection of the optimal method of neutral grounding as well as the selection of the optimal design of power lines. Today, let’s talk about MV networks with an isolated neutral and the problems it creates for universal self-supporting cable lines (UCL).

The following types of lines are used in MV networks:
✅ Overhead line with “bare” wires.
✅ Overhead line with “covered” wires (left photo).
✅ Cable lines.
✅ Cable lines that can be laid both underground and above ground on poles (right photo), and therefore called Universal Cable Line (UCL).

UCL lines, also known as Multi-Wiski (Prysmian) or Torsada (Nexans), are applied in the following cases:
1️⃣ The power line route is partly underground and partly overhead. At the same time, the owner does not want to build transition points with cable terminations and surge arresters on them.
2️⃣ For quick arrange of power line for power supply of consumers.
3️⃣ For temporary power supply where, at the end of a certain period, it should be possible to quickly disassemble the line and keep it in warehouse for future projects.
4️⃣ In protected forest and park areas where it is prohibited to cut down trees there as well as to dig an open trench.
5️⃣ In populated areas, including along the facades of buildings.

The cables included in UCL have a fairly thin outer sheath on top of a metallic screen. Therefore, it is very important to monitor the 50 Hz voltage that can occur on the screen relative to the ground if the main insulation of the cable (insulation between the core and the screen) is damaged.

In a MV network with an isolated neutral, damage to the cable main insulation does not cause high currents in network, including in the screens, and therefore there are no dangerous voltages on the sheath. However, if another damage is added to the first damage (somewhere else in the network, on a different phase), then a double fault F(1,1) will occur in the network, and AC 50 Hz currents will flow, the magnitude of which may approach the magnitude of the three-phase short circuit current. In such cases, the voltage on the outer sheath can reach 5-10 kV, leading to its breakdown and violation of the tightness of the cable.

To limit the voltage on the screen, it is necessary to re-ground the screens on the intermediate towers (supports, poles) of the line. Calculations show that to achieve the effect, it is necessary to ground the screens at least every 0.5-2.0 km of the line route. Therefore, almost every cable joint must have screens which are brought out for their bonding and grounding.

This problem is discussed in Chapter 7 (Neutral) and Chapter 9 (Mixed Lines) of the book.