Medium voltage 6-35 kV grid usually do not have a solid neutral grounding, and a voltage of 50 Hz may occur on the neutral. Today let’s talk about why it may happen in the cable network even in normal operation, and how this can lead to Temporary Overvoltages (TOV) and damage to equipment.

One of the reasons for the appearance of neutral voltage in normal operation is the difference in the three capacitive phase-to-ground conductivities (Y) of phases A,B,C.
✅ This often happens if network has overhead lines, since the phase-to-ground capacitances of the upper and lower phases are not the same.
✅ This rarely happens if network consists of cable lines, since the phase-to-ground capacitances are the same for phases A,B,C. However, there is one exception – the case when a cable line has several cables per phase.

Some 6-35 kV cable lines are designed so that each phase has several cables per phase. This is done to increase the current-carrying capacity. Cable lines are usually designed with a margin, and therefore if one of the cables in the phase is damaged (for example, one of the n cables of phase A), it seems that for the time required for repair, it is possible to supply power to consumers by “reduced” cable line (in phase A, there is only n-1 cables in operation instead of n cables). However, it’s better not to do that.

Disconnecting one of the cables in phase A will cause the resulting phase-to-ground capacitive conductivity (Y) for phase A to be less than for phases B and C. This will cause a voltage to appear in the neutral. Calculation examples are given in the Table.

For instance, in the network with isolated neutral, if there are two lines No.1 and No.2, and each of them is made with n=2 cables per phase, then disconnecting one of cables of phase A of line No.1 will result in a neutral shift of 9.1% (0.091 of the phase voltage E of the network). Such a shift does not seem dangerous. However, the situation will dramatically change if there is a reactor in the neutral of the network (network has a compensated neutral).

If a reactor is installed in the neutral in a cable network, configured to fully compensate for the network capacitance (K~1), then any shift of the neutral (even less than 10%) will provoke a resonance. This means that the phase-to-ground voltages (U) in the entire network will increase, leading to insulation failures – see oscillograms. For example, surge arresters are likely to be damaged in the network.

So, in a network with a compensated neutral, it is strictly forbidden to turn on cable lines if their phases A,B,C have different capacitances which may happen:
✅ If phases A,B,C have a different number of cables per phase;
✅ If phases A,B,C use cables of different types or core cross-sections.

Considerations for protecting cable networks from lightning and switching overvoltages are also important and are discussed in Chapter 10 of the book.