AC 50 Hz cable lines are often referred to as reactive power sources. In idle (no-load) operating mode, any cable line by its capacitance generates reactive power, which creates difficulties:
✅ for a network that is forced to consume reactive power;
✅ for a cable that is heated by idle currents, especially at the beginning of the cable line, where these capacitive currents reach a maximum;
✅ for circuit breakers that are forced to switch capacitive currents;
✅ for insulation at the end of the line, where the 50 Hz voltage rises.

For these reasons, the maximum length of AC 50 Hz cable lines is about 100 km, and longer cable lines can be built only if one of the following conditions is met:
✅ the line is equipped with controlled shunt reactors;
✅ the line has been switched from AC to DC.

Returning to AC 50 Hz lines, it is important to note that in fact the line is not always a source of reactive power. Two processes are involved in the balance of the reactive power of the line at once:
✅ reactive power is generated in the capacitance (C) under the action of the operating voltage;
✅ reactive power is consumed in the inductance (L) under the action of the operating current in the cable core.

Let’s gradually load the cable line from idle (no-load) operating mode to load mode. As the current increases, we will have one by one the following three stages:
1️⃣ The line is a source of reactive power;
2️⃣ The line is balanced in terms of reactive power (does not generate or consume);
3️⃣ The line is a reactive power consumer.

Interestingly, at what load current (current in the core) is the line balanced? This happens at a current equal to the “natural line current (Inat)”. This Inat current can be calculated based on the rated voltage Un of the network and the wave (surge) impedance Zw of the line. The big table shows the corresponding Inat calculations.

It can be seen from the table that the Inat values for 35-500 kV cable lines can reach 5-10 kA, which is several times more than the actual currents that can pass in the cable core. Therefore, for most cable lines, I<Inat is valid, that is, the cable lines are not loaded enough to be balanced, and they are sources of reactive power.

Only cable lines of 10-20 kV may have currents I in cores, which are close to the Inat value. Therefore, 10-20 kV cable lines are the only cable lines that are not reactive power sources in normal operation, but are balanced.

Natural current is one of the issues where the wave impedance Zw of a cable line is important to us. There are other issues where Zw is important. We will talk about them in further posts.

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