When any element of the electrical network is energized, a transient process usually occurs. For example, when a voltage is applied to a power transformer, an inrush current of a magnetic nature occurs. The magnitude of the magnetization current of the transformer can reach several kA and sometimes causes serious concern. Today let’s compare inrush currents of transformer and cable line. I may upset transformer fans – cable inrush current can be greater, reaching 15-20 kA, and even higher (for example, if we have several cables per phase).

Switching on the transformer (TR) may occur at the moment when the AC 50 Hz network sine voltage passes:
✅ZERO – it gives the largest TR inrush current.
✅the maximum – it gives almost no TR inrush current.

The TR magnetization current is a 50 Hz current with a shape distortion. Various measures can be used to limit this current, for example, to equip the circuit breaker with controlled switching devices that provide switching near the maximum of the network sine voltage.

What about the cable line? Inrush current may also occur when the cable line is switched on. This is not a 50 Hz current, but a “rectangular” impulse (i.e., of high frequency), determined by the value of the wave impedance Zw of the cable line. The duration T of the current impulse is determined by the length of the cable line (the longer the line, the longer the duration of the impulses of inrush current). The attenuation of the inrush impulse process depends on the active resistances of the circuit elements.

The cable may be switched on at the moment when the AC 50 Hz network sine voltage passes:
✅zero – it gives almost no CL inrush current.
✅the MAXIMUM – it gives the largest CL inrush current.

The oscillograms show inrush currents in a 500 kV network at the worst moment of the switching on:
✅for TR, when the network voltage passes a ZERO.
✅for CL, when the network voltage passes a MAXIMUM.

It can be seen that the inrush current of the cable line is several times greater than of the transformer. Therefore, in my opinion, switching a HV/EHV cable line sometimes can be considered as a more serious event.

What is the danger of high inrush currents of cable lines:
1️⃣Connecting a cable line, especially a long one, can lead to dangerous voltage drops on the busbars, characteristic of a three-phase short circuit; this is a voltage cut on the busbars, damaging the winding insulation of power and instrumental transformers connected to the busbars.
2️⃣Overvoltages related to busbar transients (voltage recovery process).
3️⃣False protection operation.

How to defeat the inrush currents of cable lines? Well, it can be controlled switching, and it is optimal to turn on the line when the network sine voltage is close to zero. It is also recommended to avoid designing cable lines with several cables per phase, because the inrush current magnitude is proportional to the number of cables per phase.