Underground AC cables on land

The use of AC voltage is one of two technical options open for the construction of new power links as underground cables. That said, here too, the laws of physics – especially in relation to capacitance – dictate the extent to which we can integrate this new technology into our grid. Our top priority is always to ensure that our transmission grid provides maximum availability and reliability. Listed below are the key facts about underground AC cables at a glance:

AC voltage

  • The polarity of the voltage changes 50 times every second (50 hertz)
  • Is simple to transform to different voltage levels
  • Is suitable for transmitting in meshed grids and for supplying customers/distribution networks with electrical energy

Underground AC cable

  • Reactive power required continuously during operation – approx. 10 to 20 times more than an overhead line
  • The length of AC cables is physically restricted to just a few kilometres
  • Partial underground cabling of sections along defined pilot links legally stipulated

Capacitance of AC cables

With AC cables their capacitance first has to be charged before electricity can be transmitted. However, because the positive and negative poles change every 20 milliseconds, a charging and discharging current flows continuously in AC cables – what’s known as the “reactive power”. This is the crucial difference between AC and DC cables.

The issue of the reactive-power demand can be explained by means of a water hose analogy. That said, the inner wall of this water hose is not smooth but contains little pockets. If we now pump water into the hose from one end, these pockets are filled up first.

Mimicking the behaviour of an AC voltage, we now switch the pump feeding in the water from pump mode to suction mode and back again every 20 milliseconds. This also reverses the direction of flow of the water and the pockets are drained again. Owing to this rapid switching back and forth of the flow, only a certain length of the hose is filled with water. If the hose is too long or the filling time too short, no water flows out at the other end of the hose.

The situation is similar with AC cables: the capacitance of the cable must first be charged up before active or real power is transmitted along the cable. The constant changing of the direction of flow every 20 milliseconds means that the capacitance of a long cable cannot be fully charged. That’s why no power reaches the other end. Consequently, the length of an AC cable is limited for technical reasons.

Reactive-power compensation

The reactive-power problem can be partially alleviated by means of reactive-power compensation, which is more commonly referred to as power factor correction. This function is performed by reactors, which resemble large transformers and provide reactive power.

This correction, however, also gives rise to another physical phenomenon. If we connect up reactors and cables electrically and then connect these to an AC source, compensating currents flow continuously between these elements. This results in resonance phenomena that in an extreme case can even endanger the stability of the grid. These resonances likewise limit the maximum possible length of the cables in an AC grid. This is comparable to the effect that can occur when a large number of pedestrians walk over a bridge in lockstep: the resonances triggered by this can even cause a bridge to collapse.

The cable transfer stations, reactors and joints that connect the roughly one kilometre-long sections of the underground cable can give rise to a technically complex system.

Experience so far with AC cables

Until now, very few underground EHV AC cable routes have been installed – especially for lines with high power levels. On the one hand, this has to do with the high reactive-power demand; on the other hand, we still need to find out how underground cables fit into a close-mesh transmission grid. They will definitely not be able to replace overhead lines one to one, among other things because the two technologies behave differently in operation. For that reason, we are studying the operation of AC cables very closely and trialling them in pilot projects. Based on these experiences, we’ll then be in a position to decide to what extent underground cables can be deployed in the country’s transmission grid.

AC voltage: important for linking different network levels

With the aid of transformers, it’s simple and cheap to change the level of ac voltages. In substations, these transformers are connected to one another by means of busbars and switchgear bays and also connected to the overhead-line and cable connections. The busbars are comparable to multiway connectors, the switchgear bays to switches.