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  5. High-power energy transmission - How does underground cabling work?

High-power energy transmission - How does underground cabling work?

Author: Prof. Dr. Jochen Fricke, Cluster Energietechnik (As of: August 2016) The transmission of electrical energy at the highest voltage level by means of overhead lines has many advantages over the cable solution:  With overhead lines, the encroachment on the ground is limited to the mast areas, while for cable installations wide aisles must be cleared of plant and tree growth and kept free, and large amounts of soil must be moved. While the free-hanging  conductor cables are efficiently cooled by the air, the ohmic heat loss from the cable is dissipated into the surrounding soil and heats it, which can cause overlying fields to dry out.  Repairs can be quickly located visually and repaired on overhead lines.  In the case of cabling, once the damage has been located  the ground must be removed. The faulty section of cable must be cut out, then the resulting cable ends must be rejoined via a complex process using sleeves. Finally, cable solutions are often more expensive than overhead lines. The costs must take into account soil conditions, terrain, also repairs and dismantling, in other words, life cycle costs.

Because of the numerous citizen protests, especially against the planned new north-south "electricity highways", the German government has prescribed underground cabling  as the rule, despite the advantages of the overhead line solution.

First short distances for underground cabling

High-voltage underground cables for alternating current are already being installed on short distances in Germany: for example, by Amprion in the 380 kV, 3.6 GW power line between Wesel and Meppen. To increase acceptance among the population  partial cabling is being carried out in pilot projects at several locations near residential areas or in nature conservation areas. This is being done  e.g. near Raesfeld over a distance of 3.5 km, resulting in very considerable earthworks. The 4x3 three-phase cables  are laid in protective tubes in 2 trenches, each about 7 m wide, at a depth of about 2 m, and then grouted with liquid soil (soil with 5% aggregates) or lean concrete (Fig. 1). 

Laying of 4 three-phase transmission systems with a total capacity of 3.6 GW in the ground
Abb.1. Verlegung von 4 Drehstrom-Übertragungs-Systemen mit insgesamt 3.6 GW Leistung im Erdreich. Die lateralen Abstände sind aus thermischen Gründen erforderlich. Die schwarzen Punkte neben den Hüllrohren sind Datenleitungen.  (Bildnachweis: J. Fricke).

These dissipate the ohmic heat loss into the surrounding soil. At route constrictions, concrete mixed with highly thermally conductive graphite (Powercrete® or CableCem® from HeidelbergCement AG)  can also be used.

High effort for safe transmission

Each cable contains a core of stranded copper wires with a cross-section of 2,500 mm². The division into wires is necessary not only for flexibility but also because of the skin effect in 50Hz alternating current. This effect causes the alternating current to be partially displaced from the interior of the conductor towards the edge, which increases the electrical resistance. The insulation consists either of an oil-permeated paper winding (MI insulation) or, more recently, of highly crosslinked, particularly breakdown-resistant, extruded polyethylene about 3 cm thick. Functional and protective layers then follow on the outside.

Since the maximum cable length transported on land on a reel is about 1 km for weight reasons, the cable ends must be connected to each other by sleeves. The installation of a sleeve requires about one week of working time.

Transfer station overhead line to underground cable
Abb.2. Übergabestation Freileitung auf Erdkabel (Abbildung J. Fricke, adaptiert von Amprion GmbH).

At each of the interfaces between overhead line and cable there is a  transfer station (after all, you can't just loop the overhead line into the ground - there would be an immediate short circuit). Starting from the approx. 65 m high extra-high voltage steel lattice mast (Fig.2), the incoming 2x3 overhead lines are led to an approx. 35 m high steel lattice entry portal with lightning protection in the transfer station and fixed to it with approx. 3.6 m long insulator rods. In the following steel grid distribution portal there are busbars in which the current from the 2x3 conductor cables is distributed to 4x3 conductors. This distribution is necessary to keep the cable temperatures in the floor as low as possible. The 12 conductors are then fed at the top into the interior of the insulator columns, which are almost 9 m high and made of porcelain or GRP, and pulled down into the ground as insulated cables. The required area of a transfer station is 60mx80m. The total construction height including lightning protection is about 35m. At the transition from the cable to the overhead line section, a transfer station is passed through again, then in reverse order.  Devices for disconnection and reactive current compensation can be integrated into the transfer stations.

While the power line between Wesel and Meppen will be constructed using three-phase technology, the 800 km long German power "highway", SuedLink, is to be realized using high-voltage direct current (HVDC) transmission technology and with a voltage of 500 kV and a capacity of 4 GW. SuedLink will run in the area of the grid operators TenneT and TransnetBW.  High-voltage direct-current transmission of electrical energy over long distances is so attractive because no capacitive reactive current components arise in the process, i.e. current and voltage  remain in phase and no skin effect occurs.

HVAC - HVDC mixing system
Abbildung 3: In einem HVAC - HVDC-Mischsystem gibt es Gleichrichter-Stationen und Wechselrichter-Stationen (fette Kästchen). Wesentliche Schaltelemente in diesen riesigen Anlagen sind Thyristoren oder  IGBTs (Insulated Gate Bipolar Transistors). Der Wechsel von Freileitung auf Erdkabel und v.v. erfolgt in ebenfalls  riesigen Übergabestationen (kleine schwarze Blocks). (Bildnachweis: J. Fricke).

HVDC or HVDC transmission  only pays off from a distance of about 600  km. This is because it requires rectification of the fed-in three-phase current at the input of the DC line and reconversion to three-phase current at the end  (Figure 3). The rectifier and converter stations required for this are technically very complex and contribute about 40% to the cost of the entire DC transmission system. They require a lot of space, about 300mx230m. They include  two 20m high converter halls (one for generating the voltage +500 kV, the other for -500 kV). Further, the converter stations house transformers, switching equipment, cooling equipment and the feeders to the AC and DC grid. What is attractive about HVDC transmission is that 2 DC conductors (plus metallic return conductor)  replace 6 AC conductors (Figure 3). The underground cables have a Cu or Al core with a cross-section of about 3,000 mm² and are electrically insulated with an insulation jacket of cross-linked polyethylene about 3 cm thick. Functional layers and reinforcement follow to the outside.

Transfer stations required for partial cabling

If partial cabling is used, transfer stations must be installed at the transition from the conductor cable  to the underground cable. Similar to Figure 2, the conductor cables are fixed from the lattice tower to the insulators of an entrance portal in the station  . From there, they are inserted from above into  insulator columns about 10 m high and pulled inside them down into the ground as underground cables. The area required for a transfer station is about 2,000 m², and its height is about 35 m.  

The laying of the cables requires very extensive earthmoving (Fig.4). The width of the HVDC cable route is about 25 m. HVDC cables can be laid in one piece (for transportation reasons) only up to lengths of 1,200 m maximum. The muffing of two cable ends with joints prefabricated in the factory takes about 2 - 7 days, depending on the cable type.

Cable route for an HVDC cable system with a capacity of 4x1 GW
Abb.4. Kabeltrasse für ein HGÜ-Kabelsystem mit einer Leistung von 4x1 GW. In jedem Graben werden ein positives und ein negatives Kabel sowie eine Datenleitung geführt Der Abstand der einzelnen Gräben beträgt etwa 6 m. Die vertikalen Dimensionen entsprechen etwa denen in Abb.1. (Bildnachweis: J. Fricke).

So we can see what considerable technical effort must be made to implement cabling instead of an overhead line. But also with this solution probably with substantial citizen protests is to be counted. Therefore, it can certainly be considered a success if the SuedLink, which is urgently needed for Bavaria's power supply, can go into operation by 2025 - as planned.    

 The author thanks Mr. Arndt Feldmann, Amprion GmbH in Dortmund for information about the Raesfeld project and points to an exhibition, in which one can inform comprehensively about the pilot project by 360° panoramic view on the Amprion homepage: http://netzausbau.amprion.net/projekte/wesel-meppen/pilotprojekt-erdkabel/index.html