Partial Undergrounding

Partial undergrounding with cables can be a solution where public acceptance or environmental impact do not allow for the installation of overhead lines. Comparing to well-known conventional point to point cable connections, the consideration of further aspects is necessary as for example design of cable-to-line transition, concepts for compensation of reactive power, special switching requirements to circuit breakers and deriving maximal length of cable section.


Technology Types

For partial undergrounding, the typical ac cables can be used. The choice of cable type depends mainly on required ampacity, installation time and required cable reliability.

The cables can be directly buried or placed in protective pipes. The latter one can be laid conventionally in trench or drilled mini tunnels.

At the transition point between cable and overhead line substations are required. They are typically equipped with current transformers and surge arresters only. Thereby, the air and gas insulated solutions are available.


Components & enablers

  • EHV AC cables (in majority XLPE type)

  • Cable joints and terminals

  • Transition stations

  • Reactive power compensators


Advantages & field of application

The concept of partial undergrounding of HVAC transmission lines has become one of the solutions to reach public acceptance and obtaining building permits, even when direct financial costs can be significant.

The decisions about the use of partial underground section should be taken in conjunction with the investment costs, obstacles (roads, water pipes etc.) in the cable route and different environmental impacts during the construction phase. Moreover, further specific system aspects like shift of the network resonance frequencies or dynamic stability and the special switching requirements for circuit breakers (energising resistance, missing zero crossings) should to be considered.


Technology Readiness Level

TRL 8 - EHV pilot projects


Research & Development

Current fields of research: deriving the maximal cable length, that can be integrated into specific system, increasing the reliability of cables and joints, improving cable laying technology (in trench and tunnel).


Best practice performance

Maximum capacity: 1700 MW-2500 MW

Current rating: 1.8 (single)-3.6 kA (double cable per phase)

Voltage rating: 380 kV


Best practice application

Wesel - Dörpen, Germany

2022

Description
Part of a planned 150 km 380 kV AC-EHV transmission line will be underground. The commissioning of complete link is planned for Some parts of the transmission line (Raesfeld/Münsterland and Borken/Münsterland) are in pilot operation as junction of two busbars.

Design
• 380 kV, 3600 A, 2.6 GW cross-linked polyethylene • Length of partial underground cables ~3.4 km.

Results
The pilot project shows that it is possible to lay cables in open trench taking into account the environment friendly soil treatment.

Delft, Netherlands

2012

Description
The project "Randstad ring" connects the four largest cities in the Netherlands with two 380kV lines. About 11km of the 132km cable connection is installed as underground cable. The TSO completed it in November 2012.

Design
XLPE cable, conductor size 2500 mm² 132 joints with screen separating insulations 24 outdoor terminations with composite insulators 68 link boxes for earthing and cross-bonding of cable screens High-voltage surge arresters, current transformers, fixing material and supporting structures.

Results
Overcoming different obstacles along the transmission distance between Wateringen and Bleiswijk through partial undergrounding along 11 km of the distance.

Beznau-Birr, Switzerland

2020

Description
The project "Gäbihübel" a 380 kV AC line from Beznau to Birr in Switzerland will be performed by Swissgrid. The construction was planned to start in 2017 and be realized around 2020. Also, a distribution grid 110/16 kV will be laid underground.

Design
Length of underground cabling 1.3 km 2 m depth 380 kV AC, XLPE 2x 150 mm^2^ Cu max. short-circuit current 63 kA.

Results
Forrest obstacles in the area are overcome and the views are preserved in the area.

Pogliano, Italy

2006

Description
The project "Turbigo-Rho" connects the north sub-urban area of Milan. The total length of this meshed line is about 28 km and consists of a 380 kV overhead line single circuit at both ends with a double underground cable circuit of 8,4 km (16 km in total) in the middle, between transition stations of Pogliano and Ospiate. The operator Terna completed it in end of 2005.

Design
8 km of 380 kV double circuit XLPE cable, conductor size 2000 mm² Cu-Milliken; Power capacity: 2100 MVA; Nominal current: 1600 A; Distance between the circuits: 6 m In operation since beginning 2006; PD and DTS monitoring systems installed.

Results
Main design criteria adopted for choice of the route for the overhead line is to avoid or to minimize the crossings of protected areas (environmental constraints, landscape issues, etc.), urbanized areas (even where residential buildings are planned for the near future), and those zones presenting soil instabilities (flooding mud, erosion, overflowing). Anyway, due to the necessity to cross a highly urbanized area and to fulfil the stringent requirements in terms of environmental impact (electric and magnetic field limitation, visual impact, etc.) an underground XLPE cable system was decided to be installed throughout the whole sensitive area.


References

References

[1] Tennet. Partial Undergrounding complementing EHV overhead lines to accelerate grid extensions: the EnLAG opportunity. [Link]

[2] Europacable. Partial Undergrounding, The Europacable concept of partial undergrounding. [Link]

[3] NKT. The Randstand380 Zuidring project. [Link]

[4] ENTSO-E. Feasibility and technical aspects of partial undergrounding of extra high voltage power transmission lines. [Link]

[5] Amprion. underground cables in the transmission grid. [Link]

[6] Amprion. Project description: Wesel -- Meppen. [Link]

[7] Consentec. Technische, planerische und regulatorische Bewertung der Erdkabel-Projekte nach EnLAG und BBPIG. [Link]

[8] Swiss Grid. 380-kV-Leitung Beznau -- Birr: Teilverkabelung «Gäbihübel» kann realisiert werden. [Link]

[9] Swiss Grid. Netzprojekt Beznau -- Birr [Link]

[10] Variant Market Research. High Voltage Cable Market Overview. [Link]

[11] National Grid. Undergrounding high voltage electricity transmission lines. [Link]

[12] Cigrè 2006 B1-302: The new Turbigo-Rho 380 kV transmission line: an example of the use of underground XLPE cables in a meshed transmission grid. [Link]