Underground transmission cable technology

Over recent decades, people have become more aware of the detrimental environmental and socio economic effects of pylons and other above ground electrical infrastructure and this has lead to pressure to the undergrounding of existing and new lines in some areas. Overhead distribution voltage cables in the Dedham Vale for example have been undergrounded.

Underground cable technology is developing rapidly to match the demands from people to hide higher voltage overground transmission systems. Distribution network voltage cables up to 132kV are regularly put directly underground and the 220kV cables installed by National Grid are usually installed in tunnels. 

However the big challenge is the higher voltage cables such as the 400kV that would be needed on the Bramford – Twinstead connection. Recently, National Grid has been installing 400kV cables in tunnels as has been seen around east London for the 2012 Olympic Games site.

The picture (right) shows a three-meter diameter bored concrete lined tunnel containing a new 400kV cable system with space for a future 2nd 400KV system on other side of the tunnel. This type of installation has been used in North London Ring from St Johns Wood to Elstree, a total length of 20 Km and the Olympic Park area.

The first underground cables were particularly complicated and involved the cooling of the cable with oil pumped through the pipe (the large voltages create a huge amount of heat) this process was used in the Vale of York project. New cable technology is now focussed on the XLPE design (Cross Linked Poly Ethylene) 

400kV XLPE underground cable within a 3 metre diameter tunnel under London.

These polymer insulated cable systems are becoming a mature technology and current developments are mainly concerned with increasing transmission voltages without significantly increasing the diameter of the cable. Today, the maximum voltage is 550kV ( a 40km underground 550kV project is already installed in Japan).

These cables have a diameter of 15 – 18 centimeters, are extremely heavy and only so much can be rolled onto a single cable roll (currently 800m to 1km before a join is required. There is economic pressure to increase the lengths of a single length cable as joints are costly and bring reliability risks.

Increasing the length of cable between joints reduces cost and complexity. Joining the cables involves a significant amount of specialist work whether the cables are laid in tunnels or installed by direct burial. This photo is of a joint on the 14km 400Kv Madrid Airport link.

There are in essence two ways to install underground cable, in tunnels as we have seen or by direct burial. Despite being used in other countries including Denmark and Japan, direct burial was formerly not favored by National Grid.

However, National Grid now plan to use this technique to install XLPE cables as part of the Bramford to Twinstead, Somerset and Mid Wales projects.

This man standing in front of a 400kV XLPE underground cable drum gives a sense of the challenge that surrounds its handling and installation 

400kV XLPE underground cable joints near Madrid Airport

Installation involves excavation of a wide trench before cable laying and back filling with material that controls water content, flow and heat dissipation. The ground is then reinstated above this.

For the underground sections of the Bramford to Twinstead Project, 12 or 18 cables will be laid in parallel, depending on the transmission capacity of the cables that are then available. 

The lesser number of 12 higher capacity cables would allow a narrower trench and therefore lower environmental impact.This would logically also reduce the number of joints and the amount of civil engineering to install them and would therefore prospectively reduce overall costs.

400kV XLPE underground cable being laid in Japan using the direct burial technique.

400kV XLPE cable structure