npower’s Wayne Mitchell going underground

Should our power cables go underground? Calls for our network of power supply cables to go underground is nothing new. But the recent storms and floods have only added weight […]

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Should our power cables go underground?
Calls for our network of power supply cables to go underground is nothing new. But the recent storms and floods have only added weight to the argument, as not only will this approach create less of an impact on the landscape, it means powerlines will be less susceptible to weather damage.

At face value, undergrounding does seem like an obvious route to improve the robustness of the network. Such an approach would also improve social amenity by hiding transmission and distribution infrastructure beneath the ground. But inevitably, there are both technical and economic considerations.

Air is in fact a fairly good insulator – so good that suspended cables do not typically require any further covering and are just bare conductor. This contrasts with buried cables which require both insulation and often steel armour protection too. However, more significant is the air’s ability to dissipate heat.

The challenge of heat
By their very nature, power cables are designed to be low resistance, but the transmission of hundreds of megawatts of power at high voltages nonetheless generates significant heat. Suspended in the air, this heat is easily given up to the atmosphere. But buried underground, and covered with thick insulation, this presents an engineering challenge.

Typically underground cables are much thicker to cope with the higher heat load. This obviously requires larger volumes of increasingly expensive commodities such as copper and aluminium. Some underground cables are even water cooled or have cooling fans and ventilation systems when installed in ducts.

National Grid says that to match the thermal performance of a 400kV double circuit overhead line, as many as 12 separate cables in four separate trenches may be needed, resulting in a work area up to 40 metres wide.

Aside from land use and heat loss considerations, there is also the far more involved issue of cable access for any potential repairs or replacement over time. All of these issues add both cost and complexity.

Billions of investment required

Indeed, a 2012 report from Parsons Brinckerhoff and the Institution of Engineering and Technology analysed the whole life costs of installing and maintaining new high voltage transmission circuits underground, under sea and over ground.

Produced for the now defunct Infrastructure Planning Commission, one of the main findings was that the cost of installing new power infrastructure underground, using existing technologies, is always more expensive than installing overhead lines.

Facing tough questioning from the Commons Energy Select Committee just a few weeks ago in their enquiry into ‘Power disruption due to severe weather’, distribution companies emphasised this point. The Committee, looking into the power network failures that hit much of the country due to storms over Christmas, heard that burying the UK’s transmission and distribution networks would cost tens of billions of pounds.

Preferred option in special cases
There are, of course, situations where undergrounding cables is the preferred option. For example, the proposed new nuclear power station Hinkley Point A requires a transmission line upgrade to deliver its output to the UK’s demand centres. The route of the existing line passes through the Mendip Hills Area of Outstanding Beauty. As part of the proposals, some five miles of the new 400 kV line would be buried, while the existing 132kV line will be removed, freeing the area from the site of pylons for the first time in more than four decades.

Similarly, in February 2011, National Grid launched a seven-year project to upgrade London’s high voltage power network using a total 32 km of deep underground tunnels – up to 60 metres below street level – carrying 400kV cables.

However, these examples are clearly exceptional, and that is perhaps the point too. The series of storms and floods that has hit the UK and disrupted the power network is believed by many to be just that, exceptional.

Undergrounding the country’s power network is certainly technically feasible and it would clearly resolve some reliability issues under similar circumstances. But with consumers already concerned about challenging energy prices and the prospects of further rises, rerouting the power network would result in some pretty major additional expenses – and I can’t imagine that this would be popular.