Transmission line report shows overground cable to be cheapest

IET oversees report released today comparing costs of over and underground cables

IET briefing

An independent report studying the cost of overhead verses underground transmission cables was launched today with endorsement from the IET. The report reflects six transmission options for the Infrastructure Planning Committee (IPC) to consider when planning for the development of the UK's existing power infrastructure.  

The report deduced that when comparing overall lifetime costs, overhead transmission cables are the most economical technology. For example, 75km of direct-buried AC underground medium capacity (6380 MVA) cable will incur a lifetime cost of £1414.3million, whilst its overhead equivalent will cost £299.8million. 
"The report is intended as a generic tool, as a costing reference for the government to make a decision on introducing new underground or overground cabling." says Mark Winfield, Principal Consultant Engineer at Parsons Brinckerhoff and lead author of the report. 

Parsons Brinckerhoff won the tender to produce the report for the IPC, requesting and analysing data from 24 manufacturers and distributor sources. Amongst those contacted included ABB, Siemens and Northern Ireland Electricity.

The report differs from a previous, similar report launched by KEMA, due to the depth of the research and its international scale, which was based on comparative data from 24 countries. Data gathered from KEMA's previous report was also submitted to help support the costing study. 

The six transmission technologies included in the study were overhead lines, direct-buried underground cables, tunnel-buried underground cables, direct-buried gas insulated lines (GIL), tunnel-buried gas insulated lines and high voltage subsea cables. The report addresses the need for an extended transmission network due to increased power generation from sources such as renewables and nuclear new-builds, but the effect emerging technologies such superconductivity will have on cost were not considered.

"We have DECC support, and a much more reliable set of international supplier and manufacturing data than before." says Professor Mike Sterling, president of the IET. "We were previously not looking at whole life costs, and the variability on these reports are large. We anticipate there will be shifts in costs as the cost of underground cabling has decreased over recent years. In the 1960's underground cabling cost 20-30 times more to lay than overhead cables, now they cost ten times as much."

GIL is a relatively new technology explored in the report and would be a new venture for the UK in an underground capacity, joining parts of Europe and the Middle East who already utilise the technology.

"GIL is used in the UK in small above-ground sections, but is not widely used." says Winfield, "This is to preserve the safety of the public and to prevent tampering of the technology. It can kill if drilled into, so its just not safe having it sitting above ground where the public can touch it."

Variables such as fixed building costs are highlighted by the report, which specify costs that remain unaffected regardless of route length. For example, in the case of underground cabling, extras such as cable terminations and accompanying compounds must be funded regardless of the length of the power lines. Medium capacity DC subsea cables top the list for highest fixed building costs, incurring £1376.3million per 75km. These costs are influenced by factors such as research, assessments, landing costs, materials and logistics.
The report also advises how the variable length of a power route affects construction costs, taking into account the complexity of each technology. The variable build costs of a Hi capacity 3km AC overhead line for example, makes up 42.5 per cent of the overall £12.7million needed to build the line, but the variable costs jump up by over £20million when the line is extended to five times the length.
Operating costs are also studied, showing the value of costs of operation over the 40-year lifespan of the transmission equipment in each technology.  Operational costs considered include operations and maintenance, as well as power and energy losses.
The report also highlights to government the impact of sensitive factors on each technology. Overhead line costs are most likely to be affected by factors such as circuit loading and route length, the latter of which could affect upper and lower costs of overhead lines by up to 67 per cent. Whilst obviously not affected by circuit loading, the installation and operation of underground cables are also affected by installation base costs, system material costs and terrain.

The report does not give weighting to environmental, aesthetic, space or security impacts, but does acknowledge these factors must be considered. The aesthetic and environmental benefits of using a more expensive technology such as underground cabling is their invisibility, but direct-burying cables also restricts farming, new builds and the planting of new trees above ground.  Winfield says encasing the cables in a tunnel would mean the ground above is made available again, but this incurs higher costs and more intensive installation.  

Sterling said the report employed a neutral stance on environment and aesthetics, advising the government only of the costs and variables of each technology: "We are leaving it up to the government to decide whether they believe it will be worth paying substantial amounts of money to preserve the environment."


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