Institute for Energy, Drive and Environmental Technologies
Ilmenau, 13 October 2011
Meta study on the properties of
overhead lines and underground cabling
Executive summary
13 October 2011
Scientific responsibility:

Dipl.-Medienwiss. Dorothee Arlt

Dr.-Ing. habil. Alexander Novitzkij

Prof. Dr.-Ing. Dirk Westermann

Prof. Dr. Jens Wolling
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Introduction and demarcation
In the preparatory and authorisation phase of grid expansion measures for the transmission
network, it is common for a variety of technology options to be fiercely debated on a wide
range of levels. The discussion primarily focuses on the comparison between overhead lines
and underground cabling. In this context, recent years have seen the appearance of a large
number of publications addressing the problems of technology comparison both generally and
with reference to specific expansion projects.
This meta study aims to examine and aggregate current findings on overhead lines and underground cabling by analysing existing primary studies and projects. While a meta study does
not provide any direct comparison between existing studies, it enables a statistical comparison
of individual key figures. The object of this meta study is therefore to throw light on the available body of scientific evidence concerning power line expansion.
It is not the aim of this meta study to validate the set of studies examined or to provide any
supplementary analysis in the form of new examples, case studies or new calculations. Moreover, it is in the nature of a meta study to abstract information from a variety of particular features from those individual studies. Only the data documented in the selected studies was
used to present the content of the criteria examined, with no additional data being collected.
The metastudy focuses on the transmission network levels (220kV and 380kV).
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Methodology
The study was conducted at the Technical University of Ilmenau's Institute for Energy, Propulsion and Environmental Technology by an interdisciplinary team consisting of electrical engineers and social scientists.
Since the results of the study are attracting a great deal of public and (supra)regional interest,
Swissgrid set up an Advisory Study Council representing all stakeholder groups in Switzerland
concerned with grid expansion projects. The tasks of the Advisory Council were to deliberate
on the study findings, and to confirm their comprehensiveness and differentiality. The objective
performance of this scientific study did not fall under the remit of the Advisory Council.
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Study set and approach model
The meta study examined studies on grid expansion in the extra-high-voltage transmission
network that were published in the period between 2000 and the beginning of 2011 in European countries with an infrastructure comparable with that of Switzerland. In all, the studies comprised 176 documents. From these, two study sets were created to reflect as closely as possible the current state of scientific knowledge. One set covers general studies which examine
the properties of overhead power lines and underground cabling according to scientific methods and without any project focus. A second set covers project-based studies which discuss
the effects of overhead power lines or underground cabling using specific expansion projects
as examples.
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The list of criteria used to examine the properties is based on the Swiss "Overhead versus
Underground" Test and Evaluation Model at the 220/380-kV level, which was adapted to meet
the needs of the meta study and consolidated with the approval of the Advisory Council. The
catalogue covers metrically saleable criteria such as losses, lifecycle, reactive power requirement etc. The second group of criteria covers ordinally saleable criteria such as impact on the
landscape, flora and fauna. To analyse these, a codebook was developed as an instrument for
data recording, containing a description of all ordinally-scaled criteria and their characteristics.
This enabled the criteria to be measured.
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Main findings
Preamble
Only two of the project-based studies cover 90% of the criteria relevant for a technology decision. Fewer than one third cover 80% of all criteria and one quarter of all studies take fewer
than 50% of the criteria into account for a technology decision. There remains a clear need for
a standardised, generally binding process and legally permissible reference framework (guidelines, criteria) for the socio-economic evaluation of overhead power lines and underground
cabling. This must be broad-based and transparently communicated.
Line length
The lengths of alternative overhead and underground line routes are not always identical. The
alternative underground cabling routes are often shorter. This is due both to the higher cost of
underground cabling and to possible restrictions on the route for overhead lines.
Losses
The meta study examines only the statistics for capacity and energy lost in the lines for technical reasons. Losses depend on the load and layout of a line. A correct layout is fundamentally one based on an attempt to ensure an economic optimum of underground cabling and overhead lines. For this reason, taken in isolation, both underground cabling and overhead power
lines are economically optimised, although individual loss factors may vary between underground cabling and overhead lines. The general studies show that load-independent losses
are smaller in overhead power lines than in underground cabling, a conclusion that is supported by the project-based studies.
By contrast, overhead power lines exhibit higher load-dependent losses than underground
cabling under identical operating conditions. Only a few studies cover the additional throttling
and shielding losses which occur in underground cable routes, although according to the evaluation results, the number of throttling losses is higher than voltage-dependent losses. Shielding losses may be greater or smaller than voltage-dependent losses. The findings reveal a
lack of any standardised reference frame for conducting loss analyses. Further research and
deliberations are required in this context.
Operation/security of supply
An analysis of the parameters for frequency of outage, repair time and non-availability shows
that overhead power lines are restored to normal operation faster than underground cables.
While the frequency of outage is slightly higher for overhead lines, repairs to underground ca-
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bling take longer. The non-availability of underground cabling can therefore be between 40270 hours higher per year than that of overhead lines.
If one were to consider only the criterion of lifecycle (useful life), then the choice would always
be for overhead lines. The lifecycle of underground cabling is 20 to 60 years, compared to 40
to 120 years for overhead lines. There are no internationally applicable regulations in this area.
It is important in this connection to note that there are no corresponding empirical values for
cross-linked polyethylene (XLPE) extra-high voltage cables, hence such values can only be
estimated at present.
Operating costs
Rather than presenting new calculations on the cost criteria, the aim of this metastudy was to
provide an overview of cost factors which were drawn up in the analysed studies. The studies
are not predicated on transparent, uniform operating and financial assumptions. Costinfluencing factors such as electricity prices, lifecycle, interest rates, depreciation periods or
load profiles and losses are not uniformly applied in the studies examined, something which is
also true of determinants such as changes in material prices over the depreciation periods.
Three cost criteria were reviewed: Investment costs, operating costs and total costs. Only 30%
to 35% of the studies address the criteria of operating costs, including losses and total costs
over the useful life, while 60% examine the criteria of investment costs. A macroeconomic cost
review that also factors in the value of surface area, soil or landscape was not carried out,
since these were either ignored or insufficiently reviewed in the studies examined.
In 60% of the studies, only the costs of losses was included under operating costs, although
maintenance, servicing and non-availability costs also fall under this category. Non-availability
costs have a major influence on operating costs. Operating costs are essentially dependent on
current electricity prices. However, given that the costs related to non-availability are the dominant factor on the overhead/underground operating cost ratio, a rise in electricity prices is of
less significance here.
In most cases, only the investment costs, rather than the total costs were examined over the
useful life. The higher the voltage level, the greater the difference in investment costs – up to
a factor of 13 (excluding extreme values).
In terms of total costs, underground cabling is generally more expensive than overhead power
lines. Although the difference in costs may sometimes be insignificant, additional costs up to a
factor of 19 may be incurred, depending on the project. The effective amount of total costs
incurred over the useful life depends on the specific project. Generally speaking, the examined
studies show that total costs over a set observation period of 80 years are higher for a cable
system than for an overhead line installation.
Landscape
For this frequently studied criterion, the aim was to determine whether effects on the landscape, in particular in relation to the visibility of overhead lines and structures, were reviewed
and evaluated. Overhead lines are rated very negatively, although underground cabling is also
negatively evaluated to some extent. Direct comparisons of the effect on the landscape always
come out in favour of underground cabling.
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Flora and forests
Here the aim was to determine whether effects on the flora during the construction and operating phase, for example through deforestation, settlement, interference with wetlands, were
discussed and evaluated. The criterion was more frequently examined in the operating phase
than in the construction phase. Both underground and overhead lines have a negative impact
on flora in both phases (construction and operation). Only sparse data exists for the purposes
of a direct comparison. The available data favours overhead lines in the construction phase,
but the findings for the operating phase are contradictory.
Fauna
Here the aim was to determine whether effects on fauna during the construction and operating
phase, for example through electric shock, loss of refuge zones, migration barriers, were discussed and evaluated. The criterion of fauna was more frequently studied in the operating
phase than in the construction phase. Overhead lines are negatively assessed both in the
construction and operating phases, in some cases very negatively. For underground cabling,
this finding primarily applies to the construction phase, with negative assessments occurring
less frequently for underground cabling in the operating phase.
Soil
Here the aim was to determine whether effects on soil during the construction and operating
phase, for example through soil shifting, soil compaction, soil warming or contamination, were
discussed and evaluated. The effects on soil of underground cabling were studied in particular
during the operating phase. Both underground cabling and overhead lines are negatively assessed in both phases, but underground cabling to a greater extent. Direct comparisons between underground cabling and overhead lines favour overhead lines in both phases.
Noise
The aim here was to determine whether noise caused by construction work and during operations was examined and evaluated, i.e. noise in the construction and operating phase. The
criterion was reviewed in particular for 380-kV overhead lines in the operating phase. There
was virtually no assessment of the criterion of noise during the operating phase. For underground cabling during the construction phase in particular, this criterion was negatively assessed. Little direct comparison was made between underground and overhead lines in both
phases.
Electrical and magnetic fields
Here the aim was to determine whether the effects of magnetic fields on humans during the
operating phase, for example in the form of sleep disturbance, illnesses, influence on pacemakers etc., were discussed and evaluated. This criterion is discussed very frequently and in
most cases there are no clear assessments (neutral, contradictory or non-existent evaluations), with only one negative assessment of overhead lines for Switzerland. In a direct comparison, underground cabling comes out better than overhead power lines.
Public acceptance
The aim with this criterion, discussed in at least a quarter of the studies, was to determine
whether statements on the population's acceptance of a planned line, such as protests or ap-5-
peals, were discussed and assessed. In a great many of the studies, overhead lines are negatively assessed, while underground cabling is accorded a better rating when directly compared.
Faults during the operating phase
This frequently-studied criterion was examined to assess the inclusion and evaluation in the
studies of faults caused by deliberate or unintentional human intervention or environmental
factors which can result in outages during the operating phase or have negative effects on the
environment. Overall, the assessments view overhead power lines less negatively than underground cabling. However, in fewer than half the studies was either type rated negatively. In
direct comparisons, the studies arrive at contradictory results, with more studies concluding
that the effects of overhead line failures are less serious than faults in underground cabling.
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