Proposed Beauly to Denny 400kV Overhead Transmission Line
6
Chapter 6
Alternatives
Alternatives
6.1
Introduction
6.1.1.1
This chapter describes the alternative strategic options which were considered in the early
stages of the project and it refers to the technical information which informed SHETL and
SPT’s view that the line would be overhead. Chapter 7 presents the design statement and
Chapter 8 summarises the routeing process and the decisions that were made in arriving at
the proposed route for the overhead transmission line forming part of the project. The
alternatives that were considered for substation locations are summarised in Chapter 9.
6.1.1.2
There are various means of achieving upgrades to a transmission system including overhead
and underground options. Alternatives to the proposed project have been considered at
several stages in its development including:
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•
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Initial consideration of strategic options;
Consideration of whether the line should be underground or overhead;
Appraisal of routeing options for the overhead line in the preferred corridor taking account
of routeing criteria set out in the design statement.
6.2
Strategic Options
6.2.1.1
A variety of strategic alternatives to achieve the required transmission capacity were
considered by SHETL in the early stages of the project. These included using the existing
275kV route from Beauly to Foyers and then building a new route from Foyers to Fort
Augustus. The existing 132kV route between Beauly, Fasnakyle and Fort Augustus would be
retained. This option was rejected by SHETL on technical and financial grounds as there
would have been considerable generation constraints during off peak periods and it would also
limit possible future capacity increases.
6.2.1.2
A variety of other routes in the west to connect Beauly to the SPT area were also considered.
In general, these were technically feasible, but were more costly. In addition, many of the
routes were remote from the sites of renewable activity and thus additional radial transmission
lines would have been required to harvest the generation.
6.2.1.3
The main alternative to the Beauly-Denny route was to reinforce the 275kV infrastructure
between Beauly and Blackhillock/Keith and down the east coast via Kintore, Tealing and
Westfield in SPT’s area. However this option was rejected for the following reasons:
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6.2.1.4
The east coast option involved construction of more new 400kV overhead tower lines
(300km between Beauly and Keith, Kintore and Tealing and Tealing to Kincardine as
compared with 220km from Beauly to Denny) to achieve the same capacity increase.
The east coast option is remote from many of the renewable generation planning
application locations and consequently, additional radial transmission lines would be
required to harvest the generation.
An economic assessment, on a net present value basis, of the east and west routes
indicated the Beauly to Denny route was the lowest cost option.
Therefore, in autumn 2002, SHETL commissioned ASH Design + Assessment to undertake an
initial investigation of broad corridors for the proposed overhead transmission line in an area of
search defined by SHETL which broadly followed the existing 132kV corridor between Beauly
and Bonnybridge (see Section 5.2). As part of the study, a significant quantity of baseline
information was collected and consultations were undertaken with the Local Planning
Authorities, SNH and others. A preliminary analysis of the information that was gathered was
carried out, and preliminary search corridors of a uniform width either side of notional routes
were defined.
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1
6.2.1.5
The findings of the study were reported in a Preliminary Options Report (see Chapter 5).
6.2.1.6
A review of the Preliminary Options Report by SHETL concluded that the preliminary study
was an invaluable data source, but it was agreed that the routeing study should not be
constrained by the preliminary corridors which were identified.
6.3
Undergrounding High Voltage Transmission Lines
6.3.1
Introduction
6.3.1.1
SHETL owns a transmission network2 comprising around 4,790km of overhead lines, mostly
carried on steel lattice towers, and 60km of underground cables. SPT owns a transmission
network comprising around 3,740km of overhead lines and 217km of underground cables.
6.3.1.2
A recent Commission of the European Communities Paper3 concluded that land based
underground cables account for only 2.4% of the total 220kV to 400kV network in Europe.
6.3.2
Technical Overview of Comparison between Overhead and Underground
Introduction
6.3.2.1
An overhead line conductor or an underground cable simply provides an electrical link
between two points. For transmission network purposes, using an underground cable or an
overhead line can achieve the link. Technically the difference between the two is the type of
insulation used and the practicalities of designing, constructing and operating a safe system.
6.3.2.2
In an underground cable, the insulation is provided by a manufactured medium, such as fluid
impregnated paper or cross-linked polyethylene (XLPE) that surrounds the conductor. The
mechanical protection of the cable is provided partly by a sheath that surrounds the insulation
and partly by burying the cable below ground.
6.3.2.3
In an overhead line, the insulation is provided by glass, porcelain or polymeric insulators and
by the air surrounding the conductor. The mechanical protection and electrical clearances are
maintained by supporting the conductors high above the ground in the air.
6.3.2.4
For transmission underground cables at 275kV and 400kV, the main advantage in their use is
that once buried, the cables are not visible, although the cable trench may be for some years
as the vegetation re-establishes. There are only two practical insulation mediums currently
available at these voltage levels: fluid filled and XLPE. Each of these insulation mediums has
specific issues for an operator.
6.3.2.5
A section in the document4 produced by National Grid Company (NGC), the licensed
transmission operator for England and Wales titled "A Question of Insulation", discusses these
issues, outlining the practical implications which arise from the laying of underground cables
and the construction of overhead lines. SHETL and SPT endorse these and the key points are
summarised in the following sections.
Fluid filled cables
6.3.2.6
Fluid filled cable routes need to be profiled to ensure the fluid pressure within the cable is
maintained within its design operating limits. If the cable route has large changes in levels then
additional stop joints and pressure tanks are needed along the route to divide the cable into
separate hydraulic sections. The fluid used is mineral based oil.
1
ASH for SHETL Preliminary Options Report (2002).
2
In Scotland, transmission voltage is 132kV and above.
3
Commission of the European Communities paper on “Undergrounding of Electricity Lines in Europe”, December 2003.
4
‘A Question of Insulation’ National Grid Company, NGC (2004).
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6.3.2.7
Chapter 6
Alternatives
Advantages:
•
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Fluid filled cables have a proven reliability record over many years.
The fluid in the cables is under positive pressure and provides a means of monitoring the
state of the cable.
•
Fluid filled cables very rarely develop electrical faults: usually a fluid leak warns the
operator of a potential failure.
Disadvantages:
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Fault location and repair times can take many weeks.
High fault repair costs.
The fluid used in modern cables, although biodegradable, is classified as a List 1
controlled substance under the Groundwater Regulations 1998.
Fluid leaks from cables into the ground in proximity to ground water extraction bore holes
or other areas where watercourses are used for water supply have resulted in
prosecutions by the Environment Agency. The use of this technology may not accord
with the increasingly regulatory approach of UK environment legislation and agencies.
Vehicular access is needed to the pressure tank positions at all times in order to maintain
the circuit in service and gain access for repairs.
There is an additional cost of installing stop joints and fluid pressurising tanks. Tanks are
typically needed at least every 4 or 5km and for large changes in profile.
XLPE
6.3.2.8
Cross-linked polyethylene (XLPE) cables have a solid insulation. XLPE insulation has been
used for low and medium voltage cables for over 30 years but only recently at 132kV, 275kV
and 400kV. The cables are single core and can be laid in trefoil or flat formation.
Advantages:
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XLPE cables are environmentally safe when installed and require only minimal
maintenance for sheath testing and cross bonding maintenance.
Lower capital cost than fluid filled cables.
Disadvantages:
•
•
•
•
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Usually there is no indication of a possible failure before the event.
Specific spares are required to match existing cable dimensions.
There is a risk of possible double circuit failure due to rise of potential on cable sheath.
Fault repair times can take several weeks.
High fault repair costs.
There are a limited number of installations to validate reliability. NGC, which has the
largest transmission network in the UK, still has not approved XLPE for widespread use
at 400kV and some overseas installations have not proved reliable.
6.3.3
Advantages of Overhead
6.3.3.1
NGC has produced a document which also provides an authoritative engineering overview of
the comparison between overhead lines and underground cables.
6.3.3.2
In relation to installation, when considering the merits of overhead line versus underground
cable at transmission voltages, the following summarises the position stated within the NGC
document:
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5
Overhead or Underground? – The National Grid Company approach, 1995.
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6.3.4
275kV and 400kV underground cables have the advantage of incurring lower power
losses. However, cable installations have a far higher charging current than their
overhead line equivalent (up to 40 times for 400kV), which if left uncompensated will tend
to increase cable and wider network losses. More significantly, uncompensated cable
installations will cause voltage issues for both cable insulation and quality of supply,
especially when switching the cable in and out of service. The practicalities in providing
the required level of compensation are onerous, both in terms of cost and the
environment, given that there could be several compensation substations required along
the cable route.
Underground cables limit the potential for future upgrade. Whereas a 275kV overhead
line originally designed for 400kV operation can readily be upgraded to 400kV, a 275kV
underground cable cannot, and would need to be replaced, or alternatively 400kV
underground cable installed from the outset.
An underground cable circuit may consist of a number of separate cables laid at
nominated spacing to satisfy thermal requirements relating to the circuit loading, which
means that the cable installation occupies a considerable width.
Overhead lines only require tower positions typically 300 to 400 metres apart, allowing
them to cross over many obstacles following a relatively direct route. Crossing of natural
features such as rivers, small bogs and wet areas, and man made features such as
roads, railways and canals is made easier and with less disruption.
Although cables are buried, it is important that the ground surrounding the cables is
stable and has good thermal characteristics. In certain areas of Scotland, rocky and
boggy ground conditions are more likely to be encountered on the route, making it
difficult to install the cable and gain access for repairs and maintenance.
Weather may restrict access to the cable in very wet spells or prolonged periods of snow.
Repairs to cable require excavations and the provision of clean and dry conditions for
jointing.
Advantages of Underground
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6.3.5
Chapter 6
Alternatives
Less visually intrusive than overhead.
Less liable to experience transient faults caused by lightning, trees etc.
Less danger of contact by third parties to overhead conductors.
Undergrounding a Section of Overhead Line
Introduction
6.3.5.1
Undergrounding a section of overhead line involves terminating the overhead line at a tower
and connecting the overhead line conductors to underground cables: this joint is referred to as
a sealing end. If the cable section is at the end of the line, then only one terminal tower is
required. If it is in any other part of the line, then two terminal towers are required. Terminal
towers are the most structurally complex and therefore the bulkiest and most visually intrusive
of the towers in a family of towers. The sealing ends can either be on a platform on the tower
or in a separate fenced compound, either of which adds to the visual intrusion of the terminal
tower. This makes it very hard to site these in such a way as to minimise visual impact.
Electrical Considerations
6.3.5.2
Most faults on overhead lines are transient. This means that the cause of fault disappears
very quickly; typical examples are lightning or wind blown debris. To facilitate the quick
restoration of the transmission system, overhead line circuits are usually fitted with an
automatic reclose scheme that restores the circuit within a few seconds after the fault has
been detected. Faults on underground cables are usually permanent, such as third party
damage or manufacturing / design failure. Auto reclose schemes are not usually installed on
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circuits with underground cable sections, mainly due to the danger that the person causing the
damage may still be within the vicinity of the cable and also because of voltage depressions
on the system. There will also be other technical and operational issues that need to be
considered.
Routeing Considerations
6.3.5.3
As a result of the different environmental effects of the two different types of construction, a
mixed overhead line and underground cable route involves different routeing considerations
from either an all overhead line, or an all underground cable route. In particular, identifying
an appropriate location for terminal towers and sealing end compounds can be a key
determinant of a route.
6.3.6
SHETL and SPT's obligations
Obligations
6.3.6.1
SHETL and SPT are required by the Electricity Act to develop and maintain an efficient, coordinated and economical system of electricity transmission, to facilitate competition in the
supply and generation of electricity, and to have regard to the preservation of amenity and
fisheries (see Section 2.2). To fulfil these requirements, SHETL and SPT take into account
economic, operational and environmental factors to assess the advantages and
disadvantages of overhead lines and underground cables.
The Economic Factors
6.3.6.2
It is estimated that at 275kV and 400kV voltage, underground cables are between 10 and 25
times more expensive to construct than overhead lines (based on a km of 400kV double circuit
overhead line costing £750,000). The actual cost ratio will vary depending on the length and
nature of the cable section, size of cable and number of terminations. The unit cost of short
lengths will tend to be more expensive due to the high cost of terminating equipment.
6.3.6.3
A survey carried out by a major international organisation (CIGRE 1996) shows that these
figures are typical of world experience. Other reports have indicated lower cost ratios on
specific projects within Europe. However, these lower ratios often depend on other factors
such as the use of XLPE cable, reduced capacities or simple installation conditions. More
recently, a 5.7km underground cable installation at 400kV equated to £10million per km.
Discussions with leading cable manufacturers have also confirmed the above ratios as a
realistic cost comparison.
The Operational Factors
6.3.6.4
The Licences under which SHETL and SPT operate require them to build a secure network
capable of withstanding specified fault contingencies without loss of supply. Therefore each
strategic link in the network must comprise of at least duplicate circuits, each capable of
individually carrying 100% of the required maximum load. This is to enable maintenance and
fault repairs, minimising the constraints on the load carrying capacity of the network.
6.3.6.5
Faults on overhead and underground networks are of a different nature. For overhead lines,
many faults, such as lightning, are temporary and only last a few seconds. Sustained faults on
overhead networks are usually relatively easy to find and repair and can normally be fixed
within hours. For underground networks, sustained fault rates are generally higher and repair
times are considerably longer, running into weeks and months, and repair costs are
considerably higher, often amounting to hundreds of thousands of pounds. It also takes time to
locate underground cable faults, whereas overhead line faults are usually easily identified.
6.3.6.6
Research undertaken by French and Japanese engineers indicates that statistically, EHV
cable network fault rates are around 0.20 / 100 circuit km / year. Over the 40-year design life
of a cable network, this equates to 8 faults / 100 circuit km. Data from the Council of Large
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Electric Systems (CIGRE) 6 indicates that worldwide, fault rates for EHV fluid filled cables were
1.6 /100 circuit km / year. Both figures exclude damages caused by third parties. Whilst these
figures are low and for short lengths of cable have little significance, as the circuit length
increases, the network implication become more problematic.
6.3.6.7
In considering the on-going operating costs of a cable network as compared with an overhead
line network, SHETL's operating experience of its existing 1,558km of 275kV tower lines gives
annual fault and maintenance costs per 100km/year of £30k to £40k. This compares to an
anticipated cost of £40k to £320k per 100km/year for fault repairs on a 275/400kV cable. This
cost range is calculated from the fault rates in paragraph 6.3.6.6 combined with a cost of
£200k for a single two joint repair.
6.3.6.8
Due to the nature of the failure mechanism in XLPE cables, there is an increased likelihood of
both circuits in duplicate routes failing at the same time. The international view for security of
supply is that on strategic cable routes that are direct buried, fluid filled cable designs are still
the preferred choice despite environmental implications.
6.3.6.9
The electrical operating characteristics of a cable compared to an overhead line are also
different. The cable charging current (MVAr's produced by the uncompensated cable) will
cause the voltage to rise appreciably. While the charging current for a short length of cable
(<10km) could readily be compensated, the charging current for 200km of 400kV cable would
be in the region of 10,000MVAr and would cause a 35% voltage rise.
The Environmental Factors
6.3.6.10
The environmental debate on overhead compared to underground generally focuses on the
visual impact of the steel lattice towers. A 400kV tower can be over 50 metres in height.
SHETL and SPT acknowledge that transmission towers and their associated conductors are
visually intrusive and as such, can have an effect on visual amenity.
6.3.6.11
SHETL and SPT, through the choice of a good route and the process of environmental impact
assessment, which includes considerable consultation, seek to mitigate the effect on visual
amenity that any new or upgraded line will have. This process will also mitigate other
environmental effects.
6.3.6.12
However, it should be noted that transmission voltage underground cable installations also
have an effect on the environment due to the width of the construction area necessary, the
physical and visual impact of the cable sealing end compounds and terminal towers, and the
number and depth of cable trenches (typically 25m wide, with four separate trenches of
1300mm depth, for a high capacity 400kV double circuit installation). Underground cables also
place restrictions on use of land after installation. Some examples of this are given below:
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6
Cable installations take up to five times longer to complete than equivalent overhead
lines.
Disturbance to flora and fauna, drainage, archaeological sites and land use can be
significant as a result of excavations for the cables. Reinstatement after construction can
be difficult, particularly as a result of ground and drainage disturbance.
The on-going land use above the cable route has to be restricted, particularly in relation
to tree planting, and deep agricultural operations. No buildings are permitted over the
cables. Due to the need to export and import large volumes of material throughout the
length of the cable route during construction, suitable access tracks, if not in existence,
would need to be built. Some permanent access tracks are required for ongoing
operation and maintenance requirements.
Excavations for the repair of cables and access for associated plant causes ongoing
disruption.
It is currently considered that strategic cable routes should be constructed using fluid
filled cable technology. The topography of the Scottish Highlands would make a fluid
filled cable installation very onerous and may increase the risks of fluid loss from the
cables.
Survey of the performance of HV AC Systems (Electra 137), Council of Large Electric Systems (CIGRE), August 1991.
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6.3.6.13
If an existing transmission overhead line were to be dismantled in the future, then only the
tower foundations below ground would remain as a permanent effect, whereas a transmission
cable would be left in-situ with a proportion of the fluid being unable to be drained out.
6.3.7
Summary
6.3.7.1
Both economic and operational factors normally support an overhead line approach to
transmission circuits.
6.3.7.2
Visual amenity can support an underground approach. However, other environmental factors,
such as natural and cultural heritage considerations, the effect on the hydrology and the
difficulty and length of time to re-establish ground cover, weigh against the use of cables.
These factors primarily relate to the construction phase and restrictions on land use on the
cable route once installed. On balance, an overhead approach is to be preferred.
6.3.7.3
It is therefore SHETL and SPT's approach that in situations where an exceptionally high value
is placed on visual amenity, and where no suitable alternative route for an overhead line can
be identified, they will consider laying underground transmission cables.
6.3.7.4
This approach is similar to that adopted by the other licensed transmission operators in the
UK.
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