SC B3
103
CIGRE SC B3 Berlin 2007
http : //www.cigre-berlin-2007.org
SUBSTATION DESIGN IN A CHALLENGING REGULATORY ENVIRONMENT
C. Twomey
ESB International
Ireland
SUMMARY
ESB (Electricity Supply Board) Networks has an extensive (15+) programme of 110kV/MV
substations to be built within a regulated budget by 2010.
A comprehensive cost-reduction study was undertaken which included a review of current
practice in a number of other utilities and an enquiry to a wide range of manufacturers based
on an open functional specification.
The tenders received were evaluated against a full range of cost factors including material
cost, installation costs and lifetime costs.
The outcome of the study was a decision to standardise on the use of 110kV gas-insulated
equipment (either hybrid modules or conventional GIS) for new 110kV substations and also
to use this equipment as far as possible in extensions or refurbishment of existing AIS
substations.
A further outcome was a decision to replace conventional buildings with prefabricated
enclosures for various functions e.g. 38kV GIS, MV switchboards, control/relay rooms,
battery rooms, store rooms where it makes economic sense to do so.
KEYWORDS
Hybrid, substations, cost reduction, prefabricated equipment
colm.twomey@esbi.ie
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Background
The Irish electricity system had a maximum demand of 5042 MW at the end of 2006. The
network which is owned by ESB contains four 400kV stations, twenty seven 220kV stations
and approximately one hundred and thirty 110kV stations. The 400kV network comprises two
lines connecting a 900MW generating station on the west coast to the main load centre of the
capital city of Dublin on the east coast. The 220kV network is the main transmission network
to which about 66% of generation is connected. In addition, due to the dispersed pattern of
development in Ireland much of the 110kV network outside Dublin is operated as part of the
transmission system (with about 16% of generation capacity including most of the wind
generation connected to it) rather than as a sub-transmission system.
The historical transformation ratios to distribution voltages are 110/38kV and 38/10kV. In the
mid 1990’s it was decided to introduce a new voltage level of 20kV with the long term aim of
moving to a single MV voltage replacing both the 38kV and 10kV levels.
At this stage most of the 10kV network with the exception of urban cabled networks has been
upgraded to 20kV. The original intention was to freeze the 38kV system with all future
transformation to be directly from 110kV to 20kV. However due to the dispersed nature of the
load it has proved uneconomic to do this in all cases so a certain amount of 110/38kV
development has continued.
Since 1996 Ireland has undergone an unprecedented rate of economic development with
demand for electricity growing continuously. The average rate of load growth since 1996 has
been of the order of 5% and this rate of load growth is still continuing at present.
Meeting this load growth has required a corresponding network development effort from
ESB.
During this period one 400kV station, six 220kV stations and over thirty 110kV stations have
been built. The ongoing construction programme includes the construction of at least four
110kV stations per year.
Traditionally ESB carried out all detailed design in house and used ESB staff for all
construction work. Meeting the concentrated workload of this programme in addition to an
extensive refurbishment of the MV network could only be done through the extensive use of
installation contractors and also a limited introduction of turnkey projects for a series of urban
110kV/MV GIS stations.
This process was also complicated by the need to respond to the change in environment from
a single national vertically integrated utility to a liberalised regulated system mandated by EU
legislation which has taken place over this period.
ESB is at present operating in its second 5 year (2005-2010) price review period which has
imposed significant cuts in allowed capital and operating costs.
This has resulted in a challenge to:
- Deliver the required programme
- Review critically every aspect of the historical project delivery method
- Maintain an absolute commitment to quality
- Reduce the project delivery cycle
To meet these requirements a comprehensive cost reduction study was undertaken. This study
covered the whole field of various issues associated with the substation construction project
colm.twomey@esbi.ie
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life cycle. The initial stage of the study identified the following areas as being the most
promising for cost reduction.
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
Use of prefabricated equipment and buildings
Reduction in site dimensions
Reduction in construction time
Reduction in engineering costs
Optimisation of individual design issues
Reduction in life cycle costs
Use of mobile plant
Review of site purchase commercial issues
Review of construction contract arrangement
Review of material purchase tender strategy
This paper will describe the outcome from the work carried out in areas 1-6.
Existing standard designs
110/38kV (Station dimensions are 90m x 84m)
- 110kV: Single stranded busbar with AIS equipment. The station is laid out and
equipment rated where appropriate for an ultimate development of six feeders, two
63MVA transformers and a sectionaliser bay. All disconnectors are manually operated.
- 38kV: single tubular or solid busbar with phases in a vertical formation. The layout
allows for an ultimate development of six feeders, two transformer bays, a house
transformer bay and a sectionaliser bay.
The initial station development is frequently in the form of a tail-fed 31.5MVA transformer
with a small number of 38kV bays. The station may be then extended by a 110kV loop-in
with the installation of a busbar. A second transformer could follow in due course and the
transformers could eventually be replaced by 63MVA units.
110kV/MV (Station dimensions are 63m x 37m)
- 110kV: A tubular copper busbar with AIS equipment. The station is laid out as a H-type
layout of two feeder bays and two 20MVA transformer bays with a sectionaliser bay. All
disconnectors are manually-operated. The transformer secondary voltage is selectable as
10kV or 20kV. The design of the station allows for a possible uprating to 31.5MVA
transformers. It does not allow for additional bays.
- MV (where MV is 10kV or 20kV): Single busbar metal-enclosed switchgear rated for
operation at 10kV or 20kV in a building sized for an ultimate capacity of 20 feeders. The
initial installation is fitted with the number of feeders immediately required.
110kV/MV (Station dimensions are 34m x 34m)
- 110kV: Single busbar GIS. The normal initial layout is two feeders and two 20MVA
transformer bays and a sectionaliser bay with space left to add an additional feeder at
each end of the busbar. The transformer secondary voltage is selectable as 10kV or 20kV.
The design of the station allows for a possible uprating to 31.5MVA transformers.
- MV (where MV is 10kV or 20kV): Single busbar metal-enclosed switchgear rated for
operation at 10kV or 20kV with space for an ultimate capacity of 20 feeders. The initial
installation is fitted with the number of feeders immediately required.
colm.twomey@esbi.ie
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The HV & MV equipment is installed indoors with transformers either indoors or in opentopped enclosures.
All HV or MV equipment is bought on the open market in the form of 3-5 year framework
contracts for each item.
The default standard is that of an AIS station. GIS is normally used only in urban areas or in
high pollution locations e.g. coastal generating stations.
ESB experience with the two technologies may be summarised as:
Advantages
AIS
GIS (25 year history)
Lowest material prices as best value option
is bought for each item
Excellent service history
Freedom from long-term dependence on
any particular supplier as layouts allow for
use of equipment from any supplier in new
construction or in individual item
upgrading or replacement
Short construction time
Small site size
Disadvantages
Normal AIS maintenance issues,
particularly with disconnectors
High initial cost
Large site size
Corrosion issues on exposed
flanges
Construction time
Awkward to extend or
modify once the particular
equipment has gone out of
production.
Cost reduction study
The cost reduction study focussed on the AIS 110kV/MV station as this comprises the vast
majority of currently foreseen new station construction. As this is already a very basic H
arrangement it was not considered worthwhile to look at alternative configurations.
The process began with a research phase in liaison with various manufacturers to identify
possible options. Many different designs are available on the market today as suppliers,
recognising the current pressures on utilities, are promoting various approaches to cost
reduction.
Nothing was ruled out at this stage so information was collected from suppliers and visits
were made in conjunction with suppliers to utility substations in 7 countries – France,
Germany, Hong Kong, Italy, Japan, Portugal and UK.
ESB itself has experience since 2000 with some of these new solutions in the form of:
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- Compact AIS on a single frame used in the refurbishment of a generation-associated
110kV station in a restricted site (Fig. 1)
- GIS modules used in the extension of a small generation-associated 110kV station on a
very restricted site (Fig. 2)
Fig. 1 Compact AIS on a single frame
Fig. 2 GIS modules
- GIS modules used to provide 110kV mobile bays (Fig. 3)
- GIS modules used as part of relocatable capacitor banks (Fig. 4)
Fig. 3 110kV mobile bay
Fig. 4 Relocatable 110kV capacitor bank
- GIS modules used by IPP contractors in the connection of new wind farms and of a new
generation station to the ESB network
Layout studies were carried out using:
- Existing AIS (as a baseline)
- Rotating circuit breakers
- Withdrawable circuit breakers
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- Outdoor GIS
- GIS modules
- Compact AIS on single base frame
Possible layouts proposed by suppliers in their publicity layouts were reviewed and modified
in cases where it was felt that they were based in over-optimistic assumptions. A particular
example of this was proposed layouts for adjacent bays using open terminal connections using
a bay spacing based on electrical clearances rather than maintenance clearances.
Comparative cost estimates were produced using budget prices from suppliers (where
available) or cost information from ESB records or experience. These estimates also used the
suppliers’ estimates for the effort required to install the equipment modified where
appropriate to reflect ESB experience.
The study showed that savings of the following order appeared to be feasible:
Total Project cost:
Required site area:
Construction time:
0% to 19%
25% to 46%
25% to 42%
Use of standard prefabricated modules over a series of stations should lead to a reduction in
detailed design costs for subsequent stations. It will certainly lead to a reduction in
procurement (as opposed to material) costs due to most of the 110kV equipment in a station
being bought on a single order.
The level of saving achievable was highly dependant on the cost of land. In general the
equipment cost was higher than the AIS equipment but this additional cost was
counterbalanced by the reduced land cost, reduced civil works cost and the impact of the
shorter construction time.
As mentioned above many of these stations would be under the operational control of the
Transmission System Operator so close contact was maintained with this organisation during
the study.
The best results seemed to be obtainable from the use of Mixed Technology Switchgear
(MTS) either in the form of GIS modules with an AIS busbar or GIS modules using a GIS
busbar i.e. outdoor GIS.
These designs appeared to offer the lowest lifetime cost due to all operating contacts being
enclosed under SF6 rather than exposed to atmosphere, assuming of course that the GIS
corrosion issues previously experienced by ESB have indeed been solved as claimed by the
various suppliers.
They also have the advantage of being offered by most or all of the main suppliers. This
should ensure competitive pricing and also provides insurance that the design layout will not
be dependant on a single supplier.
One possibility raised by the lower visual profile of this equipment compared to traditional
AIS was the possibility of using it in semi-urban locations where previously the only suitable
alternative was indoor GIS in an expensive building.
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Pilot Project
The next step was to confirm the results of the study with actual tendered prices.
An enquiry using a basic functional specification was issued in December 2005 as a pilot
project for the next 3 planned green-field semi-urban stations. Tenders were received for all of
the equipment types covered in the feasibility study. The tender evaluation process confirmed
the conclusions of the study.
A graphical representation of the evaluation outcome is given below in Fig 5, based of course
for Irish civil works and labour costs. The minimum and maximum savings achievable at land
prices of €300 and €500 per sq. m. are given.
Fig. 5 Variation in Station cost with land price
The consistent level of saving achieved compared to the traditional AIS can be clearly seen as
can the increased percentage saving achieved as land costs increase.
As can be seen from Fig. 5 there are very small differences in station cost resulting from the
use of any of the tendered options. These small differences emphasise the importance of
careful evaluation to ensure that the selected option is the best fit to a utility’s particular
requirements.
The reduction in site dimensions achieved compared to a traditional cable-connected AIS
110kV/MV station is shown below in Fig 6.
colm.twomey@esbi.ie
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Fig. 6 Reduction in site dimensions
Orders were issued for:
- MTS of the GIS busbar type (for two stations of the basic H type, see Fig 7)
- MTS of the AIS busbar type (for one larger station requiring four transformer bays, see Fig
8).
Fig. 7 MTS (GIS Busbar type)
Fig. 8 MTS (AIS Busbar type)
Feedback from the planning permission process has been favourable to date due to the
reduced visual impact of the smaller switchgear and also from the decision to change to a 3m
high boundary wall rather than the previously-used combination of a 2.8m high palisade fence
and extensive landscaping. See Fig. 9
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Fig. 9 Elevation of MTS Station Frontage
The smaller site size was also helpful during site acquisition negotiations with developers.
Main enquiry
The experience and information gained during the pilot project was used in the preparation of
a detailed specification for a full term contract. One outcome from the pilot project was a
decision to restrict the scope of the enquiry to MTS type equipment only. As many of the
stations would be part of the transmission system the specification was also signed off by the
TSO.
An enquiry was issued to cover the equipment required by ESB for substations up to 2010 and
tender evaluation is in progress at present.
Prefabricated enclosures
A parallel process was carried out to investigate the benefits achievable from replacing
conventional concrete buildings by prefabricated enclosures delivered with all internal wiring
completed. The perceived benefits are a reduction in civil work and a reduction in site
construction time.
ESB’s previous experience with prefabricated
enclosures was of containers built to ESB design
used to provide distributed control and protection
(one container per bay) facilities in a number of
new and refurbished 220/110kV stations
equipped with digital control systems. (See Fig.
10)
Fig. 10 Distributed control & protection
container
The first step in the process was to include an option with the current MV switchboard
contract for a number of the boards to be delivered prewired in suitable enclosures for use in
38V/MV stations. Matching enclosures were also delivered to the assemblers of ESB control
and protection cabinets for installation and interwiring of the control and protection cabinets.
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The first installation of this equipment was carried out in early 2007. See Figs. 11 & 12
Fig. 11 Side view of MV enclosure
Fig. 12 End view of control & MV enclosures
As in the case of the 110kV HV equipment the experience gained in this pilot project is being
used to prepare detailed specifications for an enquiry for all of the enclosures required for use
in 38kV and in 110kV stations. Particular issues of concern are the durability of the structures
in Ireland’s notoriously wet and humid climate and the maintenance of the existing level of
security provided by conventional buildings.
The durability issue is addressed by an explicit requirement for a 45 year lifetime and for a
very high quality surface treatment which was assessed as being adequate by a metallurgy
expert.
Security was addressed by a requirement for a 3mm thick steel external skin and for
appropriately specified doors, windows and external fittings
It is expected that the enquiry process will be completed during 2007 and a decision will be
taken then on which of the enclosure requirements will be fulfilled by prefabricated
enclosures and which will be more economically met by conventional buildings.
Conclusions
A full review of existing practice has resulted in design changes which are expected to
produce cost savings of up to 18% in 110kV substation construction. These savings should be
obtained without a need for any dramatic changes in operational practice.
As suppliers are continuing to innovate in this area it is expected that further savings will be
achievable in the future. Further studies will be carried out before the next term contract
enquiry to ensure that these savings are captured.
One particular approach which looks very promising is the disconnecting circuit breaker. As it
now appears that this product will be offered by more than one supplier in the near future it
will become more attractive as ESB is normally quite wary of getting into a situation where a
particular layout is dependant on a single supplier.
colm.twomey@esbi.ie
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colm.twomey@esbi.ie
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