Think of a primary substation and you’ll probably envisage a vast outdoor compound with massive transformers connected to overhead powerlines and bank after bank of circuit breakers and disconnectors. This is not always the case says Stephen Trotter, ABB’s director of power systems projects for the UK
Traditional substations have provided excellent service over many years, and many are still being constructed today. However, when it comes to planning substations in urban areas there is an ever increasing demand from utility customers to minimise the space required, not just because of the cost and availability of land, but also to reduce their visual impact on the local environment. New high voltage technology offers the ideal solution in the form of gas insulated switchgear (GIS) which enables substations to be ‘shrunk’ into about 20 per cent of the space required by a traditional design, and housed indoors or even buried underground.
Until the 1970s, air insulated switchgear (AIS) was the type most commonly in use for substations. AIS requires large distances between earth and phase conductors and therefore a good deal of space. This means that for higher voltages – typically above 36kV - this type of installation is only feasible outdoors.
The situation changed when SF6 (sulphur hexafluoride) became available as an insulating medium in switchgear enclosures in order to reduce phase to earth distances. The advantages of GIS compared to AIS are as follows:
• Less space requirements, especially in congested city areas, saving on land costs and civil works
• Low visibility buildings can be designed to blend in with local surroundings
• Less sensitivity to pollution, as well as salt, sand or even large amounts of snow
• Increased availability and reduced maintenance costs
• Higher personnel safety due to enclosed high voltage equipment and insignificant electromagnetic (EM) fields.
A direct comparison of the component investment for identical switchgear configurations will suggest that the GIS variant is more costly than the AIS solution. However, this does not necessarily show the true story. The capability to install a GIS substation within a significantly smaller site – typically up to 80 per cent smaller - enables it to be located close to the load centres, providing a far more efficient network structure at both the HV (high voltage) and MV (medium voltage) levels. As a result, both the investment and operating costs are reduced.
Sites large enough for new AIS substations are seldom available, and when they are their cost is usually extremely high. But it is not just the smaller size of the site that can make GIS the lower-cost option: GIS is also the more economic alternative when expanding or replacing existing substations. An inner city site that has been used previously for an AIS installation could be sold or rented out and the income used to finance the new substation. The compact nature of GIS enables an HV transformer substation to be fully integrated in an existing building, which may only have to be increased in height or have a basement added.
Port Ham shrinks from view
Central Networks’ £12 million replacement Port Ham switching station, which has recently been constructed by an ABB and Balfour Beatty consortium on the banks of the River Severn, near Gloucester, provides an ideal example of the advantages of the GIS approach.
Port Ham is a grid supply point. It takes electricity at 132kV from the National Grid substation, a few miles away at Walham, and feeds it into the Central Networks distribution network. Through a network of primary and secondary substations, this network feeds over 240,000 customers in Gloucestershire, Herefordshire and much of south and east Worcestershire.
The original outdoor station, built in the early 1950s, had experienced above average load growth, to a peak load of 672MVA. The AIS equipment had reached the end of its useful life, so in 2002 Central Networks decided to completely rebuild the facility to ensure continued reliability of supply, as well as providing scope for further load growth.
Initially, the project was tendered in the expectation that the AIS would be replaced on a like-for-like basis. However, in consultation with the ABB and Balfour Beatty consortium, Central Networks decided building a new indoor GIS switching station would offer a number of important advantages, at around the same overall cost. A key benefit was that ABB’s state of the art compact ELK-04 (GIS) switchgear solution could be condensed into just one-fifth of the space used by the existing station. Port Ham is in an important nature conservation area. So the smaller switchgear allowed Central Networks to meet planning concerns by housing the station in a low-profile building designed to blend in with the local environment.
In addition to saving space, GIS also offered two further advantages. Firstly, circuit downtime could be reduced, as the new GIS circuits were constructed with the existing units still in service. Downtime was limited to the rerouting of the network connections. This was a crucial factor, because of the critical position of Port Ham in the supply network. Secondly, the GIS was constructed outside the existing live compound, considerably reducing health and safety risks to personnel working on site.
One of the major project challenges was the soft ground – on the flood plain of the River Severn – which required major foundation work before construction could begin. In just over 10 days some 120 cast concrete piles were driven down 15 metres to the bedrock. The building itself has been raised on stilts to ensure that the switchgear is at least one metre above the predicted level of the once in 100 years flood level.
The new indoor switching station comprises 20 bays of GIS switchgear: 12 feeder circuits; four National Grid incomers; two bus couplers; and two bus sections. The size of the investment and the strategic importance of Port Ham made it a flagship project for Central Networks.
NEDL’s Norton substation
A similar approach was adopted when NEDL needed to replace its 132kV substation at Norton, near Stockton on Tees, that interconnects the National Grid and NEDL’s distribution network.
The new indoor GIS substation, completed in 2005, occupies just one sixth of the space of the old AIS substation. It is rated at 540MVA, and features 20 bays of switchgear (four of which have been transferred to National Grid) with four incoming circuits fed by Supergrid transformers and 14 outgoing circuits, two of which feed local grid transformers.
The GIS switchgear concept has been taken to its logical conclusion in ABB’s Barbana 132kV/20kV transformer substation in the centre of Orense, Spain. The 132kV switchyard, comprising two cable feeder bays and one transformer bay, has been constructed entirely underground and concealed beneath a park. This design requires forced cooling, which inevitably entails unwanted fan noise. But damping features or low-noise fans can be expensive. Instead a waterfall has been created. This acts as a heat exchanger to dissipate the heat created by the transformer while the sound of the falling water also drowns out the noise from the fans.
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