AES Kilroot – reaffirming reliability via relocation1 January 2010
How AES Kilroot Power contributed to meeting the challenge for flexible power generation on Northern Ireland’s electrical system by importing under-used GTs from a sister facility in Africa.
In 2007 AES Kilroot Power Ltd, the operator of a 538 MW dual coal/oil fired power plant saw the opportunity to install open cycle gas turbine generators at its existing Kilroot facility, situated near Carrickfergus, Northern Ireland. As part of the 25 000 strong worldwide AES Group, AES Kilroot saw the potential, with its sister plant in Itabo, to relocate two under-utilised Frame 6 OCGTs from the Dominican Republic to Northern Ireland. These units were originally installed in 1997 but had accumulated only a low number of operating hours.
Feasibility studies, undertaken in early 2007 by AES, confirmed that such a decision would be economically attractive. The studies also confirmed that Kilroot power station would be a suitable site owing to its spare flue capacity, available electrical connection, access to fuel storage and in-house OCGT operating experience. By late 2007, the decision was finally taken by AES’s management to relocate two of the three existing units with a target commissioning date of 31 March 2009.
The Frame 6 units at Itabo
The existing three OCGT units, as shown in Figure 1, are GE Frame 6B gas turbine generators (model PG 6551-B) with an ISO rated electrical output of 38.45 MW. They were originally commissioned in 1997 with the following design features:
• Modular units suitable for outdoor location
• Operating as peak lopping units
• Firing on distillate fuel oil
• Operating at 60 Hz system frequency
• Complete with a 9 metre exhaust stack.
In mid-December 2007, AES engaged Mott MacDonald Ireland Ltd (MMI), based in Dublin and part of the worldwide Mott MacDonald Group, as the design engineer for this challenging project. Shortly afterwards, MMI undertook an extensive condition survey of the units in the Dominican Republic and produced a specification outlining the necessary technical requirements for rehabilitating and refurbishing these units to operate at Kilroot, which covered:
• Repair of minor corrosion.
• Up-rating each unit to 42 MW
• Modifying the combustion system to allow for the injection of demineralised water as a primary abatement technique for NOx control.
• Replacing the existing gearbox to allow for operation at 50 Hz.
Main technical considerations
Environmental studies commissioned by AES had identified the need for the relocated units to exhaust through two spare ducts in the existing 100m high four-flue stack. A consequence of this decision was that the units were located to a position in front of the storage facility as shown in Figure 2.
The selection of this area for siting presented some technical challenges including:
• Identifying, diverting and relocating existing buried services
• Routing of the exhaust ducts from the relocated OCGTs and connecting to the flanges of the existing spare flues
• Demolition of some old structures, including removal of a disused buried fuel oil tank
• Running a 275kV underground cable from the new HV step-up transformer to the existing Northern Ireland Electricity substation. This cable route crossed existing services and passed through the 275 kV NIE switchgear building.
From the outset, and due principally to the tight time constraint for completion, AES, as the overall project manager, decided to approach the procurement on a multiple contract basis with the main contracts being;
• Relocation, refurbishment, transportation, erection, testing and commissioning of the Units.
• Construction of the civil works.
• Supply and installation of the 275/11.5/11.5 kV step-up transformer.
• Supply and installation of the 275 kV underground power cable.
• Supply and installation of the mechanical systems
• Supply and installation of the electrical systems
• Modification of the existing 275 kV switchgear within the NIE substation
Although drawings of the existing stack were available it was decided that a detailed survey of the connecting flanges on the spare flues be carried out to ensure that the new exhaust ductwork connected correctly.
As a cost saving measure, the Itabo fuel oil transfer pumps were to be relocated to Kilroot, so it was also necessary to carry out studies to ensure that they were suitable for the new installation and could accommodate the electrical up-rating mentioned above.
The fuel oil and demineralised water pipework was run on an existing pipe rack which required careful co-ordination with the existing services.
The high voltage element of the installation consisted of a 275/11.5/11.5 kV transformer and switchyard to facilitate the connection to the NIE owned 275 kV substation.
Several major technical challenges were encountered.
• The specification for a three winding 275/11.5/11.5 kV step-up transformer with dual 11 kV windings for the connection of both OCGTs arising from (i) space constraints and (ii) economic reasons.
• Determining the optimal transformer impedances to limit the short circuit fault contribution on the new 11.5 kV switchgear.
• The optimal transformer tap changer positions that would allow the full power factor transformer range over the entire voltage range to be achieved.
• The selection of a route for the 275 kV cable that would minimise the diversion of existing external services and allow for the presence of asbestos, access ways and energised 275 kV equipment within the 275 kV NIE substation. This involved careful planning and co-ordination among many parties.
• The design of a new LV switchboard fed from a new 50 Hz unit auxiliary transformer to include frequency inverters that would enable the existing OCGT motors, fans, distribution boards and control panels to remain operational at 60 Hz.
• The design needed to accommodate the intricate routing of four cores per phase of large diameter MV cabling within a constrained space and through the oil-tight transformer bund.
• The transformer bund and fire wall was required to satisfy the requirements of NFPA 850 and to comply with the terms of AES’s insurance requirements for a deluge system.
• The design of the protection philosophy allowed incorporation of all the project electrical equipment (generators, three-winding transformer, 275 kV cable and 275 kV busbar) with the limited roster of existing protection instrumentation (CTs and VTs) in both the NIE substation and the OCGTs.
Transport to Kilroot
The equipment was dismantled in the Dominican Republic and shipped to Belfast. As it was a modular design the various blocks could be transported by road from Belfast to Carrickfergus using specialised heavy load moving trailers and associated traction units (Figure 3).
Civil design and construction
The geotechnical investigation, completed in April 2008, revealed that the soil was of low load bearing capacity and therefore, in order to provide the required support and to achieve the minimum deflections required by the design criteria, piling was adopted to support both the turbine generators and the step-up transformer foundation block.
Mott MacDonald had full responsibility for all the civil and structural design work. This included the use of specialist dynamic design methods involving finite element techniques for the turbine/generator blocks. The ambitious timeline proved to be a challenge and the prolonged curing periods required for the foundation proved to be a major element in the critical path of the project timeline.
One particular problem arose in relation to the design of the foundations for the new exhaust duct, particularly where they were located in close proximity to the existing 100 metre high stack (piled) foundation and the recently completed new turbine generator foundation blocks. Detailed analysis completed by Mott MacDonald concluded that adopting traditional foundations would require substantial and deep excavations which would not be practical within the confines of a small site and would also compromise the demands of working to a tight programme. It was, therefore, decided to install mini-piles to support the exhaust duct foundations and this solution also ensured that the foundations would be independent – no loadings would be transferred from the new foundations to either the existing stack or the new generation block foundations.
The machines were ready for no-load testing at the beginning of March 2009. With mechanical tests complete, the machines were synchronised to the grid during April 2009. Both machines had to undergo a rigorous set of tests to prove their performance capability for operation in compliance with NIE Grid Code characteristics. The units were finally commissioned and operating in the market in April 2009. Figure 4 shows the final installation.
Health and safety aspects
Near-miss and incident reporting was monitored throughout the project with great emphasis being placed on contractor adherence to AES safety guidelines. This was a major challenge considering the tight confines of the construction area, the large number of contractors at site and the need to maintain an existing power station in full production. The non-occurrence of any major incidents highlighted the success of the stringent health and safety guidelines applied to this project.
Detailed environmental studies were undertaken at the outset of the project particularly in relation to noise and emissions. In the event no environmental issues arose either during construction or commissioning.
During commissioning emission testing was carried out to ensure the machines complied with the specified environmental limits.
Teaming Mott MacDonald, as design engineer, with AES Kilroot staff handling all aspects of programme co-ordination, monitoring, and overview proved to be a successful strategy. Significant technical challenges encountered during the tight timeframe were successfully met by this team. In addition the approach to procurement adopted by AES proved to be fruitful. Excellent lead times were achieved on major components, which in turn led to a reduction of previously conceived installation times and consequently an overall cut in the critical path of the programme.