The vast scale and form of the buildings to be protected adds to the design complications of any services. As well as helping to recreate the skyline in London's Docklands, the 44-storey Cesar Pelli-designed Citigroup Tower has prompted an expansion of the electrical circuit protection supplier's product range to cater for the increased power demands that such buildings have.
Delta Electrical won the £2.8 million contract for the supply and installation management of its MEM branded switchgear by tender. The company is on the list of approved suppliers for Canary Wharf and was hired by the project's electrical contractor Phoenix Electrical.
Delta was previously involved in the construction of Citigroup's adjacent building on the scheme, which was completed in late-1999, and is currently working on several other projects in the development. But at the moment nothing compares to the scale of work in which it is involved in the Citigroup and adjacent HSBC towers – the former will include the company's largest ever switchgear order to date, and the latter its largest ever busbar order.
Powering up
For Citigroup, Delta provided a wide range of medium and low voltage switchgear for the project, some of which was specially designed to cater for the needs of the giant structure, the first of these being the packaged substations.
Two power supplies feed the separate halves of nine double-ended packaged substations serving the building from basement, roof and mid-level electrical plantrooms. Four of the substations provide landlord's services, the other five supply the tenants' services. These have a 3·5 MVA 11 kV/415 V cast resin transformer at each end, which are each capable of supplying up to 4·5 MVA under forced cooling.
The selection of transformers by Delta was: "A project managed reason for cost and specification," explained the company's Martin Fox. "We can provide transformers from a number of manufacturers, including inhouse companies. In this case we went to an outside company to provide the transformers."
The transformers feed into a 6300 A overhead busbar system. A busbar interconnector is positioned between each pair of tranformers to separate the supply when needed. "The packaged substations are generally run with the busbar open and all the transformers live," explained Delta's project manager Neil Hopkins, "so that in the event of a transformer going out of service the switchboard can be reconfigured so that the whole switchboard can be fed from the other."
The transformers convert the 11 kV incoming supply from the hv switchboards into a 415 V supply, which then feeds into the busbar system via two manually-operated MEM air circuit-breakers. The busbar system is the main means of power distribution and supplies the sub-distribution panels throughout the building.
The switchboards were another area that needed special design attention to fulfil the needs of the project. Martin Fox explains: "The variance here from standard components is in the busbar current rating. In the Citigroup building, the incomers are on 6300 A, so this is a 6300 A distribution system, whereas buildings normally run on 4000 A. These are farily high current ratings, but we do know that the trend is going towards the higher current switchboard.
"This is because more output is required. More use of computers means a need for more cooling, so air conditioning plant needs to be bigger. This demands more electricity, so the transformers get bigger. Ultimately the increase in size is for the support systems for the computers, not the computer systems themselves."
This 6300 A rating was outside the company's standard scope of supply at the start of the project so the products had to be specifically developed.
A design team was set up to create the new switchboards for the London site, utilising the company's team of 15 design engineers on the switchboard division. "We worked out what we could use and developed the design based on our 4000 A switchboard," explained Fox. "We designed them from concept through to the completed design that has been installed and handed over using our Asta certified switchboard systems," he added.
The design task was increased as the client specified the use of ABB circuit-breakers, and it was decided to incorporate these within the MEM switchboards. Delta worked with ABB to develop a system where the three air circuit-breakers could be stacked vertically within the panel. This helped fit the panels into the allocated plantroom space while meeting the specification requirements.
"This was quite a difficult feat and something that we had to develop specifically for the job," states Fox. But this creation will not be used once and forgotten. "The three-tier air circuit-breaker system is special, but will now become part of our portfolio," confirms Fox. "Its an evolving design process – once you've built something like this you know that you can, so we then move onto the next system, which is slightly different." The company is now offering these panels as standard units in its specials range.
The system is protected in a number of ways. The transformers are connected to a winding temperature indicator, and an alarm condition and shutdown are signalled if overheating occurs. The internal air circuit-breakers have overcurrent protection, short circuit protection, and earth fault protection included, and the subcircuits also have their own protective devices fitted.
In the event of a mains failure, essential power supplies will be met by a system of standby generators.
Project co-ordination
Delta subcontracted the installation work on the project and provided the management of the process. The company will be responsible for the commissioning of the final installed system, doing the final tests and issuing handover certificates to the client, Canary Wharf.
With such a large project to manage in a relatively short timescale constant contact was called for. "The building is being completed in 17 months, where it would have traditionally taken about 3 years," reported Martin Fox. "We would have meetings covering technical, commercial, contractual and programming with Phoenix Electrical and Canary Wharf, the ultimate client, at least once a month. In the early part of the job we probably had a meeting once a week so that things could be finalised," he added.
This was necessary for several reasons. Aside from the close involvement with the intial design work, there was a huge amount of logistical co-ordination needed in order to simply get the materials and labour to the correct place at the right time in order to keep to schedule.
Simply to get the equipment into the building – the transformers alone weigh around 6·5 tonnes – required a high degree of planning, so that it was carried out on time, safely and within the programme requirements.
The transformers and their enclosures were delivered to site separately. Enclosures were supplied in flat-pack form, with front, back, sides and bases supplied as separate items. The enclosure tops were supplied complete with busbars in place, the sections being prefabricated at MEM's Birmingham facility.
Some plant was craned into the building through what ultimately became the external wall, while smaller parts could be transported to the higher floors using hoists. Offloading and hoist times were allocated to each construction team member at weekly meetings so transportation had to be carefully co-ordinated and planned between the factory and the site, and from arrival on site to the place of installation to ensure that times were not missed and the ultimate deadline is met.
Source
Electrical and Mechanical Contractor
