Heather Johnstone, Senior Editor
On the windswept north shore of the Bristol Channel, west of the city of Cardiff, stands the Aberthaw coal fired power station. At full capacity the power plant generates 1500 MW of electricity for the UK’s national grid system, which is enough power to meet the needs of around 1.5 million people.
One of the two spare generator transformers for Aberthaw
Aberthaw, which has been in full commercial operation since 1971, currently generates around a third of Wales energy needs. Its operator RWE npower, which is a division of the German energy giant RWE Group, has long recognized its importance, and in recent years has made a number of significant investments in the power plant.
With the UK’s adoption of the European Union’s Large Combustion Plant Directive (LCPD), Aberthaw was due for closure because it did not meet the emission limits set out by this legislation. However, in 2004 RWE npower decided against mothballing the power plant, and instead invested over à‚£100 million ($197 million) in installing a state-of-the-art flue gas desulphurization system that gave Aberthaw compliance with the LCPD, which came into force in January of this year.
Hot on the heels of this investment came a à‚£45 million turbine upgrade programme to replace the steam turbines on all three of Aberthaw’s 500 MW generating units. Installation of the new equipment began in 2007. Two units are now complete, with the third completed this year.
In October 2007, the power station took centre stage when RWE npower announced plans to design and build the first carbon dioxide capture pilot plant in the UK, with the first phase, which could be fully operational by 2010, located at Aberthaw. This means that Aberthaw will serve as a crucial test-ground for the potential of carbon capture and storage technology as a means to generate low-carbon energy.
Clearly Aberthaw power station has now secured a long operational life. However, the power plant has not been without incidents. One of the most serious occurred in early 2006 when two out of its three generator transformers failed within a month of each other.
The first transformer to fail was the Unit 7 generator transformer, which happened in February 2006. The timing of this failure could not have been worse because it occurred right in the middle of the peak revenue period for RWE npower. Thus, the company faced extreme time pressures to restore the transformer generator to operation as quickly as possible. Although RWE npower had insurance, the lost generation capacity was costing a significant amount of money, so the insurance company was encouraging a quick resolution.
RWE npower quickly issued a competitive tender for the work, and AREVA T&D successfully secured the project as the main contractor. The scope of the project involved the removal of the old transformer and installation of RWE npower’s only spare transformer generator. The spare was located in a storage facility in Goole in the north of England, some 380 km away from Aberthaw, so AREVA T&D was also responsible for coordinating the transportation of it to the power station.
However, while work began on the first project, Aberthaw’s Unit 9 generator transformer unexpectedly failed, but this time the situation was compounded by the fact that RWE npower did not have a second spare transformer. After searching within and outside the UK a suitable spare transformer was located at Scottish & Southern’s Peterhead power station on the northeast coast of Scotland. RWE npower then negotiated a commercial deal with Scottish & Southern to purchase its spare transformer and to provide it with a new spare transformer.
Because time was so critical RWE npower decided to expand the scope of the original project with AREVA T&D to cover the transportation and the installation of the Peterhead transformer generator. Furthermore, in an effort to keep capacity loss at Aberthaw to a minimum RWE npower and AREVA T&D agreed upon an ambitious time schedule of 26 days for the removal and installation of each transformer. Furthermore, RWE npower offered AREVA T&D a financial reward framework whereby AREVA T&D would receive a specified amount (up to a maximum) for each day it delivered the installation ahead of schedule.
One of the biggest logistical challenges AREVA T&D faced was coordinating the transportation of the spare generator transformers to Aberthaw, primarily because of their large size. From Goole and Peterhead the transformers had to be transported by both sea and land, i.e. transportation by boat to the port of Barry in south Wales and then the land movement from the port to the power station; all of which required special permission. To ensure that all went smoothly AREVA T&D worked with Abnormal Load Engineering, a company that specializes in the transportation of large and heavy loads.
This proved to be a smart decision because by the time the first spare transformer from Goole arrived, the old generator transformer had been removed and the bay was ready to receive the spare.
Ensuring equipment is fit for purpose
An important part of AREVA T&D’s role as main contractor was to assess the technical suitability of the new spare transformers and their auxiliary equipment to ensure they were fit for purpose, i.e. were they going to be able to do the intended job once fitted at Aberthaw. This detailed assessment involved checking for a variety of factors, including whether the transformer had the correct voltage ratio, impedance, MVA capacity, etc, and confirming what its actual rating was, in other words, how many kW could it actually accommodate and identifying whether it would export the same or less electricity compared to the original unit.
This process, however, was hampered to a certain extent because the specification records of the original transformers were incomplete. But, with the help of RWE npower, AREVA T&D was able to make the necessary judgements and assessments to confirm the suitability of the units.
The biggest technical challenge AREVA T&D faced was ensuring that all the civil, mechanical and electrical interfaces between the power station and the new spare transformers would match up, and more importantly if not, devising an engineering interface solution.
So during the course of the installation work a number of interface issues did arise. As AREVA T&D began to map out the dimensions/footprint of each of the new transformer to the old space, some civil limitations came to light. In one case, the new spare transformer was bigger than the original unit and therefore would be physically sitting on piece of concrete that the old one had not sat on. Thus, working with RWE npower, AREVA T&D had to ensure that the new area of concrete would take the weight of the new transformer.
Similarly with the cooler system for the transformer it was important to ensure that the flow rate of the cooling oil was compatible with the new spare transformer. It was discovered that this was not the case, so the cooler pipework had to be completely redesigned to make sure the oil flow fell within the tolerances of the spare transformer.
Finally, AREVA T&D had to redesign the transformer control system to make it compatible with Aberthaw’s existing control system. This involved AREVA T&D control system engineers working from 30-year old transformer and power station drawings, to produce a design for a new control scheme that enabled the spare transformer to be controlled from the power station control room in the same way as the old one. This operation had to be done for both units.
Despite the project being a steep learning curve for both AREVA T&D and RWE npower, both Unit 7 and Unit 9 generator transformers were successfully installed. More significantly the project was completed ahead of schedule – 22 days compared to the original 26 days, which meant that RWE npower was able to save significant lost generation revenue.
Exceeding a tight deadline
The most important reason that the project fell within the agreed timeframe was down to effective planning, not only making a plan but also being able to adapt and modify it to take advantage of opportunities for programme improvement. As it turns out AREVA T&D ended up completing the job in a very different sequence of events than originally intended because it was able to react to changing circumstances.
One particular example relates to the installation of the bushings in the first transformer. AREVA T&D had originally intended to install the bushings and fabricate the cooler pipework simultaneously as soon as the transformer was put into its final position, which was inside a building. However, it became apparent that it was not going to be possible to do both jobs together because of space constraints.
However, to avoid having to do the jobs one after the other, AREVA T&D decided to build up the transformer as much as possible outside of the building, which involved using a different method of installing the transformer. At the same time, while the bay was empty the fabrication sub-contractors were able to build up the cooler pipework. Thus, when the transformer was brought into the bay it was well advanced in terms of its build. Following this experience, AREVA T&D implemented this change in sequence for the installation of the second transformer.
Another important reason that the project was able to be completed well within deadline was the close, long-standing working relations AREVA T&D has with all its sub-contractors, so for example when RWE npower asked the company to work through the night AREVA T&D was able to ask the same of the sub-contractors.
An emergency plan in place
Building on the experience and knowledge it gained at Aberthaw, AREVA T&D now offers a service to power plant operators where it will conduct a study at each of their facilities to determine the suitability of their spare transformer portfolio and devise an emergency response plan. This involves examining whether its spare equipment will fit and whether the interfaces will be a problem. Furthermore, in the circumstances where a power generator does not have the resources to hold spare equipment AREVA T&D can help it arrange access to a suitable unit from a second party in the event of unit failure.
One of the two spare generator transformers for Aberthaw
According to AREVA T&D, the power industry has reacted positively, and both British Energy and E.ON have placed enquiries for such an emergency response plan.
The author would like to thank Iain Tait of RWE npower and Tony Garnett of AREVA T&D for their invaluable contribution to the article.