When two of its three generator transformers at the 1.5 GW coal fired Aberthaw power station failed in quick succession, owner RWE found itself under extreme pressure to restore the 600 MVA, 275/22 kV units to service as quickly as possible without compromising technical quality and safety.


When RWE experienced the failure of one of its three generator transformers at the 1.5 GW coal fired Aberthaw power station in South Wales, it was placed under great pressure to restore the 600 MVA, 275/22 kV transformer as quickly as possible. When fully operational, Aberthaw is generating the power supply for some 1.5 million people, therefore RWE had to ensure that disruption to generation was kept to an absolute minimum, and that losses also were minimised during a peak revenue period. A competitive tender was issued and Areva T&D secured the contract – to replace the generator transformer on as short a timescale as could be managed without compromising safety. Areva and RWE agreed on a demanding schedule of 26 days for the removal and replacement of the unit.

The first stage of the project was to remove the failed equipment from its bay. It was then the responsibility of the Areva team to visit the site where the back up transformer was located, in Goole, in northern England, and evaluate its overall condition, including the ancillary equipment, to ensure that it was fit for purpose. Having given the replacement transformer a clean bill of health, Areva arranged to move the unit to Aberthaw using land and sea transport. While the unit was in transit, the installation team spent time inspecting the interfaces at the power station. This included looking at the physical connections that would be made to and from the new transformer and identifying any issues that would have to be addressed to ensure rapid link-up and commissioning once the unit was delivered.

But within two weeks of the project’s starting, RWE experienced a second generator transformer failure at the power station, leaving only one fully serviceable unit producing electricity. RWE gave Areva the job of sourcing an additional back up transformer within the transmission and distribution industry, effectively doubling the size of the project overnight. Once a second transformer was located, the Areva team undertook a thorough inspection to determine the suitability of the unit before organising transportation and logistics, and addressing any possible interface issues that they might face.


The first failed transformer being removed from its bay


As the principal contractor, Areva was ultimately responsible for ensuring the smooth running of the project. One of the key requirements in guaranteeing its overall success was that the main contractor should ensure effective health and safety provisions, teamwork, co-operation and transfer of information among all the subcontractors involved.

Replacing the generator transformers with the available spares also created a number of technical challenges linked to the physical, mechanical and technical interfaces. First, Areva had to undertake an assessment of the technical suitability of each potential replacement transformer. This included establishing whether it was the right voltage ratio and the correct impedance, what power rating it and its cooling system could accommodate and whether or not it would affect generating capacity compared to the original unit if it were to be put into operation.

There were also concerns surrounding the lack of records for the transformers which were being replaced, in terms of their capabilities. Through the various assessments and evaluations conducted, Areva and RWE had to make a number of judgements on what the original specifications of the transformers were, in order to determine if the replacement units were suitable. When it came to control systems, Areva’s control system engineers were able to re-design the control scheme to ensure that the spare transformers being installed would be fully controllable from the power station by assessing the drawings of the transformers as well as the original drawings of the power station.

It was also necessary to ensure that there was enough physical space to accommodate the different size of transformers, and that safety clearances were not infringed. One of the spare transformers was actually larger than the existing unit, with a larger footprint that encroached onto areas of the concrete floor that had not previously been load-bearing. As a result, Areva had to undertake a structural assessment to ensure that the existing concrete plinths, floor and sub-structure would be capable of supporting the additional load without remedial strengthening works.

Another technical challenge the team faced was ensuring that the flow rate of the oil within the cooler would be within the tolerances required by the replacement transformers, as the cooler is a fixed part of the power station and therefore cannot be modified. Consequently, the emulsifier pipework needed to be re-designed and modifications were required for the interface of new cooler pipework as well as the low voltage connections. Cooler flow characteristics were also calculated to ensure that the spare transformers did not overheat. Because the footprint had changed, positioning of the pipes and nozzles needed to be altered.


Installation of the first replacement unit

The strict time constraints placed on the project also meant that the additional pressure put on the project to be completed quickly led to conditions on site that were often difficult, owing to the large number of personnel working there throughout the day. This forced Areva to cope with the challenge of striking the most effective balance between getting the job done quickly, but more importantly, safely.

Planning flexibility

It is a tribute to the expertise and seamless co-ordination of all the contractors on site that Areva’s team was able to complete the project in just 22 days – four days ahead of schedule. This was achieved through rigorous planning but also by having shared vision, clear expectations and trust throughout the project. At an early stage Areva brought all of the subcontractors together and broke down each part of what needed to be done in order to effectively manage the re-design and modifications required to safely bring the spare transformers back online.

The team recognised the importance of having a plan in place at the start of the project, but also the significance of being able to adapt and modify the plan in order to take advantage of opportunities for project improvement. By means of such flexibility and agility, Areva were always able to continue moving forward with the project even if a certain part of the plan did not work out as expected – putting the project on hold was not an option.

For example, Areva’s original plan was to build the transformer, and then install the bushings once it had been put into its final position within the power station building. However, it became apparent that it would not then be possible to bring a crane into the building, install the bushings and actually manufacture the cooler pipe work all at the same time. As a result, the team built the transformer as much as they possibly could outside the building which, in fact was much safer than trying to build it in a confined and restricted space where none of the other subcontractors would be able to work. With the transformer still outside, the bay inside was clear for the fabrication team to build the cooler pipework. Once this work had been carried out, RWE was able to see the project take a giant leap forward when the transformer physically went into place due to the pipework’s having already been completed. By going through this voyage of discovery with the installation of the first spare transformer, Areva was able to adopt the changes in the original plan for the second unit, saving even more valuable time.

Safety management

As Areva was the contractor responsible for the safety management of the project, its site manager made sure that everyone kept to their allocated, working area and was able to work with zero risk to his or her safety. An important consideration, for instance was that the scaffolding team were present on site at all times. Documentation covering method statements, risk assessments and a comprehensive health and safety plan was required to be in place for every subcontractor involved, and daily meetings were conducted in order to discuss progress and provide an opportunity to highlight any areas of concern to ensure that any health and safety and environmental issues were dealt with quickly and efficiently.


The exceptional management of the project, especially the excellent team co-ordination and time management, deeply impressed RWE and exceeded the management team’s expectations. The total commitment by Areva to monitor and control every phase of the project led to early completion which saved the power station from losing significant revenue.