On the same day, on-site oil samples were taken for laboratory analysis and initial diagnostic measurements (measurement of winding resistances and transmission ratios, SFRA measurement) were carried out. The results suggested that the damage inside the transformer was too great to put it back into service. A replacement was inevitable, and was quickly found in the form of an available reserve transformer in Gösgen.
Replacing the transformer
Within a day, two assembly teams were formed for the disassembly and for getting the transportation ready – one team in Gösgen and one in Gerlafingen. To ensure that the maximal permissible transport weight was not exceeded, each of the roughly 40 tonnes of oil were decanted into conveyable cisterns. The disassembly of the transformers then took place over the weekend, to facilitate the transportation of the heavy loads on the roads. The replacement was completed in just two or three days after the damage occurred and the assembly of the mounting parts (feedthrough, expansion tank, radiators etc.) were carried out in a two shift operation. Both the low-voltage and high-voltage cable and pipe connections as well as the complete secondary technical connections had to be adjusted to the control system and the protection. At the same time, oil processing had to be carried out continuously for 48 hours.
Commissioning and conversion concept
After a prescribed runtime of 36 hours and checking all protection and monitoring devices, the new transformer was commissioned just ten days after the damage occurred. The biggest challenge of replacing this transformer was the enormous time pressure to keep the disruption to the steelwork's production to a minimum while not compromising on occupational safety and quality. This was achieved particularly because, when initially procuring the reserve transformer and knowing how important the transformer in Gerlafingen was, Bouygues E&S EnerTrans Ltd. already created or procured all the plans and schemata as well as the necessary components for the possible event of a replacement as a precautionary measure.