Large-scale manufacturing facilities in remote areas often rely on independent power generation sources to deliver the vast amounts of energy needed for operations. Turbo generators are often the cornerstones of these power sources. However, if they suffer unforeseen downtime, this reliance can result in a 100% loss of production.
The breakdown of the 55 MW generator was caused by strand-to-strand faults that eventuated into a stator connection phase ground fault. Due to the excessive amount of winding oil contamination from a leaking bearing labyrinth seal, the ground fault flash resulted in a stator fire. As this was the only power source for the facility, production was halted, risking the delivery of orders to customers. Any order obligations would need to be met by other facilities, placing stress on the cement producer’s regional operations. To resolve this, root cause analysis and a temporary repair was requested by the cement plant management, followed by a full stator rewind at a more convenient time to suit their production commitments.
Sulzer revealed that the failure was attributed to the stator winding design having parallel stranding, which caused high circulating currents. The tracking across the surface of the stranding resulted in excessive winding temperature, eventually instigating the ground connection phase fault.
Sulzer was awarded the repair and upgrade project for the scope of winding and incorporating a 360° Roebel bar.
To get the plant temporarily operational as soon as possible, Sulzer completed a partial repair to the failed stator windings. This allowed the cement producer to work through their material stockpile and meet existing orders.
Meanwhile, Sulzer worked on re-designing the new stator windings. The 360° Roebel bars would eliminate future circulating current problems compared to the original designed parallel conductors. However, this required overcoming numerous engineering challenges due to the short length of the stator core.
Firstly, the OEM did not offer Roebel bars for this generator model. Precision reverse engineering was required to ensure that the new top and bottom bars with a 360° Roebel would fit. Finally, the replacement bars would have to be engineered and manufactured specially for this generator. These requirements were accomplished in-house by Sulzer’s engineering and design team.
As part of this work, Sulzer Indonesia collaborated with its Birmingham Service Centre in the UK. Using data collected from the stator and existing top and bottom bars, the Birmingham centre reverse engineered the winding to incorporate a 360° Roebel. With this complete, engineers precision manufactured a half stator section to confirm the correct fit of winding pitch to expedite installation on-site. Sulzer produced 48 top bars and 48 bottom bars in 10 weeks.
To ensure a continuous workflow and minimise the duration of the winding process, the bottom bars were shipped first, followed by the top bars. This phased delivery allowed the Sulzer Indonesia team to complete the generator rewind sooner, as waiting for a single shipment would’ve increased the project timeline by several days. Furthermore, this scheduling ensured that the project could be completed within a planned shutdown period of 10-12 weeks, reducing disturbance to their production operations.
The benefits of switching to Roebel bars were apparent. Winding temperatures were reduced, allowing the generator to be run more efficiently, providing energy cost savings to the cement plant. In terms of reliability, electrical circulating currents were all but eliminated, reducing the risk of any future premature failures. Reduced winding temperatures also meant less thermal stresses on the winding insultation, improving service life. With the generator up and running, the pressure on the customer’s other facilities was alleviated too, with the plant now operating at full capacity.