MechChem Africa May-June 2022

Complex breakdown repair of 30 MW generator rotor Marthinusen & Coutts (M&C) recently repaired the rotor of an aging 30 MW generator, which had failed during operation at Sappi’s Saiccor pulp mill in Umkomaas, KwaZulu-Natal. Rudi Els, general manager of the Power Generation & Large Motor facility at M&C tells the story.

arcing having occurred between one of the coils and the coil retaining ring. Severe arcing on the rotor body landing had also occurred. Tests conducted by the investigation team established that the arcing had been caused by negative phase sequence currents. “In addition, there was an open circuit in one of the rotor coils that had resulted from thermal cycling fatigue, as well as damage to thewindings causedby overheating, resulting from the overhang insulation having shifted over time, thereby blocking ventilation ac cess to the windings,” says Rudi Els, general manager of the Power Generation & Large Motor facility at M&C. The repair procedures carried out on the rotor to enable the generator to be returned to service in the mill included: • Complete rewinding of the rotor using new locally-manufactured windings to replace the old. The rewinding process, which included brazing a total of 2 500 joints, took fourweeks toperform, work ingdouble-shifts throughout that period. • Manufacturing two new coil retaining rings (CRRs) by a reverse engineering process, including drilling 132 ventila tion holes in each. “These were made

of forgings that we arranged to have manufactured in Germany according to our specifications. Our sister division ACTOMTurboMachines thenmachined the forgings and drilled the ventilation holes in the CRRs,” Els explains. • Designing and manufacturing a damper winding for the rotor to prevent any recurrence of arcing on the rotor body landing and on the CRRs. The rotor had not originally been fitted with a damper winding. “The introduction of a damper winding meant we also had to drill ven tilation holes in the damper circuit to line upwith theCRRs’ ventilation holes,” said Rudi. The rotor damping circuit winding was de signed by Sebenzana Consulting and fabri cated by M&C subcontractors. The repair process was concluded by per forming a high-speed balancing procedure on the rotor. It was returned to the Sappi plant a week before Christmas last year and recom missioned at the plant on Christmas Day. ”This once again demonstrates M&C’s widely-recognised capability to perform complex repairs inquick time,”Rudi concludes. www.mandc.co.za

O ver a period of less than five months, Marthinusen & Coutts (M&C) has performed compre hensive repairs to the rotor of an aging 30 MW generator, which had failed during operation at Sappi’s Saiccor pulp mill in Umkomaas, KwaZulu-Natal. The Jeumont Schneider 31250kVA2-pole generator had been operating in the mill for over 30 years. M&C took delivery of the 12 t rotor at its Power Generation & Large Motor repair facility in Benoni for investigation and repair in August last year. The scope of work by M&C was moni tored and witnessed at hold points by client third party rotating equipment specialist, Sebenzana Consulting, and the Sappi Mill’s Regional Head Office Engineers. The investigation revealed a zero value for the insulation resistance in the rotor, due to

Sappi’s 30 MW generator rotor undergoing dimensional checks in M&C’s 140 t precision lathe that can handle 40 t work pieces that are 11 m long with a swing of 3.2 m.

10 ¦ MechChem Africa • May-June 2022