In the largest repair of this nature in Africa, Marthinusen & Coutts recently refurbished 11 of the full set of 14 rotor poles of Motor Generator Unit Three at Eskom’s Ingula Pumped Storage Scheme on the Drakensberg escarpment. The rotor poles were extensively damaged during a fault condition at the station, and the return to service of the unit was of national importance.
Working closely with stakeholders, an extensive local programme of testing, dismantling, inspection and repair was conducted by Marthinusen & Coutts within extremely tight time-frames to accommodate the criticality of the project. A technical audit confirmed that facilities at the company’s 8000 m² main workshop in Cleveland, Johannesburg, and its 12 000 m² Power Generation division in Benoni, were up to the daunting task. All 14 main rotor poles were then collected from Ingula Pumped Storage Scheme and dispatched to the Benoni works. Each pole weighed 12 tons, with the coil alone weighing in at over two and a half tons.
It was vital that Marthinusen & Coutts determined not just the electrical status of the windings, but the health and physical integrity of each entire rotor pole (body and coil). They were therefore subjected to insulation resistance and inter-turn insulation tests, comprising both impulse and power frequency inter-turn insulation tests. Extensive visual inspections were also conducted. The test results were analysed, resulting in the decision to refurbish 11 of the 14 poles; two coils were accepted as healthy and the station owned a spare universal coil.
To facilitate the grinding procedure necessary to remove the support side brackets from the main rotor poles, a customised heavy duty jig was manufactured. After the severity of the damage was determined, various techniques and repair concepts were tabled and discussed in detail. It was finally agreed that the coils be stripped of their existing inter-turn insulation layers and the coils be reinsulated, heat cured using 2500 A, including pressing at up to 1000 tons, and re-assembled to the main poles. To fast track the process, Marthinusen & Coutts prepared custom parallel operations at the two facilities by separating the dirty and clean processes, with full clean conditions areas for the critical pressing and heat curing processes.
Manufacturing the tools and equipment for the work demanded substantial engineering and sourcing capacity. Among the items to be custom made were various tables and jigs to help move the coils in and out of the burn-out oven. A coil removal rig to remove coils safely from the main pole body was built, as well as two heavy duty consolidation rigs with a set of 1000 ton hydraulic jacks and their dedicated power control unit. Marthinusen & Coutts also designed and constructed a dimensionally accurate dummy pole centre to help ensure correct fitment of coils to the main poles. Other items included a 2500 A direct current controlled rectifier to assist with the curing cycle, heavy duty turning rigs, lifting assemblies and a magnetic link for inter-turn insulation testing.
The scope of work included pyrolising the coil insulation in a controlled oven environment at 300°C and removing the existing insulation. The copper stacks were cleaned and polished, new insulation was pressed and prepared, and all new insulation kits were manufactured with pre-impregnated Aramid paper. All pole coils were reinsulated and thermo-electrically cured with a 1000 ton consolidation pressure, after which they were cleaned and tested.
Work on the main pole bodies included removing the existing insulation and checking and testing the copper damper winding and series connectors. Weld scars were cleaned and polished and new insulation kits were supplied and fitted. In the final assembly and test stage, Marthinusen & Coutts repaired the existing support brackets to as-new OEM specification and tested them to meet international standards. Coils were fitted to the rotor poles and the coils sealed onto the main poles. Finally, the finished rotor poles with coils were tested, crated and transported back to Ingula by Marthinusen & Coutts. Unit 3 is now operating to full capacity. The entire project was carried out under extreme pressure, in record time, and according to all the OEM specifications and requirements for new rotor poles.
For more information contact Richard Botton, Marthinusen & Coutts, +27 (0)11 607 1700, [email protected], www.mandc.co.za
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