MechChem Africa February 2017

Synchronous condenser reassembly and recommissioning

Marthinusen & Coutts was awarded a repair contract at a dc/ac converter station in the Democratic Republic of Congo: for the synchronous condensers, critical components of a power station that feeds the energy-intensive Copperbelt in the Katanga Province.

S uccessful completion of challenging electrical rotating machinery refur- bishment projects has long been the hallmark of Marthinusen & Coutts, a division of ACTOM. It is this reputation that secured Marthinusen & Coutts the contract to complete the reassembly and recommis- sioningof avery largesynchronouscondenser in the Democratic Republic of Congo (DRC). The repair contract was awarded to Marthinusen & Coutts early last year by an international electricity equipment company currently engaged in an upgrade project on the hydroelectric power supply system for the DRC’s power utility, Société Nationale d’Électricité (SNEL). Richard Botton, divisional chief executive atMarthinusen&Coutts, says that this order was placed on Marthinusen & Coutts by an international OEM who is a global leader in electrical engineering. The synchronous condensers are critical components of the dc/ac converter station in Kolwezi in theDRC, which converts dc power transmitted on a 1 700 km transmission line

provider withOEMcapabilities and backup that led the customer to award the contract to Marthinusen & Coutts instead of to the international OEM. It is always critical for operationstohavethislevelofsupportavail- able, providing skilled technical assistance with very quick response times,” he adds. The removal of the poles required a two- day shutdown during which Marthinusen & Coutts assisted the mine maintenance personnel. The poles were delivered to Marthinusen & Coutts’ Cleveland facil- ity where thorough investigative work was undertaken on them. After the results were known, the best method to improve and partially rectify the surface insulation of the remaining 60 poles, was investigated,” adds David Motloung, design engineer at Marthinusen & Coutts. A two-part epoxy spray treatment and repeated insulation resistance tests under extremely wet coil conditions were used to achieve this. Motloung says this was to simulate the worst case conditions on site with the ul- timate purpose of evaluating a method de- The repair work undertaken on the 90 t rotor of the 70 MVA condenser involved conducting a thorough inspection of the ro- tor forging and bare rotor by Marthinusen & Coutts’ on-site repair team. Thiswas followed by the refurbishment of thebearings and test- ing andfitting newsalient poles that hadbeen manufactured earlier by the international company to replace the original salient poles. Marthinusen & Coutts is also responsible for the balancing of the rotor. The heavy rotor runs at 750 rpm ina compromised installation from a converter at the Inga hydroelectric power station on the Congo River in the north. The power fromtheKolwezi converter feeds the energy intensive Copperbelt in the Katanga Province. The synchronous condensers supply the necessary reactive power, which cannot be transmitted via the dc transmission line or providedby the converter station. The inertia of the rotating assembly of the condenser provides the necessary energy to stabilise the power system in the region, which aids the overall stability of the grid.

Marthinusen & Coutts Lead Field Services Technician, Wynand Willemse applies finishing touches to one of the SNEL 70 MVA synchronous condensers completely refurbished in the DRC. froma foundation stiffness point of view. “We are employing the services of arguably the top balancing specialist in South Africa who will carry out the dynamic balancing on-site,” Botton says. Marthinusen & Coutts’ understanding of local conditions in Africa, and specifically the DRC, provides amajor advantage. Skilled technical personnel from Marthinusen & Coutts’ centre of excellence in Cleveland, Johannesburg, were deployed to site and the condenseriscurrentlybeingrecommissioned, “overseenbyourcustomerandSNELofficials,” Botton concludes. q

Rewind and refurbishment of three gearless mill drive rotor poles A recent project which showcased Marthi­ nusen & Coutts’ capability was the rewind andrefurbishmentofthreegearlessmilldrive rotor poles for a platinummine in the North West Province. The enormous 17.5 MW, 4 220 V, 2 531 A, 12 rpm, 5.8 Hz grinding mill motors are 15 m in diameter, are driven by cyclo-converters that supply the motors with a variable frequency to control their torque, and allow the speed to be controlled for optimummetallurgical processing. vised by Marthinusen & Coutts to improve the insulation of the remaining poles still fitted to the machine. Motloung says that after studying the design of the pole coil it was decided to use a different conductor for the rewind as this would increase the insulation integrity. Test results confirmed that a thicker coil insulation did not cause it to run at a higher temperature due to the reduced heat trans- fer, and there was a negligible impact from the reduced copper cross sectional area. q

Rob Melaia, engineering and technical executive at Marthinusen & Coutts, says that these are among themost critical large special high profile motors in the world. There are only two OEMs worldwide that manufacture these enormous machines, which have an assembled mass of 1 000 tons each. “The rotor poles were subjected to overheatingdue toanoperational error, and we were approached by the mine to assist,” Melaia says. “While this contract could be seen as a simple procedure, we believe it is the immediate access to a large local service

Marthinusen & Coutts’ rotating machines design engineer, David Motloung, records photographic evidence of tests on one of the enormous gearless mill drive rotor poles recently refurbished by the company.

14 ¦ MechChem Africa • February 2017