MechChem Africa February 2017

MechChem Africa

•

February 2017

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

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.

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.

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

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-

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.

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.

Synchronous condenser

reassembly and recommissioning

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.

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.

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.

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