Bringing power to Skorpion
The commissioning of Anglo Base Metals' US$450M Skorpion Zinc plant
in Namibia, which has an installed power capacity of 120 MVA, has increased
the country's total power consumption by some 25 %.
Furthermore, double the plant's capacity has been
installed in the Namibian power supply authority's switchyard to maintain
a high level of power availability to the plant, with expansion capability
for future prospects in the region.
Electricity supply for Skorpion Zinc presented a number
of logistical challenges with power having to be brought to the plant
over a distance of 260 km across a harsh, unforgiving terrain. Nampower,
the Namibian power supply authority, provided the infrastructure, while
the joint venture between BATEMAN and SNC Lavalin, the Skorpion Project
Joint Venture (SPJV), coordinated the power requirements and was responsible
for the detailed engineering of the electrical reticulation on this
high-tech integrated mine, refinery and residential township development.
For the project, Nampower established the Kokerboom
400 kV tee-off switchyard to tap into the national grid near Keetmanshoop,
the Obib 400/66 kV stepdown switchyard near Rosh Pinah, and the Skorpion
66/11 kV switchyard at the plant. In addition, Nampower constructed
a 400 kV overhead powerline over a distance of 260 km between the Kokerboom
and Obib switchyards, as well as two parallel 66 kV overhead powerlines
between Obib and Skorpion.
For Nampower to bring power across the Huib-Hoch mountain
plateau, pylons, including the pouring of concrete, were erected by
helicopter.
The main power consumer on the plant is the electrowinning
cellhouse, which takes its power from four identical rectifiers, each
with a rated output of 55 kA at 370 VDC and interconnected via a 220
kA aluminium busbar system. The rectifier transformers take their power
at 66 kV from the Skorpion switchyard.
A significant advantage of using four rectiformers
(rectifier and associated transformer) rated at 55 kA is that this size
enabled manufacture and repair, if necessary, in nearby South Africa.
In addition, the number of units facilitates on-line maintenance, since
the plant can still run at full production without interruption, with
one rectifier out of service for maintenance.
Due to the remoteness of the location, the rectifier
transformers had to be transported completely oil filled, each with
a mass of 90t. Because of the poor road conditions, the rectifiers were
transported on air-suspension trucks, and the transformers by means
of special multi-wheel trailers. At times, over the last 160 km of the
route, the vehicles had to travel at a speed of
only 25 km/h.
Two 55 MVA, 66/11 kV transformers provide power to
the rest of the plant via an 11 kV reticulation network, with further
transformation down to 3,3 kV and 525 V. The transformation from 11
kV is done at a number of substations, strategically positioned around
the plant at the load centres of the various process areas, to feed
approximately 760 motors.
The plant lighting and small power requirements are supplied at 400
V distributed from a ring of mini substations, which were also initially
used to supply the plant with all its construction power requirements
and area lighting for safety and security.
The normal power supply network is supplemented
with a number of self-contained, diesel-alternator emergency power generating
sets to ensure an uninterrupted power supply to strategically important
processes. Air-conditioning and air-filtration units were built into
the substations to counteract the harsh climatic conditions, which include
high ambient temperatures of up to 45 ºC and severe dust storms.
For further information, please contact Mubanga Mofu, Manager, Electrical
Engineering, on +27-11-899-2790 or email basemetals@batemanengineering.com.
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