Power failures at underground mines – which over the years have resulted from factors ranging from lightning strikes to load shedding – pose significant safety risks.
Mobile standby power solutions have demonstrated how sound and innovative engineering can optimise asset utilisation and reduce capital expenditure.
This article first appeared in Mining Review Africa Issue 4 2018.
Working with a gold miner, Zest WEG Group engineered an emergency power generation solution for its operations in the Free State.
The solution provided allows key activities to continue should power go down. These activities include the evacuation of workers from underground, air ventilation and mine dewatering.
When designing the mobile solution it was necessary to factor in the scenarios to cover the possibility of power being cut to one shaft, a number of shafts or a complete area-wide blackout.
“Our approach is always to design a mobile power generation solution that exactly suits each customer’s requirements and this is exactly what we achieved for this gold mining operation,” says Alastair Gerrard, Zest WEG Group Energy Solutions executive.
“Provision of this particular solution meant also ensuring its quick and safe integration with the existing electrical infrastructure at each shaft, which are not all the same.”
Gerrard explains that an in-depth technical audit of the various shafts was conducted to confirm the details and constraints associated with each operation. The audit further confirmed the power requirements and number of generators needed at each shaft.
Leveraging its depth of expertise in power generation technologies, Zest WEG Group designed and constructed four mobile diesel generator sets, each mounted on an engineered trailer so that they could be quickly and easily moved from one position to another.
This optimised capital expenditure, as it obviated the need for a dedicated power generation facility at each shaft.
The rated capacity of each of the mobile generators is 2.5 MW, generating electricity at 6 600 V. Depending on the power demand of each shaft, the units can be synchronised in pairs, or used as combined sets of three or four.
Cooling was another important engineering aspect, due to the compact nature of the mobile generator’s design. “Our solution here was a 10 blade radiator fan to accommodate the demanding cooling requirements, as it was necessary to move 48 cubic metres per second of air through the generator,” says Gerrard.
“Engineering analysis confirmed that increasing the number of blades allowed less heat to be generated at the tips of the blades, as the fan could be run at a slower speed while still delivering the required performance.”
There was also a fuel storage requirement for the diesel engines, each consuming about 560 litres per hour of diesel at full load. This was addressed in two ways: each trailer was designed with a small 500 litre intermediate fuel tank on the mobile generator, while separate custom engineered fuel pup-trailers with 4 500 litre fuel tanks cater for the balance of the fuel requirements.
“An advantage of the pup-trailers is that they can be used independently of the mobile generator, or can be connected to the back of the mobile generator and pulled in tandem,” says Sollie Herbst, general manager at Zest WEG Group Energy Solutions Division.
“This allows flexibility and a significant time-saving, as the mobile generator can be sent immediately to the next shaft with sufficient fuel capacity, while the pup trailers are refilled independently. They could also be used as normal fuel bogeys for other purposes on the mine.”
To protect against environmental contamination from fuel spillages, the fuel tanks supplied were double-skinned in a self-bunded configuration. Another innovation was the cable reel, holding the cable that would take the power from the mobile generator to the shaft.
“We designed a flexible cable reel to be located within the mobile generator, with a quick medium voltage coupler system to allow mine staff to easily plug in to the integration systems installed at each shaft,” he says. “Use of a slip-ring allows the cable to stay on the reel, with one side always connected to the synchronisation switchgear on the trailer.”
To deter theft and enable secure operator access to the generator controller, Zest WEG Group developed a three-door system.
“The first outer door is lockable and only accessible by authorised operators,” says Gerrard. “The second inner door incorporates the controller interface, allowing the operator to control the machine, while the third inner door only permits further access for maintenance and fault-finding when required.”
All components are contained within a custom-designed aluminium enclosure, with sound attenuation material to keep noise levels down to the specified requirements. For ease of maintenance, it is split into two compartments, so that the portion over the engine and alternator compartment can be removed for easier access during scheduled major services.
In line with its emphasis on partnership and delivery standards, Zest WEG Group engaged the customer closely in the commissioning process.
“Together with the mine personnel, we set up the mobile generators at each shaft as required, connecting them up and simulating the operation to ensure that the power requirements were met,” he says.
“This was also a useful opportunity to train the relevant staff in operating the system, where they could see how to apply the processes that we had documented with them.”