Ampcontrol is leading the way, developing the world’s first 11kV longwall electrical system.
Since longwall mining was first mechanised in Australia in 1963, the industry has seen significant evolution in technology and electrical voltage increases, leading to increased safety, production and output. Since 1992 the standard supply used on longwall faces and AFC’s in Australian mines has been 3300 Volts (3.3kV), with 1,100 Volts (1.1kV) only occasionally used up until 2002.
Providing for a step change in AFC capacity, Ampcontrol commissioned the world’s first 11kV AFC for use on a 405m coal face in the Hunter Valley mining region of New South Wales Australia.
Why 11kV?
During the project scoping phase for the AFC electrical system, one of the key problems identified was the fact that the AFC would require three electric motors, each rated at 1.6MW to achieve the desired coal clearance performance and the required longwall face width of 405m.
At 3.3kV this level of power demand would result in significant issues including; excessive voltage drop, significant reduction in motor torque, increased short circuit levels exceeding the rating of existing 3.3kV switchgear and the requirement for impractical, large power cables to be installed.
Due to the perceived risks associated with the use of higher voltages in the hazardous zone, such as increased arc flash risk, earth faults and short circuit levels, 3.3kV has been the traditional industry standard used on longwall installations in Australia and countries including the United Kingdom, Russia, South Africa and China, with 4.16kV used in the USA.
Restricting the supply voltage to longwall AFC’s to 3.3kV causes a variety of limitations to the capacity of the AFC’s motors including pull out torque and the overall operability of the equipment.
At 3.3kV the maximum operationally functional motor available was practically limited to 1200kW due to current demand and size limitations. If more than 1200kW was required the only option available was to install a larger number of smaller rated motors, increasing installation costs and using significant space in the maingate and tailgate underground which is often at a premium on modern longwall faces.
Dependent on the system fault level, often when 3.3kV AFC motors are started the voltage drop exceeds 25% thereby reducing the overall torque performance of the motor. As motor torque is proportional to voltage squared, a 25% voltage drop during starting would result in a 43% reduction in torque.
When voltage drop exceeds 25% it has the potential to stall motors resulting in equipment damage. This voltage drop phenomena is often manifested during heavy loading events such as face ‘slabbing’, where the AFC load increases rapidly. Depending on the responsiveness of the coupling control system, both of these events can lead to motor stall, and in extreme events chain breakage.
The fault levels in a 3.3kV system using 1.6MW motors exceed the contactor short circuit ratings available. 3.3kV contactors are only available rated to 9,000 Amps short circuit capacity, however many systems have risen to include short circuit levels in excess of 16,000Amps, resulting in the risk of equipment failure and arc faults in service.
The power infrastructure required for 3.3kV is potentially too large for the restricted space available underground on longwall faces. The 3.3kV transformers required are very large, as are the 3.3kV cables required. These issues create increased risk of equipment damage during relocation, increased risk of cable damage and increased manual handling issues.
Excessive voltage drop can also impact on control systems which are required to operate at a significantly reduced voltage level. Typically when voltage drop exceeds 25% and the starting cycle is extended for greater than 10 – 15 seconds the automation systems are prone to shut down. This shut down requires the hard reset losing of the control system and can result in 15 to 20 minutes of lost production time.
Ampcontrol engineers knew these inherent limitations and problems associated with 3.3kV could be alleviated by introducing an 11kV system to the AFC.
The 11kV project
Critical to the success of this project was proving the safe use of 11kV in the hazardous zone through extensive engineering design review, first principle calculations and verification testing.
Each piece of equipment was designed in accordance with New South Wales Department of Primary Industry (NSW DPI) guidelines, all relevant Australian Standards and NSW OH&S regulations; with particular focus on the key electrical risks including the limitation of the touch and step potentials, Arc Fault prevention and restriction of short circuit faults. The relevant sections of the equipment were also certified to IECEx standards for use in the hazardous zone.
To meet the challenge of designing such a large system, Ampcontrol customised numerous existing products, as well as developed a number of world first 11kV products to suit the application and hazardous area environment present in coal mining.
Ampcontrol designed and manufactured the following:
- 2 x 5 MVA 11kV/3.3kV Substations
- 2 x 11kV Distribution and Control Boxes (DCB’s) for the AFC
- 2 x 3.3 kV Distribution and Control Boxes (DCB’s) for the Beam Stage Loader, Crusher and Shearer
- 2 x 240V Main Gate Control and Monitoring Enclosures (TG CME’s)
- 2 x 3.3kV Pump Station Distribution and Control Boxes (DCB’s)
Technological firsts
The engineering, design, manufacture and successful delivery of the project required a number of technological firsts to ensure the electrical system not only met the customer’s exacting specifications but also met the stringent safety requirements of the NSW DPI.
11kV protection scheme
To support the system, a purpose built 11kV protection scheme was developed. The scheme comprised of overload, short circuit, earth fault, earth continuity, earth leakage, earth fault lockout, frozen contact and loss of vacuum protection features, plus inclusion of a 5000V DC pre-start insulation test function, all in accordance with Australian Standard AS 2081-2000 and IEC standards for intrinsic safety. The protection scheme was custom designed for the project as there was no comparable product on the market anywhere in the world and incorporated patented Ampcontrol drive recognition technology.
The system was subjected to special testing to verify the performance of the protection scheme in the event of a cable crush. Cable crush and the resulting potential for earth fault were deemed to be an area of high probability for failure in the longwall application. The testing identified the new 11kV protection scheme, earth fault protection scheme and 11kV cable design delivered an equivalent level of safety to the existing industry standard 3.3kV system in normal operation.
Intrinsically Safe LED (IS LED) Face Lighting System
Incorporating the very latest in LED lighting technology, a completely new IS LED face lighting system was developed as part of the overall longwall project. This product was developed by Ampcontrol Group Member Company, Burn Brite, who pioneered the original Intrinsically Safe (IS LED) Lighting System which was launched to the mining market in 2009.
Enhanced safety was achieved through increased lighting levels and improved targeted lighting across the 405m wide face and operating at extra low voltage IS 12V DC as opposed to the previous 110V AC flameproof systems. The slim line housing design allowed complete integration into the roof support canopy which increased personnel access and eliminated head injury and physical damage.
Another benefit of the Intrinsically Safe design is that following the removal of the flameproof Exd style housing, the maintenance costs are almost completely removed.
Increased Safety
There are several key safety features of the 11kV electrical system which contributed to the overall safety of the work environment. In line with the increasing safety requirements of the NSW DPI, individual outlet visible disconnect load break isolators and fault make earth switches were incorporated into all load supply outlets. This provided a safe and identifiable means for conducting all electrical maintenance on the equipment.
A High Integrity Isolation System was developed to provide a single isolation point for the safe and effective isolation of the complete longwall face 11kV and 3.3kV drive systems for non electrical works. This system was designed and assessed to have a Safety Integrity Level (SIL) of 3 and comprised a SIL rated Programmable Logic Controller and associated hardware. The entire control ‘system’ was independently certified to achieve a level of safety equivalent to only one failure in every 100,000 years.
A SIL2 rated distributed Remote Isolation and Communication System was also used across the 405m wide longwall face. Utilising Ampcontrol’s proprietary Integrated Monitoring And Control System – iMAC signal line control system, VCA & VAA, Digital Voice Communication system and face lockout isolation stations, the entire control ‘system’ was independently certified to achieve a SIL2 rating.
The SIL rating of both systems enabled mechanical maintenance works to be completed without the need for routine, time consuming high voltage isolations and the involvement of electrical personnel.
Increased production
The use of 11kV in lieu of 3.3kV has resulted in the system’s ability to deliver more energy to the face which in turn enables larger machines to be run with higher output levels at the longwall face.
With production output paramount to the financial success of the overall project, the potential in AFC tonnage capacity is 35% greater than the traditional 3.3kV longwall.
The new longwall system commenced full production in October 2010 and has already provided improved electrical performance during both starting and face slabbing events on the AFC. It has met and exceeded all commissioning targets and the electrical system has proven to be reliable in service.
This project represents an exciting new chapter in longwall mining development in Australia with worldwide opportunities. Not only has the technology provided a step change in AFC capacity, it has delivered increased production rates along with the potential for increased coal face widths.