…3,300V/50Hz electrical supplies are essential for >500kW motors
Torque is proportional to Voltage squared
WRT Copper conductor there is an Amps per mm2 limitation thus a weight and cable flexibility problem when CSA gets large
The May 1969 Mining Technology Official Journal of The Association Of Mining Electrical & Mechanical Engineers UK
Summed it up pretty well in the paper
COAL FACE VOLTAGE -the choice
By J F Wilkie C. Eng. FlEE. MIMinE. Member.
=>> Can PM you a copy to post on the Board for comparison if you do not have access to it.
Down side of voltages 3.3kV and above depending on the geometry, voids, dew point, dielectric material quality and E Field Stress Grading you get Corona and other partial discharge concerns leading to incipient insulation degradation with time…X/R for fault levels tends to be higher (less damping) and the system capacitance is also larger so transient overvoltages during switching and network faults are more of a problem.
The paper catches the 1998 point in time Motor sizes, those quoted are small in comparison to current 2014 Longwalls…my experience as follows
1996 6.6kV Distribution Backone
AFC was 2 x 261/522kW 3.3kV pole changing motors
Shearer was Electra 600 2 x 275kW 3.3kV Cutter Motors
2 x 5/6.6MVA 66kV/6.6kV Txf at Pit Top
2000 11kV Distribution Backbone
AFC was 3 x 750kW 3.3kV Motors
Shearer was 2 x 550kW 3.3kV Cutter Motors
2 x 25MVA 66kV/11kV Txf at Pit Top
& per my post to
CMPB re High Voltage Faces2014 11kV Distribution Backbone
AFC 3 x 1650kW AFC 11kV Motors (for 400m wide)
Shearer 2 x 860kW 3.3kV Cutter Motors
?2 x 25MVA 66kV/11kV Txf at Pit Top
The kW increase from 1998 to 2014 is interesting because the 11kV (or 13.8kV in USA) Distribution Backbone voltage has not changed, hence the use of strategically positioned boreholes for Distribution Backbone cables to reduce the cable lengths to the longwall block length…with 4km long 400m wide blocks 5m gate roads and 30m MG/TG pillars you have added 1320m of Backbone cable to extract the 4th longwall block so assuming you get your main headings roads driven you can get say eight (8) longwall blocks from one side of the main headings and if your mining lease is big enough and the goaf pressure arch impact on the main headings pillars is not brutal another eight (8) longwall blocks from the other side of the main headings before requiring another borehole. (obviously all backbone cable CSA mm2 and DOL start dependent ie it is critical that the electrical supply system designers complete power system load flow and motor starting assessments)
Electronics and horizon control have advanced significantly, 256kB USB Drive was $140 in 1996, 32GB USB Drive is now $6..…corporate memory is even cheaper..…accountants often suggest that "Technical Memory" is disposable during a corporate restructure……
Fault levels and X/R ratios have risen….unfortunately the flameproof enclosures have not kept pace WRT internal arcing fault pressure rise for Class 1 CH4 environments….Tic…Tic…Tic. not sure that MSHA research is as well funded as it used to be, UK Coal does not have the $ or the industry to support the necessary research & ACARP is dead and Australia has ASX listed companies focusing on maximising shareholder returns.....the question of who will fund the required research has been asked......
WRT "Longwall mining will be the dominant underground coal mining method"
=>> You need good roof, good floor, minimal faulting or dyke intrusions into the coal & a relatively flat seam, minimal folds or undulations……mid block longwall relocations are costly & not fun……some consider them character building others enjoy playing dodgem cars with an Eimco Chock Mule....while their production bonus is at $0.
=>> Open cut dominates where where stripping ratios are not cost prohibitive...you can mine multiple seams concurrently & blue sky does not fall in....visit the Hunter Valley NSW or Bowen Basin QLD on pamraeN or htrae elgoog.
As retreat Longwall extraction rates increase CH4 challenges mean Ventilation to ensure dilution becomes critical , CH4 make in the Goaf can be considerable, predrainage of Longwall blocks for CH4 may be mandatory/required. CO2 Outburst can also create challenges…surface subsidence on surface utility infrastructure can be highly destructive consider a 400m wide face in 4.5m of coal, single pass longwall dependent on overlying strata and depth…vertical shear surface deformation of 1.5-2m has been observed…now that is an instantaneous landscape change
Rob
