Design and Implementation Issues of Active Front End Based Systems in Mining Draglines

The primary operating objective of mines is to optimize the cost per ton of material moved to remain profitable. In other words, this means moving the highest possible payload per hour while minimizing operating costs over the lifetime of the machines. Draglines remain a popular choice among other excavation machines because of their high productivity and low cost per unit moved. Siemens provides Active Front End (AFE) drives based on IGBT technology for the dragline operation. AFEs do not rely on passive line commutation and are actively switched at a much higher frequency to precondition the incoming power before it reaches the DC link. This DC voltage is the used to drive the IGBT inverters which power the hoist, drag and propel motions of the dragline. The challenge in designing such a system lies in the fact that multiple staggered AFEs on a single transformer secondary winding that control multiple DC buses. Unless each and every component is identical and the system is balanced, there will be circulating currents. This paper presents such a system along with preliminary field results. Specific solutions for eliminating circulating currents and other protection features are discussed.