A coal mining drag-line machine in an open-pit mine consumes 0.92 MVA at a 0.8 power-factor...

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A coal mining "drag-line" machine in an open-pit mine consumes 0.92 MVA at a 0.8 power-factor lagging when it digs coal, and it generates (delivers to the electric system) 0.10 MVA at a 0.5 power-factor leading when the loaded shovel swings away from the pit wall. At the end of the "dig" period, the change in supply current magnitude can cause tripping of a protective relay, which is constructed of solid-state circuitry. Therefore, it is desired to minimize the change at the current magnitude. Consider the placement of capacitors at the machine terminals and find the amount of capacitive correction (in kvar) to eliminate the change in steady-state current magnitude. The machine is ener- gized from a 36.5 kV, three-phase supply. Start the solution by letting Q be the total three- phase megavars of the capacitors connected across the machine terminals, and write an expression for the magnitude of the line current drawn by the machine in terms of Q for both the digging and generating operations. A coal mining "drag-line" machine in an open-pit mine consumes 0.92 MVA at a 0.8 power-factor lagging when it digs coal, and it generates (delivers to the electric system) 0.10 MVA at a 0.5 power-factor leading when the loaded shovel swings away from the pit wall. At the end of the "dig" period, the change in supply current magnitude can cause tripping of a protective relay, which is constructed of solid-state circuitry. Therefore, it is desired to minimize the change at the current magnitude. Consider the placement of capacitors at the machine terminals and find the amount of capacitive correction (in kvar) to eliminate the change in steady-state current magnitude. The machine is ener- gized from a 36.5 kV, three-phase supply. Start the solution by letting Q be the total three- phase megavars of the capacitors connected across the machine terminals, and write an expression for the magnitude of the line current drawn by the machine in terms of Q for both the digging and generating operations.

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