Phenomena That Affect the Calculation of the Inductance and Resistance of Mine Track/Trolley Systems

The measurement and calculation of the resistance and inductance of dc track/trolley systems for mine power systems has been of great interest over the last few years because the system time constant influences dc fuse performance in a critical manner. Several important phenomena are frequently overlooked when making calculations of the resistance and inductance of the track/trolley system. Among these phenomena are saturation of the iron in the rails, the increase of internal inductance of the rails caused by the presence of ferromagnetic material, the division of current between the track, the track feeders and the earth, the impact of track bonds and cross bonds, the influence of bonds between sections of trolley, and the division of current between trolley and trolley feeders. These phenomena may be thought of as falling into two classes: current division and nonlinearities due to properties of materials. This paper will present analytical results and arguments with numerical examples to establish the conclusion that return current flow occurs primarily through the earth and track feeders as opposed to the track and that nonlinearities caused by the iron in rails can greatly affect the system inductance if the rail is the only path for current flow. The paper will also discuss the order of the correction needed in track/trolley resistance that is necessary to account for the resistance of bonds. The result of the corrections is to increase both the system resistance and inductance in a way that the system time constant is not dramatically affected.