Improvements in Fault Current Calculation Techniques for DC Systems

Since 1970 at least 12 men have died from underground coal-mine fires which were started due to undetected high-resistance faults on trolley systems. Methods which have been proposed to determine these dc fault currents more accurately involve the assumption that the magnitude of the fundamental-component current on the ac side of the rectifier is equal to the magnitude of the current on the dc side of the rectifier. Since this assumption breaks down when the fault occurs at the terminals of a rectifier, a new nonlinear equation set is proposed which eliminates this assumption. The equation set is formulated to account for dc fault current contributions from dc machines and three-phase bridge rectifiers. Newton's method is used to solve the set of nonlinear equations. The use of rectangular form for all voltage variables and elimination of current as a variable provides more reliable and faster convergence than previous methods. A new phenomenon is identified in the results presented. Under certain fault conditions, dc motor voltages may be of sufficient magnitude to reverse bias electrically close bridge rectifiers and hence prevent them from contributing to the fault current.