Active length of cable lines with uncoated metal sheaths as long grounding conductors

Abstract

Power cables that have sheaths, usually lead, and are manufactured with bitumen impregnated hessian which degrades over time, act as long grounding electrodes or inject fault current directly into the surrounding soil. As a consequence of this, the MV lines made of this type of cable spontaneously form a very large and complex grounding system around the high-voltage/medium-voltage (HV/MV) distribution substations. The area encompassed by this type of grounding system is much larger than the area occupied by the HV/MV substation itself. Due to this, the grounding system exhibits excellent performance, including a very low value of impedance to ground. The problem with this type of grounding system appears only at the design stage of the HV/MV substation, when safety conditions inside and in the vicinity of this substation should be confirmed by calculation. This paper provides improvements in accuracy to the only existing practical method developed for the analysis of this type of grounding system. The proposed improvement enables a more accurate estimation of active cable line length, grounding impedance of the cable line, the urban area covered by this type of grounding system, as well as grounding impedance of the entire grounding system. Copyright © 2010 John Wiley & Sons, Ltd.