Detection of Irregular Sheath Current Distribution for Diagnosis of Faults in Grounding Systems of Cross-Bonded Cables

Sheath currents in high voltage cable circuits have proven to be one of the most convenient indicators for assessing cable health. The sheath currents are regularly distributed with fluctuation in a certain range under normal conditions, while the principle of distribution characteristics is still unclear when the cross-bonded grounding system of the cable circuit is abnormal. This paper classifies the common defects of grounding system into three types of circuit topologies and analyzes the correspondence between grounding system conditions and sheath currents. A fault diagnosis method is proposed to detect irregular distribution of sheath currents by quantifying the difference of sheath currents in each of the minor sections and the average currents of each sheath loop. Sensitive analyses are provided to evaluate the criteria, clarifying the influence of fault resistance in each defect. The results show that the method can be very effective when the ratio of the fault resistance to the earth resistance is less than 7.6 for a short-circuit defect in metallic sheath, or less than 1.2 for a short-circuit defect between metallic sheath and ground. The performance of the method is also shown to be very efficient by evaluation with the data collected on site.