Two-Sheath Loop Short Circuit Defects Detection in High-Voltage Cable Systems Using Sheath Current Phasors

The joint is the weak point of HV (high voltage) cable insulation systems; creep discharge between insulation layers of the cable joint, due to moisture intrusion, is one of the main defects leading to single-phase grounding. Carbonization on the insulation interface after creep discharge would lead to a short-circuit defect in the sheath loops and result in abnormal sheath current. In this study, a novel diagnostic criterion using the phasor difference of sheath currents at both ends of the same circuit is proposed. The coupling effect between the sheath and the conductor under defect conditions is considered, and the original lumped parameter model of the cable circuit is optimized. The cable parameters are further corrected using a genetic algorithm. The diagnostic criterion comprehensively accounts for the adverse effects of unequal cable segment lengths, load current fluctuations, grounding impedance, and phase voltage variations. When the phase angle fluctuation of the phasor difference is within 10° and the defect impedance is below 100 Ω, the defective joint can be accurately diagnosed by this method. The conclusion has been validated through PSCAD simulations, with a diagnostic accuracy above 97%. Even under 20 dB noise interference, the error increase remains within 2%.