Impact of Insulation Degradation Length, Severity, and Boundary on Cable Defect Localization Using Broadband Impedance Spectrum

Insulation degradation is a common type of minor defect in cables and often exhibits a large spatial scale. Broadband impedance spectrum (BIS), as a frequency-domain reflectometry (FDR) technique, is effective for diagnosing and localizing such defects. However, most existing studies assume concentrated defects and overlook the effects of defect length, severity, and boundary, which are critical in the context of insulation degradation. This article systematically investigates these factors in coaxial cables through rigorous theoretical derivation, supported by simulation validation. The results show that while the location of the defect start remains unaffected, the localization of the defect end is shifted. This shift increases linearly with defect length and severity and further propagates to subsequent discontinuities along the cable. Moreover, defect boundaries introduce additional shifts, which are related to the integral of the relative permittivity variation across the boundary region. The boundary also reduces the amplitude and broadens the width of the associated localization peaks. The applicability of these conclusions to coaxial power cables is discussed, and a method is proposed for identifying long spatial-scale defects based on BIS measurements from both cable terminals. These findings provide theoretical insights and practical guidance, serving as a supplement to existing localization methods based on BIS.