Development of Severity and Location Indices Based Condition Monitoring Scheme for Underground Cables by Impedance Spectroscopy

This work presents the development of a condition monitoring scheme for detection, severity assessment and localization of small cavities in cable insulation by impedance measurement in frequency domain. A high frequency model of cable incorporating the frequency dependence of cable parameters is developed to analyze the impedance changes during the occurrence of cavities in the insulation. The simulation and analysis of impedance with cavities in a 11 kV, 95 mm
²
cable reveals the parameters such as Severity Index (SI) and Location Index (LI) which shows gradual changes with change in size and location of void and have potential scope to be used for severity assessment and localization of cable deformations. Thus based on the dataset of SI and LI, the condition of cables while in service can be predicted. It is experimentally validated for deformation such as cavities at the outer surface of insulation in 11 kV XLPE cables having conductor areas of 95 mm
²
and 150 mm
²
. The reliability of the proposed method for detection and localization of cavities inside the insulation near cable joints is validated by measurement and analysis on 11 kV, 95 mm
²
XLPE cables with artificially created voids at the insulation region near cable joints.