Tangent delta extraction of cable joints for aged 11kV underground cable system

Tangent Delta, also called loss angle or dissipation factor (δ) is a measurement technique to determine the quality of cable insulation. It utilizes the phase shifts caused by the displacement current induced due to the existence of impurities in the insulation or stress on the cable. More than 50% of the underground cable breakdown is due to cable joint failures and also aged bulk cable failures. Current Tangent Delta measurement used is an overall or bulk assessment technique where it is not able to differentiate the responses of each cable system component (i.e. joints, terminations and cables). However, there is no current method is available to extract the tangent delta of joints from bulk defective cable using HFAC or other commercial techniques. It is important to distinguish the cable and its accessories response within the cable system for the ease of maintenance action. Therefore this experimental approach is an attempt to distinguish the responses of each component made using High frequency Alternating Current (HFAC) Tangent Delta measurement technique. This paper presents the extraction method for cable joint of 11 kV defect conditioned cable sample induced with carbon black (CB) impurities in its insulation. The technique will implement on defective cable with various combination of two joints of 20 meter length. The tangent delta of cable joint from bulk tangent delta measurement will be determined using higher frequencies (>100Hz) at voltages of 1 kV. The current (I), voltage (V) and phase (θ) were obtained in this experiment using HFAC measurement setup. The phasor diagram is then constructed where the actual joint phase is extracted using the basic trigonometric functions. Hence, the tan delta values of extracted joints were obtained through the calculations.