The Interaction Between Electric Field and Partial Discharges Simultaneously Detected in a HVDC Cable Under Operating Conditions

The reliability assessment of HVDC systems greatly depends on the insulating materials aging level and, among these, the cable insulation layer plays a fundamental role. With this in mind, in this work, the correlation between the electric field distribution and the triggering and evolution of PD in a DC cable containing an internal air void defect and subjected to normal operating conditions has been investigated. The aim is to demonstrate that the PD activity under DC depends on the electric field distribution which, in turn, is related to the conductivity gradient that varies with load. In a previous paper, the field distribution in a cable specimen was simulated. Here, instead, the field profiles have been experimentally obtained starting from the measured space charges detected simultaneously with the PD activity. For the charge detection and for the PD monitoring, an innovative PEA cell and a PD acquisition system have been used, respectively. Measurement results highlight that the PD behavior depends on both the electric field distribution and the time constant
$\tau $
. Specifically, during the first 10 minutes of the beginning of the test, the field in the outer cable radius passes from 12 to 17 kV/mm and it is maintained around this last value until 30 minutes. PD are triggered after 1 minutes from the start of the test, corresponding to a PDIEF of 12.6 kV/mm calculated in the healthy cable section. The detected PD activity shows a PDRR with maximum value equal to 110 pulse/min at the beginning of the test. Whereas, after 15 minutes, the electric field variation dE/dt is approximately zero and the PDRR, that is maintained only by
$\tau $
, results around 10 pulse/minute.