In practice, High-voltage (HV) cables are occasionally exposed to impulse and superimposed transient conditions, which may initiate partial discharges (PD) temporarily. Whether such PDs persist under AC voltage after the transient conditions have vanished, is at focus in the research described in this paper. Since for cross-linked polyethylene (XLPE) cables the accessories are weak links in the HV cable insulation system, we investigated the PD behavior of an artificial joint defect in a HV cable model under impulse and superimposed voltages. By applying a dedicated PD measuring system it was found that, the impulse and superimposed voltages can initiate PD in the artificial defect, which under local electrical field conditions can persist for some time. The different parameters of the applied voltages have different effects on the PD behavior.

Introduction

By facing the expanding power grids, underground power cables are installed more and more as an alternative to overhead lines [1,2]. By 2006, 57% of all cables in service are cross-linked polyethylene (XLPE) insulated cable, and 70% of all accessories are extruded types [3]. Nowadays extruded cables, especially XLPE cables, increasingly dominate new installments. This is due to their higher efficiency brought by the lower dielectric constant, higher operating temperature and easier manufacturing and installation process [4]. A failure investigation based on approximately 170 individual power cables in service over the period from 1997 until 2014 [5], learned that around 69% of the failures occurred in the cable accessories. More than half of the failure causes of XLPE cables in the sample pool (74%) occurred in the cable accessories due to installation related defects. The latter weak link of the insulation system is at focus in the present experimental investigation about transient effects. In service, power cable systems are occasionally subjected to abnormal superimposed transient conditions, where impulses are superimposed on the normal AC voltage. One example is a mixed overhead line and cable system, in which a switching action may lead to high frequency oscillations, causing large superimposed voltage impulses [6]. Cable systems are well designed to withstand the normal operating and transient conditions, however in such cases the insulation system faces exceptional superimposed transient overstress conditions. Usually, most defects in the cable insulation are detected through PD test during commissioning tests or maintenance tests.