Selectivity of damped ac (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded W cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan delta measurements. Damped ac voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous ac voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan delta measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded W cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with dc. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.