PD test procedure as after laying acceptance test on HV and EHV polymeric cable systems

During the last two decades the use of XLPE polymeric cables for High Voltage and Extra High Voltage transmission lines has grown significantly, due to increased electrical congestion of metropolitan areas and the proven reliability of XLPE power transmission cables. High quality extrusion processes and cable materials, along with more stringent factory acceptance tests, are at the basis of this increased reliability. Extruded cable drums, as well as cable accessories, are PD tested in the factory in order to check for the presence of insulation defects. However, when cable systems are installed on site, jointing is one of the most important steps of the overall laying process to achieve a high reliability. At this stage, small defects can be introduced, causing PD activity at operating voltage once in service, and leading to failure in relatively short times. For this reason, the best practice to obtain an effective after-laying commissioning test is to perform both HVAC test and PD test. PD test can identify smaller defects which can be missed by HVAC withstand tests. This paper discusses the main differences between different test procedures (e.g. simultaneous and sequential) and test voltages, which are used to check the installed cable system condition. Furthermore, general considerations on the requirements that a PD testing device needs to fulfill are presented, along with two working cases where an improved PD system with separation and identification tools was able to find PD activities which were not highlighted during the HVAC test. Voltages of 1.2-1.4 U₀ seems to be effective to assess insulation condition during HVAC test. If an effective technology is used, both simultaneous and sequential PD tests are able to spotlight minor defects in XLPE cable insulation system.