Review of field acceptance hipot & PD testing of medium voltage underground cables
There are some well-defined guides and standards regarding factory testing of medium voltage shielded cables in laboratories provided by IEEE, IEC, ANSI/ICEA, and CSA. These guides cover AC withstand and PD testing of XLPE and EPR cables and accessories, such as 50/60Hz AC Hipot and PD tests which are routinely conducted on every reel of cable produced at the factory. All terminations, splices, and separable connectors are tested with more or less the same test method using different test levels and timing.
Approaches to the forensic failure investigation of medium voltage polymeric cables
Medium voltage (MV) polymeric cables are critical assets in modern power systems, since their introduction, approximately five decades ago. More recently, there has been a growing emphasis on the maintenance testing of polymeric cables, with some of them being prone to premature failures. This paper covers the various techniques available for the forensic failure investigation of polymeric cables through practical examples. Some of these include diagnostic electrical tests such as Insulation Resistance, AC Hipot & PD, and nonelectrical procedures such as visual inspection, optical microscopy, and differential scanning calorimetry (DSC). The recommended order of performing these techniques is also presented, based on the suspected failure mode. This serves as a practical guideline for utilities, towards identifying the type of cable failure sustained in the field. Diagnostic electrical testing (post-failure) helps establish the most probable cause of failure associated with polymeric cables and/or their accessories. A very useful, but less explored technique in polymer morphology is the DSC, which can be used to reveal the thermal history of a polymeric cable since its processing. In this regard, a detailed case study is presented to highlight the use of a DSC thermogram for determining the maximum temperature experienced by a polymeric cable and the potential source of heat.
In search of optimum chest radiography techniques.
A nation-wide study of chest radiography in Sweden had previously revealed a large variation in the physical and technical factors involved, in particular the radiation dose. In this study the image quality could not be assessed unambiguously. The aim of the present study was to try to establish a correlation between visual grading of radiographs and physical and technical factors in order to find the optimum chest X-ray system. The physical and technical performance of 24 chest units was evaluated. Radiographs were taken of an anthropomorphic chest phantom supplied with test structures simulating various pathologies. Image quality was assessed by visual grading analysis of the radiographs. The physical and technical parameters of the units rated best were used to exemplify good radiographic practice. The results were in agreement with the recommendations issued by the Commission of the European Communities (CEC). It was also shown that low radiation dose is compatible with high-quality radiographic imaging of the chest.