Cable Testing, Commissioning and Monitoring
Polyethylene is a ‘non-forgiving’ insulation system. Thus, close attention must be paid to partial discharge activity during factory testing, commissioning, and online service. On-site measurement techniques have to cover the needs of an aging polymeric cable distribution net as well as an increasing application of polyethylene extra high voltage cables.
High Voltage (HV/EHV) Cables
Cables are generally factory-tested. The accessories of high voltage (HV) and extra high voltage (EHV) cables are usually also pre-tested. However, mechanical forces during the laying, hidden imperfections, and flaws caused by improper handling, for instance, require partial discharge commissioning tests. Ideally, the cable accessories of such transmission-class cables are equipped with embedded sensors. Power Diagnostix introduced this cost-effective principle in 1994 and numerous cable manufacturers have implemented it since then. The ICMsystem, especially if enhanced with the FOsystem for optical isolation, offers powerful tools for the analysis of the cable insulation system. Additionally, different preamplifiers and embedded or external spectrum analyzers complete the instrumentation.
Medium Voltage Cables
As with HV and EHV cables, the ICMcompact is used for shop-floor production testing on medium voltage cables and ICMflex applications include HV/MV cable acceptance testing, MV/HV cable onsite testing and cable joints and accessories (laboratory/onsite). During the past decade numerous cable manufacturers have changed to this instrument during the modernization of their test room. A large population of service-aged medium voltage or distribution-class cables has reached their projected service life, as polymeric cables were increasingly introduced since the 1970ies. The ICMcompact is available in a ready-to-use package for field testing and partial discharge location with any external high voltage source. Especially the combination with very low frequency (VLF) sources and the combination with variable frequency resonant test sets provide light weight and cost-effective solutions to keep the distribution grid reliable.The package includes a combination of T-filter and coupling unit to reduce the high frequent disturbance signals from the HV source.
Generally, partial discharge monitoring can take care of any high voltage insulation system to detect incipient breakdown and on-going degradation. The defect mechanisms for cable accessories and for the cable itself differ due to different material properties, for example. When monitoring EHV cables systems, a comparable long prewarning time applies to EPR and EPDM accessories, while the cable insulation of a 400 kV XLPE cable, for instance, has a much shorter prewarning time. Further, an installed monitoring system can be used of course also for the commissioning testing of the cable system. Besides using fiber optic TCP/IP interconnection, accessing the instruments via WLAN offers an effortless installation.