Partial Discharge Testing - MV, HV, and EHV Cables and Cable Accessories
Partial discharge (PD) testing of medium-voltage (MV), high-voltage (HV), and extra-high-voltage (EHV) cables and their accessories starts with quality control of newly fabricated systems at the factory. It continues during the commissioning process to check the assembly of all components, and it can be a complementary part of a cable monitoring installation.
Cable testing, commissioning, and monitoring
Polyethylene is a ‘non-forgiving’ insulation system. Therefore, close attention must be paid to partial discharge (PD) activity during factory testing, commissioning, and in-service monitoring. On-site measurement techniques need to cover both the needs of an ageing polymeric cable distribution network and the increasing application of polyethylene extra high voltage (EHV) cables.
High voltage (HV/EHV) Cables
Cables are generally factory tested and the accessories of high voltage (HV) and extra high voltage (EHV) cables are usually also pre-tested. However, mechanical forces during cable laying, as well as hidden imperfections and flaws caused by improper handling, require partial discharge commissioning tests.
Ideally, the cable accessories of such transmission-class cables are equipped with embedded sensors. Megger 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 analysing the cable insulation system. Additionally, the instrumentation can be completed with different preamplifiers and embedded or external spectrum analysers.
Medium voltage (MV) cables
As with HV and EHV cables, the ICMcompact is used for shop-floor production testing on medium voltage cables. ICMflex applications include HV/MV cable acceptance testing, MV/HV cable on-site testing, and testing of cable joints and accessories (both in the laboratory and on-site).
Over the past decade, numerous cable manufacturers have adopted this instrument during the modernisation of their test facilities. A large population of service-aged MV or distribution-class cables has reached their projected service life, as polymeric cables were introduced in the 1970s.
The ICMcompact is available in a ready-to-use package for field testing and partial discharge location with any external HV source. This is particularly effective in combination with very low frequency (VLF) sources and, when combined with variable frequency resonant test sets, it can provide a lightweight and cost-effective solution to help maintain the reliability of the distribution grid. Additionally, the instrument includes a T-filter and coupling unit to reduce the high frequency disturbance signals from the high-voltage source.
Partial discharge (PD) monitoring
Generally, partial discharge (PD) monitoring can be used to detect incipient breakdown and ongoing degradation in any high-voltage insulation system. However, the defect mechanisms for cable accessories and the cable itself may differ due to variations in material properties.
When monitoring extra-high-voltage (EHV) cable systems, a relatively long prewarning time typically applies to ethylene-propylene rubber (EPR) and ethylene-propylene-diene monomer (EPDM) accessories. In contrast, the cable insulation of a 400 kV cross-linked polyethylene (XLPE) cable may have a much shorter prewarning time.
An installed monitoring system can also be used for the commissioning testing of the cable system. Besides using fiber-optic TCP/IP interconnection, accessing the instruments via wireless LAN (WLAN) can offer a more convenient installation.
Instruments and Special Sensors
Coupling capacitors, as used for factory testing and for offline testing, are not applicable for online testing. Megger offers several types of external sensors for partial discharge (PD) measurements.
The differential foil sensor (DFS) makes use of the stray capacitance of cross-bonding joints and can provide comparable low-noise measurements at higher frequencies. The CT100 is a clamp-on high-frequency (HF) current transformer that can be used to pick up partial discharge signals on ground leads and on cross-bonding connections.
Additionally, Megger can provide customized sensors, including Rogowski coils, tailored for specific applications. The company also assists with the design and implementation of embedded sensors.
