TRANSMISSION AND DISTRIBUTION
CABLE CAPACITY A
New testing methods for cables significantly reduce the risk of in-service failures
n effective site commissioning test of an HV cable system is one which reliably identifies
laying and installation related life- limiting defects while, also, not introducing damage into the cable system. The combination of near power frequency AC withstand and partial discharge testing for commissioning testing (site acceptance testing) of HV cable systems prior to energisation has been performed since the 1990’s. Near power frequency (NPF) is defined as the frequency range of 10 - 300Hz where breakdown and PD inception voltages vary less than 10% of that at 50/60Hz (CIGRE TB 841). The methodology is field proven globally for identifying life-limiting defects and, therefore, significantly reduces the risk of in-service failures post energisation in any HV cable system. Unlike other voltage waveforms, NPF tests apply realistic, operationally-related electrical AC stresses continuously to the cable system end-to-end. Field data acquired over more than 20 years of testing has shown the importance of applying constant, continuous voltage at NPF for 60 minutes (or more) for identifying
Inter-array offshore cable testing
laying installation defects in HV cable systems (CIGRE TB 728). However, it is imperative that voltage
levels and durations are as per Clause 16.3 of IEC 60840, IEC 62067, and IEC 63026 and that PD testing is performed as per recommendations in CIGRE TB 728. Specifically, data collected over more than 20 years has shown that, given the distribution of break-down voltages and partial discharge inception voltages measured, it is important that test levels for 66kV-115kV cables are at 2U0, where U0 references the rated line-to-ground voltage, and that for 132kV and above the test level is 1.7U0 for an effective test (CIGRE TB 728). For platform- and inter-array cable
systems rated 138kV and below, it is important that distributed partial discharge monitoring is performed continuously in conjunction with the withstand test. This particularly the case for terminations, where the AC withstand test may not result in dielectric failure during the 60 minutes withstand test and thus life limiting defects will be identified via PD. Distributed PD tests further allow for pre-maturely terminating a test (meaning that failure under test
is avoided), which then reduces repair times, costs and allows for quick re- commissioning testing following repair. Today’s solutions for offshore
commissioning testing of IAC systems involve modular, DNV certified resonant test sets. Each component weighs less than four metric tonnes and can be stacked to minimise space requirements on the OSS. For PD measurements, sensors and battery-operated monitors connected to the fiber-patch panel in each WTG, can be installed while terminating, thus minimising efforts and costs for testing. Similarly, for testing of export cable
systems, modular mobile test systems exist, allowing for testing of up to 100km lengths of 275kV rated cable systems as per IEC recommendations. For export cable systems, PD measurements should be obtained on the termination end as well as on the transition joints.
Authors: Mark Fenger, Austin Radford, Raed Ayoob & Michael Heiting
For more information visit:
www.kinectrics.com
20
www.engineerlive.com
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