VLF Testing Research Articles

Electrical field distribution on the cross-linked polyethylene insulation surface under partial discharge testing

Cross-linked polyethylene (XLPE) insulated power cables are widely used in transmission and distribution networks. The enhanced localised electrical field/stress can result in surface discharges (SD), i.e. partial discharges on the surface of the dielectric. To measure discharges, very low frequency (VLF-0.1 Hz or lower) excitation has emerged as an attractive alternative to the conventional power frequency (PF- 50/60 Hz) testing as it significantly reduces the required reactive power from the test supply. As the discharge process mainly resulted from the enhanced electric stress; it is necessary to understand how the electric field distributes on the XLPE dielectric surface when it is exposed to a high voltage AC excitation. In this study, the distribution of surface electric field before, during and after a PD event at VLF and PF test voltage is investigated. Finite element analysis (FEA) based numerical simulation shows that the space charge dynamics cause the differences in the field distribution and supports the experimental results.

VLF Testing of High Voltage Cables, State of the Art

A new testing method for testing long and extra-long high voltage cables, called Differential Resonance Technology (DRT) is presented. The working principle, the application, as well as the several challenges for onsite testing in an on- and offshore environment are described.

An Amendment of the VLF tanδ Criteria to Improve the Diagnostic Accuracy of the XLPE-insulated Power Cables

VLF diagnosis technology is introduced in IEEE Std 400 and proposed as evaluation criterion in an effective way of detecting water tree which mainly causes the failure of XLPE insulated cables. In order to inspect the accuracy of the VLF method for XLPE insulated power cables in Korean distribution system, diagnosis for 41 cables which were being serviced in the fields has been carried out and they were removed for AC breakdown voltage test after. Regarding the 41 cables, it was hard to confirm any relation between the VLF values and AC breakdown voltages and also water tree in the insulation was not detected. However, the other cables were failed several days after the diagnosis of the 41 cables. Water trees were found and their VLF values were also much higher than the criterion of IEEE standard. It has been ascertained that we need to change the IEEE criteria in order to improve the accuracy of detecting water trees by additional analyzing of field examples of failure and case studies from overseas countries and therefore amended VLF test voltage and evaluation criteria have been proposed.

High voltage VLF testing of power cables

This publication describes a laboratory test program conducted with the objective to develop a test that would replace the existing DC withstand test. The article describes the methodology used to establish the voltage duration and magnitude of VLF (0.1 Hz) high voltage field tests suitable for crosslinked polyethylene (XLPE) insulated power cable. The results show that the voltage breakdown of laboratory aged XLPE cable at 0.1 Hz is approximately equal to that at 60 Hz, that proof tests at 0.1 Hz cause very little damage to the cable, and that 0.1 Hz testing appears to be a satisfactory alternate to DC testing. Preliminary values are suggested for voltage magnitude and time duration of cable acceptance, maintenance and proof tests at 0.1 Hz for XLPE cable rated up to 35 kV. A program is underway to similarly evaluate samples of service-aged XLPE cable; as well as to demonstrate the use of the preliminary test values at typical utility installations.

Field testing of HV power cables: understanding VLF testing

DC testing of solid dielectric insulated cables has been shown to have limited usefulness. Currently, the two VLF test techniques, the author has found, seem to be the most promising alternatives for testing service-aged power cable systems with extruded dielectric insulation. >

Field testing instrumentation

The author reviews the progress of instrumentation for testing installed electrical power apparatus up to the present. He also discusses current work and the possibilities for future development. Particular attention is given to DC insulation resistance tests, DC field testing, AC insulating power factor testing, VLF testing, and partial-discharge testing in the field.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>