The Application Research of Very Low Frequency Diagnostic Methods in Acceptance Tests on New Cable Systems of Medium Voltage

Tangent delta (TD) measurements using very low frequency (VLF) sinusoidal waveform in aged cable system have been proved sensitive to water tree degradation and insulation aging. Whereas, the application of VLF in acceptance tests on new cable systems of medium voltage is still on research and development. This paper presents an experimental study of the capability of the main VLF testing methods for newly installed 10kV XLPE cable systems in acceptance tests. Compared with oscillation wave excitation, the different waveforms including sinusoidal (Sinus) and cosine-rectangular (CR) of VLF voltage were employed to stress the nine samples of artificial defects of different types. Various characteristics of TD measurement and partial discharge (PD) measurement were analyzed. The results showed that VLF-Sinus voltage has a distinctive ability of detection for a wet or lossy accessory through TD or PD measurements. It is recommended to employ a combination of the monitored VLF-Sinus withstand with TD measurement and the PD measurement under damped alternative current (DAC) or VLF-CR as field diagnostic tools in acceptance test for newly installed cables.