Use of capacitive couplers for partial discharge measurements in power cables and joints

Zhong, L., Xu, Y., Chen, G., Davies, A. E., Richardson, Z. and Swingler, S. G. (2001) Use of capacitive couplers for partial discharge measurements in power cables and joints. pp. 412-5 .

Abstract

Partial discharge (PD) measurements are universally accepted as a technique giving some indication of the state of the insulation in high voltage apparatus. Cable end users are keen to adopt an on-line pd monitoring during commissioning of systems. However, because of noisy environments and the problems of interference the conventional methods are difficult to implement. As a consequence, ultra-high frequency (UHF) / very high frequency (VHF) techniques of on-site detection of partial discharges have been developed. However, to apply the techniques in the field, a thorough understanding of the principle and factors which may affect the signal is essential. This paper presents the principle of VHF partial discharge measurement in cables and cable joints using the capacitive coupler. The frequency response of the coupler has been analysed based on its equivalent circuit and compared with the measured response using the network analyser. The calibration method of the measuring system is also discussed. The discharges in a 66kV cable and a 132 kV cable containing a joint have been detected using this method. It has been revealed that the minimal discharge detectable is less than 1pC.

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