Special requirements of high frequency current transformers in the on-line detection of partial discharges in power cables

The insulation condition of power cables is closely associated with partial discharge (PD) [1]. Early signs of cable insulation failure such as from electrical treeing are revealed by PD detection [2], and insulation failure will eventually lead to breakdown [3]-[5]. For AC cross-linked polyethylene (XLPE) cables, online PD detection is used to determine potential defects in the cable, and high frequency current transformers (HFCT) are most commonly applied for convenient and noninvasive detection. Conventional PD measuring techniques have been specified in IEC 60270 [6], and cable factory tests are based on this method. When PD detection is conducted on cables in service, some special issues such as noise reduction and dynamic range have to be dealt with [7]. Complex electromagnetic interferences and noise may significantly affect detection results by reducing the detection sensitivity [8]. Some high frequency components of PD signals cannot be covered by the detection bandwidth of a measuring instrument because the upper limit frequency is not high enough [9]. One more challenge is that a long cable can cause significant attenuation of PD signals [10]-[12], which will decrease the signal amplitudes to be detected. Therefore, the PD measuring instruments for on-line detection for cables have to satisfy some additional requirements.