High performance broadband capacitive coupler for partial discharge cable tests

Partial discharge (PD) cable field tests require the use of high frequency sensors coupled to the cable for PD apparent charge evaluation. PD sensors can be coupled to the cable by magnetic field, like high frequency current transformers, or by electric field, like capacitive couplers. If high sensitivity and high bandwidth are required, then capacitive couplers need to be employed since high frequency current transformers exhibit a limitation concerning bandwidth and gain. This paper summarizes some capacitive couplers topologies, their drawbacks and their benefits, and finally proposes a topology based on high voltage capacitors and operational amplifiers. The topology described in this paper allows obtaining a capacitive coupler with a low cutoff frequency in the range of a few kHz and a high cutoff frequency in the range of tens of MHz. The bandwidth achieved with this topology makes possible a reliable PD pulse wave shape record, useful for mathematical post processing, and exhibits a good sensitivity, improving PD field test measures, in both off-line and on-line tests.