Practical frequency response characterization of a test circuit for partial discharge measurements

When digitally acquiring partial discharge (PD) signals, the bandwidth, the parameters of the measuring circuit and the arrangement of the test set-up play a role in preventing the signals from being affected by oscillations, distortions and noise. In this paper, a compact design of test set-up for PD measurements and a method for characterization of the frequency response of the measuring circuit is proposed. The frequency characterization was based on the induction of a fast current pulse in the closed HV loop of the measuring circuit. Frequency domain analysis and the use of a RLC model led to a practical estimation of the circuit inductance L. Experimental results showed that the resonance produced by the circuit inductance and the combined capacitance of the test object and the coupling capacitor C can produce resonance peaks within the bandwidth of the measuring PD sensor, which results in large oscillations. In addition, it was shown that adding a critical resistance Rc damps the oscillations, thus reducing the error in parameter computation.