Dielectric Spectroscopy under Extremely High Voltages

The current-integral method (namely, the Q-t method) applies a step-like high voltage to an insulating system and integrates the current response. The response is displayed as a function of time. It has an advantage of reducing noise effects and accurately measuring the conduction current that is normally very weak. The Q-t method proposed at present displays the voltage appearing between two electrodes in the time domain and estimates the displacement current and conduction current from its incline. The transient response of the voltage indicates the process of dielectric polarization, and the frequency response can be obtained by applying appropriate digital signal processing. In this study, to facilitate the Fourier transform, a leakage resistor is connected in parallel with the integrating capacitor, and a finite time constant is given so that the wave tail of the response converges to zero. By normalizing the obtained signal using the transfer function of the system, the results were estimated to be equivalent to the conventional current integration method with an infinite time constant. If the response is recorded over a sufficiently long time, the conduction current can be estimated from this result. At the same time, the frequency dependence of the impedance obtained by signal processing was evaluated. The impedance trajectory and phase spectrum suggest that the boundary between the transient response including the displacement current and the steady-state region of the conduction current can be approximately discriminated. In addition, it was shown that this methodology can estimate the frequency dependence of the complex permittivity under very high voltages.