Impedance and nonlinear dielectric testing at high AC voltages using waveforms

This paper presents the application of a waveform technique that can determine the complex impedance and nonlinear response of dielectric composite films at high ac voltages using a data acquisition (DAQ) card and virtual instrumentation. The voltage waveforms are Fourier transformed from the time domain to the frequency domain to obtain the fundamental and higher order harmonic responses as complex phasor quantities. The specimen impedance is determined by performing complex algebraic calculations. It was found that the conventional fiber-glass reinforced epoxy resin laminates exhibit a flat impedance characteristic, nearly independent of voltage, up to near breakdown conditions. At near breakdown conditions, the second-harmonic response starts to decrease, indicating a dielectric softening of the material. At higher voltages, nonlinear dielectric behavior is dominated by the third-harmonic response. In contrast, the impedance of dielectric hybrid materials made of organic resins and high-dielectric constant ceramics decreases continuously with increasing voltage. The drop in impedance is accompanied by a reversible transformation from a dielectric to resistive character. The field-induced nonlinear dielectric effects are mainly due to polarization reversal of the high-k filler, which is manifested by a large third-harmonic response. The near breakdown conditions in such materials can be inferred from the second-harmonic response, which diminishes when the ac electric field erases the residual static polarization. The presented testing procedure represents a compatible extension of the existing standard test methods for dielectric breakdown but is better suited for testing thin-film materials with a high-dielectric constant. The results demonstrate that the voltage withstanding condition can be inferred from the impedance characteristic and the nonlinear dielectric response without ambiguity.