An Improved Calibration Method for the Measurement of Space Charge Inside Insulating Materials

Accurate measurement of the space charge inside insulating materials facilitates reliable assessment of insulation performance. In this paper, an improved calibration scheme based on the pulsed electro-acoustic (PEA) methodology is proposed for space charge measurement. First, according to the transmission-line model of the measurement system, the simulation results indicate that the acoustic impedances of the upper electrode and the sample may affect the recorded waveforms. Then, taking into account the actual situation that the acoustic impedances of the semiconducting electrode and the sample are different, an improved calibration method is presented by analyzing the propagation process of the acoustic waves. Comparison of the results further indicates that the material of the semiconducting electrodes may affect and lower the accuracy of the traditional PEA method. In contrast, the recovered results by the proposed method are more accurate and reliable. The improved method is thereby applied to calibrate the signals from polyimide (PI) samples. Combining with the simulation results, it is concluded that different semiconducting electrodes render sensible impact on the accumulation rate and process of the space charge. Furthermore, two groups of calibrated results under the same electric field are used to verify the effectiveness of the proposed method. Since the electrode material may be an influential factor in the measurement, it should be properly chosen according to the practical applied conditions of the insulation samples to achieve a better evaluation of the charge accumulation process.