Resolution Improvement in Space Charge Distribution Measurements using Step-like Voltage

To observe space charge distribution directly on an oscillogram without deconvolution, space charge measurements for 1 mm thick LDPE sheet are performed not with the very short pulse voltage but with the step-like voltage. In this system, a transducer consisting of a PVDF and an amplifier acts as a CR high-pass filter. The impulse response can be monopolar pulse because it corresponds to the differentiation of the pressure wave generated by the step-like voltage e(t) with a very long tail. In this paper, the influence of the rise time of the step-like voltage and the thickness of the PVDF sheet on the impulse response is experimentally studied. The half-width of the impulse response becomes short with reducing the rise time because de(t)/dt increases as the rise time decreases. On the other hand, the half-width cannot be short by reducing the PVDF thickness. This is because the cut-off frequency of the CR high-pass filter becomes low with decreasing the PVDF thickness, so the half-width for the 9 µm thick PVDF becomes longer than that for the 30 µm thick PVDF. We conclude that this method, with appropriate controls for the rise time of the step-like voltage and for the cut-off frequency, is very convenient for direct observation of the space charge distribution on a thick sample such as power cables.