Masumi Fukuma

Simultaneous measurements of space charge distribution and external circuit current up to electrical breakdown in LDPE film

A space charge distribution and an external circuit current were simultaneously measured up to electrical breakdown in a 100 /spl mu/m thick low-density polyethylene (LDPE) film with a semi-conducting electrode under dc voltage by using pulse electro-acoustic (PEA) method. We estimated an electric field distribution and a conduction current from these experiments. An electric field distortion due to a positive charge accumulation was observed at room temperature near the breakdown field. At 60/spl deg/C and 90/spl deg/C, however, an electric field distortion due to negative charge was observed, but the electric field distortions were not so large even near the breakdown field. The estimated conduction current reached to about 10 mA/m/sup 2/ before the electrical breakdown in each measurements. The obtained results suggest that the LDPE film becomes quasiconductive for negative charge at the field near breakdown and that the electrical breakdown is affected by the conduction current rather than the maximum electric field within the sample in the LDPE film.

Decrease in Charge of Sharp-Edged Ellipsoidal Particles by Self-Discharge

The saturation charge of conductive sharp-edged ellipsoidal particles imparted by field charging is measured and found to be in agreement with the theoretical value. The saturation charge decreases, however, when the charging electric field exceeds a certain value. When the decrease in the saturation charge takes place, a corona discharge is observed from the particle at its one edge facing downstream in the ion flow. This discharge is referred to as the self-discharge. A theoretical initiation condition of the self-discharge is calculated, considering the field around the ellipsoidal particles. The theoretical initiation condition of the self-discharge is checked by an experiment to measure the electric field strength to initiate the decrease in the saturation charge of two conductive ellipsoidal particles with a = 3.5 mm focal length, ¿o = 0.068 eccentricity, and a = 1.87 mm, ¿o = 0.1. The theoretical initiation condition of the self-discharge for various sizes of ellipsoidal particles is also calculated to discuss the possibility of the self-discharge taking place with crushed or agglomerated particles.

Proposal of numerical analysis model of acoustic wave propagation and generation on PEA method

The pulsed electroacoustic(PEA) method has been widely used to observe space charge distribution in polymeric dielectric materials. In PEA method, a pulsed electric field is applied to specimen to generate an acoustic wave which amplitude is in proportion to the space charge profile. The acoustic wave is detected by piezoelectric device in the PEA system. The space charge profile can be reproduced by detected the acoustic wave profile. However, when a specimen has a different acoustic property from that of the upper electrode, detected acoustic wave contains other acoustic wave due to the difference of the acoustic property. The acoustic wave due to the difference of acoustic properties makes measurement error in the space charge measurement. The measurement error has been discussed by using theoretical equations of transmission and reflection. This method, however, could not succeed to clarify the spatial and transient acoustic phenomena. In this paper, a new numerical analysis model has been proposed to discuss the measurement error in metal-polymer composite dielectric materials in space charge measurement.

Two-dimensional structure of charge packets in polyethylene under high dc fields

The puzzling behavior of space charge inside solid dielectrics has been revealed by space charge measurement techniques. In particular, positive and negative “charge packets” propagating through the bulk with a repetitive character are thought to be related to prebreakdown mechanisms. Here, we investigated the space charge distribution in a lateral direction by using our recently developed two-dimensional pulsed electroacoustic setup and discovered that the structure of the charge packets is not homogeneous in the lateral direction. This could change drastically the common view as regards their generation and propagation mechanisms as well as their relationship to breakdown.

Computer analysis on transient space charge distribution in polymer

This paper presents the computer simulation of the transient space-charge distribution in degassed and non-degassed cross-linked polyethylene (XLPE) which was measured by a pulsed electroacoustic method by Li and Takada (1991). The simulation was made on the transport of both bulk-excited and electrode-injected carriers from anode and cathode by using a hopping model where the trapping and the recombination of the bipolar carriers are also taken into account. The following items are considered in analysis; the bulk-excited carriers, electron injection from the cathode, hole injection from the anode, carrier transport, carrier traps, trap densities and carrier recombination. The simulation was made by fitting the calculated results to the experimental data of the time variation of the space-charge distribution in XLPE. The results show that the carrier trapping and carrier injection are very important factors to explain the time variation of space-charge distribution in degassed and non-degassed XLPE. The cross-linked byproducts increased carrier mobility due to reduced trap site.<<ETX>>

Numerical analysis of dielectric breakdown in polypropylene film based on thermal and electronic composite breakdown model

DC electrical breakdown of polypropylene film was studied experimentally and theoretically. The temperature region was divided into two regions, and qualitative analysis of experimental results suggested an electronic breakdown in the low-temperature region and a thermal process in the high-temperature region. A novel breakdown model was proposed by introducing the electronic-breakdown criterion with thickness dependence in the bulk into the thermal-breakdown process due to the transient conduction current. The model could successfully explain the breakdown characteristics, not only in the low temperature, but also in the high-temperature region. This model suggested that the electronic breakdown occurs by hetero space charge at the anode at about 100 degrees C. Experimental results of charging and discharging currents and breakdown tests with different combinations of electrode metals at 100 degrees C supported the prediction of this breakdown model.<<ETX>>

Short interval space charge and external circuit current measurements in LDPE film in high temperature region

The pulsed electroacoustic (PEA) method has been used to measure space charge phenomena in polymers. We have developed a new transient PEA system, and observed space charge profiles of low-density polyethylene (LDPE) films at 1 ms intervals during breakdown teats at various temperatures. In order to clarify the relationship between the space charge and external circuit current at intrinsic electrical breakdown of polymer insulating films, we have developed an external circuit current measuring unit for the transient PEA system. This paper shows space charge dynamics and external current observed during a short time breakdown test of a 100 /spl mu/m thick LDPE film at 90 /spl deg/C. The experiments revealed that positive charge injection is one of the most important factors affecting the intrinsic breakdown of LDPE in the high temperature region.

Measurements of conduction current and electric field distribution up to electrical breakdown in LDPE film

In this report, we estimated a conduction current and an electric field within a sample from the time dependence of space charge distribution and an external circuit in a 100 /spl mu/m thick polyethylene (LDPE) film. The space charge distribution and the external circuit current were simultaneously measured up to electrical breakdown under DC voltage by using pulse electroacoustic (PEA) method. Electric field distortions due to positive charges were observed under high applied electric field. However the electric field distortions due to positive charge were decreased near the breakdown field. From the estimated conduction current, it shows the external circuit current was almost the conduction current near the breakdown field.

Measurement of acoustic property for signal recovery in PEA method

Recently, the acoustic-impedance microscope has been developed by the application of pulsed electro-acoustic (PEA) method for the biomedical sample observation. The acoustic-impedance microscope can measure the difference of acoustic impedance in thickness direction in materials. When measuring the space charge distribution of a polymer insulating material by using the normal PEA method, the acoustic waves to obtain the space charge profile are detected by piezoelectric device in the system.

Development of very short interval space charge measurement system on PEA method

In order to investigate transient space charge formation in dielectric material, some short interval space charge measurement systems were developed. However the interval of space charge measurement was limited by the frequency of the pulse generator and recording method of the digitizing oscilloscope. Recently, a low cost long memory digitizing oscilloscope and a high speed switching semiconductor are available by the development of semiconductor technology and computers. We have developed a 10/spl mu/s interval transient space charge measurement system. Using this system, 10/spl mu/s interval space charge distribution was measured in low density polyethylene (LDPE) film in 1ms duration. The space charge distribution up to electrical breakdown was also measured in a 100/spl mu/m thick LDPE film at 90/spl deg/C under very rapidly varying applied voltage to investigate space charge behavior just before breakdown in insulating polymer.