Masamitsu Kosaki

Electrical Conduction of Poly (Vinyl Fluoride)

The relation between d c conductivity evaluated from one-minute current after a voltage application and reciprocal temperature bends notably above 50°C supposedly due to an ionic layer build-up near an electrode and also due to the clean-up of charge carriers. The existence of the ionic layer is supported by the a c conductivity calculated from dielectric loss vs. 1/ T curve and also by the current maximum in the current-time curves after the reversal of applied voltage polarity. The apparent activation energy of conduction E c is about 23 kcal/mole. The mobility of charge carriers is estimated (µ≃2×10 -8 cm 2 /volt. sec at 102°C) from an initial slope of current-time curves assuming the clean-up effect of charge carrier and its activation energy E µ is obtained as 16 kcal/mole. The difference of E c and E µ may give rise to a discussion of the origin of ions.

Detection of Joule heating before dielectric breakdown in polyethylene films

Local Joule heating in polyethylene films was detected at room temperature before dielectric breakdown under DC field. When a high-temperature point appeared in a film, the conduction current increased with time. The final breakdown occurred at the point of the highest temperature. This concurrence of the breakdown point and the Joule heating point suggests that a thermal process takes part in the DC electrical breakdown of polyethylene films at room temperature and that there are weak points leading to the breakdown in the film where the current density is above the average. Almost the same results were observed for ethylene-vinyl acetate copolymer films, although the local Joule heating and the electrical breakdown occurred at lower electric fields than those for polyethylene films. >

New approach to diagnostic method of water trees

A novel method for the detection of high-field AC conduction loss current containing harmonics in polymeric insulating materials is presented. The method is based on the use of the unbalanced operation of a current-comparator-type capacitance bridge and the deconvolution technique of signal processing. This technique was experimentally applied to the diagnosis of water trees in CV power cables by detecting the waveform of the high-field AC conduction current of the cables with and without water tree degradation. The distortion of the current waveform measured for the water-tree-degraded cable appeared to be larger than that observed for nondegraded cables. This result might be related to the nonlinear electric field dependence of the AC conduction loss current around water trees.<<ETX>>

Evaluation of ethylene-propylene rubber as electrical insulating material for a superconducting cable

Mechanical and electrical properties of ethylene propylene rubber (EPR) pieces were studied in comparison with polyethylene (PE) in the cryogenic temperature region to examine the possibility of its use as solid electrical insulating material in superconducting cables. The mechanical properties of EPR are preferable to those of PE mainly because of its low shrinkage. The electric strength and dielectric loss tangent of EPR are comparable with those of PE at liquid helium temperature. Based on the encouraging results, an extruded EPR-insulated cable was fabricated and its 15-m length was cooled down to liquid helium temperature successfully. Partial discharge experiments at that temperature showed good electrical characteristics. This is a breakthrough in terms of the electrical insulation design of cryogenic cables. >

Solid insulation and its deterioration

Abstract The failure of power apparatuses and electronic devices usually occurs in an insulation part which sustains electric field rather than a conductor part which carries electric current. The performance of the solid insulation which is supporting conductors at high potential, therefore, determines the life of every apparatus and device. The capability of solid electrical insulation at cryogenic temperatures is generally excellent, and especially in the case of organic polymers. It can be named “electrical super insulation” from the criteria of electrical insulation designers at room temperature or above. These aspects will be reviewed in this paper.

High-field dielectric properties and AC dissipation current waveforms of polyethylene film

The authors have developed a new method for the detection of AC dissipation in dielectric materials under high AC field. Using this technique, high-field dielectric loss properties and their relation to AC dissipation current of low-density polyethylene (LDPE) films are measured. Near room temperature, tan delta of a LDPE film has small electric field dependence. In this temperature region, the dissipation current shows saturation around the voltage peaks in phase even with high-field application. At higher temperatures, however, the high-field tan delta tends to have a strong field dependence, and the AC dissipation current deforms and is nonlinear near the voltage peaks. The nonlinearity of the AC dissipation current decreases under the decrease of tan delta with the voltage application period. These results show that the large increase of tan delta in the high-field and high-temperature region is related to the loss due to DC-like carrier motion. >

Research and development of electrical insulation of superconducting cables by extruded polymers

This is a review to show the importance of electrical insulation in ac superconducting cables. An attractive superconducting cable has to be designed for the voltage range of 60-270 kV. Therefore, a reliable insulation design is imperative. Two types of insulation have been compared. One is composite insulation, namely laminar paper or plastic tape, impregnated with coolant. The other is solid insulation of extruded polymer on the conductor. The composite insulation has a lengthy history and, in the constructions to date, has been the insulation design used for superconducting cables. Some prototype superconducting cables with this type of insulation have been developed and successfully tested. However, the partial discharge in butt gaps may affect their long-term reliability. The solid insulation, on the other hand, can separate the coolant from the electrical insulation and can exploit the benefit of the super electrical insulation characteristics of polymers in the cryogenic region. Some attempts have been made to use this design with liquid nitrogen and liquid helium. One example incorporating extruded ethylenepropylene rubber (EPR) for insulation was found to satisfactorily go through the cool-down to a liquid helium temperature and to endure the simultaneous voltage and current tests. EPR, and possibly some other polymers, seem to be promising materials for solid insulation in the cryogenic region.

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.

High field dissipation current waveform of polyethylene film obtained by new method

Non-polar polymers, such as polyethylene (PE), polypropylene (PP) and so on, are widely used as insulators for power cable and capacitor. In ac high field region, it is known that carrier injection from electrodes is getting dominant, and then the dissipation current waveform shape begins to show a nonlinear property due to complex conduction mechanism, that is, charge injection, space charge effect, recombination and so on. To study the conduction mechanism under ac high field, a new dissipation current waveform observation method which enables detection of the harmonies with high accuracy is developed. By using low density polyethylene (LDPE) film, high field dissipation current waveform observations from room temperature to 90/spl deg/C are carried out. Through these series of experiments, very unique properties which indicate the space charge effect under ac high field in high temperature are obtained.

High-field dissipation current waveform in e-beam-irradiated XLPE film at high temperature

We developed a detection method of ac dissipation current waveforms combined with a fast rising ac ramp voltage application as a new technique to evaluate the high-field dielectric properties and the dissipation current waveform over a very short time range. This measurement was carried out on electron beam irradiated crosslinked polyethylene film. At high temperature the high field tan /spl delta/ and ac dissipation current show nonlinear electric field dependence and the dissipation current during decreasing field becomes larger than that during increasing field, suggesting the contribution of carrier injection from electrodes. The dissipation current waveforms in the high temperature region show the peak in the first part of each half cycle of a full waveform, and fast decrease in the latter part. This result is probably due to the hetero and home space charge formation caused by the injected carriers from the electrode.