Hiroaki Uehara

Barrier effect of treeing in composite insulating materials with heat-adhesive interfaces of different polymers

A polymer/polymer composite system is widely used in high-voltage equipment and power cables. The interface of this composite system plays an important role in the electrical degradation, such as electrical treeing or partial discharge, of the composite system. Therefore, it is important to study the effect of an interface on the electrical treeing of polymer/polymer-composite insulating materials. In this paper, we describe the tree growth and complete breakdown characteristics of specimens with a polymer barrier film layer molded in ethylene vinyl acetate copolymer (EVA), in which the interface of EVA and the barrier film is perpendicular to the electric field under an AC voltage. It is found that with a barrier film, the tree growth is retarded on the barrier film, which then punctures the film, or develops along the edge of the film. It is also found that the pressure of decomposing gas in the tree channels plays an important role in tree propagation.

Pre-breakdown conduction in polymeric films

Fundamental understanding of carrier mobility-related pre-breakdown phenomena in dielectrics provides insights into high field transport phenomena as well as associated aging and onset of charge injection instability. A system for measuring resistive current through a planar dielectric film during a linear ramp voltage to breakdown has been developed to address the limits of conventional steady-state approaches in which the sample typically fails prior to achieving steady state current at around sixty percentage of the breakdown field. With this technique, pre-breakdown conduction in polypropylene, polystyrene and polyethylene-terephthalate thin films with varying molecular structures, crystallinity, chain orientation were studied extensively under room and elevated temperatures. The ability to measure resistive current up to breakdown will advance the fundamental understanding of conduction mechanisms in polymeric dielectrics and provide a basis for material engineering for improved high field performance.

Three-dimensional tree simulation of composite system considering an interface perpendicular or parallel to the electric force line

We carried out a three-dimensional tree simulation of a composite system with an interface perpendicular or parallel to the electric force line based on a dielectric breakdown model (DBM) considering the growth probability. It was found that an interface perpendicular to the electric force line exhibits a positive barrier effect and that an interface parallel to the electric force line exhibits a negative barrier effect.

Directional properties of positive impulse tree propagation in oriented PET and PP barrier films molded in EVA

In this paper, we describe the directional properties of positive impulse tree propagation at an ethylene vinyl acetate copolymer (EVA) base polymer/oriented barrier film (polyethylene terephthalate (PET) and polypropylene (PP)) interface. It was found that, for biaxially oriented PET and PP barrier films, after a positive impulse tree enters a film, the tree develops preferentially in both directions of the machine direction (MD) and transverse direction (TD) drawings of the barrier film. It was also found that, for uniaxially oriented PP barrier films, after the tree enters a film, the tree develops preferentially in the direction of the MD drawing of the barrier film. These results indicate that the preferential direction of positive impulse tree propagation in oriented PET and PP films is closely related to the microstructures of oriented molecular chains in PET and PP barrier films.

Temperature characteristics of water tree propagation in a wide temperature range using XLPE sheets

Water tree propagation depends on temperature. However, water tree propagation in a wide temperature range remains to be elucidated. In this study, we investigated the temperature characteristics of water tree propagation from (−20°C to 80°C in cross-linked polyethylene (XLPE) sheets. The water tree observed has a radial structure with a fanlike or human-hand-like shape at low temperatures (−20°C∼room temperature), but a dense-ball-like or sphere shape at high temperatures (50°C ∼ 80 °C). On the basis of the results of our microscopy observation of a water tree, we considered water tree propagation in two temperature ranges: low temperatures and high temperatures. Furthermore, for the growth mechanism of water trees, we suggested that an electromechanical factor is dominant at low temperatures, whereas an electrochemical factor in addition to an electromechanical factor is dominant at high temperatures.

Propagation characteristics of water trees using full and half-cycle rectified voltage waveforms

In this paper, we describe the propagation characteristics of water trees in cross-linked polyethylene (XLPE) using full and half-cycle rectified voltage waveforms. The results show that water trees under the application of a positive full-cycle rectified voltage propagate considerably further than those under other waveforms, regardless of the voltage frequency and application time. Moreover, water trees under a negative half-cycle rectified voltage propagate more than those under a sinusoidal voltage.

Effects of ions and waveforms on water tree initiation and propagation

In this paper, we describe an investigation of the effects of ions and waveforms on water tree initiation and propagation utilizing a new water electrode method. The results indicate that ferric ions contribute to water tree initiation and propagation. Moreover, a water tree was initiated when the waveform of a pulse simulating an inverter waveform was applied for 2 weeks even at room temperature, in spite of no ac voltage application.

Three-Dimensional Fractal Analysis of Real Electrical Trees in Polyethylene

In this paper, real electrical trees in polymeric insulating materials were discussed from the viewpoint of their fractal characteristics. Experiments on real electrical trees were carried out in the needle to plane electrode geometry under ac voltage application. Samples in this investigation were made of cross-linked polyethylene (XLPE). Two methods of reconstructing three-dimensional (3D) patterns of real electrical trees were developed. One is the computerized tomography method (CTM) and the other is the serial sectioning method (SSM). The relationship between tree characteristics and 3D fractal dimensions of tree patterns was studied. The experimental results showed that the spatial tree patterns have a fractal characteristics and that the tree characteristics can be characterized by fractal dimensions.

Density of bulk trap states in polymeric films

Fundamental understanding of carrier mobility related prebreakdown phenomena in dielectrics provides insights into high field transport phenomena as well as associated aging and onset of charge injection induced instability. A system for measuring resistive current through a dielectric film during a ramp voltage to breakdown has been developed to address the limits of conventional steady-state approaches in which the sample typically fails prior to achieving steady state current at around sixty percent of breakdown field. Prebreakdown conduction in polypropylene, polystyrene, and polyethylene-terephthalate thin films with varying molecular structure, crystallinity, and chain orientation were studied under room temperature as well as elevated temperature. Space-charge-limited current spectroscopy is used in the data analysis to extract more information from the experimental data. Critical transport parameters, such as prebreakdown density of bulk trap states, were extracted to provide a quantitative basis for comparative study and material engineering for improved high field performance.

Polarity effect of water treeing using new water electrode method

In this study, we investigate the polarity effect of water treeing, as a first step toward applying an inverter waveform, in cross-linked polyethylene using a new water electrode method. The results show that water trees under the application of a positive full-cycle rectified voltage propagate considerably further than those under the application of a negative full-cycle rectified voltage or an ac voltage. Therefore, it is clarified that there is a polarity effect on water tree propagation. Moreover, it is also clarified that the effect of dc bias is small.