T. Umemura

Electric Conduction Phenomena and Carrier Mobility Behavior in Dielectric Fluids

Electric conduction phenomena and carrier mobilitybehavior in mineral base electrical insulating oiland various kinds of synthetic base dielectric fluidshave been investigated in the low voltage region andhave been compared with our results in these fluidscontaining various kinds of oil-soluble impurities.It was shown that the impurities bring about someionic conduction in the fluids, and that the carriermobilities of the fluids containing the impuritiesnot necessarily seem to be governed by Waldens rule,depending on the types of the fluids and the impurities.In contrast, the fluids with industrial puritiesobey the rule. Furthermore, it was suggestedthat an electric conduction mechanism in the fluidscontaining the impurities seems to be closely relatedto a carrier mobility mechanism in these fluids.Thus, it appears advisable that the electricalbehavior of the artificial impurities dissolved in thedielectric fluids should be dealt with apart fromthat of the natural impurities.

Morphology and Electrical Properties of Biaxially-oriented Polypropylene Films

BO-PP films have fairly different morphologies to those of non-oriented PP films. These morphological differences were found to originate from the orientation of the crystalline and amorphous phases. The orientation of the amorphous phase causes a pronounced decrease observed both in ionic conduction and in dielectric loss. Two types of BO-PP films were found to have fairly different morphologies: the tubular type seems to exhibit a network structure made of oriented crystalline zones which are linked with slightly entangled amorphous chains, while the tenter type shows a spherulite-deformed structure made of a highly oriented cyrstalline phase connected with elongated and highly oriented amorphous chains. Despite their dissimilar structures, the two BO-PP films studies were found to have generally the same dielectric behavior except for the activation energy of the conduction and the dielectric relaxation processes.

Impurity Effect of the Dielectric Properties of Isotactic Polypropylene

Two dielectric relaxation peaks, the crystalline and the primary relaxations, were observed in the temperature-dielectric loss curves in isotactic polypropylene. Among them the primary relaxation ß was studied in detail using different kinds of polypropylene films and their films purified with acetone. It was found that ß-relaxation consists of two peaks, denoted ß1 and ß2, being consistent with the dynamic mechanical characteristics. The lower temperature peak ß1 is assigned to the amorphous phase, which is rich in atactic chains, and the upper temperature peak ß2 is ascribed to an antioxidant contained in the amorphous phase which is rich in isotactic chains. The antioxidant also causes an increase in dielectric loss in the higher temperature region. The extraction of impurities from polypropylene films with acetone reduces the magnitude of the primary relaxation peaks.

Accelerated-Life Testing of Power Capacitor Dielectric Systems

The degradation behavior of various dielectric systems for power capacitor use is discussed. Accelerated life testing of prototype capacitors and their constituent dielectric materials are described in detail. The composite polypropy lene/kraft paper system was estimated to have a life time of more than 30 years in service and its degradation behavior was predominantly determined by that of the kraft paper. Moreover, the all-film type system has a longer lifetime than that of the composite system.

Phenyl Methyl Silicone Fluid and Its Application to High-Voltage Stationary Apparatus

Phenyl methyl silicone fluids of low phenyl radical concentrations and low viscosities are characterized by their extraordinary gasabsorbing property in high electric fields. Their physical, chemical and electrical properties were investigated and compared with those of dimethyl silicone fluids. The corona resistant characteristics and applied voltage vs. life expectancy relationships were studied, using model capacitors consisting of the silicone fluid and polypropylene film capacitor paper insulation system, and heatresistant transformer models consisting of the silicone fluid and Nomex Aramid paper. It was found that the phenyl methyl silicone fluids greatly improve the corona resistance property and the life expectancy in high voltage fields over the dimethyl silicone fluids, although both fluids are similar in their general characteristics. As a result, it is suggested that the phenyl methyl silicone fluids can be used to construct reliable high voltage stationary apparatus which is small in size.

Electrical Conduction in Synthetic Insulating Liquid

Electric conduction and dielectric loss of synthetic aromatic insulating liquid for capacitor use were studied. The dielectric loss was found to originate from the ionic conduction and to depend on the concentration of impurities in the liquid. The conduction phenomena of the liquid can be divided into two temperature regions. In the low-temperature region, below about room temperature, the temperature dependence of conductivity is governed mainly by ionic carrier mobility. In the high-temperature region, however, it is characterized by thermal dissociation 0of impurities as well as carrier transport process. It was also established from the identical activation energies for mobility and viscosity that the carrier mobility is governed by the viscosity of the liquid. When an artificial impurity is dissolved in the liquid, a reduction in activation energy of the conductivity is observed in high temperature and the conduction mechanism is mainly dominated by the carrier mobility; no appreciable change was obtained in the low-temperature region.

Maxwell-Wagner Dielectric Polarization in Polypropylene Film/Aromatic Dielectric Fluid System for High Voltage Capacitor Use

The chemical interaction between biaxially-oriented polypropylene film and diarylalkane has been investigated by studying the dielectric properties of the polypropylene film impregnated by the fluid. It was found that the ß dispersion in its dissipation factor-temperature relationship is related to the segmental motion of the amorphous phase existing in the film, and that the a dispersion is connected with the Maxwell-Wagner type dielectric polarization at the surface of the crystalline phase in the film. Furthermore, it was demonstrated that the dissipation factor at the ß dispersion temperature correlates well with the dissolution of the film into the fluid, and that the dissipation factor at the a dispersion temperature correlates closely with the oil absorption of the film immersed in the fluid.

Dielectric Behavior of Solid/Liquid Insulation System

Dielectric loss of the PP/oil system was decreased by high voltage application beofe the dielectric measurement. The extent of decrease depends on the magnitude and the duration of the prestress voltage. This decrease in dielectric loss is caused by trapping of the impurities on the interface of the PP/oil. We found that the dielectric loss tends to go back up to the initial value with time after the voltage has been removed. The experimental results can be interpreted in terms of trapping of ions if the following process are assumed: in the very low voltage region, ionic carriers are accumulated near the interface without trapping; and as the voltage increases, they are trapped near the interface, causing a decrease in ionic concentration in the thin oil layer.

Thermal-Aging Behavior of Bo-Pp Films

The variation in electrical properties and morphology of biaxially-oriented polypropylene (BO-PP) films with thermal aging is discussed, using two kinds of BO-PP films. It was found that the thermal aging behavior of those two types of films depends on their initial morphology. The thermal aging gives rise to an increase in the degree of cyrstallinity due to crystallization of a part of the amorphous phase in BO-PP films. Crystal re-organization was also in-dicated, resulting in the improvement of the crystal perfection and the thermal stability. The increase in crystallinity enhances selective morphological change and also selective degradation in the amorphous phase. The morphological change due to the thermal aging manifested itself clearly in dielectric and electrical conduction properties in the form of an increase in primary relaxation loss peak and an increase in ionic conduction loss.

All-Film Power Capacitor with Folded Electrode Foil

All-film type power capacitors with folded electrode foil have been developed and was clarified to have an excellent electric properties and also a reliable lifetime of more than 30 years in service field. According to the electric field calculation, the folded foil can reduce the electric field at the electrode edge and provide much higher rated electric stress than the plain non-folded foil, resulting in a size reduction of power capacitors with low dielectric losses, higher partial discharge inception voltage and also higher reliability.