Yasuaki Ishioka

Degradation of Insulating Materials of Transformers

Insulating papers and pressboards must have high dielectric strength and high tensile strength. However, these materials are gradually degraded due to thermal stress, oxygen, and moisture. Because the tensile strength decreases owing to degradation, the degradation of insulating material may affect the life of transformers. In our laboratory, through accelerated tests using models of oil-impregnated insulating systems, changes in characteristics of insulating cellulose materials have been investigated. These data were compared with data obtained from insulating papers of transformers with long service life. From these investigations, good correlation was found between the amount of gas generated from insulating papers in insulating oil and the retention of tensile strength and of degree of polymerization. Using this correlation, the degree of degradation of insulating papers in transformers may be known from the amount of gas. The average characteristic curves of insulating papers from transformers coincides with the degradation curve of 90 ° C ob tained in experiments. And if the life of insulating papers is considered to be equal to the life of transformers, the life of transformers is between 20 and 40 years.

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.

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.

General Properties of Mixtures of Paraffinic Insulating Oil with Alkylbenzenes

Mixtures of a paraffinic-base electrical insulating oil for oil-filled transformer use with a branchedchain type alkylbenzene with high viscosity, a branchedchain type dodecylbenzene and a straight-chain type dodecylbenzene were evaluated for improving the characteristics of a paraffinic mineral oil for transformer use. It has been demonstrated that blending the alkylbenzenes in the paraffinic oil brings about some improvements in its chemical, physical, and electrical properties. Moreover, it has been concluded that a modified paraffinic oil which contains 50% of the branched dodecylbenzene gives overall characteristics almost equivalent to the naphthenic transformer oil which is practically employed.

Study on decomposition gas for diagnostics of gas‐insulated transformers

Concentrations of population and business activities result in high energy demand in urban areas. This requires the construction of underground substations. Oil-free, nonflammable, nonexplosive equipment is recommended for underground substations. For this reason, gas-insulated transformers have been developed. A diagnostic method for gas-insulated transformers is thus required. This paper provides an experimental survey of the main components of decomposition gas generated by various faults in gas-insulated transformers carried out through simplified model tests. The phenomena of overheating and partial discharges are modeled, taking the actual materials related to each fault into account. For example, CO, CO2, and aldehydes are produced by overheating of pressboards and PET films. The amount of gas produced increases with rising temperature. While various gases are produced from a partial discharge, the principal components are SO2 and SOF2. These results will be used to develop a diagnostic method for gas-insulated transformers.

Heat-stability of mixtures of paraffinic transformer oil with alkylbenzene

Effects of blending alkylbenzenes in a paraffinic base transformer oil were extensively investigated, paying particular attention to their influence upon its heat-stability. Oxidation stability tests of the oil specimens, tube aging tests of Kraft paper/oil electrical insulation systems for oil-filled transformer use and accelerated aging tests of the distribution transformers consisting mainly of the same electrical insulation systems were carried out and their test results were compared with those obtained by use of an actually used, naphthenic mineral oil for oil-filled transformers. It has been pointed out that blending the alkylbenzenes in a paraffinic base oil improves its pour point, gassing tendency and oxidation stability to some extent. As a result of the tube aging tests conducted in a closed nitrogen gas atmosphere it has been clarified that blending the alkylbenzene in a paraffinic base oil improves the degree of deterioration of the Kraft paper, although the paraffinic base oil appears more stable than the alkylbenzene blended one because of having difficult access to oxygen. Moreover, judging from the results of the accelerated heat-aging tests for the distribution transformers, it has been demonstrated that the dissipation factor of the alkylbenzene blended paraffinic oil increases less than that of the pure one along with the aging period, and that this tendency is definitely delineated by the aid of the relationship between the dissipation factor and the neutralization value of the oil specimens. Thus, it has been concluded-that the alkylbenzene blended paraffinic oil is considered an transformer oil equivalent to an actually used, naphthenic mineral oil.