Electric field relaxation technology for suppressing the local accumulation of electric field is commonly used in high voltage equipment that are used for electric power system and the like. One electric field relaxation method for high electric field portions involves dispersing a powder having current-voltage nonlinear resistance characteristics in a polymer matrix that is an insulator, and using this varistor powder/polymer matrix composite as a varistor.
For example, Patent Document 1 discloses a technique of using a polymer matrix in which is dispersed a ZnO varistor powder for the electric field relaxation of terminals or joints of power cables. Patent Document 2 discloses a technique of using a polymer matrix in which is dispersed a ZnO varistor powder for the electric field relaxation of polymer bushings. Further, Patent Document 3 discloses a technique of using a polymer matrix in which is dispersed a ZnO varistor powder for the electric field relaxation of lightning arresters.
Also, it is stated in Non-Patent Document 1 that the operating voltage of a varistor powder/polymer matrix can be controlled by baking a ZnO varistor powder used for electric field relaxation for 4 to 20 hours at a temperature of from 935° C. to 1320° C., or by changing the amount of ZnO varistor powder in the polymer matrix.    Patent Document 1: Published Japanese Translation of PCT Application No. 2000-503454    Patent Document 2: Published Japanese Translation of PCT Application No. 2004-522259    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-348596    Non-Patent Document 1: Strumpler et al., “Smart Varistor Composites,” Proceed. 8th CIMTEC Ceramic Congress & Forum New Material Symposium, pp. 15-22
Incidentally, increasing nonlinearity and raising the withstand voltage of a varistor powder are even more important with a varistor powder that is used for obtaining a varistor powder/polymer matrix (electric field relaxation composite) that can exhibit an excellent electric field relaxation effect in a high electric field portion.
Specifically, to improve the withstand voltage characteristics of an electric field relaxation composite, it is necessary to raise the operating voltage of a ZnO varistor powder, and to improve the electric field relaxation effect of an electric field relaxation composite, it is necessary to enhance the current-voltage nonlinear resistance characteristics of a ZnO varistor powder.
However, with the prior art discussed in the above-mentioned patent documents and non-patent document, the varistor powder had inadequate withstand voltage and nonlinearity, and when it was applied to a portion of extremely high electric field, the electric field relaxation composite had to be made thicker in the electric field direction, or the proportion of varistor powder (the electroconductive ingredient) in the electric field relaxation composite had to be reduced, in order to raise the withstand voltage of the electric field relaxation composite.
In this case, if the proportion of varistor powder (the electroconductive ingredient) in the electric field relaxation composite is reduced, this creates a problem in that the current-voltage nonlinear resistance characteristics deteriorate in an electric field relaxation composite containing a varistor powder with nonlinear resistance characteristics, and the electric field relaxation effect of the electric field relaxation composite is diminished.
Furthermore, even if the proportion in which the varistor powder is contained in the electric field relaxation composite is not reduced, it is important that the ZnO varistor powder have excellent current-voltage nonlinear resistance characteristics in order for the electric field relaxation composite to exhibit an excellent electric field relaxation effect.