Yuzuru Kamata

Development of low-permittivity pressboard and its evaluation for insulation of oil-immersed EHV power transformers

The development of a low-permittivity pressboard obtained by blending polymethylpentene fiber with cellulose fiber is described. The permittivity is 3.5, which is lower than that of conventional pressboard. With this pressboard, insulating characteristics of intercoil models were investigated under lightning impulse and AC voltage conditions. It is demonstrated that a spacer fabricated from this pressboard material increases the partial discharge inception and breakdown voltages up to 30% compared with conventional pressboard spacers. >

A singular flashover path observed on the surface of synthetic-resin-bonded paper cylinders immersed in transformer oil under switching impulse voltage conditions

In order to clarify basic surface flashover phenomena in transformer oil under switching impulse voltage, the flashover characteristics on four kinds of large insulating cylinders are studied for positive lightning impulse of 1.2/50 mu s, positive switching impulse of 500/3600 mu s, and 50 Hz alternating voltage. The insulating cylinders are made from material containing about 40 wt.% phenol resin and 60 wt.% kraft paper. A singular surface flashover path is observed under switching impulse voltage which includes a partial jump of the discharge channel around the HV electrode. The peak value of the flashover voltage for the switching impulse is nearly equal to that of a 50 Hz voltage; this was not expected before the experiment, because breakdown voltage of switching impulse for oil-immersed insulation is generally intermediate between the lightning implulse and AC voltage. The singular flashover path of the switching impulse is explained by an electric field analysis taking account of the remaining charge of 100 pC/mm/sup 3/ which has the same polarity as the applied voltage and is spread out along about 10 to 40% of the distance between the HV electrode to the grounded electrode. >

Development of low dielectric constant pressboard

High breakdown strengths of oil-impregnated insulating materials are expected and the insulating design is improved if the dielectric constant of cellulose material is close to that of insulating oil. This paper presents a low dielectric constant pressboard which can be obtained by blending polymethylpentene fibre with cellulose fibre. The dielectric constant is 3.5, which is a very low value compared with the 4.7 of conventional pressboard. By using the new pressboard, decreased insulating distances and smaller transformer sizes are expected.

Dielectric breakdown strength of GFRPs under mechanical stresses in cryogenic liquids

The dielectric strength of GFRPs (glass-fiber-reinforced plastics) under combined application of electrical and mechanical stresses was investigated in liquid He, liquid N/sub 2/, and silicone oil at room temperature. With the combined application of tensile and electrical stresses in cryogenic liquids, the dielectric breakdown of GFRPs occurred prior to mechanical breakdown when the applied electrical stress was lower than the DBS (dielectric breakdown strength) of GFRPs without tensile stress. In the case of tensile stress application under constantly applied electrical stress, the DBS and GFRPs decreased markedly. The reason for the large DBS decrease of GFRPs was the occurrence of microcracks in matrix resins because of their brittleness at cryogenic temperatures. Combining GFRP with polyimide film in cryogenic liquids lessened the decrease in DBS. Compared with tensile stress application, the influence of compressive stress on the DBS of GFRPs was very small.<<ETX>>

Mechanical Properties of Low Thermal Contraction GFRP

Liquid helium duct spacers in large superconducting magnets must have high compressive strength to withstand large electromagnetic forces. They must also show a low thermal contraction, in the direction perpendicular to the laminations, to maintain windings stability at liquid helium temperature. To attain higher glass content or higher density, we adopted glass roving cloth with mineral fillers and unidirectional woven glass cloths. E-,S-, and R-glass cloths were examined as reinforcements and epoxy and bismaleimide triazine, as matrix resins.

Developmetn of transformer insulation utilizing new low-permitivity pressboards

A pressboard providing both low permittivity and good mechanical properties is required as for insulating material in oil-immersed transformers insulation. Using a newly developed pressboard, insulation characteristics of intercoil models are investigated under lightning impulses and AC voltage conditions. it is demonstrated that a spacer fabricated from the new pressboard material (permittivity of 3.5) increases the partial discharge inception and breakdown voltages up to 30%, compared with those of conventional pressboard spacers (permittivity of 4.7).