Hisashi Morooka

Recent trends of low-viscosity silicone liquid immersed transformers

In this paper we report on “verification of arcing in insulation fluid” and “fire resistance evaluation of insulation fluid” for accident spread prevention measures, as recent trends in low-viscosity (kinematic viscosity 20 mm/s2, (20cSt) at 25°C) silicone liquid immersed transformers. The “verification of arcing in insulation fluid”, which simulated transformer internal short-circuit accidents, indicated that protection coordination similar to that for oil-immersed transformers can probably be applied in silicone liquid immersed transformers as well, and also clarified the produced gas patterns. The “fire resistance evaluation of insulation fluid”, which simulated transformer disaster accidents, quantitatively evaluated the fire resistance and self-extinguishing performance of 20cSt silicone liquid.

Insulating system properties for generator stator coils

The novel mica-epoxy insulating systems for generator stator coils that cause less harm to the environment have been developed. The high-voltage stator coils are generally constructed with copper conductors and insulating layers. The insulating layers made from mica flakes and epoxy resins are widely used because of the excellent insulating and heat-resisting properties. The mica-epoxy insulating layers are generally manufactured by vacuum impregnation. It is required that epoxy resins have the appropriate viscosities for impregnation, and cured resins have high electrical, mechanical, and heat resisting properties. Present impregnating epoxy resins have about a months pot life. Therefore, we studied the chemical structures of the epoxy resins and the curing agents, and used the latent accelerator, and developed the new impregnating epoxy resin system that have a six-fold longer pot life than the present resin so that we could reduce the waste resins. Moreover, by studying the conditions of the coil manufacturing processes by using the Taguchi method, we optimized the processes for the new insulating systems, and applied them to a 390 MVA class generator.