NTRODUCTION!he concept of mica based insulation and its application in igh voltage rotating machines has been developed and > p h i z e d within the last 50 years and it has reached a level .:here further improvements seem difficult to achieve. The .omponents of the main wall insulation are mica paper (about ;5 % by weight), resin (25 %) and glass fabric or other .:'pport materials (10 %) [ 11. Unfortunately all these materials which have excellent electrical insulating properties -have a datively low thermal conductivity.n turbo generators with direct cooling systems -where the %or bars have hollow conductors cooled by hydrogen gas or d e r -the heat generated in the conductor is transmitted :ircctly to the coolant and the thermal conductivity of the ,oin wall insulation has little influence on the efficiency of he cooling system. he situation is different in turbo generators with indirect .ooling systems, where the steel core of the stator is cooled by ydrogen gas or air and the electrical losses generated in the .onductors produce heat which has to pass through the main :,all insulation to the coolant. Indirectly cooled turbo ,tiierators are increasingly used because they do not require d i a r y equipment as used in direct cooling systems. The .aerator construction can he kept simpler and the operational diability and maintainability can be improved. However, the Aca insulation acts as a thermal barrier restricting the filciency of the cooling system and thus limiting the power ?nsity and capacity of the generator. The range of thermal mductivity of mica insulation is between 0.25 and 0.30
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