Life expectancy of liquid-nitrogen impregnated, taped cable insulation

Tests are described on lapped taped cable models impregnated with liquid nitrogen to investigate discharge activity and insulation life. A number of polymer materials are discussed with an emphasis on high density polyethylene. For all the polymers a life expectancy-stress relationship of the form t Eⁿ = Constant is obtained. The exponent “n” is seen to be strongly dependent on the pressure of the impregnant and also the effects of tape thickness are discussed. Variation in the rate of rise of the applied voltage for polyethylene tape is seen not to affect the life expectancy provided the initial high discharges are withstood. The application of this information to cable design is discussed.