Transient behaviour of helium-cooled current leads for superconducting power transmission

Based upon time dependent differential equations, numerical calculations are presented which allow, first, the determination of burn-out conditions for a helium cooled current lead. Secondly, the response of the lead to an overload current may be examined, both during the overload and during subsequent time when the normal current again persists. Representing the temperature dependent resistivity and thermal conductivity of the copper conductor as a function of its purity (residual resistivity ratio), the results are parameterized by dimensionless mass flow rate, current and residual resistivity ratio. Defining the burn-out temperature as 500 K and an operating current as 85 per cent of the burn-out current, it is shown that although all residual resistivity ratios between five and 100 are suitable, there is a limit on the mass flow rate number which can be utilized. This results from secondary temperature surges occurring after the overload, the conditions for which are rather complicated. It is concluded in particular that, within the permissible range of mass flow rates, no damage should result from a 10X current overload for periods up to 0.2 seconds. (author)