A coupled electrothermal simulation approach is introduced.An ungraded 550 kV-system arrester is modeled including a nonlinear thermal conductivity model for the arrester air gap.A novel nonlinear heat conduction model for the radiative heat transfer in annular cylinders is proposed and successfully evaluated. Continuous operating voltage simulations provide a steady-state temperature distribution. Good agreement to a measured temperature distribution is obtained. Furthermore, a thermal energy handling capability of arresters can be investigated with the proposed arrester cooling rate. This parameter is computed from the simulations based on the IEC operating duty test with varying impulse intervals. The cooling rate does
not depend on the absolute temperature, which is found in the top segment. Rather, the middle segments experience a significantly stronger deterioration of the cooling rate for short
impulse intervals. This effect might lead to thermal instability originating in these segments for higher pulse intensities.