Laboratory investigations of the electrical performance of ice-covered insulators and a metal oxide surge arrester

This review paper summarizes the main results of the laboratory investigations conducted at Tampere University of Technology (Finland) on ice-covered insulators and a metal oxide surge arrester (MOA). The most important factors affecting the dielectric strength of high voltage (HV) insulators in freezing rain conditions are the resistivity of the freezing water, the length and the number of icicles between insulator sheds and electrodes and the state of the ice deposit (i.e. how wet it is). The studies verify that most of the commercially available HV insulators may flashover even with a normal phase-to-ground ac voltage in rather light salty (or otherwise slightly contaminated), but long-duration freezing rain conditions. The ac strength of a clean insulator string under rime ice during melting period was approximately 75% of the ac strength in dry conditions. Accordingly, the ac strength of the contaminated (ESDD approximate to 0.3 mg/cm(2) NaCl) insulator strings during the melting period of rime was on average 65% of the AC strength in dry conditions. The icing of an MOA consisting of two or more units in series may have harmful effects on the electrical performance of the MOA. With ac voltage stress an unevenly ice-coated MOA may be thermally stressed due to the leakage current transition from the ice covering of one unit to the interior of another unit not covered with so much ice. With switching impulse current surges the residual voltage across an ice-covered unit rather easily causes an external flashover. This leads to a sudden increase in the surge current: with the MOA of two units studied the current: with the MOA of two units studied the current peak reached as much as twice the peak value in normal operation without ice covering.