The impact of repetitive combined voltages with low and high fundamental frequencies on the ageing of cast resin

Due to the increased unbalanced loading of electrical grids, the penetration of power electronics and an increased number of switching phenomena in low and medium voltage grids, it is obvious that electrical insulation systems are no longer subjected to only a 50 Hz, purely sinusoidal voltage. The existence of fast transients, switching impulses or in general combined voltages with at least two clearly distinguishable frequencies affects the ageing mechanisms and expected lifetime of electrical insulation systems. Some typical fast transients superimposed on the main 50 Hz voltage are generated in the laboratory by using a Tesla transformer and supplied to a commonly used cast resin insulated system. One of the most important conclusions confirms an accelerated accumulated ageing mechanism of the resin due to the combined fast transient pulses. This specific ageing is also modelled by an equivalent ageing effect using only a 50 Hz sinusoidal voltage having a crest value of 127 % of the highest peak value ever measured for the superimposed fast transients.