The ageing of cast resin subject to repetitive combined voltages of low and high fundamental frequencies

Due to the increased unbalanced loading of the electrical grids, the penetration of power electronics and the increasing number of switching phenomena in low and medium voltage grids, it is obvious that electrical insulation systems are no longer subjected to only the 50 Hz, purely sinusoidal voltages. 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. 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 that there is indeed an accelerated accumulated ageing of the resin due to the combined fast transient pulses. This 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 measured for the superimposed fast transients.