Experimental method for investigations on the influence of superimposed AC/DC field stress on the flashover performance of polluted outdoor insulators

In hybrid AC/DC overhead transmission lines outdoor insulators are exposed to superimposed AC and DC field stress. As the pollution flashover performance of insulators under pure AC and pure DC field stress differs, the impact of superimposition of field stress is not predictable and has to be investigated on experimental basis. A suitable test set up for such investigations based on simplified specimen geometry is developed. As the test bench uses hybrid AC/DC test voltage for hybrid field generation several protection measures concerning saturation of ferrite core based inductances, overvoltage and overcurrent are implemented. Experimental validation of suitability of the test set up regarding pollution tests is carried out and presented. Further, first pollution flashover tests under hybrid stress with homogenous pollution layer featuring constant Non-Soluble Deposit Density and varying Salt Deposit Density are performed, the results of which are presented as well. Applying hybrid AC/DC stress increases the peak flashover voltage compared to uniform DC stress. Depending on the AC/DC ratio of the electrical stress, the peak flashover voltage can also exceed the uniform AC peak flashover voltage.