Corona discharges caused by water droplets on the surface of polymeric insulators can affect the long-term reliability of the component by lowering the surface hydrophobicity encouraging surface discharges. It is well-known that droplet-initiated discharges behave differently under AC and DC field stress affecting the hydrophobicity of polymeric outdoor insulation materials. The presented investigations study the effect of hybrid field stress on partial discharges at singular water droplets. The behavior differs depending on the direction of the electric field as also known for uniform AC or DC fields. Tangential oriented field stress is more critical to aging phenomena due to a significantly lower pd inception level. By high speed analysis of the droplet deformation the oscillation frequency is linked to the acting electrostatic forces depending on the AC and DC component of the excitatory electric field. Thus, the mechanical oscillation model is extended for hybrid field stress.
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.
Keywords-outdoor insulation; pollution flashover voltage; hybrid AC/DC overhead line; articifical pollution testI.
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