In order to obtain the corona characteristics of electrodes under positive DC and AC composite voltages, the experiment platform of series circuit was established by using a stainless rod-plane electrode, in which rod electrode was applied with positive DC voltage, and plane electrode was applied with AC voltage. The change of ultraviolet photon counting was used as the corona criterion and the characteristics of corona inception voltages were studied. Besides, the influences of composite voltages and geometrical size on the electric field distribution were also investigated. It was found that, in the same atmospheric conditions and electrode arrangement, there exists the minimum corona inception voltage under a certain combination of AC and DC voltage. On the basis of the simulation results of electric field, the variation of composite electric field strength agrees well with that of corona inception voltage under composite voltages. The maximum electric field strength of electrodes decreases with the increase of rod radius or air gaps when fixing the composite voltages of AC and DC.

The impact of composite AC-DC voltage on characteristics of corona discharge in rod-plane air gaps

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.

http://ieeexplore.ieee.org/iel7/6979383/6995725/06995875.pdf

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

Flashover characteristics are closely related to the electrical field distribution on the surface of oil-paper insulation. In this study, the effects of electrical field and DC voltage components on the creepage flashover characteristics of oil-paper insulation under combined AC-DC voltage (CADV) are investigated using two electrode models. The results demonstrate that when the DC voltage component is large, the flashover voltage under the influence of strong parallel electric field component is approximately 1.2 times higher than that under the influence of strong normal electric field component. When the DC voltage component value increases under the influence of strong normal electric field component, the value of corresponding AC voltage component of the creepage flashover voltage exhibits a decreasing trend. It is believed that due to the strong normal electric field component, the electrons intensify the collision on the surface of the insulating pressboard. Under the influence of the DC voltage component and due to the presence of space charge on the insulating pressboard, the distribution shape of carbonized particles on the surface of the insulating pressboard after creepage flashover is different. Furthermore, the instantaneous energy release during creepage flashover under CADV is higher than that under AC voltage, and the greater the DC voltage component value, more severe is the damage to the fibers on the insulating pressboard surface.

Effect of Electric-Field Components on the Flashover Characteristics of Oil-Paper Insulation Under Combined AC-DC Voltage

Flashover characteristics are closely related to the electrical field distribution on the surface of oil-paper insulation. In this study, the effects of electrical field and DC voltage components on the creepage flashover characteristics of oil-paper insulation under combined AC-DC voltage (CADV) are investigated using two electrode models. The results demonstrate that when the DC voltage component is large, the flashover voltage under the influence of strong parallel electric field component is approximately 1.2 times higher than that under the influence of strong normal electric field component. When the DC voltage component value increases under the influence of strong normal electric field component, the value of corresponding AC voltage component of the creepage flashover voltage exhibits a decreasing trend. It is believed that due to the strong normal electric field component, the electrons intensify the collision on the surface of the insulating pressboard. Under the influence of the DC voltage component and due to the presence of space charge on the insulating pressboard, the distribution shape of carbonized particles on the surface of the insulating pressboard after creepage flashover is different. Furthermore, the instantaneous energy release during creepage flashover under CADV is higher than that under AC voltage, and the greater the DC voltage component value, more severe is the damage to the fibers on the insulating pressboard surface. 

The scale down models of pre-compressed mineral oil insulation system have been studied under surface flashover of the surface of insulation. The experimental investigations have been dealt with for conventional AC, DC and superimposed voltages of AC/DC at different electrode gap distances. Two types of models using cylindrical embedded electrodes and stuck electrodes on the surface are employed. The studies relating to different conditions of the insulation system namely dry pressboard, impregnated pressboard in air medium, impregnated pressboard in oil medium are detailed. The surface flashover voltages under AC, DC and superimposed voltages have predominant effect on small electrode gaps causing sufficient damage to the surface of insulation. The time to failure of insulation due to discharges are presented. The discharges are more severe on dry pressboard as compared to oil impregnated pressboard.

K. Dwarakanath

Papers

Surface flashover studies on precompressed pressboard insulation under superimposed AC/DC voltages