Study On Discharge Performance And Model Of Short Air Gaps With Strong Rain

According to the running conditions of UHV transmission lines, heavy rain poses asignificant threat to the safety operation of power systems. Although the dischargeperformance of air gap in rain conditions has been investigated, the dischargeperformance of air gap in heavy rain conditions is rarely considered. Therefore, thebreakdown performance of rod-plane and rod-rod air gap in heavy rain conditions aswell as the breakdown process and breakdown model has been maily investigated in themulti-function artificial climate chamber and field in this paper.AC and DC breakdown performance of rod-plane air gap with the separation of0.2-0.7m in heavy rain conditions have been investigated in the climate chamber. Theeffect of rain intensity, water resistivity and ambient temperature on the breakdownvoltage has been studied. It is shown that the AC and DC breakdown voltage of shortrod-plane air gap are decreased linearly with the increase of rain intensity and increasedlinearly with the increase of ambient temperature. The effect of water resistivity on thebreakdown voltage of short air gap is very small. AC and DC breakdown performanceof short rod-plane air gap in rain conditions have been studied in field. The results showthat the AC and DC breakdown voltage of short rod-plane air gap under natural rainconditions are decreased linearly with the increase of rain intensity. The decreasingpercectage is13.6%and9.3%as the rain intensity ranges from0to4mm/min. Basedon the test results in climate chamber and field, the correction method for the ACbreakdown voltage of short rod-plane air gap under heavy rain conditions is proposed. Itis shown that the difference between the calculation results obtained by this method andthe test results is less than5.7%.The AC breakdown performance of short rod-plane air gap in the presence of awater stream has been investigated for the first time. It is shown that the presence of awater stream decreases the breakdown voltage of short rod-plane air gap by40%compared with the dry conditions. The effect of velocity and intial diameter of waterstream on the breakdown voltage are obtained, and it is indicated that AC breakdownvoltage of short rod-plane air gap in the presence of a water stream has an expotentialrelationship with the wate stream velocity and a linear relationship with the initialdiameter.By using a high speed camera, the effect of electric field on the breakup characteristics of a water stream has been investigated. The results show that the criticalbreakup length of a water stream, the size of water droplets and the breakup timedecrease with the increase of applied voltage. It is found that water droplets formed bythe breakup of a water stream fall away from the axis with the applied voltage higherthan40kV due to the fact of the helix motion of the water stream under the effect of theelectric field. The mathematical relationships between the breakup length, the size ofwater droplets, the breakup time and the applied voltage are obtained, and the effect ofwater stream velocity and initial diameter on these relationships is analysed.Additionally, it is found that the number of water droplets and the radial velocity ofwater droplets are affected by the applied voltage. As the water stream velocity keepsconstant, these two factors would increase with the increase of applied voltage.The breakdown images of short rod-plane air gap in the presence of a water streamare captured with a high speed camera. It is observed that the spark, originating from thenozzle electrode, propagates along the water stream surface, and then transvers theremaining air gap. It is found that the water droplet in close vicinity of the tip of thewater stream elongates in the direction of the water stream and forms a projection with avery small radii curvature, which leads the spark to develop toward it. According to thisbreakdown process, the model to calculate the breakdown voltage of rod-waterstream-plane air gap has been established. The model consists of two parts, one is thewater stream-droplet gap and the other is the droplet-plate gap. It is shown that thedifference between the calculated results and the test results is less than12%.