Study On Discharge Characteristics Of Grading Ball/Ring To Ground Gap In Converter Station Under Rain Condition

UHV transmission project can achieve high capacity power transmission and cross regional power grid interconnection,and improve the security of power system stability.However,with the increase of the voltage level,the size of the grading ball/ring are also further increased.Under the rain,the surface area of the larger grading ball/ring has the effect of rainwater collection which can reduce the switching impulse voltage.So we have to choose the ball/ring as the research object,to study the discharge characteristics of the gap under the rain.The distribution of the raindrop on the surface of the ball electrode and its effect on the corona discharge process under the rain were studied.The study shows that the shape,size and position distribution of the rain will be changed by corona discharge,and the rain will reduce the corona voltage which also has a polarity effect,corona discharge intensity varies with the size of the ball,and increases with the increasing of rainfall.The results of electric field simulation show that the maximum field strength of the rain drop at the surface of the ball electrode can reach 7.7 times that of the drying.The results of positive and negative polarity calculating with the improved photo-ionization model are less than 7.4%and 5.3%respectively.The positive switching impulse test of the rod-plate and the ball-plate under the conditions of drying and raining with the gaps of 10-60cm is carried out.Research shows that:the rain causes the ball-plate gap positive switching impulse U50 reduced more than rod-plate gap;The influence of precipitation on rod-plate gap breakdown voltage is not obvious;When the rainfall is 0.5-3.9mm/min,the diameter of 10cm sphere electrode,the clearance distance 30cm and Uso decrease by 15.8%?32.6%compared with the dry condition.When the rainwater conductivity is 100?2000?S/cm,with the increase of gap distance,the Uso reduction of ball-plate and rod-plate gap decreases.The rain will make the discharge delay shorter than the trend under dry,and guide the trend of the development of discharge.The positive switching impulse characteristics of large scale grading ball/ring to ground clearance under UHV Engineering were obtained under the rain.The study shows that:the larger the grading ring size,the U50 reduces more greatly under rain;With the increase of rainfall,the U50 of the ball to ground gap is reduced and the impact of the size of ball on the breakdown voltage is not obvious.When the rain is 2.25mm/min and the ball to ground gap is 4m,in the process of discharge the velocity of leader is about 3.2cm/?s,and there will be a negative up-leader before the breakdown.When the corona starts,the current value is about 2.4A,and the duration is about 1?s.Before the leader start the current pulse is about 1.7A.Before the gap breakdown,the amplitude of the current pulse is about 2.8 A,which is greater than the corona onset and the leader initial current.Finally,an improved predischarge model is proposed based on the test results,and the calculation error is less than 6%.