| Gas-insulated switchgear(GIS)is the key equipment of power grid and is widely used in power grids.GIS is used in all the 1000 kV(ultra-high voltage,UHV)projects and there have been more than 60000 intervals of GIS in operation in the State Grid.Thus,the operation reliability of the GIS is very important to the safety of power supply.However,in recent ten years,the abnormal discharge fault of the insulator occurred frequently during field operation.Accurately determining the cause of the failure is difficult owing to serious ablation of insulators following flashover;thus,proposals for effective prevention measures are lacing.It is a serious concern of the equipment manufacturer and power operation departments and urgently needs to be further studied.In this paper,through fault statistics and investigation,it is found that small metal particle attached to the insulator surface may be the main cause of abnormal discharge.The particle length is 0.5 mm-5.0 mm and the operating voltage will not exceed 1.3 p.u.when the fault occurs.However,in the previous studies,the metal particle used is usually longer than 10 mm,and when the particles with smaller-length are selected,the applied voltage is 2.0 p.u.and even higher.Obviously,this does not match the field operational condition and cannot provide a satisfactory explanation for the actual flashover phenomenon in the field.In this paper,small particles are used to carry out systematic research on the induced discharge of metal particles on the insulator surface under operating voltage conditions.The main research work and innovative achievements are as follows:(1)The sensitivity of partial discharge(PD)pulse current method is analyzed and the measures to improve the sensitivity are investigated.A distributed pulse current measurement method is proposed,which can detect the PDs of multiple insulators simultaneously and identify the insulator with PDs in multi-insulators effectively.The detection sensitivity of the proposed method is 0.2 pC,which is 50 times higher than the conventional method.(2)The PD routine test system based on distributed pulse current measurement method was constructed.The PD factory screening tests of 62 basin insulators and 115 post insulators were carried out in the factory.The PD level of the insulators with intact surface condition is obtained,and the apparent charge is smaller than 0.1 pC,which indicates that the intact insulator will not induce surface discharge during operation.(3)The PD characteristics of metal particles of different lengths(?=0.5 mm,L=0.5,2,and 5 mm)at different positions(near the grounded electrode,at the middle of the insulator surface and near the high voltage electrode)on a real spacer surface were studied.It is revealed that the PD under the operating voltage of small particle is weak and sporadic,and the smaller the particle,the less the discharge repetition rate and the lower the pulse amplitude.When the length of particle is 0.5 mm,the maximum apparent charge is as low as 0.04 pC and the discharge repetition rate is as low as 3 times/h.(4)The movement characteristics of metal particles attached to the convex surface of horizontal insulator and concave surface of vertical insulator during switching operation are studied for the basin insulator used in the air chamber of GIS moving parts.It is found that the metal particles attached to the insulator surface are easy to move into the gap between the insulator and the GIS enclosure under the impact vibration.The movement of metal particles is simulated by a magnet,and the surface flashover induced by particle motion is studied.The occurrence and development process of surface discharge induced by particle motion under the operating voltage of small particles are revealed.It is found that the discharge repetition rate is less than:1 0 times/s,and the apparent charge of PD is low,less than 0.3 pC,when the metal particle is at the meddle of the insulator surface.In the process of particle movement,the discharge repetition rate and apparent charge have no obvious change.After the particles move into the gap between the spacer and the enclosure,the discharge repetition rate increases suddenly,which can attain and exceed 1000 times/s,and the apparent charge of PD also increases suddenly,which can exceed 50 pC.It is indicated that the movement of metal particles can lead to surface flashover.(5)The PD characteristics of metal particles of different lengths(?=0.5 mm,L=0.5,2,and 5 mm)at different positions(near the grounded electrode,at the middle of the insulator surface and near the high voltage electrode)on the real 1100 kV post insulator surface were studied,and the surface charge accumulation is also investigated using the dust figure technique.It is the first time to observe the phenomenon of charge accumulation on the insulator surface and the occurrence and development process of surface discharge induced by charge accumulation with small metal particle under operating voltage.It is found that 0.5 mm and 2.0 mm long metal particles do not cause charge accumulation,but 5.0 mm long metal particles can result in significant charge accumulation on the insulator surface.The length of charge accumulation extending to the electrode can reach 180 mm,which is much longer than the particle length.It indicates that the metal particles will cause significant charge accumulation as long as they attain a certain length.It is also found that in the process of surface discharge induced by charge deposition,the larger the apparent charge of PD is,the longer the charge accumulation extends to the electrode,and the greater the possibility of surface discharge is.It can be concluded that the accumulation and diffusion of surface charge generated by PDs of metal particles can lead to surface flashover.In summary,metal particle attached to the insulator surface may cause flashover during operation.The longer the particle length,the greater the possibility of flashover.The mechanism of discharge can be explained as follows:one is surface flashover due to the electric field distortion caused by metal particle movement;the other is surface flashover due to the charge accumulation caused by PDs. |
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