Research On Flashover Process Of High Voltage Composite Insulator And Diagnosis Methods On IT

To ensure high voltage insulators’ working safely and reliably; it is valuable in both academic and practical field to develop research on both their flashover causes and prevent-flashover. This paper takes long rod suspension composite insulators online as objects of study, of whose unknown reason flashover causes and prevent-flashover methods based on UV imagery are researched to solve several key problems as followings:(1) Traditional insulator flashover mechanism theories all think the insulator flashovers are caused by the surface arc creepage. As the result, all the theories can’t explain the cause of flashover without arc trace, which is classified as’unknown reason flashover’. By summarizing the factors of the time, the place, the climate condition which are easy to cause’unknown reason flashover’, the material and type of the insulators, and the characteristics of insulator after flashover, this paper put forward a hypothesis that the ’unknown reason flashover’ is a kind of air gap breakdown between two electrodes of insulator rather than real surface flashover. The cause of the dielectric strength decline of the air gap is the electric field distortion caused by the space charge electric field which comes from particles partially dissolved in water nearby the insulator. To testify the exactness of the new hypothesis, the air gap between two electrodes of insulator is simulate by the air gap between a pair of rod-plane electrode with the same length. Finite-element analysis software ANSYS is used to calculate the electric field of insulator’s surface and air gap between two electrodes with or without the space charge electric field, and the feasibility of the simulation is testified by comparing electric field non-uniform coefficients. To calculate the electric field of bar-plate electrode air gap more accurately, a new long-gap discharge calculating model called’Leader Channel Plasma Transport Model’is put forward by summarizing both the advantages and disadvantages of the existing models. The plasma parameters in leader channel and electric field distribution are obtained by solving the model. Based on calculating the electric field, the influence to air gap electric field caused by space charge electric field is calculated by combining the space charge calculating model, and the conclusion that’If the space charge electric quantity achieved1012cm-3order of magnitude, the space charge electric field and the applied voltage electric fieldjnfluenced the air gap electric field together, which made the air gap electric field enhance, and the breakdown was able to happen in the air gap first’.(2) It is discovered by comparing the PF flashover(or breakdown) voltage and50%impact flashover(or breakdown) voltage of insulator surface and air gap with specific electrode structure with the same pollution level that the latter may be less than the former, which is caused by the influence of the different pollution condition between insulator surface and air, the nonuniform pollution on the insulator and so on. In other words, there is a certain probability that the breakdown occurs in the space channel. To prove this conclusion, put clean insulator into heavy polluted air gap, then, apply PF voltage on the insulator until the flashover(or breakdown) occurs. It is discovered that the probability of space breakdown under the experiment condition is7%. The biggest experiment error source is the difference between the pollution on the insulator surface and in the air around.(3) To measure the spacing distribution of plasma produced by corona discharge on the top of insulator and local arc discharge under atmospheric conditions, both Langmuir Probe Method and Spectrum Analysis Method are used to diagnose the plasma parameters. Exactness of the hypothesis is testified by comparing diagnostic and calculating results.(4) To improve the UV method whose defect is the lack of fault detecting criteria after imaging, prevent-flashover methods based on UV imagery are researched. UV radiation intensity is chosen as the UV characteristic parameter, and UV imager is improved to measure the UV radiation intensity. Using the new long-gap discharge calculating model which is deduced in part (1), the ’Three Relations’ among the three key parameters--plasma parameter, UV radiation intensity and electric field are deduced, which are ’relation between plasma parameter and UV radiation intensity’,’relation between plasma parameter and electric field’ and ’relation between plasma parameter and UV electric field’. Based on this, the prevent-flashover criterion of insulator based on UV imagery is obtained:When a FXBW10-70composite insulator under the initial running condition of terminal voltage0, standard atmospheric pressure, temperature27℃, humidity80%RH, ESDD0.3mg/cm2, NSDD2mg/cm2whose valid insulation distance achieves the min value can bear max terminal voltage, its UV flux density is3x10-9(W/m2), the corresponding electron average energy is2.47eV. If its UV flux density is higher than3×10-9(W/m2), the insulator is faced with flashover dangerous. The concept of plasma energy level is made to obtain prevent-flashover criterion of insulator based on it by building the relation among plasma energy level and the former three parameters, which makes better for field test.