| Owing to the special electrical characteristics of liquid dielectrics,which are between those of gas and solid,research on its discharge characteristics is always scientific hots pot.However,there are masses of factors influencing the liquid discharge process which includes physical and chemical processes from micro to macro.So far,there are still lots of liquid discharge phenomenon and mechanism without being analyzed fully.Therefore,it is quite necessary to study the law of liquid discharge further on theory and experiment.The development process of liquid discharge is quite complex,but experimental results show that liquid discharge process produces a strong optical signal,that its discharge channel is random and that the whole structure of the discharge has a fractal characteristics.Branching phenomenon in liquid discharge channel is studied and analyzed through fractal theory and spectral method.In this paper,one mathematical model describing the branching phenomenon in liquid medium discharge channel,the subject investigated of whose was pure transformer oil,was built on the basis of discharge and fractal theory.Evolution process of discharge path on a needle-plane electrode in transformer oil was numerically simulated,the whole process was studied,in which discharge channel was triggered from branches,branches grew up into a tree crown and liquid dielectric was broken down caused by needle-plane gap,and the effects of development probability index and applied voltage on the development process of the discharge channel,using this model.As is shown by results,fractal dimension of discharge image decreases with the growth of development probability index;The longer the time is,the longer the breakdown path and the discharge channel are,the more the quantity of electric charge in the channel is,and the faster the velocity of streamer development is;With the growth of the external voltage,fractal dimension of the channel,the quantity of electric charge and the average velocity of streamer development are increasing,while discharge breakdown delay is decreasing,these facts are consistent with the experimental results in literature reported.In order to gain feature information of spatial form evolution of liquid dielectric discharge channel and to determine the value of development probability index in the parameter of discharge branches,experiment system for studying dielectric liquid discharge was built and discharge information of the needle-plane electrode in transformer oil under DC high voltage was collected.Simulation model was verified and completed by means of observing development path of the discharge channel and calculating fractal dimension.What is shown by results is as follows : The facts are demonstrated that discharge channel is not only along the axial line,but it develops brightly and irregularly into a highly conductive one with a number of branches;The similarity between simulated images and experimental images is very high,and the fractal dimension of experimental images in the range of 1.16-1.58 coincides basically with the fractal dimension of simulated images in the range of 1.2-1.46,which verifies the rationality of the branching model for liquid dielectric discharge channel;When development probability index ?=0.02,simulation results are closer to experimental results;It can be inferred by spectral analysis that the production of hydrogen ions and carbon ions is much higher than that of any other metal ions;The number of photons radiated during discharge increases with the growth of applied voltage leading to the branching phenomenon of discharge channel stronger,which confirms the conclusion simulated. |
PDF Full Text Request