Abstract
In recent years, the investigation of leader tortuosity in long air discharge is brought into focus, for it is the main factor that influents the 50% breakdown voltage and can be used to validate and test the discharge modes in the design of long air gap dimension. In order to find out whether the test data of the whole leader channels are accuracy to represent the leader propagation paths, a discharge test of a 4 m rod-plane air gap subjected to switching impulse voltage is carried out. And an observation platform is set up to synchronously record the voltage waveforms, the discharge currents, the leader propagation processes and the whole leader channels. A new method is proposed to subtract the final-jump part from the whole leader channel by identifying the leader tip height at the initiation of the final jump (LTH for short) from the CCD image. Then the test data of LTH and the leader propagation path are statistically obtained. And probability density functions (PDFs for short) of three different angles between consecutive segments of the leader propagation path are used to quantitatively describe the leader tortuosity. According to the analysis of the results, the mean value of LTH is nearly half the gap length in the gap axial direction. And an obvious difference can be discovered when comparing PDFs of the leader propagation paths with those of the whole leader channels. A conclusion to account for the difference is deduced that the leader paths during the final jump are much straighter than those during leader propagation. In the end, the present work recommends that the final-jump part should be subtracted from the whole leader channel when the test data of leader tortuosity are used in the numerical model for positive long air discharge.
Highlights
Air is the main insulation media in electric power grid
In the design of air gap dimension, care has to be taken in accurately evaluating the 50% breakdown voltage (U50%) and its statistical variation [1]–[3]
OF RECONSTRUCTION OF LEADER PROPAGATION PATH In order to efficiently obtain the whole process of breakdown discharge phenomenon, the impulse generator was set to a crest value of 1.3 MV, 6% higher than the measured gap U50% for a 4 m rod-plane electrode arrangement, to efficiently get a total of 17 recorded shots of the breakdown events
Summary
Air is the main insulation media in electric power grid. Relevant experiments and theoretical model analysis shows that leader tortuosity and its fluctuation are the main factors that influent the 50% breakdown voltage and its statistical distribution [4], [5]. Positive polarity and rod-plane configuration, which has some advantages in theoretical analysis and industrial application, is used as the most critical situation for an air gap from the dielectric point of view [6]–[9]. As early as 1970s, extensive laboratory work by Les Renardières group on positive long rod-plane air gaps depicts properly the chronological sequence of the breakdown discharge which consists of three main phases: first corona formation, leader progression and final jump [7]–[10]. While the corona front at the leader tip reaches the earthed plane, the final jump is initiated and breakdown is inevitable
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