Statistical Analysis of Critical Flashover Voltage for small airgap

Technical Report from the year 2011 in the subject Engineering - Power Engineering, grade: A, Atlantic International University (Niger Delta University), course: SAC 672, language: English, abstract: Knowledge of electric fields is necessary in numerous high voltage applications. Electrode geometries that are encountered in practice or under laboratory experimental condition are very many but the rod-plane arrangement with rod tip of various shapes (cone, needle, sphere, co-axial cylinder etc) are more common (Onal, 2004). These gaps are subjected to several types of voltage shape so that breakdown and withstand voltage have to be ascertained.Several models for explaining the observed characteristics of the fifty percent breakdown of long gaps have been put forward by many researchers. Some theories or models used experimental data to obtain numerical estimates for some factors. Even with the varying assumptions the results were similar because they were based on the same avalanche and streamer theories.Based on the critical flashover (CFO) voltage of long gap put forward by Paris (Begamudre, 2008) there was need to relate this study to small gaps. This was particularly necessary since calculation of small gap field distribution is easier because the controlling factors could be checked.The most important factors by which engineers describe the breakdown voltage of various gaps are the critical flashover and the withstand voltages. The critical flashover (CFO) voltage is the voltage that yields flashover of 50 percent of the number of shots given probability of voltage flashover (V50%).The withstand voltage which is critical to the designer could be determined in various statistical form.Due to large number of variables involved in insulation design especially the use of long gaps tends towards a statistical procedure because each of these variables has its own characteristics probability of occurrence either alone or in conjunction with other variables.Also the probability of most events occurring simultaneously is also remote. Therefore a flashover once in many operations is allowed. The most usual case is to allow one flashover in 100 switching operations. This could form the basis for design of airgap insulation.Some designers use 0.2% probability of flashover (1 in 500 operations). In insulation design and operation, the withstand voltage is of outmost importance. Thus, the low probability region of flashover, 0.1% or 0.2% must be obtained from flashover probabilities of higher values .It is necessary to note that the flashover probability of a given gap distance with voltage follow a nearly Gaussian or normal distribution.