| The breakdown characteristics of liquid medium is the key to affect the safe and reliable operation of the power equipment, which greatly influenced by the inside electric field distribution characteristics of the liquid medium. In high-voltage direct current transmission system, the liquid medium withstands the influence of the complex electric field, such as ac, dc, ac/dc composite, the polarity reversal voltage, especially in the converter transformer, oil-paper insulating under the influence of impulse voltage. Based on Kerr electro-optic effect in the impulse electric field of high speed measurement of liquid medium, because of the low sensitivity, it is typically used for larger Kerr constant liquid medium, as for transformer oil of smaller Kerr constant the electric field is difficult to measure. So, develop a new impulse electric field measurement system, grasp electric field measurement technology in liquid medium of the low Kerr constant, research methods of electric field measurement of liquid medium. Double light beam interference fringe method used in the electric field measurement, image processing to extract the accurate and effective spatial information to calculate electric field intensity is very important. Focusing on the present situation and the necessity of the interference fringe image processing, develop interference fringe image processing method and realize intelligent fringe image processing, it plays an important role to realize fringe image processing and minimize measurement errors.In the paper, a double light beam interference fringe measurement system of Mach-Zehnder Interferometry is built, and measurement sensitivity is high, and it can be used impulse electric field measurement of transformer oil, whose Kerr constant is so small. The method has some advantages of interference fringe method: fast response, non-contact, low interference, high speed measurement, high sensitivity etc. Develop corresponding trigger circuit to control the trigger of the measurement system. Interference fringe figure can be captured used by trigger circuit, and transient real-time measurement is realized.Interference fringe is extracted by SUSAN filter and two-dimensional derivative binary diagram method in interference image processing. First of all, some kinds of filter processing for interference fringe figure is researched, and the advantages and disadvantages of the various methods is explained by using quantitative and qualitative methods in this paper. SUSAN filter is chosen as the interference fringe image filtering method. In addition, the two-dimensional derivative binary diagram method and the refinement of interference fringe to extract the fringe center-line. And select a target fringe in the interference fringe image instead of the whole interference fringe image; extract the space coordinate information of target fringe.The measurement of impulse electric field is realized in a liquid medium of the propylene carbonate based on interference image processing. The results between theory field curve and electric field curve consistent, and the error coefficients of several groups of experimental results are mostly less than 10%. Besides, the measuring sensitivity is 7.2 times higher than that of single beam interference fringe method, and the sensitivity of double light beam interference method can also be improved. |
