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AC electric air arcs in medium voltage (MV) distribution networks, including railway catenary, photovoltaic power generation systems, and traditional distribution networks, can cause insulation damage accidents. Although various studies have been performed on the macroscopic characteristics of the MV AC arc in the air, the research on the physical properties of the arc, such as temperature field and particle composition, is relatively lacking. This work deals with diagnosing the temperature and particle composition of the arc under laboratory conditions based on the arc generation and the moiré deflection diagnosis systems. Based on the experimental results, there are three typical stages in developing the MV AC arc: the initial, transition, and stable combustion. The temperature during the stable combustion of the arc is between 1500–2100 K, while the fluctuation period is half a power frequency cycle. The particle components of the arc are mainly composed of O2, N2, and O. Different from the DC arc, the AC arc exhibits a zero-crossing extinguishing phenomenon, while the gap temperature is still high after extinguishing the arc, and the insulation has not been fully restored. This research provides a method for diagnosing MV AC arcs in the air under laboratory conditions and initial values for the arc modeling.

A morphological filter is proposed to obtain a highaccuracy roundness measurement based on the fourparameter roundness measurement model, which takes into account eccentricity, probe offset, probe tip head radius and tilt error. This paper analyses the sample angle deviations caused by the four systematic errors to design a morphological filter based on the distribution of the sample angle. The effectiveness of the proposed method is verified through simulations and experiments performed with a roundness measuring machine. Compared to the morphological filter with the uniform sample angle, the accuracy of the roundness measurement can be increased by approximately 0.09 μm using the morphological filter with a nonuniform sample angle based on the fourparameter roundness measurement model, when eccentricity is above 16 μm, probe offset is approximately 1000 μm, tilt error is approximately 1″, the probe tip head radius is 1 mm and the cylindrical component radius is approximately 37 mm. The accuracy and reliability of roundness measurements are improved by using the proposed method for cylindrical components with a small radius, especially if the eccentricity and probe offset are large, and the tilt error and probe tip head radius are small. The proposed morphological filter method can be used for precision and ultraprecision roundness measurements, especially for functional assessments of roundness profiles.

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