A three-dimensional physical model for calculating the corona onset voltage of a large-diameter alternating-current shielding ball under different ambient conditions

Abstract

For an alternating-current (AC) shielding ball with a large radius of curvature, a three-dimensional (3-d) physical model for the corona onset voltage (COV) of the large-diameter shielding ball under different ambient conditions was established according to numerical calculation in concert with the theory governing gas discharge. At first, distribution of the spatial 3-d electric field around the shielding ball was calculated by using the ring charge simulation method. Then, a calculation criterion suitable for the COV of the shielding ball was derived and the area factor of the geometric absorption function for photons applicable to such structures was proposed in detail based on the theory of secondary photoelectron emission. Moreover, tests on COVs of shielding balls with different diameters were conducted in an extra-high voltage (EHV) AC test-base. The test results show that the model was found to show a favourable effect in calculating the COV of a large-diameter shielding ball under different ambient conditions. Finally, in combination with the thickness of the ionosphere and effective ionization coefficient, the influence and mechanism of action of various parameters (such as atmospheric pressure, humidity, and temperature) on the COV of the shielding ball were investigated.