Calibration Of Capacitive Probe For Insulator Surface Charge Measurement

In GIS, an insulator surface is the weakest part of a solid-gas insulation system. Knowledge of the insulating properties of solid insulator surfaces and the influence of their profile is thus very important. A major factor that influences the dielectric strength is the charge deposited on the surface of the solid insulation. These surface charges accumulated on the surface of dielectric may reduce the flashover voltage of an insulator specimen. So it is very important to get the information about the charge distribution on the surface of insulator.In the field of high voltage, capacitive probe method is one of the basic methods for evaluating surface charge distribution. Based on the capacitive probe theory, the main factors such as thickness of insulator, the clearance between the probe and insulator surface, charges accumulated on the inner surface of insulator, volume charge density of insulator and etc. that influence surface charge accumulation are discussed in the present paper. Theoretical analysis shows that the relationship between the charge induced on the probe and surface charge on the insulator is very complicated.Based on 3-D surface charge method, the probe response matrix is presented. Employing the theory of Takuma, the electrical field calculation times is reduced from N2 to N. An indirect formula to get the accurate integrals for 3D constant potential elements is proposed. Compared with Gauss integration method, this new method is simpler; furthermore, nearly singular and singular integral can also be evaluated by this formula.In this paper, an image processing technique is introduced for the calibration of surface charge measurement. Based on this method, the relationship between the surface charge density and the probe output can be treated in the spatial frequency domain through Fourier analysis. The Wiener-filter technique is used to suppress the excessive amplification of the noise during the calibration. The performance of the method is compared quantitatively with conventional calibration method in terms of the solution time and accuracy. The results show that this new method is more accurate than the previous method and the time taken by the calibration is also less than that of the previous method.