Study On Influence Of External Electric Field On Error Characteristics Of Capacitor Voltage Transformer

Capacitor voltage transformer(CVT) was easily affected by the electric field for having no electromagnetic shielding. With the decrease of the load and increase of voltage grade, capacitance of capacitive voltage divider became much smaller while its size increased gradually. Both of those caused prominent effect of the electric field on measurement error which was directly related to the accuracy of electric energy metering and the reliability of relay protection devices in power system. Therefore, to study the effect of electric field on the measurement error was of great significance to ensure safety and reliable operation of power system.(1)By analyzing the basic structure and principle of CVT, the thesis adopted two models to analyze the influence of stray capacitance on the error. 1)Through modeling the stray capacitance uniform distribution calculation, the paper respectively analyzed the influence of stray capacitance both to the ground and to the high side on the effect of measurement error to capacitive voltage divider. Meanwhile, the thesis deduced the error expression under this model and put forward three methods on error reduction. 2)To truly reflect the effects of stray capacitance on the error, the stray capacitance non-uniform distribution calculation model was firstly proposed. And according to the degree of model simplification, two methods were given. For the simple model, by deducing the function of stray capacitance expressions, the voltage distribution could be calculated with analytical solution. While for the complex model, the charge simulation method could be used to calculate the stray capacitance matrix, and combining with the node voltage method, the numerical solution could be obtained. The methods involved in both the two models could be applied to other high-voltage electrical equipment.(2)From the perspective of the circuit, the qualitative analysis was made upon the mechanism of the surrounding objects on the influence of CVT measurement error. The result showed that it was the combination action of three factors. The error increased with the increase of object height while with the increase of distance, the error reduced. Both of those were related with potential. The error was linear with the object potential.(3)Relevant simulation model was formulated by using finite element software(Ansoft Maxwell 14.0) from the perspective of the field to study the influence of measurement error caused by the height, voltage and distance of the surrounding object. The simulation analysis was also made in actual installation about the length, diameter and installation angle of high-voltage wire on the influence of the error, which could provide theoretical support for the actual installation and on-site guidance on error reduction. In order to provide a reference for the choice of the interphase distance, the simulation model of the three-phase CVT was made to study the influence of interphase distance to the measurement error.(4)Finally, the thesis set up the experimental platform to analyze the effect of electric field on CVT measurement error and the experimental results validated both theoretical analysis and simulation. At the same time, the causes of the error was also analyzed.