| As the ultra-high voltage DC networks are rapidly developing in China, the demand of utilizing gas insulated equipment such as gas insulated transmission line(GIL) and gas insulated switchgear(GIS) in DC system is becoming more and more strong because of their advantages like compact, environment friendly, stable, etc.. Different from AC field, DC electric field doesn’t change direction. It will cause charged particles in the field moving along the field lines and accumulating on the surfaces of dielectrics. When the quantity of accumulated charges reaches a certain level, it will cause the distortion of the original field and will influence the insulating properties of the dielectrics. As the voltage level are getting higher and higher, surface charge accumulation on insulator has become one of the most important problem which restricts the utilization of gas insulated equipment in DC system. To solve the problem, the accumulation and decay behaviors of the surface charges and its mechanism should be studied firstly. In this thesis, the accumulation and decay behaviors of the surface charges are studied. The main work and achievements are as follow.① A measurement and experiment platform for the study of surface charges on polymer dielectric surface under different gas environments is designed and constructed. Firstly, according to the common features of real insulators, the experimental model witch include screen electrodes are designed. As the simulation shows the field features of the experimental model are consistent with the real ones. Secondly, a measurement system with 3-D movement control is designed to solve the problem in surface charges measurement. Results show the system can provide precise measurement witch has laid a solid foundation for the further experiment.② The dynamic characteristics of the surface charge accumulation are studied in experiments. Utilizing the experiment platform, charges accumulated by direct charging, flashover charging and corona charging are studied. The surface charge distributions are acquired using the charge simulation method. The viewpoint of ambipolar accumulation is proposed to reveal the the microscopic features of accumulated charges. The results show under different charging methods, surface charges are main from gas discharge. The polarity of accumulated charges is consistent with the voltage polarity. The charges with opposite polarity on the surface are not formed during the main charging process. They are formed by a secondary discharge witch is caused by the former accumulated charges.③ The dynamic surface charge decay characteristics under different conditions are studied. It is found after different charging processes, the charges will decay in different rate. Charges accumulated by flashover will decay more rapidly than by direct charging. Charges accumulated by intense corona will decay more rapidly than by dim corona. That is to say charges accumulated in ambipolar process will decay more rapidly than unipolar ones. During all decay process, local charge density increase can only be found on samples treated by flashover of intense corona.④ The influence of temperature on surface charges and its mechanism are studied. When the temperature rises, the energy barriers of charge traps on dielectric surface will drop. Charges accumulated in ambipolar accumulation will de-trap and neutralize with opposite charges around them more frequently. Charges accumulated in unipolar accumulation have much more less opportunity to neutralize. It results in that ambipolar accumulated charges are more sensitive to temperature.⑤ The influence of surface charges on surface flashover and its mechanism are studied. Results show direct charging and dim corona charging have no effect on flashover. Flashover charging can raise and stabilize the subsequent flashover voltage. Intense corona charging will make the flashover voltage drop at room temperature. As temperature rise, the effect decreases. Net accumulated charges influence the flashover by modifying the original field. Ambipolar accumulated charges influence the flashover by interact with charges in the flashover path. |
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