Research On Non-linear Conductive Characteristics And Mechanisms Of Polymer Composites Filled With Inorganic Filler

Because of the excellent mechanical properties and electric insulation performance, polymers such as polyolefin have been widely applied in the fields of electrical and electronic insulation. And blending modification technology of polymer makes it possible to develope functional insulating materials. Through adding inorganic semiconductor and ferroelectrics with non-linear conductivity and\or relative dielectric constant to polymer matrix, non-linear composite insulating materials can be prepared by blending technology, whose conductivity and\or relative dielectric constant changes with external electric field. These materials are also called intelligent insulating materials, for they can uniform the electric field distribution by self and then improve the dielectric strength of insulation structure. At present, they have been widely used in the anti-corona structure at the end of winding bar in large generator and the wrapping type terminals of distribution cable. In the future, more forceful and intelligent power grid will be constructed, which put forward the higher and higher requirements for insulation performance of power equipment. Therefore, these non-linear materials will have extensive applied space. For instance, they could be applied in the insulation structure of DC ultra-high voltage casing and cable. So experimental research on the conductive characteristics and influence factor of the composite materials formed by blending the polymer with non-linear inorganic filler has important engineering significance to guide the development and application of the non-linear composite materials. Meanwhile, study of conductive mechanism of non-linear composite materials has important theoretical significance to improve and enrich the theory of dielectric. First, the composite materials with non-linear conductive characteristics have been prepared by adding silicon carbide (SiC) and zinc oxide (ZnO) with different attributes to low density polyethylene (LDPE) matrix. Through the DC conductive characteristics measurement, it has been found that the filling concentration of inorganic fillers showed percolation characteristics, that is the filling concentration had no significant influence on the conductive characteristics of the composite when the filling concentration was low, and when the filling concentration reached or exceeded a certain critical concentration, the filling concentration had apparent influence on the conductive characteristics of the composite, and the conductive percolation characteristics of the composite become more obvious with the higher non-linear degree of inorganic filler. At the same time, in a certain filling concentration range, the relationship between the conductivity of the composite and the macroscopic average electric field presented straight line or broken line form in the double logarithmic coordinates and broken line form indicated that the non-linear conductive coefficient of the composite have changed at different electric field strength. And the critical macroscopic average electric field under which the non-linear conductive coefficient change will move to low electric field with increase of the filling concentration. The reasons for the phenomenon was that the inner electric field of the composite increased with increase of the filling concentration at the same macroscopic average electric filed, and the inner electric field was the direct factor to determine the change of the non-linear conductive coefficient. Furthermore, the conductivity and non-linear degree of the composites with nanometer silicon carbide as filler was higher than that of composites with micron silicon carbide as filler.Then, to further study the conductive characteristics of composites and lay a foundation for the conductive mechanism of composites as well as, it has been researched that the effect of temperature on the non-linear conductivity of composite materials. The results showed that the conductivity of composites increased with temperature, and in double logarithmic coordinates, the conductivity at high temperatures were higher than that at the low temperature regardless of the low field area or high field area, indicating that the conductance of composite materials exits thermal excitation always. Based on the above experimental study, combined with microstructure of the non-linear composite material and the classical theory of dielectrics, three-step conductance model of the composites have been proposed, namely carrier transited the interface through the filler, and then transited the interface through the polymer matrix and entered into the filler, the conductance process of composite can be completed by repeating this process. As the conductivity of filler was much greater than that of polymer matrix, the electric field within the polymer matrix and the filler surface field were the direct factor to determine the conductive mechanism of composite materials. According to the proposed three-step conductance model, combining with the relationship between the inner electric field of polymer matrix, the filler's interface electric field and the filler concentration and the applied electric field, the approximation theoretical formula between the conductivity of the composite and the macroscopic average electric field have been derived. The approximation theoretical formula qualitatively explained the relationship between the composite non-linear conductance and electric field strength, temperature, and filler particle size and filling concentration. Change trend of non-linear conductive characteristics of composites predicted by theory is consistent with experimental results.In order to explore the feasibility of changing macroscopic dielectric properties by regulating the microscopic structure of the composite material during the hot procedure, the electric pre-stress effect on non-linear composite materials has been researched. It has been confirmed the existence of the electric pre-stress effect on composite materials. Namely, its macroscopic electrical conductance property can be changed by exerting the electric field on the material during the hot procedure. Its function equates increasing the concentration of the filler.At last, the effect of non-linear barrier prepared by non-linear composites on the electrical tree growth and breakdown time of insulation was studied through experiments. Experimental results show that the growth of electrical trees can be effectively retarded by non-linear barriers and the life of XLPE insulation is extended. Meanwhile, simulation and analysis have been made on the electric field distribution at the cable terminal insulated by the non-linear materials. It is found that the electric field strength of the field control cone at the cable terminations has been reduced significantly when non-linear insulation is used as outside insulation of the traditional cable terminal structure. Results of experimental research on non-linear barriers and simulation and analysis on electric field at the cable termination with non-linear outside insulation lay the foundation for the application of non-linear insulation in the more insulation structure.