As energy demand, voltage level and operating temperature increases, technical demands on compact insulation design have incresed. The electric field utilization factor in the actual electric power facilities, is uneven due to the structure and shape of the conductor, so electric field grading techn...
As energy demand, voltage level and operating temperature increases, technical demands on compact insulation design have incresed. The electric field utilization factor in the actual electric power facilities, is uneven due to the structure and shape of the conductor, so electric field grading technology is required. In order to reduce local high electric field, electric field relaxation technology is applied to high voltage insulation systems. The types of electric field relaxation technology are as follows ; first, the electroded grading second, condenser grading third, the permittivity fourth, resistive grading. Since such electric field control techniques have different advantages and disadvantages, applicable field control techniques are limited depending on the target power device. In recent years, two novel electric field control techniques (FGM: functional grading material) have received attention. In the first approch, the a functionally graded material is a material in which the magnitude of the dielectric constant continuously changes in one direction. In the other approch, the conductivity (resistivity) increases exponentially with the electric field. This material is made of a composite material by filling SiC (silicon carbide) having nonlinear I-V characteristics as an additive in a base material such as epoxy resin. SiC/epoxy composites have a problem of large leakage current in a low electric field and relatively low nonlinear coefficient in high electric field. In order to improve these problems, in this study the ZnO microvaristor/epoxy composites, which have recently been attracting attention as electric field relaxation materials, were fabricated and their electrical conduction and dielectric properties were measured. The results are summarized as follows. First, the electrical conduction characteristics of ZnO microvaristor/epoxy composites showed a lower leakage current and a larger nonlinear coefficient than the conventional electric field relaxation materials. The nonlinear conduction characteristics of the inorganic/epoxy composites were confirmed to depend on the kind, content and dispersion of the filler in addition, the main nonlinear conduction mechanism was confirmed by electron hopping or tunnel effect. Second, the dielectric properties of the ZnO MicroVaristor/Epoxy composite showed that the real part ε' and the imaginary part ε“ showed a temperature dependence and gradually decreased with increasing frequency. This result can be understood as Maxwell-Wagner-Sillars (MWS) theory. In the case of specimens filled with ZnO microvaristor at high sintering temperature, the ε' and ε“ increased. The increase of ε' is considered to be due to the decrease of the grain size in the grain of the ZnO microvaristor filler, while the increase of ε“ is thought to be due to the increase of the conductivity due to the decrease of the intergranular in the ZnO microvaristor particle. Third, the circuit simulation using the Maxwell-Wagner capacitor model, GIS spacer insulation design and electric field simulation on bushing with new field relaxation material are performed to verify the effect of field relaxation. As a result, it was confirmed that the electric field relaxation effect of ZnO microvaristor/epoxy composites with higher nonlinear coefficients was excellent than those of conventional field relaxation materials. In summary, the important point of this study is that ZnO microvaristor/epoxy composites have low leakage current and high nonlinear coefficient in lower electric fields than conventional nonlinear resistive materials, so electric power loss and electric field relaxation effect.
As energy demand, voltage level and operating temperature increases, technical demands on compact insulation design have incresed. The electric field utilization factor in the actual electric power facilities, is uneven due to the structure and shape of the conductor, so electric field grading technology is required. In order to reduce local high electric field, electric field relaxation technology is applied to high voltage insulation systems. The types of electric field relaxation technology are as follows ; first, the electroded grading second, condenser grading third, the permittivity fourth, resistive grading. Since such electric field control techniques have different advantages and disadvantages, applicable field control techniques are limited depending on the target power device. In recent years, two novel electric field control techniques (FGM: functional grading material) have received attention. In the first approch, the a functionally graded material is a material in which the magnitude of the dielectric constant continuously changes in one direction. In the other approch, the conductivity (resistivity) increases exponentially with the electric field. This material is made of a composite material by filling SiC (silicon carbide) having nonlinear I-V characteristics as an additive in a base material such as epoxy resin. SiC/epoxy composites have a problem of large leakage current in a low electric field and relatively low nonlinear coefficient in high electric field. In order to improve these problems, in this study the ZnO microvaristor/epoxy composites, which have recently been attracting attention as electric field relaxation materials, were fabricated and their electrical conduction and dielectric properties were measured. The results are summarized as follows. First, the electrical conduction characteristics of ZnO microvaristor/epoxy composites showed a lower leakage current and a larger nonlinear coefficient than the conventional electric field relaxation materials. The nonlinear conduction characteristics of the inorganic/epoxy composites were confirmed to depend on the kind, content and dispersion of the filler in addition, the main nonlinear conduction mechanism was confirmed by electron hopping or tunnel effect. Second, the dielectric properties of the ZnO MicroVaristor/Epoxy composite showed that the real part ε' and the imaginary part ε“ showed a temperature dependence and gradually decreased with increasing frequency. This result can be understood as Maxwell-Wagner-Sillars (MWS) theory. In the case of specimens filled with ZnO microvaristor at high sintering temperature, the ε' and ε“ increased. The increase of ε' is considered to be due to the decrease of the grain size in the grain of the ZnO microvaristor filler, while the increase of ε“ is thought to be due to the increase of the conductivity due to the decrease of the intergranular in the ZnO microvaristor particle. Third, the circuit simulation using the Maxwell-Wagner capacitor model, GIS spacer insulation design and electric field simulation on bushing with new field relaxation material are performed to verify the effect of field relaxation. As a result, it was confirmed that the electric field relaxation effect of ZnO microvaristor/epoxy composites with higher nonlinear coefficients was excellent than those of conventional field relaxation materials. In summary, the important point of this study is that ZnO microvaristor/epoxy composites have low leakage current and high nonlinear coefficient in lower electric fields than conventional nonlinear resistive materials, so electric power loss and electric field relaxation effect.
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