Study On Space Charge Characteristics Of LDPE Nanocomposites With Different Nanoparticles

ABSTRACT:The rapid development of the power industry put forwards higher requirements for reliability and performance of solid insulation. Study on the higher performance solid insulation is the urgent need of practical engineering application. As an insulating material widely used in AC transmission fields, LDPE is limited by space charge problem under DC electrical fields. Nanocomposites have attracted a widely spread attention because of promoting electrical performance by increasing its breakdown strength, space charge injection suppression and control of carrier conduction properties. This thesis aims to explore space charge suppression mechanism in nanocomposites by studying space charge distributions, trap characteristics and conduction currents of LDPE and its nanocomposites, which provides test data and theoretical support for development and evaluation of insulating material used in HVDC cables.SiO2/LDPE, ZnO/LDPE and MgO/LDPE nanocomposites are prepared by melt bending method. The microscopic-morphology, structure composition and thermal properties of nanoparticles and nanocomposites are investigated by SEM, FTIR and DSC, which can confirm nano-composite effect. Surface hydroxyl radicals of nanoparticles have an important influence on the dispersion of nanoparticles in the LDPE matrix, which affects effective area and interaction at the interface between nanoparticles and LDPE matrix.Nano-doping improves electrical properties of nanocomposites compared to LDPE. Firstly nano-doping brings new dipole moments into nanocomposites, which improves relative permittivity and dielectric loss of LDPE nanocomposites. Results of TSC measurements show that in spite of having little influence on trap depth, nano-doping effectively increases trap quantity in LDPE nanocomposites, which is related to the dispersion of nanoparticles in the LDPE matrix. Results of conduction current measurements show nano-doping affects transport of carriers in the nanocomposites. On the one hand, the introduction of nanoparticles suppresses the impurity ionization, which reduces carrier quantity. On the other hand, traps produced by nano-doping capture carriers, which accelerate attenuation rate of conduction current. Both of these effects reduce conduction current in the nanocomposites. In addition, nano-doping with ZnO and MgO nanoparticles raise the threshold voltage of SCLC. Space charge behavior of LDPE and its nanocomposites under different DC electric fields is deeply studied. Space charge suppression model based on interface trap proposed previously is analyzed via theoretical analysis and simulation in order to confirm whether the model is suitable for space charge behavior of LDPE and its nanocomposites in this thesis. Simulation results show traps at the interface between electrodes and insulating material modulate on the charge injection from electrodes. Numbers of interface trap could capture charge injected from electrodes, which lead to generation of charge layer at the interface between electrodes and insulating material. The charge layer obviously decreased electrical field besides electrodes. Reduction of electric field not only suppresses impurity ionization but also improves injection barrier height from electrodes. Higher injection barrier height leads to lower charge injection rate. As a consequence, space charge accumulation is suppressed.