Jihuan Tian

Effect of nanoparticle surface modification on breakdown and space charge behavior of XLPE/SiO 2 nanocomposites

This paper focuses on the effect of nanoparticle surface modification on the DC breakdown and space charge behavior under DC endurance test of XLPE/SiO2 nanocomposites. A titanate (TC) and a vinylsilane (VI) coupling agents, both of which contain non-polar functional groups, were used as surface modifiers for SiO2 nanoparticles. FESEM results clearly show that TC and VI coupling agents improved nanoparticle dispersion within XLPE matrix compared with unmodified nanocomposites, where occurred severe agglomerations larger than 1 μm in size. The improvement of nanoparticle dispersion was due to the increase of surface hydrophobicity of SiO2 nanoparticles. In addition, it was found that surface modification improved DC breakdown strength under different temperatures compared to XLPE or unmodified XLPE/SiO2 nanocomposites. XLPE/VISiO2 nanocomposites possessed the highest DC breakdown strength and relatively low dispersibility. The results of space charge measurements under DC endurance test reveal that the introduction of SiO2 nanoparticles could not effectively suppress the injection and movement of space charge until organic surface modification, which was believed to contribute to the formation of more and deeper trap sites considering the better dispersion of SiO2 nanoparticles within XLPE matrix. The space charge results also show that breakdown during DC endurance test usually occurred after the largest electric field passed, which is believed to be the degradation of dielectrics caused by the formation, movement, accumulation, and dissipation of space charge. Finally, the lower electric field distortion of modified XLPE/SiO2 nanocomposites was considered to decrease the degradation of XLPE/SiO2 nanocomposites.

Effect of nanoparticle surface modification on charge transport characteristics in XLPE/SiO 2 nanocomposites

This paper focuses on the effect of nanoparticle surface modification on the charge transport characteristics in XLPE/SiO2 nanocomposites. A titanate coupling agent (TC9) and a 3- (Methacryloyloxy)propyltrimethoxysilane (KH570) were used for the surface modification of SiO2 nanoparticles. It was found that both KH570 and TC9 coupling agents improve the nanoparticle dispersion compared with unmodified SiO2 nanoparticles. The improvement in dispersion was found to be due to increased surface hydrophobicity of the treated SiO2 nanoparticles. In addition, it was found that the surface modification improved the DC conductivity, dielectric characteristics, and space charge properties as compared to XLPE or XLPE/SiO2 nanocomposites without surface modification. The results of the TSC measurements showed that the introduction of SiO2 nanoparticles into XLPE increased the trap density and produced more trap energy levels. Improving the nanoparticle dispersion was found to further increase the corresponding trap depth and trap density. The trapped homocharge formed an independent electric field and reduced the effective electric field, which reduced charge injection and increased the charge injection barrier height. Therefore, the space charge formation in the material bulk was suppressed.

Simulation of bipolar charge transport with trapping and recombination in polymeric insulators using Runge–Kutta discontinuous Galerkin method

The widely used QUICKEST method with ULTIMATE flux limiter is not capable of solving the charge transport problems with a very steep wavefront accurately, due to the wide stencil adopted. Furthermore, the splitting process of separating the convection and the reaction terms in the method introduces additional errors. To solve such problems accurately, a novel numerical method based on the Runge?Kutta discontinuous Galerkin (RKDG) method is introduced in this paper, which has high-order resolution and weak correlation between cells. The bipolar charge transport under dc voltage in solid dielectrics with trapping and recombination is simulated using this new method. The results of charge profiles provided by the method are obviously different from the simulation results in the existing literature. The method was verified by problems with analytical solution and experimental observations.

Simulation of space charge dynamics in low-density polyethylene under external electric field and injection barrier heights using discontinuous galerkin method

In this paper, a novel high-order accurate numerical method was proposed to simulate the space charge transport in solid dielectrics. It is based on a unified framework, discontinuous Galerkin (DG) method, to solve both charge concentration and electric field. It is easier to implement and more efficient than the combined DG and boundary element method (BEM) with equivalent numerical accuracy. With this new method and symmetric model parameters for low-density polyethylene (LDPE), space charge concentration and electric field distribution under different external electric field and injection barrier heights were obtained. The simulation shows that above an electric field threshold, large oscillations appear in the time-dependent maximum electric field strength and the total charge quantity. This phenomenon can be considered as a sign of a relatively larger quantity of charge injection compared to the low field regime. When the Schottky injection barrier height in the simulation is raised, this electric field threshold also increases in order to maintain a certain amount of carrier injection.

Gas generation characteristics of transformer oil under combined AC-DC voltages in the process of arcing

As an important equipment, convertor transformer plays a crucial role in ultra high voltage direct current (UHVDC). How to improve the safety running level of convertor transformers is a key problem to be solved imminently. Compared with AC power transformers, the valve side windings of convertor transformers will endure combined AC-DC voltage. In these conditions, the gas generation characteristics in oil for convertor transformers will be more complicated. In this paper, according to the operational principle of convertor transformers, the ratios between AC and DC voltage on valve side windings in the star connection and triangular connection were 77.1% and 83.7% respectively. Based on the above results, 65 kV peak voltages in 5 different AC and DC voltage forms were applied to the needle-plate electrode model for 2 hours. The discharge current and voltage waveforms were recorded and the arc current pulses had a short lasting time about 1.2 ms. Then the mean discharge energy was calculated at 0.1 to 0.3 J. At the same time, the gas generation characteristics under combined AC-DC voltage were obtained by using the gas chromatographic analysis method. The variations of amounts and generation rates of characteristic gases were measured with time during 2 hours, which provided fault judgment criterions and experimental experience. The experimental results showed that the amounts and generation rates of H2, C2H2 and total hydrocarbon under the pure DC voltage or combined AC-DC voltage were more than those under the pure AC voltage. It might illustrate the fact that effects of different voltage forms would lead to different gas generation characteristics could not be ignored.

Efficient Analysis of Electromagnetic Fields near Lightning Channel by Using SIBC in FDTD

This paper presents an efficient finite difference time domain (FDTD) algorithm to calculate the transient electromagnetic fields above conductive ground near lightning channel. Significant computational time and memory savings are achieved by using surface impedance boundary condition (SIBC) to avoid simulating the underground region. Numerical results are shown to validate the proposed method.

An accurate adaptive method for drawing 2-D electric lines of force

This paper presents a novel adaptive method for drawing 2-D electric lines of force of electrostatic field. The proposed method is based on high order Taylor expansion technique. By adaptively controlling the drawing step length, the proposed method can meet high drawing precision requirement. The principle, implementation and prerequisites of the proposed method are discussed. Numerical examples are given to validate the proposed methodpsilas accuracy.