Yanchao Sha

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.

Partial discharge characteristics in oil-paper insulation under combined AC-DC voltage

Heretofore, unlike HVAC transformers, there is not much experience in both insulation design guides and test methods for ultra HV converter transformers. This special insulation phenomenon is very prominent due to the complication of voltage applied on valve-side windings of converter transformers. There is a severe lack of empirical research on partial discharge in oil-paper insulation under the combined ACDC voltage. The study in this paper provides a timely research into the issues. Nine possible configurations, which intent to simulate different defects in the transformer, have been investigated. The PD inception voltages varied ratios of combined AC-DC voltages were conducted. The discharge magnitude and its relation with the applied voltage for selected four common configurations were obtained during voltage increasing and decreasing respectively, which presented the PD initiating process and also reflected the insulation behavior of dielectrics. Besides, the Finite Element Analysis Soft was used to propose the effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on PD characteristics, which is very lacking in the previous references. The roles of DC and AC component in PD characteristics were discussed in the context of the relevant work in literatures.

Influence of voltage reversal on space charge behavior in oil-paper insulation

The presence of space charge is a serious threat to the reliability of insulation material under voltage polarity reversal. The results of space charge evolution in oil-paper insulation during external voltage polarity inverse at room temperature were presented in this paper. A mirror image effect charge was observed in the steady state, but the formation of it was affected by the first polarizing, which could cause a change of electrical property. The results showed that the mirror image effect was independent of the involved nature, type and dynamics of charge. Experiments of different reversal polarities and periods indicated that the slow decay of heterocharge retained from previous homocharge injection was the cause of electric field enhancement. And the reversal period therefore had an effect on voltage polarity reversal, namely the shorter the reverse period, the higher the stress enhancement. The reliability of oil-paper insulation could be influenced by the polarity reversal because charge injection and distribution were unpredictable, and space charge modified and in return was affected by the field as well.

A study on electric conduction of transformer oil

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<;0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohms law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm<;E<;1.33 kV/mm), ln (I/E2) appears to be proportional to E-1, which meets the field emission equation. At high electric field stage (E>1.33 kV/mm), the obtained I-V2 characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well.

Electrical properties of oil-paper insulation affected by water conductivity during paper-making process

Developments of ultra-high voltage grid in China require better insulation materials, and that forces the transformer presspaper and pressboard manufactures to pay more attention to the papermaking process to enhance their product qualities. In this study, pure water and three kinds of sodium chloride solutions with conductivities of 100 μS/cm, 500 μS/cm and 1000 μS/cm were respectively used to prepare presspaper samples. By vacuum impregnation, four kinds of oil-paper samples were acquired. Test results showed that relative permittivity and tand of oil-paper specimens first varied little then increased a lot with the increasing of water conductivity; DC breakdown field decreased while AC breakdown field changed little. To confirm the differences were due to water conductivity, ion chromatography was used to determine the sodium and chloride ion contents of the presspaper samples, we found that the ion contents increased with electrical conductivity. This indicates that water conductivity does have effects on most electrical properties of oil-paper insulation and lower conductivity can bring better insulation performances.

Surface flashover characteristics of oil-paper insulation under combined AC-DC voltage

The valve side winding of the converter transformers withstood several types of voltage. This paper investigated the effects of voltage component on the surface flashover characteristics of oil-paper insulation under combined AC-DC voltage in different ratio. The whole flashover process of surface flashover of oil-paper insulation under combined AC-DC voltage was studied by using two types of electrodes. The experimental conclusion was shown that the flashover voltage increased with the DC ratio in the combined AC-DC voltage component. Flashover voltage decreased significantly when the radial electric field increases in the electric field component. Furthermore, electric field distributions versus combined AC-DC voltage in different ratio were calculated by Finite Element Analysis (FEA). Due to the different distribution characteristics of DC electric field and AC electric field, when the DC ratio was higher, the surface flashover location tended to pressboard surface. Conversely, when the AC ratio was higher, it tended to the oil gap between the electrodes. Moreover, based on high-speed camera, it was used to capture emitted light produced by surface flashover, record flashover initiation and propagation process in small oil gap between the electrodes.

Charge transport in thermally and electrically stressed oil-impregnated paper insulation

The relation between ageing and space charge is a hot area, but there is little direct evidence. In this paper, oil-paper samples were exposed to 20-kV/mm DC electrical field, 130°C temperature or both 20-kV/mm DC electrical field and 130°C temperature separately. Charge transport through them was analyzed at intervals by the pulsed electroacoustic (PEA) method. We find that the quantity of trapped charge increases with stressing durations, no matter what kind of stress oil-paper is exposed to. That indicates that energy release through space charge movement, such as charge trapping/detrapping and recombination, is another reason for traps generation besides deterioration caused by thermal ageing. However, because the difference in cellulose degradation mechanism among them, charge transport dynamics are thereby different under different stressing conditions. Space charge accumulates faster inner electrothermally stressed oil-paper than that inner another two kinds of oil-paper, and the sign of trapped charge is positive if oil-paper is electrically or electro-thermally stressed. The latter behavior is discussed in terms of electromagnetic dynamics due to cavity within oil-paper once there is external electric field.

Influence of temperature on developing process of surface flashover in oil-paper insulation under combined AC-DC voltage

In order to study the influence of temperature on surface flashover behavior with oil-paper interface under combined AC-DC voltage, a discharge test assembly was established to investigate the influence of temperature on developing process of surface flashover of oil-paper interface from 25 °C to 90 °C. Based on impulse current method, the process of surface flashover of oil-paper interface under combined AC-DC voltage was studied by electrodes configurations that simulated the tangential electric field and radial electric field of surface of pressboard, in which the voltage was increased step-by-step. Meanwhile the partial discharge (PD) spectrums at whole flashover stages were studied. Moreover, high speed camera was used to record the whole process of surface flashover which was divided into 4 stages: (1)Inception stage, (2)Streamer stage, (3)Intermittent breakdown stage, (4) Surface flashover stage. The experiments results shown that flashover voltage and PD inception voltage at high temperature (HT) was lower than the voltage at low temperature (LT). The average amplitudes of PD at LT were lower than at HT. The growth rate and repetition rate of PD at HT was faster than at LT. It was easier to develop the flashover at oil-paper interface under combined AC and DC voltage at HT. So the flashover voltage was lower at HT. the surface of pressboard was destructed severely at HT, the track of surface discharge was obvious at HT.

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.