Fubao Jin

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.

Space charge dynamics at the physical interface in oil-paper insulation under DC voltage

Space charge has received much attention recently because an understanding of space charge in oil-paper insulation is useful for the design of converter transformers. This paper presents a study of space charge dynamics at the physical interface using the pulsed electroacoustic (PEA) method. After sample inversion, space charge in the vicinity of semiconductor electrode could be clearly observed when the sample is under depolarization. When an unexposed sample was placed next to a previously polarized one, negative charge appeared at the dielectric interface. These results indicated that there were other reasons in addition to acoustic loss that led to space charge distribution distortion. As regards the dielectric interface, it was found that an electrical double layer was formed at the interface, and the space charge sign was dependent upon the polarity of applied voltage. Space charge behavior at the interface depended on several aspects of the status of the interface, and during voltage polarity reversal, the interface was a weak point of the insulation.

Space charge behavior evolution with thermal aging of oil-paper insulation

Space charge is a threat to insulation materials and oil-paper insulation gradually degrades in service. The degradation of cellulose affects space charge formation, accumulation and dissipation, and the presence of space charge affects the performance of dielectric. In this paper, the effect of thermal aging on space charge behaviors and other properties including permittivity, conductivity, and tensile strength was investigated. Oil-paper samples were aged at a hot spot temperature (180°C). During thermal aging, space charge distribution was measured at a regular interval at room temperature. And the pulse electro-acoustic (PEA) method was used for space charge measurement. The results showed that the thermal aging at hot spot temperature caused increase of conductivity and tensile strength degradation. The permittivity increased at the beginning but then it decreased, while dissipation factor showed an upward trend. Besides these, trap distribution and carriers characteristics were also affected. The formations of space charge of unaged and aged oil paper were consequently different under low electric field. Due to the generation of traps by the degradation of cellulose, more charge, especially positive charge was trapped, but the increased traps mostly were shallow traps. Based on the Schottky model, a correlation between space charge injection and the permittivity was drawn. This, together with the appearance of positive bulk charge could be used for oil paper aging status diagnosis.

Calibration of pulsed electroacoustic method considering electrode–dielectric interface status and porosity

Pulsed electroacoustic (PEA) method is widely used, but measurement conditions not always agree with the underlying PEA assumptions. This necessitates space charge recovery; however, existing research only addresses the attenuation and dispersion in lossy media. The effects of electrode–dielectric interface contact status and porosity on the accuracy of space charge distribution are discussed in the present article. It is shown that the presence of nonlinear interface and porosity can introduce severe error. However, because the properties of acoustic propagation of waves (which are generated from charge on the electrode and in the bulk) are different, the conventional recovery algorithm is no longer suitable for calibrating the charge density. To obtain accurate space charge profiles, it is necessary to eliminate these effects. A method has been proposed which is based on the original measurement process. The validity of the proposed method was tested by reasonable post-recovery electric field distributions.

Characterization of high performance AIN nanoparticle-based transformer oil nanofluids

Well dispersed aluminium nitride(AlN) nanoparticles were synthesized by two-step ball milling coupled with surface coating. The AlN nanofluids (NFs) were prepared by dispersing as-prepared nanoparticles into transformer oil. The modified NFs is investigated as a new generation of transformer oil. The insulating properties have been investigated firstly. In the case of our AlN NFs with 0.079% <; Φ <; 0.159% (Φ means the volume concentration of AlN nanoparticles in the transformer oil), the positive lightning impulse breakdown voltage and partial discharge inception voltage (PDIV) increased around 50% and 20% compared to those of the transformer oil while alternating current (AC) breakdown voltage decreased around 20-30%. However, it can still meet the demand of the standard below 330 kV. The results show that AlN NFs have a concentration dependence of AlN nanoparticles. The amount of large particles increases with the increasing concentration of AlN nanoparticles while trap density of NFs increases first and then decreases, both of which have an opposite effect on the dielectric strength. Furthermore, the experimental results showed that the thermal conductivity is increased by 3-7% because of the addition of AlN nanoparticles.

Space Charge Behavior of Oil-paper Insulation Thermally Aged under Different Temperatures and Moistures

Moisture and high temperature are the most important factors that lead to the ageing of oil-paper insulation, but the research about space charge characteristics of oil-paper insulation does not take the combined effect of ambient temperature, moisture and thermal ageing into account. The pulsed electroacoustic (PEA) method was used to investigate the influence of moisture and temperature on space charge characteristics of oil paper at different ageing stages. The results showed that moisture could speed up formation of space charge in oil paper when water concentration was low, but the formation was restrained if the water concentration was high. At the beginning of thermal ageing, heterogeneous charge accumulation had predominance, but it gradually changed to homogeneous charge injection with ageing. It was believed that moisture concentration could speed up ageing and enhance charge accumulation on one hand, and accelerate or slow down the establishment speed of space charge on the other hand, therefore, charge accumulation type changed with ageing. The more seriously the oil-paper insulation was thermally aged, the deeper the trap energy level was, hence more space charge was trapped, which could be speeded up by increasing the ageing temperature, but the effect of ambient temperature did not fit the Arrhenius law.

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.

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.

Effects of thermal aging on creepage discharge in oil- impregnated pressboard under combined AC-DC voltage

In this study, we investigate the characteristics of creepage discharge on oil-impregnated pressboard under combined AC-DC voltage conditions using a sphere-plane electrode configuration. In our experiments, we use four oil-impregnated pressboard samples subjected to different thermal aging time. Experimental results show that the creepage discharge inception voltage (CDIV) and creepage discharge flashover voltage (CDFV) are lowest under pure AC voltage and gradually increase with higher DC voltage components. Longer aging time may contribute to higher CDIV with low DC voltage components; however, with higher DC voltage components, the effect of aging time is less pronounced. The CDFV is not significantly influenced by aging time. The length of carbonized tracks and the development velocity of the creepage discharge increase and the endurance time decreases with longer aging time and higher DC voltage components. We divide the development of creepage discharge in aged pressboard into four stages according to changes in the discharge characteristic parameters. With longer aging time, characteristic parameters exhibit less fluctuation and development velocity exhibits greater sensitivity to the DC voltage component. Our experimental results demonstrate that the characteristics of creepage discharge are significantly influenced by the age of the pressboard and by DC voltage components.