Guilai Yin

Short-term breakdown and long-term failure in nanodielectrics: a review

Nanodielectrics, which are concentrated in polymer matrix incorporating nanofillers, have received considerable attention due to their potential benefits as dielectrics. In this paper, short-term breakdown and long-term failure properties of nanodielectrics have been reviewed. The characteristics of polymer matrix, types of nanoparticle and its content, and waveforms of the applied voltage are fully evaluated. In order to effectively comment on the published experimental data, a ratio k has been proposed to compare the electric properties of the nanodielectrics with the matrix and assess the effect for nanoparticles doping. There is evidence that the short-term breakdown properties of nanodielectrics show a strong dependence on the applied voltage waveforms. The polarity and the cohesive energy density (CED) of polymer matrix have a dramatic influence on the properties of nanodielectrics. Nanoparticle doped composites show a positive effect on the long-term failure properties, such as ageing resistance and partial discharge (PD) properties of nanocomposites are superior than microcomposites and the matrix. The larger the dielectric constant and CED of the matrix become, the more significant improvements in long-term performance appear. Based on the reported experimental results, we also present our understandings and propose some suggestions for further work.

A new potential barrier model in epoxy resin nanodielectrics

The dielectric constant and the conductivity of epoxy resin nanocomposites exhibit a lower value at slight filler loading compared with the host epoxy resin. The electrical strength has an increase and presents an optimal value at filler loading of 1 wt%. The interaction zone between the nanoparticles and the polymeric matrix is considered as an independent region. Accordingly, a new potential barrier model is proposed. Based on the model, carriers are restrained in the interaction zone when nanoparticle is in an isolated dispersion, leading to a decrease in both mobility and density of carriers. As a result, the conductivity decrease and the electrical strength increase. The restriction of dipole movement in the interaction zone and the increase of free volume are collectively contributed to the reduction of the dielectric constant. With increasing filler loading, the thickness of the interaction zone extends due to the overlap of the interaction zone, even a conductive path occurs when filler loading exceeds the percolation threshold, leading to a great increase in both mobility and density of carriers. Consequently, the conductivity increase and the electrical strength decrease. The increase of the dielectric constant is chiefly ascribed to the particles.

The effect of accelerated water tree ageing on the properties of XLPE cable insulation

The influence of accelerated water treeing test (AWTT) on the properties of 10 kV cross-linked polyethylene (XLPE) cable insulating materials was investigated in this paper. The dielectric and physicochemical properties of both aged samples and unaged samples were tested. Dielectric property investigation found a new peak of the dielectric loss tangent with an activation of 0.14 eV in the low-frequency domain for aged samples. With increased ageing time, the conductivity in low frequency region of less than 50 Hz increases obviously, while it nearly keeps unchanged in the frequency above 50 Hz. Physicochemical investigation of FTIR, DSC, XRD and density measurement results indicated that AWTT ageing of XLPE insulated cables will lead to decreased crystallinity and density, and methyl group at the outer insulation layer cannot be found in aged samples. The melting temperature (Tm) was also decreased with ageing time. It was suggested that water was introduced from the outer layer of insulation and the degradation was developed by a combination of mechanical forces and electric force. During the ageing process, water will choose a path between surface of lamella and amorphous region, which consequently lead to the increase of low frequency conductivity and decrease of lamella thickness. Furthermore, a model was proposed to explain how the microstructure of XLPE cable insulation will change during AWTT ageing process. It is shown that XLPE cable insulation will be changed both in dielectric and physicochemical properties before any water tree was found, and a combination of dielectric and physicochemical methods is effective to detect the degradation of the XLPE cable insulation materials.

Frequency dependence of breakdown performance of XLPE with different artificial defects

In this paper, the effect of the applied field frequency on the breakdown performances of cross-linked polyethylene (XLPE) insulation was investigated. Three kinds of artificial defects, namely an inserted needle, a needle void and a knife void, were introduced into a 10 kV commercial XLPE cable, to simulate real defects that may be introduced during on-site service. Breakdown tests were conducted in transformer oil at room temperature using a frequency-tuned resonant test system in a frequency range from 20 to 300 Hz. It was found that the breakdown voltage of specimens with the inserted needle was sensitive to the applied field frequency, and the breakdown voltage showed a maximum value at about 240 Hz. However, for other two kinds of specimens with the needle void and knife void, the breakdown behavior shows a weak relation with the applied field frequency. The breakdown voltages of the specimens with the needle and void are 25 kV and 20 kV respectively. It was further found that side channels appeared on the flank of the main channel of the breakdown path in the specimen with the inserted needle, and the frequency dependence of fractal dimension D of the side channel in the breakdown path are similar to that of electrical trees before breakdown, which may consequently lead to the maximum value in the experimental results.

Surface flashover in vacuum and bulk breakdown in polystyrene nanocomposites

Nano-sized Al2O3 and TiO2 particles are mixed with polystyrene to prepare specimens, respectively. In this paper, the dielectric relaxation spectroscopy, breakdown strength and surface flashover voltage in vacuum of nanocomposites were investigated. Both breakdown strength and flashover voltage first increase, then decrease with increasing the fillers loading. They are also dependent on applied field waveform. However, permittivity shows a decrease at slight filler loading, and then increases. According to the experimental results, specimens with lower permittivity have higher electric strength. The permittivity, breakdown strength and surface flashover are close related to the interaction zone between nanoparticle and polymeric matrix. Moreover, it should be emphasized that there is a certain relationship between bulk electric strength and surface flashover voltage.

Breakdown performance of low density polyethylene nanocomposites

Nano alumina particles and nano titanium dioxide particles are incorporated with low density polyethylene to fabricate samples, respectively. The breakdown performance of samples is conducted in the transformer oil. Results indicate that both ac breakdown strength and dc breakdown strength of nanocomposites with appropriate particles concentration have an increase. For example, ac breakdown strength of sample with 1wt% particles has an increase of 16% for Al2O3-LDPE. DC strength of sample has a maximum increase of 35.3% at particle concentration of 2wt% for both kinds of nanocomposties. When particle concentration exceeds 2wt%, dc performance of samples incorporated with Al2O3 is almost independence of particle concentration. However, it reduces greatly for TiO2-LDPE at higher concentration. For instance, dc breakdown strength of sample with 10wt% TiO2 indicates a reduction of 26.7% compared with neat LDPE. These results mean that polarity of nano particle influences the breakdown performance of nanocomposites, especially at high particle concentration.

A new view on impulse degradation of ZnO-based ceramics

The electrical and dielectric properties of a commercial ZnO-based varistor ceramics were measured in the process of 8/20μs impulse current degradation with a current density of 1725A/cm2. The characteristic of U-I in the pre-breakdown region was mainly investigated. It was found that the varistor voltage U1mA will first increase rapidly with the times of impulse, and then remain stable and finally decrease sharply. Furthermore, a new loss peak emerges in the dielectric spectra after 600 times of impulse degradation, which suggests a new trapping behavior introduced in the degradation process. It was also found that the non-linear coefficient was more sensitive to the degradation process than varistor voltage U1mA. These phenomena were not reported before.

Influence of polymeric polarity on the interface of nanocomposites

A polar polymer and a non-polar polymer are selected as polymeric matrix. Nano-TiO2 is selected as filler and incorporated with such two polymers to construct nanocomposites, respectively. Their thermal behaviors are tested by differential scanning calorimetry. Meanwhile, their dielectric responses and ac electrical strengths are measured. Results indicate that Tg of epoxy nanocomposites is lower than that of neat epoxy resin due to the existence of nanoparticles. However, thermal behavior has no change in LDPE nanocomposites. The dielectric responses also indicate a great deal of difference between the both nanocomposites. However, ac electrical strength first increases then decreases with increasing nanoparticles loading for both kinds of nanocomposites. From the results, it seems that there is a weak relationship between Tg and electrical strength. The polarity of polymer may influence the bond strength between nanoparticles and polymeric matrix. This bond strength affects the arrangement and the mobility of polymeric molecule chain at the interface, which may cause the changes of Tg.

Low-frequency dispersion of ZnO/Bi 2 O 3 ceramics analyzing by universal power law

Recently, universal power law becomes widely used. In this paper, it was used to study the ZnO/Bi<inf>2</inf>O<inf>3</inf> ceramics. Bi<inf>2</inf>O<inf>3</inf> is the only additive of ZnO here in order to directly study the effect of Bi<inf>2</inf>O<inf>3</inf> in ZnO. ZnO samples with different content of Bi<inf>2</inf>O<inf>3</inf> were made by the traditional ceramic technique. Then the microstructure of the acid erosion surface was observed by Polarized Optical Microscope, the dielectric properties of the samples, such as conductivity, permittivity and dielectric loss, were tested by Novocontrol Broadband-Frequency Dielectric Temperature Measuring Instrument, and the activation energy was calculated by Arrhenius relation. A strong Anomalous low-frequency dispersion phenomenon was found while investigating the dielectric properties of ZnO/Bi<inf>2</inf>O<inf>3</inf> ceramics. The experimental results of the dielectric properties which are in the frequency range of 0.1Hz∼107Hz and the temperature range of 173K∼473K were systematically analyzed by universal power law. The results show that the conductivity loss is the main factor in the low frequency, while the dielectric loss is dominant in the high frequency. The higher the temperature, the more similarity of the AC I-V characteristic to the DC ones, which means that ω <inf>e</inf> can token the thermally simulated degree of electrons to the temperature, since the conductivity of the samples was generated mostly by thermally simulated electrons. The density ratio of zinc interstitials and oxygen vacancies decreased with the increase of Bi<inf>2</inf>O<inf>3</inf> content fewer than 5wt%, and then it will be oxygen vacancies from 5wt % Bi<inf>2</inf>O<inf>3</inf> content to 15wt% that dominated the dielectric properties.

Evaluation of VPI insulating materials and insulation system

Recently, independent research was carried on evaluation of domestic VPI insulating materials and consequently simulated stator. In this paper, lots of experiments were executed to verify and evaluate the VPI insulation materials and insulation system domestic. Raw materials were measured regarding thermal, chemical, electrical and dielectric properties based on panel samples. Single simulation bars produced by VPI technology were tested by means of dissipation factor, dielectric breakdown and voltage endurance. The characteristics of epoxide/anhydride impregnating resins, impregnated mica tapes and panels was compared between domestic and oversea products. This research is a systematic work regarding both of VPI insulating materials and technology, which has far-reaching significance to the future study and application.