Shengtao Li

The characteristics of electrical trees in the inner and outer layers of different voltage rating XLPE cable insulation

The statistical initiation and propagation characteristics of electrical trees in cross-linked polyethylene (XLPE) cables with different voltage ratings from 66 to 500 kV were investigated under a constant test voltage of 50 Hz/7 kV (the 66 kV rating cable is from UK, the others from China). It was found that the characteristics of electrical trees in the inner region of 66 kV cable insulation differed considerably from those in the outer region under the same test conditions; however, no significant differences appeared in the 110 kV rating cable and above. The initiation time of electrical trees in both the inner and the outer regions of the 66 kV cable is much shorter than that in higher voltage rating cables; in addition the growth rate of electrical trees in the 66 kV cable is much larger than that in the higher voltage rating cables. By using x-ray diffraction, differential scanning calorimetry and thermogravimetry methods, it was revealed that besides the extrusion process, the molecular weight of base polymer material and its distribution are the prime factors deciding the crystallization state. The crystallization state and the impurity content are responsible for the resistance to electrical trees. Furthermore, it was proposed that big spherulites will cooperate with high impurity content in enhancing the initiation and growth processes of electrical trees via the ‘synergetic effect’. Finally, dense and small spherulites, high crystallinity, high purity level of base polymer material and super-clean production processes are desirable for higher voltage rating cables.

Space charge polarization modulated instability of low frequency permittivity in CaCu3Ti4O12 ceramics

A low frequency dielectric relaxation process of CaCu3Ti4O12 ceramics, which is generally overlapped by dc conduction in traditional dielectric spectra, is detected and clarified based on an improved analytical methodology in this letter. This relaxation process remarkably contributes to instability of low frequency permittivity and it clearly demonstrates the correlation between charge migration and space charge polarization. It is revealed that conduction will be achieved if carriers migrate and combine in the electrodes, while space charge polarization will be achieved if carriers migrate to the electrodes without combination.

Large electrostrain with good temperature stability in sodium niobate based ceramics

A large electrostrain of 0.24% with a converse piezoelectric constant of 790 pm V−1 was obtained from the LiSbO3 and (Bi0.5Na0.5)(Zr0.9Sn0.1)O3 modified KNbO3 ceramics. The electrostrain showed good temperature stability with vibration within 3% in the range from room temperature to 200 °C. It can even rival the current working horse PZT4. Besides, the present ceramics exhibited a large d33 of 207 pC N−1. The enhanced piezoelectric properties were achieved by pinching the phase with different symmetries together, experimentally by doping. Here the isovalent additive LiSbO3 was employed to lower the tetragonal to orthorhombic phase transition temperature and to raise the orthorhombic to rhombohedral phase transition temperature, i.e. pinch the three ferroelectric phases together. Meanwhile (Bi0.5Na0.5)(Zr0.9Sn0.1)O3 was used as the aliovalent doping to lower the Tc. It consequently resulted in a decrease of polarization anisotropy between phases with different symmetries and thus led to a large piezoelectric performance.

A brief history and research progress on solid engineering dielectrics in China

This article gives a brief history of the research in China into solid dielectrics, covering space charge, multistress aging, varistor and capacitor ceramics, nanocomposites, optical fibers, and biodielectrics.

Comparative AC tracking and erosion resistance of micro-A1N and BN filled silicone rubber composites

Thermal degradation due to dry band arcing in the discharge region causes tracking and erosion failure of composite insulators. This work investigates resistance against tracking and erosion of silicone rubber filled with micron sized aluminum nitride (AlN) and boron nitride (BN) particles. Their performances are evaluated by comparing the average tracking length, eroded mass and thermal distribution across the tested specimens. Micron AlN (5–10 μm) and BN (5 μm) particles were acquired for fillers. For each test sample, the inclined plane test (IPT) according to IEC 60587 was carried out, lasted for 4 hours with initial applied voltage of 3 kV and then increased at a rate of 0.25 kV/h. The experimental findings indicate that remarkable resistance to tracking length and eroded mass is achieved with higher filler loadings of 20wt% and 30wt% of BN as compared to AlN. Thermal distribution results indicate that heat is mainly accumulated in the discharge region and higher BN loadings suppressed it more effectively than AlN counterparts. The maximum temperatures in the discharge region at 3.5 h of IPT are measured at 384°C, 440°C and 130°C for 0wt%, 30wt%-AlN and 30wt%-BN, respectively. It is concluded that BN provides better thermal conductivity than AlN which helps in dissipation and reduction of heat in the discharge region and improves the tracking and erosion resistance.

Origin of dielectric processes in aged oil impregnated paper

Oil impregnated paper in power transformers deteriorates in long-term operation, which leads to the variations of its dielectric characteristics. This paper focuses on investigating the generation mechanism of the dielectric processes in oil impregnated paper and their variations with thermal aging. Firstly, a set of thermally stimulated current tests with bias-voltages were conducted in distinguishing the dielectric processes and revealing their generation mechanism. Secondly, the standard thermally stimulated current spectrum and the M″ spectrum of 0–1000 h aged samples were joint analyzed in investigating the variations of dielectric processes in thermal aging process. Finally, additional experiments of moisture, acidity and microscopic observation were conducted in explaining these variations. It is concluded that three dielectric processes can be found in oil impregnated paper, i.e. the dipole polarization, the interfacial polarization and the electric conduction. These processes are all strengthened with thermal aging. The generation of molecules with strong polarity, the structural changes in insulating paper, and the generation of hydrogen ions are responsible for the strengthening of these processes sequentially. This investigation on distinguishing dielectric processes and obtaining their mechanism can promote further understanding on oil impregnated paper and provide information for insulation design in power transformers.

Effect of BN size on AC breakdown performances of PTFE/BN composites

To satisfy the rapid development of energy requirements, power transmission with higher voltage must be adopted. The large improvement in transmission voltage needs more excellent power equipment and better materials to meet its insulation requirement. Polytetrafluoroethylene (PTFE) nozzle used in high voltage circuit breaker faces the issue of breakdown and arc ablation, and thus superior performance nozzles are urgently needed. Previous work has proved that boron nitride (BN) is excellent filler, which can greatly improve the arc ablation performance of nozzles. Moreover, the effect of BN size on arc ablation characteristics has been studied. However, the influence of BN size on breakdown performance is still unknown. In this paper, PTFE filled with BN of different size are prepared to study the effect of BN size on AC breakdown performances of PTFE/BN composites. Scanning electron microscope (SEM) was used to characterize the dispersion condition of BN sheets in PTFE matrix. SEM results show that BN sheets are uniformly dispersed in PTFE matrix. AC breakdown results show that the breakdown strength of PTFE/BN composites increases with the increasing BN size. And the value of breakdown strength of PTFE/BN composites filled with 3 um BN is 63.24 kV/mm, which has been enhanced by 103% compared with that of unfilled PTFE (31.17 kV/mm). Thermally stimulated current (TSC) results show that the trap parameters have been greatly changed after BN is introduced into PTFE matrix. Based on the above experimental results, the relationship between breakdown strength and trap parameters is discussed. The improvement of breakdown performance is considered to be closely related to the change of trap depth and trap density, which can influence the charge transport properties in materials.

Study on conduction current and space charge accumulation simultaneously in Ethylene Propylene Diene Monomer film

This paper investigates the space charge distribution and external current performance of Ethylene Propylene Diene Monomer (EPDM) film simultaneously by an improved integrated PEA system. Through switching the space charge and current measurement circuits, the space charge and external current were detected alternately during the electric field of 60 kV/mm and 80 kV/mm. Small amounts of homocharges accumulated inside the sample. While, an obvious electron injection occurred from cathode (Al electrode), and they moved inside the sample. This leaded to an electric field distortion near the anode. However, hole injection seemed difficult to occur from anode (semiconducting layer). Based on the measured current density and the space charge distribution, we calculated the conduction current density distribution inside the sample. The results showed that a large conduction current occurred at the beginning of measurement, which is derived from the rapid electron injection and movement. Nevertheless, it decreased with the increase of time, indicating a uniform distribution of conduction current density (less than 1×10−6 A/m2). In addition, the dissipation energy density caused by space charge and conduction current was also estimated during the field application. It indicated that a high power density appeared inside the sample at the beginning of measurement. However, it gradually decreased with the increase of time. This information reveals the energy loss dynamics caused by space charge during the electric stress, which is of great importance to evaluate the influential mechanisms of space charge on aging and insulation failure.

Dielectric relaxation and carrier transport in epoxy resin

Epoxy resin (EP) is widely used as an insulating material in power equipment. Its dielectric relaxation and carrier transport properties are important factors affecting breakdown and surface flashover performance. The dielectric relaxation and carrier transport properties of EP based alumina (Al2O3) microcomposite were investigated through broadband dielectric spectroscopy. The glass transition temperature was measured by differential scanning calorimetry (DSC), which is about 120 °C. Gold electrodes with a diameter of 30 mm were sputtering on two sides of the samples. A broadband dielectric spectrometer (Concept 80 Novocontrol) was used to measure the dielectric relaxation properties at an ac voltage of 1 Vrms in a frequency range from 10-1 to 107 Hz at various temperatures. Above the glass transition temperature, a relaxation peak occurs at high frequencies due to the motion of molecular chains or segmental chains, and a dc conductivity resulted by the migration of charge carriers appears at low frequencies. In addition, molecular chains with different scales have different relaxation times. It was found that EP microcomposite has a very broad distribution of relaxation time. We calculated the distribution of relaxation time at various temperatures. Furthermore, the temperature dependences of molecular relaxation and dc conductivity satisfy the Vogel-Tammann-Fulcher equation. Fitting the experimental results, we obtained the Vogel temperatures and strength parameters of molecular relaxation and dc conductivity. From the Vogel temperatures, we estimated the glass transition temperature to be 117 °C, which is consistent with the DSC result. It means that free volume increases with increasing temperature, facilitating the motion of molecular chains and the migration of charge carriers.

Surface flashover characteristics of EP/Al 2 O 3 nanocomposites in different atmosphere

Because of the special surface energy of nanoparticles, the electrical, thermal and mechanical properties of nanocomposties may be improved. In general, emission of electrons at cathode leads to charging of the surface and thus results in surface flashover. Since flashover happens at the interface between dielectrics and atmosphere, different characteristics of atmosphere and dielectric may influence the emission, transport and recombination of electrons. In this paper, the experimental results of surface flashover characteristics of epoxy cast with nano Al2O3 particles are presented. Special methods to avoid nanoparticles from reuniting were introduced. DC surface flashover experiments were conducted in vacuum, air, N2, SF6 and CO2 using plate-plate electrodes. Experiments showed that in SF6 and vacuum, nanocomposites have higher surfaces flashover voltages. With the increase of filler content, the surface flashover voltages of nanocomposites first increase and then decrease in N2, air, SF6 and vacuum. Both atmosphere and filler loading have effect on nanocomposites, which depends on the property of gas. First flashover was also observed under DC voltage. The mechanism of surface flashover properties, nanoparticles content and gas were discussed.