Yalin Wang

Research of simultaneous measurement of space charge and conduction current in cross-linked polyethylene

To overcome the weak correlation between space charge and conduction current caused by individual measurement, a specialized system based on pulsed electro-acoustic method (PEA) was designed to perform simultaneous measurement of space charge and conduction current for plate sample. Firstly, electric pulse response during space charge measurement was simulated to analyze the interference during conduction current measurement caused by electric pulse. To solve this problem, fast time division technique was employed. And then, this paper presents the measurement results of space charge distribution and conduction current in cross-linked polyethylene (XLPE) sample under various electric fields at ambient temperature. Finally, displacement current density distribution was calculated according to spatial and temporal distribution of space charge. Conduction current density distribution was obtained by subtracting displacement current density from external current density. The results show that the charge packet continuously appears from anode and migrates to cathode under high electric fields above 70 kV/mm. It significantly distorts the electric field near charge packet insider XLPE. The maximum conduction density also appears at the position of charge packet. Furthermore, the calculated velocities are similar at the rear and front side of charge packet. The calculated negative differential mobility probably is the main mechanism of the charge packet formation.

Electron behavior in hydrogen atom under electric fields

Polarization, charge transport, breakdown, space charge and other electrical phenomena have been studied under a quantum level. Several theories and codes based on Schrodinger equation have been applied. However, there are inevitable defects in these methods, e.g. excessive simplification, which is not reasonable when studying electrical phenomena. This paper is to study electron behavior under electric fields under a quantum level and with as less simplification as possible. A simple case is studied, where only one hydrogen atom is bounded in a limited space with a uniform electric field. Based on Schrodinger equation, mathematical model is built. The model is a partial differential equation and analytical solution is not available. Finite difference method (FDM) is employed to solve time-independent problem and introduces an error. The error decreases when using a finer meshing. Modified Euler method (MEM) is adopted to solve time-dependent equation and introduces a similar error. All these errors are estimated and limited within a reasonable range. By studying the one Hydrogen atom case, electronic polarization is predicted. Polarization intensity under different additional electric field is calculated. Polarization speed, namely response, is calculated by solving time-dependent equation, which is usually unavailable by conventional methods. We expect the application under more complex conditions and to predict more electrical phenomena.

Space charge measurement of cross-linked polyethylene at low temperatures

This study describes a modified pulsed electroacoustic (PEA) system, which can measure the space charge distribution in dielectric films at low temperatures. The temperature around the sample is controlled by the liquid nitrogen, an electric heating coil as well as a PID temperature controller. The space charge distribution of DC stressed cross-linked polyethylene (XLPE) with the thickness of ∼300 pm is investigated at −20 °C, −40 °C and the ambient temperature (20 °C). The experiment results show that with the decrease of temperature, the accumulated space charges decrease in the same applied electric fields. Specifically, there are apparent homocharges accumulated in the vicinity of the electrodes at the ambient temperature. In addition, more positive charges disperse deeper in the bulk of the XLPE film than the negative charges, which demonstrates a stronger positive charge injection than negative charge injection. However, the injected negative charges always dominate in XLPE under various electric fields at −20 °C. Whereas almost no distinguished space charge can be observed in XLPE film during short circuit when it is subjected at −40° C. Besides, in order to investigate the space charge dynamics in XLPE at low temperatures, a simple charge detrapping model is used to analyze the composition of the trapped charges. The traps in XLPE is divided into two types, one is the trap with shallow depth, and the other is the trap with deep depth. It is found a large part of the accumulated charges at the ambient temperature are trapped in deep traps, whereas a large part of the accumulated charges at −20°C are trapped in shallow traps, which is predictable to some extent. Because the detrapping rate of the trapped charges increases with the increase of temperature. In addition, the variation of the depths of the two kinds of traps at different temperatures is also discussed.

Research on the polarization relaxation current and equivalent circuit of double-layered XLPE/EPDM

The physical interface between two dielectrics has been acknowledged as the weak point of the double-layered insulation system. In order to investigate the effect of the interface on the conduction characteristics of the double-layered insulation system, the time-varying currents of XLPE, ePdM and double-layered XLPE/EPDM are measured under various electric fields at 50 X!. Besides, it is found that the time-varying current decays exponentially, thus the nth-order RC equivalent circuits are adopted to describe the relaxation processes of the above three insulation systems. The results show that the equivalent circuits of the three insulation systems change into higher-order RC circuits with the increase of electric field. According to the equivalent circuit parameters, the relaxation time constants corresponding to space charge accumulation and steady-state resistances decrease with the increase of electric field for all the specimens. In addition, the equivalent resistance of the negative stressed double-layered specimen is smaller than the positive stressed double-layered specimen in the same electric field, which indicates a polar effect of the interface on the conduction characteristic of double-layered insulation system.

Study on conduction current characteristics of corona-resistant polyimide film before and after thermal aging

In order to investigate the effect of thermal aging on the conduction current characteristics of polyimide film, two types of corona-resistant polyimide films (represented by A and B) were thermal aged at 250 °C from 0 h to 40 h. The polarization currents and the depolarization currents of all films were measured, and the corresponding electric degradation threshold under which the current changed from ohm region to space charge limited current (SCLC) region were also calculated. The results showed that the electric degradation threshold of unaged A and unaged B were 46 kV/mm and 45 kV/mm, respectively. As the thermal aging time increased, the electric degradation threshold of A became larger, whereas that of B showed no remarkable change. Two-order exponential fitting was used to analyze the depolarization process. It is inferred that after the thermal aging, there are more traps in A. And B shows a better thermal-resistant performance than A.

Research on simultaneous measurement of space charge and conduction current for thermally aged cross-linked polyethylene

To evaluate the thermal aging effect on space charge dynamics and carrier transport characteristics in cross-linked polyethylene (XLPE), films with the thickness of ∼300 μm are thermally aged at 120 °C for 36, 84, 168 and 252 hours, respectively. And then, the simultaneous measurement of space charge and conduction current is employed by our developed system, which overcomes the defect of the weak correlation of experimental results caused by individually measurement of space charge and conduction current. Fourier transform infrared (FTIR) spectroscopy experiment is carried out to observe the functional group change due to the thermal aging. Finally, based on the simultaneous measurement results, the spatial and temporal distribution of space charge are obtained, as well as the trap energy distribution. It is shown that the carbonyl index increases with aging time, which demonstrates that oxidation occurred obviously. However, the average density of the accumulated space charge slightly decreases at the first aging stage and then rises again with the extension of aging time under various DC electric fields. As for external current density, it decreases gradually with the aging time. Besides, the traps in the samples are divided into two types, one is depicted as shallow trap, and the other is the kind with deeper depth according to the isothermal relaxation current obtained in the simultaneous measurement. With the increase of aging time, the depths of both two types of traps slightly change, while the densities of both two types of traps in aged samples are greater than that in the unaged sample. Therefore, the increase of the trap density may retard the growth of carriers in aged samples, which could explain why the external current decreases with the aging time. In addition, the increase of the trap density may restrict the injection of space charge from the electrodes, which results in the experimentally observed decrease of space charge injection depth.

Investigation of surface trap distribution in LDPE/SiO2 nanocomposite based on simultaneous observation of space charge and relaxation current

In this paper, trap distribution variation in low density polyethylene (LDPE) after filled with 1 wt% nanosilica is investigated through the charge decay after corona charging with a needle-plane electrodes system. The LDPE and LDPE/SiO2 nanocomposite were firstly polarized under needle-plane electrodes system. The electrons and holes are injected through the variation of polarization of the needle. Then the surface space charge behavior and relaxation current were measured with a simultaneous measuring equipment, which overcomes the poor correlation of separate measurement. Lastly, the apparent mobility and trap distribution are evaluated based on the decay of surface space charge and relaxation current decay. It is found that the observed surface space charge and current are associated with trap distribution from 0.855 eV to 1.000 eV during decay process from 10 s to 3600 s. In LDPE/SiO2 nanocomposite, the estimated trap density of electrons and holes is more than that of LDPE in a relatively low energy range from 0.855 eV to 0.925 eV. These traps with low energy levels as transport media for carriers may be the main reason for the higher mobility of carriers in LDPE/SiO2 than LDPE, which is testified by surface space charge decay. It is also worth noting that according to the relaxation current decay, the estimated density of the relatively deep hole traps (>0.990 eV) is more than that of electron traps in LDPE, which is not observed in LDPE/SiO2. This is in accord with other scholars molecular simulation result that most hole traps exist in intra-chain areas and electron traps exist in inter-chain areas, reflecting the different transportation characteristics of electrons and holes.