Showing papers by "Junbo Deng published in 2018"

Surface charge distribution patterns of a truncated cone-type spacer for high-voltage direct current gas-insulated metal-enclosed transmission line/gas-insulated metal-enclosed switchgear

Abstract: Surface charge accumulation on spacer is an important factor to cause surface flashover. An experimental set-up is established to measure surface potential over a truncated cone-type spacer under DC voltage in SF 6 gas at atmospheric pressure using a Kelvin vibrating electrostatic probe. The surface charge density is calibrated based on multipoint measurement technique, which takes the influence of electrostatic probe into account. The experimental results show that surface charge distribution patterns can be divided into three classes including centrosymmetric distribution, spot-like distribution, and cloud-like distribution. The symmetrically distributed charges are injected by electrode, which migrate along spacer surface through the surface conductivity. While the charges showing spot-like and cloud-like distribution are generated by the ionisation of SF 6 due to local electrical field concentrated. Besides, the hetero-charges are more easily to be saturated with different charging durations and the polarity of the net charge is the same as that of applied voltage.

Research on aging parameters of XLPE cable based on isothermal relaxation current

Abstract: Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation (type 2), which is from 0.785eV to 0.82eV, increases at low temperature (120°C) due to the breaking of molecular chain and decreases at high temperature (more than 140°C) due to the over cross-linking. The peak trap level of impurities interface relaxation (type 3) is highly influenced by aging temperature, which is most sensitive to the aging degree of power cable. It is considered that IRC can be used as a practical way to judge cable insulation in the future.Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation ...

Transient Overvoltage in 10kV Hybrid OHL-Cable System during Energization

Abstract: With increasing number of cables adopted in 10kV distribution system, numbers of OHL-cable hybrid lines occur in distribution networks. The power supply is interrupted frequently due to insulation breakdown of cables caused by transient overvoltage in distribution networks. Accordingly, characteristics of transient behavior during line energization are investigated in this paper. The amplitude and frequency of transient overvoltage with different cable shares in hybrid line are studied. In the meantime, 100 simulations are performed considering the randomness of closing timings of circuit breakers (CB). Statistical results reveal that the most severe overvoltage occurs when cable share in hybrid line is 20%. This is due to the fact that characteristics of energy redistribution during line energization are different when impedance and admittance of transmission lines change. Moreover, the dominant frequency 19.6kHz is observed. Furthermore, different load rates are considered since CB exists only at the sending end of line. Statistical results reveal that transient overvoltage is lower while load rate is higher. And the dominant frequency 15.1kHz is observed, which corresponds to the circle time 0.066ms. This work has important value for investigation of electromagnetic transient phenomenon and insulation coordination of hybrid line in 10kV networks.

Analysis of Statistical and Frequency Characteristics of Transient Overvoltage of Hybrid Cable-OHL Lines

Abstract: Since increasingly more cables are adopted in distribution line, more hybrid cable-OHL lines are used in distribution network. Due to the difference of impedance and admittance of hybrid line compared with pure OHL, the energy redistribution is affected during line re-energization. This presentation focuses on the transient overvoltage during line re-energization with the use of EMT -type program. Firstly, due to the randomness of closing timings of circuit breaker, statistical distribution of reclosing overvoltage is studied. It is found that most severe overvoltage appears when cable share is 20%. Moreover, the most severe overvoltage can reach up to more than 3p.u, and its dominant frequency is 20.67 kHz. Furthermore, the influence of source network on the characteristic of reclosing overvoltage is studied as well. This work has reference value on insulation coordination and reclosing strategy in hybrid cable-OHL line.

Influence of Metal Particles on Surface Charge Accumulation Characteristics of Spacer in SF 6 /N 2 Mixtures under Impulse Voltage

Abstract: When metal particle is attached on the surface of spacer, it will affect the surface charge accumulation and electric field, and reduce the surface insulating level of insulators. Therefore, it is important to research the relationship between attachment of metal particle and surface charge accumulation. In this paper, the influence of metal particle on surface charge accumulation characteristics of spacer in 5atm 20 % SF 6 /N 2 is researched. In the experiment, a cylindrical copper wire of 0.5mm diameter and variable length is fixed on the surface of a downsized truncated cone-type spacer, and one end of this wire is in contact with the high voltage electrode. A lightning impulse voltage with different amplitudes is applied to the central electrode for 5 times, then the surface potential of spacer is measured by a Kelvin vibrating electrostatic probe, and the corresponding surface charge density is further calculated by a reversed algorithm. The results reveal under lightning impulse voltage there are just bipolar charges around the metal particles and random charges existing. When the metal particle is over 5mm, a randomly distributed charge will result in a decrease in the amount of accumulated charge near the metal particles. In addition, the amount of bipolar charge accumulated near the metal particles increases with the increase of the applied voltage amplitude, and the accumulating range of the homo-charges at the ends of the metal particles increases as the applied voltage amplitude increases.

Insulation property analysis of cross-linked polyethylene based on density functional theory

Abstract: In recent years, molecular simulation technology, as the third mothed besides experiment and analysis, has been widely used in various fields. In the field of high voltage, molecular simulation is widely used in oilpaper and polymer insulation characteristics analysis, pyrolysis analysis and so on. In this paper, in order to clarify the electric aging mechanism of cross-linked polyethylene(XLPE), a simple XLPE molecular model is established with the ORCA package and the basic sets 3–21G, 6–31G and 6–311G(d, p) are utilized to optimize and get the most stable structure. We have chosen the band length, band gap and charge transfer within the molecule to characterize the XLPE molecule's status under electric field. The results show that the structural parameters and energy of the molecule present linear changes with electric strength. Especially, the band gap presents a downward trend, which means the insulation property of the material has decreased with the electric strength increases. These phenomena reveal the aging mechanism of XLPE molecule under electric field conditions, which is of great significance to the status evaluation of XLPE cable.

Surface Charge Accumulation Behavior on GIL Spacer Under DC Voltages in SF 6 /N 2 Mixtures

Abstract: The accumulated charge on insulator surface is an important factor to trigger surface flashover. A surface charge experimental setup as well as measurement system under high gas pressure is built. An active electrostatic probe (Trek 3455ET, Model 341B) is used to measure surface potential distribution on a downsized cone-type spacer in SF 6 /N 2 mixtures of different pressure under DC voltage. The actual charge density distribution is computed by the charge inverse algorithm, which considers the impact of the probe. The surface charge distribution shows similar shape at different pressure, including the homo-charges presenting a symmetrical distribution and a streak-like distribution. For the home-charges presenting a symmetrical distribution, they come from surface conductivity. For the streak-like distribution, they are located between central and grounded electrode. These charges come from the ionization of SF 6 due to the local electrical field concentrated.

The evolution of surface charge distribution on GIL spacer under DC voltages

Abstract: The charges accumulated on spacer surface is an important factor to cause surface flashover. To investigate surface charge distribution characteristics of GIL spacer, a downsized truncated cone-type spacer is designed and manufactured. An experimental and measurement system is established to scan surface potential of this spacer under DC voltages in SF 6 by electrostatic probe method. A calibration method with high resolution is built to compute the actual surface charge distribution. There are two classes of charge distribution observed on spacer surface, including centrosymmetric distribution (the first class), streak-like distribution and spot-like distribution (the second class). It is analyzed that surface conductivity, SF 6 gas conductivity and volume conductivity contribute to different classes of charge distribution respectively.

Space Charge Analysis of Polyethylene with Chemical Defects Based on Density Function Theory

Abstract: In recent years, first principle calculation has been widely used in various fields including condensed matter physics, theoretical chemical to get a microscopic and intrinsic understanding of different material and chemical reaction, leading to the formation of a brand new subject, quantum chemistry. In the field of high voltage and insulation, first principle calculation or quantum chemistry have been applied to areas of charge trapping, charge injection and voltage stabilizer. In this paper, in order to clarify charge trapping mechanism of polyethylene, several alkanes with different chemical defects such as carbonyl, alcoholic group have been established with the help of quantum chemistry. The results show that the trapping sites have a strong relationship with the structure of the molecular. The frontier orbitals are both localized around the defects structures and the partial density of states(PDOS) of defect fragments accounts for the vast majority of total density of states(TDOS) at frontier orbital levels. Besides, the electrostatic potential around the defects structures on the van der Waals (VDW) surface are both negative which means these places tend to be holes traps.

Comparison of time-dependent variation of surface charge accumulation in SF 6 /N 2 and pure SF 6

Abstract: SF 6 /N 2 Mixture has good insulation properties and reduces the use of SF 6 , thus it has drawn more and more attention. In addition, Surface charge accumulation on insulating materials is a major factor that greatly distorts the electrical field and reduces the insulation level. Therefore, it is necessary to research the characteristics of surface charge accumulation in SF 6 /N 2 mixture. In this paper, the difference between the time-dependent variation of surface accumulated charge of spacer in 5atm pure SF 6 and that in 5atm 20% SF 6 /N 2 is researched. The downsized truncated cone-type spacer made of 70% Al 2 O 3 filled epoxy resin and coaxial cylinder electrodes are used to model a GIL. In the experiment, ±30kV and ±40kV DC voltage are applied to the central electrode for 30–1080 min, then the surface potential of spacer is measured by a Kelvin vibrating electrostatic probe, and the corresponding surface charge density is further calculated by a calibration method. The results reveal that in both gases, the surface accumulated charge exhibits a time-dependent variation characteristic that the local fluctuates and the whole tends to saturate. Under the DC voltage of same amplitude and same applied time, the total amount of surface charge in 20% SF6/N2 is far more than that in pure SF6. However, the maximum of surface charge density in 20% SF6/N2 is not much different from that in pure SF6 under the same condition of DC voltage. Moreover, in pure SF6, the total amount of surface charge under negative DC voltage is far more than that under positive DC voltage, which means there is polar effect in pure SF6. However, in 20% SF 6 /N 2 the polar effect does not exist.