Xuguang Li

Effect of temperature on space charge trapping and conduction in cross-linked polyethylene

This paper presented the measurement results of space charge distribution and high field conduction in cross-linked polyethylene (XLPE) plate sample at various electric fields for a broad temperature range from 30 to 90 °C. The temperature effect on charge trapping and transport mechanism is discussed based on estimation of threshold characteristic and apparent mobility. It is shown that the threshold field for space charge accumulation apparently decreased with the temperature, probably associated with acceleration of charge injection and ionic dissociation. The apparent mobility of charges showed an exponential dependence on temperature, which indicates enhancement of charge detrapping and transfer rate at higher temperature. The field dependence of the space charge decreases with the temperature, results in more space charge accumulate at room temperature than at high temperature at certain high field. This behavior indicates that the space charge formation rate increases slower than the charge detrapping rate as the temperature increases. Besides, negative charge is always dominant in the sample at the measuring field and temperature range. This is probably due to stronger electron injection at XLPE-aluminum interface or more negative ion formed by dissociation of chemical species.

The Effect of Electrically Prestressing on DC Breakdown Strength in the Nanocomposite of Low-density Polyethylene / nano-SiOx

Nano-SiOx and micro-SiO2 particles were homogeneously dispersed in low-density polyethylene with double-solution method. The dc breakdown strength of the composite containing various contents of nano-SiOx was experimentally investigated, as well as the composite containing micro-SiO2. The breakdown strength of the nanocomposite is greater than that of pure LDPE. For example, the breakdown strength of the nanocomposite is 16% greater than that of pure LDPE. However, the breakdown strength increment of the nanocomposite decreases as the nano-SiOx content increases. The breakdown strength of the composite containing micro-SiO2 is lower than that of pure LDPE and the breakdown strength decrement increases as the micro-SiO2 content increases. It is very interesting that electrical prestressing enhances the breakdown strength of the nanocomposite and the more content, the more enhancements. As the nano-SiOx content reaches 3%wt, the breakdown strength of prestressed nanocomposite is 20% higher than that of non-prestressed nanocomposite. However, electrical prestressing decreases the breakdown strength of microcomposite, and it could hardly find any distinct content effect on the breakdown strength in the prestressed microcomposite. Finally, the different prestressing effects on breakdown strength in both kinds of composites are discussed with the different interface characteristic in both composites.

Effect of surface fluorination on space charge behavior at LDPE/EPDM interface

Joints and terminations in polymer insulated HVDC cable system have been identified as weak points with much higher failure probability than the cable itself. This is mainly due to space charge accumulation at interfaces between different insulating materials. According to the Maxwell-Wagner-Sillars polarization theory, the accumulation of interfacial charge depends on conductivity and permittivity mismatch. Fluorination had been reported as a surface modification method of polymer dielectrics, which may lead to remarkable increase in permittivity and conductivity, thus could be a possible way to reduce space charge at the insulation interface of cable and its accessory. This paper presents the results of pulsed electro-acoustic (PEA) measurement of space charge distribution in LDPE/EPDM double-layer sample subjected to a DC field. Original and fluorinated LDPE layer were both used to study the effect of surface fluorination on space charge behavior at LDPE/EPDM interface. The results show that a significant amount of space charge accumulated at the interface of the original-LDPE/EPDM sample. And the property of the interfacial charge changed a lot after the LDPE layer is surface fluorinated.

Space Charge in Low-density Polyethylene /micro-SiO2 composite and Low-density Polyethylene/ nano-SiOx composite with different metal electrode pairs

Nano-SiOx and micro-SiO2 particles were homogeneously dispersed in low-density polyethylene with a method of double-solution mixture. Both kinds of the composites were sputtered with Au and Ag on both surfaces as electrodes, respectively. Space charge in the composites was investigated with pulsed electro-acoustic method (PEA). It is shown that space charge distribution in the sample with different metal electrode pairs relates with the electrode polarity. The amount of hetero-polarity space charge in pure LDPE after being electrically prestressed with Au (+)~Ag(-) electrode pair is greater than that with Au (-)~Ag(+) electrode pair. However, homo-polarity space charge appears in the microcomposite after being electrically prestressed with either Au (+)~Ag (-) or Au (-)~Ag (+) electrode pair, and the amount of space charge increases with micro-SiO2 content. It also could be seen that the amount of homo-polarity space charge in the microcomposite with Au(-)~Ag(+) electrode pair is greater than that with Au(+)~Ag(-) electrode pair. With electrically prestressing, hetero-polarity space charge appears in the nanocomposite with either Au (+)~Ag(-)or Au(-)~Ag(+) electrode pair and the amount of space charge increases with nano-SiOx content. In addition, the amount of space charge in the nanocomposite with Au (-)~Ag(+) electrode pair is lower than that with Au(+)~Ag(-) electrode pair. The space charge distribution difference in all samples with different electrode pairs has close relation to electron and (or) hole injection efficiency of different electrode material, as well as charge trapping and detrapping process in the interfaces of the samples.

Temperature effect on space charge dynamics in LDPE/MgO nanocomposite under DC stress

A pulsed electro-acoustic method (PEA) of space charge measuring system was developed, which could be used to investigate space charge distribution in solid dielectric at a temperature high up to 383 K. With the measuring system, space charge dynamics in the composite of low density polyethylene (LDPE) and nano-MgO which is expected to be used as an insulation of HVDC plastic insulated cable was investigated from room temperature to 353 K in detail. It is shown that the amount of space charge decreases significantly at a higher temperature in all samples including the LDPE sample. However, it should be pointed out that the amount of space charge in samples with nano-MgO is lower than that in LDPE and the shape of space charge in the composites varies with temperature. In order to know space charge decay rate, space charge in all samples after being pre-stressed for 1800s was measured and it is found that the decay rate increases significantly as temperature increasing.

Charge carrier transportation in the composite of Nano-MgO and cross-linking polyethylene

Space charge behavior of the composite of polyethylene and nano-MgO was investigated with pulsed electroacoustic method (PEA). It is found that space charge was well suppressed as the content of nano-MgO is less than 2.0%wt. Also it could be seen that bi-polar injection occurs in the composite as electrical stress increases to 60 kV/mm. Comparing to the amount of space charge in the short-circuited pure cross-linking polyethylene, it is very low in the composite. It is more interesting that semiconductive electrode has a great effect on space charge in both cross-linking polyethylene and the composite. In order to know space charge injection threshold, dc conduction was measured at various electrical stresses. And it is found that nano-MgO increases the threshold.

Computer simulation and analysis of electric and temperature fields of HVDC cables

HVDC cables play a more and more important role in interconnecting national grids and HVDC flexible power system. This paper deals with the calculation of electric and temperature fields in HVDC cables. The calculation of electric and temperature in an HVDC cables is far more complex than the equivalent case in AC cables. This is due to the fact that the conductivity of the cable insulation materials is temperature and field dependent, due to this fact that electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive or resistive nature. Electric field distribution is also influenced by temperature field. This even may lead to an instability that causes breakdown of the cable. The calculation and analysis of the temperature field distribution and its influence on the electric fields is carried out.

Electron Swarm Parameters in SF6 and Krypton Gas Mixtures

The electron swarm growth processes in SF6-Kr gas mixtures have been studied using a pulsed Townsend method over the range 32.24≤E/N≤564.2Td (1Td=10-21Vm2), where E is the electric field and N is the gas density of the mixture. The variation patterns as a function of the density-reduced electric field of the effective ionization coefficient α, electron drift velocity Ve and longitudinal diffusion coefficient DL in SF6-Kr gas mixtures have been determined. The dielectric strength of SF6-Kr gas mixtures which varies linearly with the SF6 concentration in the gas mixtures Has also been determined.

Effect of nanosilica grain size on the trap density distribution in LDPE/silica nanocomposite

In this paper, the effect of nano-additive grain size on trap density distribution in low-density polyethylene (LDPE) and LDPE/silica nanocomposites was investigated with depolarization current. The concentration of hydrophobic nanosilica was from 1.0 to 5.0 wt% and the average particle size was 7 nm and 16 nm, respectively. The depolarization current of pure LDPE acquired from 0.1 to 3600 s is associated with deep trap depth from 0.71 to 0.98 eV. Three trap peaks are obviously observed in trap distribution evaluated from isothermal relaxation current theory (IRC). It indicates that depolarization relaxation current measured is originated from detrapping of charge in these three types of deep traps. The density of deep trap significantly reduces after introduction of nanofiller, evaluated by the IRC theory and the ε proportional to trap density. It means that the introduction of nanofiller could suppress the formulation of deep trap. Moreover, the density of deep trap is smaller in nanocomposite filled with 7 nm nanofiller, compared with that filled with 16 nm nanofiller. It indicates that the suppression of deep trap is more effective for nanofiller at smaller particle size.

The condition assessment system of XLPE cables using the isothermal relaxation current technique

The isothermal relaxation current (IRC) technology is a powerful method for assessing the condition of power cable insulation. In this paper a condition assessment system was designed to meet the diagnosis of XLPE cables insulation. The assessment system consisted of the IRC measurement system and IRC analysis system. The processes of IRC measurement, such as polarizing voltage, discharging and depolarization current measurement, were controlled by software. The third order exponential decay model was utilized in analysis software to process measured IRC data. The parameters of each kind of depolarization current component and the relationship between I*t and log (t) were showed directly in software for evaluating the aging status. Based on the empirical aging factor (A-factor), the aging level of cables insulation is obtained after data analysis. At last, one part of three-phase AC XLPE cable was measured used this system. The result shows that the system is precise and the diagnostic criteria of A-factor should be corrected to meet the XLPE cable made in china.