T. Han

Effect of ambient temperature on electrical treeing characteristics in silicone rubber

In this paper, electrical treeing was investigated in room temperature vulcanized (RTV) silicone rubber (SiR) over a range of ambient temperatures. An ac voltage with a frequency of 50 Hz was applied between a needle-plate electrode to initiate the electrical tree at different ambient temperatures. Both the structures and the growth characteristics were observed by using a digital microscope system. Obtained results show that the tree initiates from a single branch with a white gap. Four typical tree structures, namely branch, bush, pine branch and bush-pine mixed tree, were observed within the sample. The occurrence of tree structures changes with the increase of ambient temperature, in which branch tree takes up a great proportion at 30°C while bush tree becomes dominated as the temperature rises up to 90°C. Meanwhile, the cumulative inception probability within the same time decreases obviously with the increase of ambient temperature. The growth rate of the tree is closely related to the ambient temperature. It is suggested that the increase of vulcanization network density and elastic modulus with ambient temperature may have great influence on the treeing characteristics (including growth rate, fractal dimension and treeing proportion).

Tree characteristics in silicone rubber/SiO 2 nanocomposites under low temperature

Silicone rubber (SiR) has been widely used in XLPE cable accessories because of its excellent electrical and mechanical properties. The electrical tree is a serious threat to SiR insulation and it can even cause the insulation breakdown. Addition of nanoparticles into SiR can improve the insulating properties compared with undoped material. The effect of nanoparticles on tree characteristics at temperatures above 0 °C has been widely researched. However, the effect under low temperature has not been researched. In this paper, electrical treeing process in SiR/SiO2 nanocomposites was investigated over a range of low temperatures. The samples were prepared by mixing nano-SiO2 into room temperature vulcanized (RTV) SiR, with the content of 0, 0.5, 1.0, 1.5 and 2.0 wt% respectively. The experiment temperature ranges from -30 °C to -90 °C. AC voltage with a frequency of 50 Hz was applied between a pair of needle-plate electrodes to initiate the electrical tree at different experiment temperatures. Both the tree structures and the growth characteristics were observed by using a digital microscope system. The experiment results indicated that both nanoparticles and low temperature are important factors of the treeing process in SiR/SiO2 nanocomposites. The distribution of tree structures depends on the content of nanoparticles and temperature. Nano-SiO2 can repress the tree growth effectively and the optimum content with the lowest tree growth speed is 1.5 wt%. Crystallization caused by the changing temperature also influences the treeing process.

Effect of low temperature on tree characteristics in silicone rubber with different power frequency

Electrical tree is one of the main reasons for long-term degradation of SiR cable accessories used in high voltage ac applications. In this paper, an investigation of electrical tree growth characteristics in SiR samples is reported. Ac voltage with frequency ranging from 50 to 200 Hz was applied between a pair of needle-plate electrodes to initiate electrical trees at temperature ranging from 30 to -100 °C. Both the tree structures and growth characteristics have been observed by using a digital microscope system. Three typical profiles of electrical trees, namely branch, bushbranch and pine branch tree, have been observed within the samples. The growth rate and accumulated damage have been employed to analyze the treeing process. It is observed that the high power frequency will lead to the occurrence of bush-branch tree and the low temperature will lead to the occurrence of pine branch tree. Meanwhile, the tree inception probability increases from 30 to -60 °C and then decreases fast. The high frequency will improve the tree inception probability under the same temperature. During the experiment, breakdown has occurred in some samples and the width of breakdown channels is related to the frequency.

Electrical tree characteristics in silicone rubber under repetitive pulse voltage

Electrical tree in silicone rubber (SiR) is a threat to the power system. Bedsides ac and dc voltage, there are pulse voltage in power system and the electrical tree behavior in SiR under pulse voltage has not been reported. In this paper, electrical treeing process in SiR under pulse voltage was investigated and analyzed. The voltage was applied to a needle-plate electrode to initiate electrical trees. The frequency of pulse voltage was 100, 200 and 300 Hz and the amplitude ranges from 8 to 14 kV. Aspects such as the patterns of electrical tree, tree length, accumulated damage, fractal dimension and tree breakdown characteristics were studied. Results indicate that the pulse amplitude plays the leading role in electrical tree initiation, propagation and breakdown processes. With the increase of pulse amplitude, the tree structure changes and the breakdown possibility increases obviously. However, the tree length is not sensitive to pulse frequency and higher pulse frequency promotes the increase of tree density. It also has been found that tree structure is closely related to pulse polarity and the inception voltage is lower with positive pulse than that with negative.

Effect of temperature on electrical tree in silicone rubber

In this paper, room temperature vulcanized (RTV) silicone rubber (SiR) was employed as test sample to investigate the relationship between electrical tree propagation characteristics and the experiment temperature. Both the structures and growth characteristics of electrical tree in SiR were observed by using a digital camera and a microscope system. Obtained results show that electrical tree in RTV SiR is white gap tree channel, which all initiate from single branch, and the width of initiative single branch channel varies a lot with the electrical tree structure. The structure of electrical tree at experiment temperature from 30°C to 90°C in RTV SiR can be classified into four categories. They are branch, bush, pine branch and bush-pine mixed tree. The distribution of tree structures changes with the experiment. At the temperature of 30 °C, branch tree take up a great proportion, however, bush tree becomes the dominant structure when the temperature rise up to 60°C and 90°C. The probability of tree initiation decreases obviously with the rise of the experiment temperature from 30°C to 90°C. All kinds of electrical tree grow rapidly in the first beginning of the treeing propagation, and this process lasts only a few minutes. In addition, a new parameter, the treeing proportion is introduced to describe propagation characteristics of bush tree.

Tree initiation characteristics of epoxy resin in Ln 2 for superconducting magnet insulation

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which ensures the feasibility of fusion energy application. Epoxy resin has been used as the adhesives and electrical insulation in the magnet system. Since ITER device operates in the liquid nitrogen environment and is driven by a pulse power, the insulation made of epoxy resin faces the challenges of extremely low temperature and pulse voltage. This paper investigated the influence of low temperature on the electrical tree characteristics in epoxy resin. Positive pulse voltage with the frequency of 400 Hz was applied on the needle-plate geometry electrodes. The amplitudes of the pulse voltage were 10, 12 and 14 kV. In order to simulate the practical defects, a metal needle was inserted into the samples and the distance between the tip and plane was 2 mm. The experimental temperature ranged from 30 to -196 °C. The experimental results indicate that the low temperature have a significant influence on the electrical tree characteristics. Different tree structures in epoxy resin are observed with the changing temperatures. The density of the tree branches increases as the temperature decreases, while the tree growth rate presents an opposite trend. It is also revealed that the time to breakdown become long under the low temperature. However, once the breakdown occurs, a wide breakdown channels will be left in epoxy resin, which means great destruction to the electrical insulation. In addition, when the samples are under the same ambient temperature, the probability of tree initiation shows a significant increase and electrical trees grow faster in the following period with application of higher pulse voltage. The higher-energy charges may be responsible for the high growth rate.

Effects of magnetic field on electrical tree growth in silicone rubber under repetitive pulse voltage

Electric field, accompanying with the magnetic field produced by high current, can generate electro-magnetic force in HVDC system, which enormously affects the treeing process. Electrical tree behavior of silicone rubber (SiR) was investigated by application of magnetic field with repetitive pulse voltage. The samples were made of silicone rubber with a pin-plane electrode system. The inserted needle had a tip radius of 3 μm and the pin-plane distance was 2 mm. The rise time and fall time of the repetitive pulse voltage were 100 and 120 μs respectively. The pulse frequency was 200 Hz, while the pulse amplitude ranging from 6 to 12 kV was applied. The magnetic flux density (B) of the magnetic field was 100, 200 and 400 mT respectively. The results were presented from experimental investigations in order to characterize electrical tree as a function of amplitude and polarities of the applied pulse voltage. The patterns of electrical tree, tree length, accumulated damage and tree breakdown characteristics were studied. It is revealed that both the pulse amplitude and polarity have a significant impact on electrical tree growth characteristics of silicone rubber. Results show that the pulse amplitude plays the leading role in electrical tree initiation, propagation and breakdown processes. Compared with the positive pulse, it is suggested that the larger tree accumulated damage is more easily to occur with the negative pulse. It also has been found that tree structure is greatly dependent on the B of magnetic field. Results show that magnetic field promotes tree growth and accelerates the treeing process.

Effects of low temperature on treeing phenomena of silicone rubber

In this paper, room temperature vulcanized (RTV) silicone rubber (SiR) was employed as test sample to investigate the relationship between electrical tree propagation characteristics and the low experiment temperature. Power frequency voltage was applied on the SiR specimens through the needle-plate electrode with the same radius of needle tips to initiate the electrical tree at different low experiment temperatures. Both the structures and growth characteristics of electrical tree in SiR were observed by using a digital camera and a microscope system. Obtained results show that electrical tree in RTV SiR is white gap tree channel which maybe composed of silicone compounds instead of carbonized channel in XLPE. Electrical tree in SiR all initiate from single branch, and the width of initiative single branch channel varies a lot with the electrical tree structure. The structure of at experiment temperature from 0 °C to -70 °C in RTV SiR can be classified into three categories, which are branch, bush and pine branch tree. The distribution of tree structures changes with the experiment temperature. At the temperature of -70 °C, pine branch tree take up a great proportion, however, bush tree becomes the dominant structure when the temperature rise up to -50 °C and -30 °C. All kinds of electrical tree grow rapidly in the first beginning of the treeing propagation, and this process lasts only a few minutes.

Effects of nano filler on treeing phenomena of silicone rubber nanocomposites

Polymer nanocomposite has recently drawn considerable attention because nanocomposites or nanostructured polymers have the potential of improving the electrical, mechanical, and thermal properties as compared to the neat polymers [1]. In this paper, the polymer nanocomposite was made by the mixed addition of SiO2 nanoparticle with the radio of 20 nm into room temperature vulcanized (RTV) silicone rubber (SiR), with the filler content from 0 to 5 wt% respectively. The typical needle-plate electrode was employed to investigate the relationship between electrical tree propagation characteristics and the content of nano filler with the ac voltage of 50 Hz. Both the structures and growth characteristics of electrical tree in SiR were observed by using a digital camera and a microscope system. Obtained results show that the distribution of tree structures is different between base SiR and its nanocomposites specimens. The growth speed varies a lot with the content of nano fillers, and there is an obvious improvement in tree initiation time as the increase of filler content. The possible reasons for the improvement in electrical tree growth and initiation time with the addition of nano fillers are discussed.

Interface charge distribution between LDPE and carbon black filled EPDM

Cable accessories made of ethylene-propylene-diene terpolymer (EPDM) are considered to be the weakest part of HVDC cable system due to the existence of the interface between cable insulation and itself. The charges are likely to accumulate at the interface between insulation materials with different conductivity and permittivity, which may induce the occurrence of partial discharge and breakdown. Nanoparticles can be applied to adjust the interface charge behaviors. In this paper, carbon black (CB) nanoparticles were dispersed into EPDM with 0, 0.5, 1, 3 and 5 wt% respectively. The effects of nanoparticle doping on the interface charge distribution of LDPE and EPDM composite insulation were measured under -15 kV/mm. Furthermore, dielectric properties and DC conduction were introduced to discuss the suppression mechanism of carbon black nanoparticle doping. Obtained results the interface charge density can be suppressed to 0.5 C/m3 with 1 wt% carbon black nanoparticle doping, much less than the undoped and other doped groups. The mechanism of interface charge suppression is attributed to the decrease of conduction and permittivity mismatch by carbon black nanoparticle doping, which was proved by the dielectric spectroscopy and conduction current tests. The interface charge buildup depends on the time constant controlled by the doping proportion.