J. G. Su

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.

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 and nanoparticles on electrical trees in RTV silicone rubber

The operating temperature of power cable system has a great influence on the electrical performance of room temperature vulcanizing (RTV) silicone rubber. This paper investigated the effects of low temperature and nanoparticles on electrical tree in RTV silicone rubber. Samples were prepared by mixing SiO2 nanoparticles into RTV silicone rubber, with the content of 0, 0.5, 1, 1.5 and 2 wt% respectively. The experiment temperature was ranged from - 30 to -90 °C. AC voltage with a frequency of 50 Hz was applied between a needle-plate electrode. The electrical tree patterns, growth rate, expansion coefficient, fractal dimension and proportion of accumulated damage were studied. The results reveal that both nanoparticles and low temperature environment have a significant impact on the electrical tree growth. The tree patterns are branch tree or bush tree when the temperature is higher than -60 °C; however, there is only pine tree when the temperature is lower than -90 °C. Compared with - 30 °C, some tree channels are easier to propagate at - 60 °C in silicone rubber. It is also suggested that the temperature affects the expansion coefficient of electrical tree. The fractal dimension of the electrical tree increases with the nanoparticle content, while the accumulated damage of treeing area shows the opposite tendency.

Effects of ambient temperature on electrical tree in epoxy resin under repetitive pulse voltage

Chosen as an important insulation material in high voltage direct current (HVDC) cable terminal, epoxy resin bears the harsh environment of high temperature and changing pulse power. This paper reports on investigations into the effect of high temperature on characteristics of electrical tree growth. Samples made of epoxy resin and equipped with a needle-plate geometry electrode system were stressed with pulse voltage whose amplitude was 12, 14 and 16 kV and frequency was 200, 300 and 400 Hz. The ambient temperature was set to 60, 90 and 120 °C. Four types of typical tree growth process were partitioned to better describe the different growth characteristics. Typical morphology, tree length, fractal dimension, accumulated damage and expansion coefficient were employed to characterize the electrical tree. The initiation and breakdown characteristics were also analyzed. The results indicate that temperature, pulse frequency and pulse amplitude affect electrical tree growth. Higher temperature would lead to more complex tree morphology and promote the growth rate of electrical tree. When reaching the glass transition temperature, which is 93 °C in this paper, the change of internal state of epoxy resin would cause some special characteristics of tree growth. In addition, inception probability increased with temperature rising, whereas the time to breakdown decreased. Meanwhile, obtained results show that pulse frequency and pulse amplitude also play an important role in promoting electrical tree growth processes, including initiation, propagation and breakdown stages.

Effects of mechanical stress on treeing growth characteristics in HTV silicone rubber

Tensile and compressive stress, produced by the spring clamp device and the banding force after the expansion of silicone rubber stress cone, can affect the electrical properties of silicone rubber in the cable accessories. Electrical tree, one of the main electrical aging phenomena, is supposed to be related to the mechanical stress. In this paper, the treeing growth characteristics considering the tensile and compressive stress were investigated by the pin-plane electrode system respectively. The tensile rate of silicone rubber ranged from 0% to 30%, while the compressive rate varied from 0% to 50%. The results indicate that both the tensile stress and compressive stress affect the treeing propagation characteristics. The appearance probability of bush tree increases and it becomes the main tree structure at higher tensile rate, while branch tree turns into the main tree structure at higher compressive rate. Electrical tree initiation probability becomes higher and the initial time tends to be shorter under the tensile stress, while they show an opposite tendency under the compressive stress. Under tensile stress, the tree length, fractal dimension and accumulated damage become lager as the tensile rate increasing, which indicates that the higher tensile rate accelerates the treeing process. However, under the compressive stress, the treeing growth process shows a converse trend, which demonstrates that the higher compressive rate retards the electrical tree growth.

Effects of high temperature and pulse voltage on treeing process in silicone rubber

High temperature environment produced in the process of high load, accompanying with pulse voltage generated by switching surges during the switch operation of the converters in the HVDC system, has an important influence on the electrical performance of silicone rubber (SiR) in the cable accessories. The pin-plane electrode system was employed in the test to investigate the treeing characteristics in silicone rubber considering high temperature under repetitive pulse voltage. Results show that when the pulse frequency increases, the tree length and fractal dimension tend to be larger and the main tree morphologies are varied from branch tree to bush tree with application 12 kV pulse amplitude at 30 °C. When the temperature becomes higher, the tree length decreases while the fractal dimension shows a converse tendency. When the temperature is below 90 °C, both branch tree and bush tree are appeared application 9 or 12 kV. However, when the temperature is above 90 °C, the bush tree becomes the lead tree morphology at 200 Hz after 10000 pulse number.

Surface Layer Fluorination-Modulated Space Charge Behaviors in HVDC Cable Accessory

Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.