Tree In Silicone Rubber Research Articles

Macromolecular Voltage Stabilizer for Enhanced Initiation Characteristics of Electrical Tree in Silicone Rubber

Improving the electrical resistance of insulating materials is critical for the development of extruded power cables. Incorporating voltage stabilizers into polymeric materials is an effective way to inhibit electrical aging of power cables. However, small-molecular voltage stabilizers usually suffer from poor compatibility with polymers. Herein, we rationally designed and synthesized a macromolecular voltage stabilizer (anthracene grafted polydimethylsiloxanes, An@PDMS) featuring well-dispersed An groups covalently bonded to the curing agent of silicone rubber (SIR). The effects of An@PDMS loading on the electrical performance were investigated. Notably, a significant enhancement (up to 57%) of the initiation voltage of electrical tree was achieved in an optimized An@PDMS/SIR with 1.0 wt% An loading, along with improved ac breakdown strength (BDS), lower dc conductivity, less positive charge accumulation, and concomitant smaller field distortion at room temperature. The highly dispersed and covalently bonded An stabilizer provides sufficient trapping sites for high-energy holes, rendering better insulation performance. Our macromolecular design strategy for voltage stabilizers is attractive for future tailorable engineering polymers.

Temperature Gradient Affecting Electrical Tree in Silicone Rubber under Impulse Superimposed on DC Voltage

In this paper, the electrical tree growth characteristics of silicone rubber (SiR) are studied under impulse superimposed on DC voltage and a temperature gradient (TG) field. The temperature difference between high voltage (HV) and ground (GD) electrodes varies from -90 to 90 °C resulting in a TG field in the samples. The electrical tree morphology, length and accumulated damage are discussed. Experimental results show that with the increase of absolute value of TG, the electrical tree length and accumulated damage continue to increase, and the tree morphology turns from branch-like tree to bush-branch hybrid tree. The positive TG has a more obvious acceleration effect on the tree growth than does the negative TG. Under the TG, the temperature dependent carrier transport results in space charge accumulation and electric field distortion at low temperature side, which is the main reason for the different growth characteristics of electrical tree. Besides, the accumulated damage of DC superimposed with heteropolar impulse voltage is larger than that under DC superimposed with homopolar impulse voltage. It is concluded that the temperature gradient and impulse superimposed on DC voltage promote the electrical tree growth of HVDC cable accessories insulation.

Intrinsic Self-healing Characteristics of Electrical Tree in Silicone Rubber

The self-healing phenomenon of electrical tree in traditional silicone rubber (SiR) insulation material, which is called intrinsic self-healing, is demonstrated and investigated in this paper. For initial SiR samples with electrical trees, with the removal of applied voltage, it is found that some electrical tree branches naturally disappear in the visual field with time. A less branched morphology and decreasing fractal dimension of electrical tree are thus observed. The effects of temperature on the intrinsic self-healing characteristics are explored and the intrinsic self-healing process can be considered as taking place in three stages based on the temperature-dependent self-healing rate. It is proposed that the intrinsic self-healing behaviors are closely related to the gas motion in the tree channels and the retractive force of SiR, which is affected by the conformation of molecular chains. Re-growth performance of electrical tree is conducted on the self-healed samples and many tiny branches at the sidewall of main channels are found in the repeated electrical trees, while the tree length remains unchanged. Furthermore, some electrical trees vanish in self-healing regions after the re-growth process, which can be attributed to the shielding effect of dense branches and the reconstruction of some hydrogen bonds between nano-silica and molecular chains of SiR.

Electrical tree in silicone rubber: Roles of silicone grease and switching impulses

This study investigates the characteristics of electrical trees in silicone rubber (SiR) samples after soaking in silicone grease (SG) for various times and exposed to switching impulses. The results show that the tree initiation probability increases from 60 to 100 % after SG soaking for 340 h when the impulse amplitude is 38 kV and the tree fractal dimension increases from 1.16 to 1.45. A similar trend is observed under negative impulse. It is suggested that SG soaking leads to a higher tree initiation probability and a larger tree fractal dimension. In addition, isothermal surface potential decay measurements are employed to obtain the trap parameters of SiR samples. Equilibrium swelling procedures are used to test the crosslinking density. The energy level and density of deep trap both decrease with SG soaking time, which is beneficial for the transport of charge carriers injected from the needle tip and further results in more severe scission of SiR molecular chains. Moreover, the expansion of free volume originating from the broken physical crosslinking structure allows more violent impact ionization of charges to occur, which can also aggravate the scission of SiR molecular chains. Therefore, the decreased tree resistance can be found in SiR after SG soaking. Furthermore, the polarity effects can be observed during the tree initiation and propagation process, which are mainly rooted in the different distribution of hole­type and electron-type traps.

DC electrical tree initiation in silicone rubber under temperature gradient

In this paper, initiation behaviors of DC electrical tree in silicone rubber were studied under different voltage polarities and temperature gradients (temperature of needle and ground electrodes ranged from 20 to 120 °C). Electrical tree initiation probability, tree shape, and tree length were obtained via the statistics. The results show that polarity effects, where the positive DC voltage is easier for trees to initiate, appear evidently under various temperature situations. As for the temperature gradient, tree initiation voltage decreases first and then returns to a larger value with the increasing needle temperature when the ground electrode temperature is fixed at 20 °C. Both the probability of multiple-branch tree and corresponding tree length increase and subsequently decrease. However, tree initiation voltage decreases alone and tree length increases with the rise of ground electrode temperature when the needle temperature is fixed at 20 °C. Homo charge injection becomes easier with increasing needle temperature following the Schottky-Richardson approximation under ramped DC voltages. In addition, electric field strength at the tip decreases when the needle temperature is higher than ground temperature, since the conductivity is sensitive to the local temperature and electric field. It is believed that the needle temperature and the temperature gradient dominate tree initiation behaviors, where the former affects the collisional ionization and partial relaxation of silicone rubber chains, and the latter decides electric field distribution in the bulk.

Temperature-dependent electrical tree in silicone rubber under repetitive pulse voltage

Silicone rubber (SiR), which is employed as the main insulation materials in the cable accessories, faces the challenge of high temperature during the overload running process. Pulse voltage is inevitable during the switch operation process of the converters or lighting, which aggravates the operation state of the insulation materials. To understand the electrical treeing process at the high temperature with repetitive pulse voltage considering the actual operating state, the needle-plate electrode system was stressed in the experiment. In the test, the ambient temperature (Tamb) was set to 30, 60, 90, 120 and 150 °C, while the pulse frequency was 5, 20, 100, 200 and 1000 Hz. Electrical tree shapes, tree length, fractal dimension (FD), tree inception and tree breakdown probability were studied to investigate electrical tree growth characteristics. The results indicate that the tree shapes are depended not only the temperature environment but also the pulse frequency. Higher pulse frequency and higher temperature facilitate bush tree shape to be generated. With the pulse frequency increasing, the tree length becomes longer, while FD tends to be larger. However, the tree length decreases as the FD increases with the temperature elevating at the same pulse amplitude. As more hot electrons to collide the molecular chain with pulse frequency increasing, the tree inception and breakdown probability become higher. Nevertheless, the tree inception probability tends to be larger as the mechanical property of silicone rubber drops with the temperature elevating, while breakdown probability becomes lower as the shorter tree length grows for a given pulse number.

Effect of Magnetic Field on Electrical Treeing Behavior of Silicone Rubber at Low Temperature

The development of high temperature superconducting (HTS) materials promotes the development of HTS cable. With the development of HTS cable technology, superconducting insulating materials under magnetic field have been taken seriously. This paper presents the study of the relationship between magnetic field and growth characteristics of the electrical tree in silicone rubber. The test was done under -30 °C, -60 °C, and liquid nitrogen temperature. Needle-plate electrode was employed in this paper to generate electrical tree, and silver needle was employed to avoid stress generated by the magnetic field. Samples were applied pulse voltage of +12 kV and +15 kV. 0, 200, and 400 mT magnetic field density (MFD) was applied, respectively. The direction of the magnetic field and the electric field was vertical. Using a digital microscope captured the morphology of the tree. The fractal dimension and the accumulated damage were measured. It is observed that the magnetic field promotes the increase of accumulated damage and tree width increase rapidly with the increase of MFD. The magnetic field will increase the probability of the electrical tree in liquid nitrogen. This shows that the partial discharge activity in the tree channel is influenced by the Lorentz force, the treeing behavior will be different as a result.

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.

Short-Term Breakdown in Silicone Rubber Based Nanocomposites Caused by Electrical Treeing

This paper presents experimental study on short-term breakdown of silicone rubber based nanocomposites with different silicone dioxide (SiO2) nanofiller loading by focusing on the effect of electrical tree. The objective of this study was to investigate the ability of SiO2 nanofiller to inhibit the growth of electrical tree in silicone rubber until breakdown. Samples of silicone rubber based nanocomposites (three samples were filled with 1wt%, 2wt% and 3wt% of SiO2 respectively whereas the forth sample was unfilled silicone rubber), were used in this experimental study and two parameters such as electrical tree initiation voltage and breakdown time were measured. Based on the obtained results, the sample with the highest SiO2 loading has shown the highest tree initiation voltage and the longest breakdown time. Therefore, this makes SiO2 a promising material to be used as fillers in polymeric insulations for the purpose of retarding electrical tree growth.

Electrical Tree Growth in Silicone Rubber/Organo-Montmorillonite Nanocomposites under AC Ramp Voltage

This paper presents the results of the study on the electrical tree growth of organo-montmorillonite (OMMT) nanofillers in silicone rubber under ac ramp voltage of 0.5 kV per second. This study investigates the ability of OMMT to retard the growth of electrical tree in silicone rubber. Samples of silicone rubber nanocomposites (two filled with 1% and 3% OMMT respectfully while the third is unfilled silicone rubber), were used in this experimental study and the growth rate of the electrical tree and its length were observed in both samples. .The result of this study has revealed that in the filled nanocomposite sample, the OMMT acts as barrier which slows down the growth rate of electrical tree. This makes OMMT a potential material to use as fillers in polymeric insulations for the purpose of retarding electrical tree growth.