Electrical Tree In XLPE Research Articles

Characteristics Analysis of Water Needle Induced Electrical Trees in XLPE

In this paper, electrical trees initiated from water and metal needle are investigated, and the inception and morphological characteristics of water needle electrical trees are analyzed. The existing literature shows that electrical trees can be initiated from the metal needle or transformed from the water trees. But in this study, it is found that the electrical tree can be directly induced by water needle in addition to the above cases. Moreover, the comparative results also indicate that the electrical trees initiated from water needle and metal needle have different inception features and morphological characteristics. To obtain the water needle electrical trees initiation and development mechanism, the water needle tips are observed before and after the sample degas process. Besides, the electric field distribution of the metal needle tip, the water needle tip before and after pretreatment, and of tree branch tip during the electrical tree development are calculated. The observations and calculations results show that before the degas pretreatment, a micro-gas cavity is formed at the water needle tip due to the tension of the water surface. The cavity at the needle tip may cause and support partial discharge (PD), causing accumulative damage and material defects. Then the density of defects becomes sufficiently large, and a tree branch can be formed. After the branch formation, the water flows into the tree branch, and the micro-gas remains at the branch tip, which may facilitate the PD, and trees propagation, resulting in the diversified morphological characteristics of the water needle electrical trees.

Electrical treeing behavior in XLPE insulation due to content AL2O3 nanoparticles

XLPE is currently commonly used in high voltage underground cables. Several researchers recently chose several nanofillers to improve the electric tree’s strength in the polymer matrix. Alumina AL2O3 nanofiller have been utilized to investigate the effects on the electrical treeing in XLPE. The percentage concentration were used as follows with different amounts “0.3wt.” % and “1wt.”% from weight of base material. The needle-plane electrodes were used in this investigation and gap selected between needle and plane earth is 3 mm. The growth and morphology of treeing in XLPE insulation have been observed by using charge coupled device camera CCDc and microscope system. Scanning of electron microscopes SEM has been investigated the nanoparticles spread in base material. The outcomes show the tree inception voltage TIV values 12.5, and 14.8 KV “0.3wt.” % and “1wt.”%, respectively in XLPE composites that is mean the TIV increase with increase concentration nanofiller, while the tree propagation time at 2mm length increase about 40 min and 2 hours in “0.3wt.” % and “1wt.”% AL2O3/XLPE, respectively compared with unfilled XLPE, as well as the breakdown time BDT enhancement by4.347% and 13.043% for 0.3wt% and 1 wt% nano AL2O3 composites compared with unfilled XLPE insulation. And showed pictures taken with a SEM Diffusion and accumulation of nanoparticles in the XLPE material.

Characteristics of partial discharge and AC electrical tree in XLPE and MgO/XLPE nanocomposites

In order to investigate the effect of nano-doping on partial discharge (PD) and electrical tree growth in cross-linked polyethylene (XLPE), samples of 0.5 wt% magnesium oxide MgO/XLPE were prepared and an AC experiment was carried out. A color-coded diagram for PD signal was developed to describe the PD activity during the whole process of treeing. The results show that the PD intensity in positive half cycle of voltage (PHC) is larger than that in negative half cycle of voltage (NHC), and the PD magnitude in MgO/XLPE is lower than that in XLPE. Moreover, the tree length and morphology in MgO/XLPE are similar to those in XLPE. Furthermore, a finite element simulation based on the bipolar charge transport model was applied to investigate the effect of space charge on PD under AC voltage. Simulation results suggest that the characteristics of PD and tree growth may be associated with the accumulated charge around the needle tip. The distribution of negative charges in NHC would be wider than that of positive charges in PHC, which would weaken the electric field in NHC and lead to the less PD in NHC. Moreover, the range of charge shield in MgO/XLPE may be larger and is equivalent to increasing the effective radius of curvature of needle tip, which would lead to smaller electric field and lower PD magnitude in MgO/XLPE. However, the addition of nano-MgO reduces the crystallinity of material based on the differential scanning calorimetry (DSC) result. Hence, although the PD magnitude in MgO/XLPE is lower, the MgO/XLPE researched in this paper does not show significant resistance to AC tree growth.

Effect of ZnO Nanofiller in the XLPE Matrix on Electrical Tree Characteristics

The effect of nanofillers on the electrical tree behavior of polymeric materials, such as epoxy resin and polyethylene, has been investigated. Thus, research on electrical tree behaviour is extended but limited only to pure crosslinked polyethylene (XLPE) cable. Electrical tree properties are yet to be explored under various types of nanofillers in XLPE. In this paper, the electrical tree characteristics of XLPE containing zinc oxide (ZnO) nanofiller was reported. The nanofiller concentrations in XLPE were varied to 0.5, 1.0, and 1.5 wt%. Needle-plane electrodes were employed with 2 mm gap between the electrodes. A digital camera and microscope system were used to observe the structure and growth of electrical tree in XLPE. The tree inception voltage (TIV) of XLPE increased with the addition of ZnO nanofiller. The propagation length of tree growth improved compared with the unfilled XLPE. The TIV for the samples with 0.5, 1.0, and 1.5 wt% ZnO nanofiller improved by 7.69%, 14.65%, and 7.16%, respectively, relative to the unfilled XLPE.

Effect of Temperature Gradient on Electrical Tree in XLPE From 0 to −196 °C

Cross-linked polyethylene (XLPE) is widely used in a warm dielectric (WD) superconducting cable. As other insulating materials, electrical tree is a threat to XLPE. When XLPE is used in cable termination connected to a high-temperature superconducting cable and a WD superconducting cable, there will be a steep temperature gradient. With the growth of the tree, the temperature gradient will be influenced by the change of temperature. In the experiment, nine temperature gradients were prepared and applied on the samples. An ac voltage was employed to initiate electrical tree in XLPE samples. Three tree structures have been observed, and they change with the gradient. In the same tree, the branch will also change in respect to its growth. Both the tree length and width are suppressed by the decrease of temperature. With the change of temperature gradient, the decrease of temperature influences the fractal dimension and accumulated damage obviously. In the gradient 1-5, the speed of tree growth in the first 20 min is much higher, while trees in the other gradients grow slowly.

Research of grounded DC electrical tree growth properties in XLPE

Grounded DC tree is a serious threat for the safe operation of XLPE cable. In this paper, growth properties of grounded DC electrical trees in XLPE under ±40kV DC voltage were investigated and anal...

Growth and partial discharge characteristics of electrical tree in XLPE under AC-DC composite voltage

The growth characteristics of electrical trees under DC and AC voltages are quite different. In order to investigate the influences of AC component in HVDC cable system on the electrical tree properties, the growth and discharge characteristics of electrical trees under AC-DC composite voltages were studied in this paper. The results showed that the AC component greatly accelerated the developing process of the electrical trees. The influences of the positive and negative DC bias voltages on the electrical tree growth properties were quite different. The growth rate under negative DC bias voltage was similar to that under pure AC voltage and pine-branch type electrical trees were more likely to form. In contrast, the growth rate of the electrical tree increased with the increase of the positive DC bias voltage and it was much faster than the negative DC biased one. More branch-like electrical trees would form under positive DC bias voltage. When the AC component decreased, the developing process of the electrical tree was fairly tough and it was easy to form bush-like electrical trees, which were the typical conducive electrical trees with fairly small discharges. Under the positive DC bias voltage, there was a fast re-growth process of electrical tree after the bush tree formed, and the positive DC bias voltage could promote the coming of the re-growth process. However, the detected partial discharge during this process was quite small. The test results indicated that it may pose a great threat to the safety of the cable insulation if there is a large AC component in the HVDC cable system.

Mechanism for impulse conversion of water trees to electrical trees in XLPE

A lightning impulse causes substantial capacitive current in a water tree channel which, as a result of its small cross section, has relatively low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV XLPE dielectric cable indicate that an 80 kV lightning impulse can cause the water within a water tree channel to boil over a range of four orders of magnitude in water conductivity, from 10/sup -2/ S/m to 10/sup 2/ S/m. Boiling of the water in a water tree channel reduces substantially the yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial discharge resulting in electrical tree initiation.