Electrical Tree Length Research Articles

Combined Effects of Electrical and Mechanical Stresses on Insulation Breakdown—Part I: Tree Growth of Epoxy Resin

In this study, the treeing characteristics of epoxy resin were investigated by stressing the sample parallelly or vertically to the needle axis. Results show that tensile stress accelerates the tree branch growth, and under a proper compressive stress, the tree branches are closed and suppressed. However, when the compressive stress exceeds a threshold, the shear stress plays a dominated part, which promotes the tree branches to grow in a certain angle. When the mechanical stress is vertical to the needle axis, it has a significant influence on the electrical tree length. And when the mechanical stress is parallel to the needle axis, it mainly influences the electrical tree width. After insulation breakdown, cracks are produced due to the released energy from the short-circuit current. And the cracks are vertical to the tensile stress but parallel to the compressive stress. The research results in this paper are very significant for analyzing the breakdown of the tri-post insulators in GIL.

Electrical degradation dynamics of glass fiber-reinforced epoxy composites considering different fiber orientation

Glass fiber-reinforced epoxy composites (GFREC) have been widely used in the mechanical components of electrical and electronic devices because of its lightweight, high mechanical strength, excellent electrical insulation properties and corrosion resistance. The research on the electrical degradation dynamics of GFREC is of great significance to avoid the failure of electrical insulation system. Herein the composite samples were prepared using vacuum impregnation technique. The electrical degradation morphology and statistical characteristics (electrical tree length, width, expansion coefficient, breakdown probability and breakdown time) were discussed. The results show that the electrical tree will develop towards the fiber side and grow rapidly along the interface in GFREC. The interfacial orientation will affect the growth characteristics of electrical tree. When the angle between the glass fiber and the needle electrode is 45°, the composite material has relatively superior tree propagation resistance performance, lower breakdown probability and higher breakdown time. The effects of electric field distribution on the tree initiation and electrical degradation are discussed, which reveals the relationship between the electrical degradation and the fiber orientations. The electric field-driven tree growth (FDTG) model is used to explain the growth of trees along interfaces with different orientations from the perspective of dynamic electric field distribution.

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.

Effect of Bipolar Square Wave Voltage with Varied Frequencies on Electrical Tree Growth in Epoxy Resin

An electrical tree in epoxy resin induced by a bipolar square wave voltage at varied frequencies has been investigated. The morphology, growing characteristics, fractal dimension, and initiation proportion of anelectrical tree at 10 kV for a frequency ranging from 50 Hz to 20 kHz was studied. The results show that electrical trees are all branch-like in epoxy resin. It was revealed that there are three regions in the propagation of an electrical tree: 50 Hz~2 kHz (region A), 2~4 kHz (region B), and 4~20 kHz (region C). When imposed frequencies are increased from 50 Hz to 20 kHz, the number of tree branches, growing rate, and initiation proportion increase in region A; the number of tree branches and initiation proportion increase, while the growing rate decreases, in region B; and the growing rate and initiation proportion increase while the number of tree branches decrease in region C. The electrical tree length and width increase linearly with treeing time when frequencies were lower than 2 kHz, while increasing exponentially with treeing time when frequencies are higher than 4 kHz. The dielectric power loss of epoxy resin under varied frequencies is computed by an expansion in Fourier series, and the results indicate that the dielectric power loss increases with increasing frequency. The characteristics of an electrical tree are ascribed to a synergistic effect of the dielectric power loss of epoxy resin, space charge, and partial discharge under a bipolar square wave field.

Effects of Temperature Gradient on Electrical Tree Initiation and Breakdown Phenomenon in XLPE Under Harmonic Superimposed DC Voltage

In this article, the temperature of the needle tip and the ground electrode were changed to obtain different temperature gradients. The harmonic superimposed DC voltage was also simultaneously applied to study the electrical tree initiation, growth and breakdown characteristics in cross-linked polyethylene (XLPE) under the temperature gradient. The tree inception voltage, tree structure, tree length, accumulated damage (AD) and breakdown time were used to analyze the electrical tree degradation process. It was found that the greater the temperature gradient, the easier it is for electrical trees to be initialed which is related to the different charge behaviors affected by the trap distribution indicated by surface potential decay (SPD) results at different temperatures. With the temperature gradient increasing, the electrical tree length increases firstly but then decreases slightly when the temperature of the ground electrode is fixed, but the electrical tree length and AD increase linearly with the temperature gradient increasing when the temperature of the needle tip is fixed. The temperature rise of the ground electrode has a more serious impact on the safety of the insulation compared with the temperature rise of the needle tip. Positive DC bias voltage is more likely to cause damage and breakdown than negative DC bias voltage under the temperature gradient. These results indicate that the temperature gradient and harmonic superimposed DC voltage will promote the electrical growth in cable insulation, so special attention should be paid to the safety of electrical equipment under such special conditions.

Effect of silicone grease on electrical tree degradation of silicon rubber under AC electric field

Silicone grease (SG) is usually applied on the interface between the insulation of cable and cable joint to enhance the sealing of interface. However, in long-term operating process, SG will gradually diffuse into silicone rubber (SiR) matrix and cause the electrical performance degradation of SiR, which can even result in the failure of cable joints. The commercial HTV SiR with functional silica filler was used in this study, and the electrical tree characteristics SiR samples with different SG coating times were investigated. The crosslinking density and trap distribution were analyzed to explain the transformation of electrical tree with different coating times. It was found that the electrical tree initiation voltage dropped from 13.35 to 10.46 kV and the fractal dimension increased from 1.25 to 1.52, while the electrical tree length showed an almost constant trend. The morphology of the electrical trees gradually changed from branch-type to bush-type. The decline of crosslinking density from 1.92 × 10−4 to 1.76 × 10−4 mol/g is attributed to the breaking of crosslinking structure between silica and SiR matrix, which can lead to the expanding free volume and decreased deep trap density. More high-energy electrons can be generated, which aggravates the degradation of molecular chains and further results in the decreased electrical tree initiation voltage. In addition, the denser morphology of electrical tree is considered to be caused by the more growth direction for electrical tree provided by the expanding free volume.

Polycyclic Compounds Affecting Electrical Tree Growth in Polypropylene Under Ambient Temperature

Polypropylene (PP) has great potential to be used as recyclable high-voltage direct current (HVDC) cable insulation material. Electrical tree is the key factor that leads to cable insulation failure and limits the increase of HVDC transmission voltage level. In this study, three different polycyclic compounds, namely 4,4'-bis(dimethylamino)phenylazo, 2-hydroxy-2-phenylacetophenone and 4-phenylbenzophenone, are added into PP matrix with the addition content of 0.1 wt%. The electrical treeing experiments are conducted under impulse superimposed DC voltage at 30, 70 and 90 °C. Then the polycyclic compound with the best effect on inhibiting electrical tree is selected, and its filling ratio is further changed from 0.1 wt% to 0.3 wt% to get the optimal inhibition effect. It is found that the addition of 2-hydroxy-2-phenylacetophenone can inhibit the electrical tree degradation. However, the addition of 4-phenylbenzophenone and 4,4'-bis(dimethylamino)benzene will increase the electrical tree length and accumulated damage. As the content of 2-hydroxy-2-phenylacetophenone increases, the electrical tree length and accumulated damage decrease firstly, and then increase. The effect of polycyclic compounds on the electrical tree degradation is affected by the relative polarity of impulse superimposed DC voltage and temperature. The experimental results reveal that adding 0.1 wt% 2-hydroxy-2-phenylacetophenone polycyclic compound to PP can effectively inhibit the electrical tree degradation, which has great potential for application in recycle HVDC cables.

Effects of Temperature Gradient on Electrical Tree Growth and Partial Discharge in Silicone Rubber Under AC Voltage

The Joule heat produced by the current in conductor decreases in the radial direction, producing a temperature gradient in the insulation. This existing temperature gradient can affect the electrical tree phenomenon in insulation materials because of the temperature dependence of insulation material, charge migration and tree growth. Different temperature gradients were obtained by controlling the temperature of high voltage (HV) and ground electrode (GD) sides. The electrical tree was recorded by a microscope, while the partial discharge (PD) was observed simultaneously. For better understanding the effects of the temperature gradient on electrical tree inception, the trap distribution in silicone rubber (SIR) with different temperature was analyzed by isothermal discharge current (IDC) method. With the fixed temperature in HV or GD side, it was found that the electrical tree inception voltage decreased linearly with increasing temperature in either side. The IDC results indicate that the increasing temperature contributes to the de-trapping of charges and lowers the inception voltage. Besides, the electrical tree length and accumulated damage gradually increase with the temperature gradient. The observed PD amplitude and quantity also increase with the temperature gradient, which facilitates the growth of electrical trees. These results indicate that the tree growth is promoted with an increase in the temperature gradient of the cable insulation, posing a threat to the safety of the cable system.

Electrical Tree Growth Characteristics in Epoxy Resin With Harmonic Superimposed DC Voltage

HVDC converters can generate harmonic voltage, which distorts the DC voltage quality and threatens epoxy resin insulation reliability. In this paper, the electrical tree growth was investigated in epoxy resin under harmonic superimposed DC voltage. The DC voltage ranged from -20 to +20 kV and harmonic frequency ranged from 50 to 450 Hz. This experiment was carried out with needle-plate electrodes system. The effects of DC amplitude and harmonic order were characterized by electrical tree length and accumulated damage. Results show that the electrical tree varies with different combinations of DC amplitudes and harmonic orders. The tree length and accumulated damage experience a non-linear trend with the rise of harmonic order. The low-order harmonics bring more damage to epoxy resin under superimposed voltage. The time to breakdown shows a minimum value at 3 rd harmonic superimposed voltage. The DC amplitude has an acceleration on the electrical treeing process. With higher DC amplitude, the electrical tree breakdown happens immediately when the electrical tree reaches the plate electrode. The charge transport process accounts for the tree initiation characteristics under different superimposed DC amplitudes and harmonic orders. Meanwhile, “reverse tree” has been found in epoxy resin sample. The field-driven tree growth model has been employed to interpret reverse tree growth from the perspective of dynamic electric field distribution.

Coconut Shell Powder and Electrical Tree Inhibition of Solid Insulator

Electrical tree phenomenon commonly occurred in solid insulator material resulting from the inconsistency of stresses during the manufacturing process. The treeing phenomenon will degrade the characteristic of insulator and may lead to breakdown. The use of filler in the manufacturing process in order to enhancing the insulation property is a popular approach. This paper presents the use of coconut shell powder (CSP) filler in epoxy resin for inhibition of electrical tree growth. The particle sizes of CSP varied from 62, 75 and 125 µm. The CSP is filled with the ratio of 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0 and 3.0% by weight. We aim to observe the effects of varying the combination of CSP particle sizes and the ratio of epoxy resin for an inhibition of electrical tree growth. The electrical tree was tested by applying AC voltage of 15kV to the test sample and conducted for 360 minutes in this experiment. When the epoxy resin using CSP filler of 1% by weight is fixed for the particle size of 62, 75 and 125 µm, electrical tree length is at 1.24mm, 1.382 mm and 1.78 mm respectively. Whereas, when the particle size of CSP at 62 µm is fixed, a tree length is 1.98 mm and 1.24 mm for the ratio of 0.1 and 1% by weight respectively. Our study demonstrates that the epoxy resin using CSP filler at a ratio of 0.1% by weight can shorten length of electrical tree when compared with a ratio of 1.0% by weight. The electrical tree length was affected through the changes of permittivity by addition of CSP compound.

Electrical tree propagating characteristics of polyethylene/nano-montmorillonite composites

Electrical tree occurring in polymer insulation limits the applications and reduces the service lifetime of power equipment. To improve electrical tree resistance in polymer insulation, polyethylene (PE) nanocomposites with nano-montmorillonite (MMT)- layered fillers are compounded and investigated in terms of electrical tree characteristics. By an organization process, intercalation and coupling agents are used to modify the inorganic-layered MMT nanofillers in sequence. Polyethylene/nanomontmorillonite (PE/MMT) composites with different MMT contents are prepared by melting intercalation. Fourier transform infrared spectrophotometry is used to determine the chemical characteristics of the MMT in different surface-modification stages. The crystalline morphologies and MMT distributions in the composites are characterized by polarizing microscopy and scanning electron microscopy. The electrical tree propagating characteristics and corresponding conduction properties of the PE and PE/MMT composites are investigated. Results show that the electrical tree resistance of PE/MMT composites significantly improves. The electrical tree length decreases and the fractal dimension increases in PE/MMT composites compared with pure PE. After performing electrical tree initiation, conductive current slightly increases in composites but decreases in PE. This result suggests that the characteristics and conductive mechanism of electrical tree differ between PE and PE/MMT.