Electrical Tree Growth Characteristics Research Articles

DC-Impulse Voltage-Dependent Electrical Tree Growth Characteristics of Epoxy Resin in LN2

This paper focuses on the electrical tree growth characteristics in epoxy resin with dc-impulse voltage caused by line fault and switching surges in liquid nitrogen (LN2). Electrical tree initiation tests were conducted on molded epoxy resin with a needle–plate system. The test samples were stressed with a dc voltage amplitude of 10 kV and an impulse voltage amplitude of 24 kV at 77 K. With a wide range of voltage frequencies and different impulse polarities, four different waveforms of dc-impulse voltage were used as applied voltage. Result shows that the electrical tree tends to grow longer when the dc and the impulse are of the same polarity. The impulse frequency plays a crucial role in tree initiation and growth process. The tree initiation probability increases with the frequency rising from 50 to 400 Hz in epoxy resin. Different charge transportation processes are discussed based on four different voltage waveforms. The collision ionization and charge recombination have a decisive effect on electrical tree initiation. The impulse voltage is worth regarding in the operation of a superconducting 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.

Electrical tree growth and partial discharge in epoxy resin under combined AC and DC voltage waveforms

The effect of DC bias on electrical tree growth and partial discharge (PD) characteristics in epoxy resin (Araldite LY® 5052/Aradur® HY 5052 supplied by Huntsman) is investigated using three waveforms: AC, AC with positive DC bias, and AC with negative DC bias. Needle-plane samples are used. Tree growth is shown to be accelerated by the combined effect of DC and 50 Hz AC, beyond the AC growth rate. Positive and negative DC biased tests result in 62% and 54% reductions in average time to breakdown, respectively. Different tree structures and stages of development are associated with different partial discharge characteristics, with thick dark tree branches associated with high PD magnitudes, whereas fine tree channel growth is linked with PD magnitudes below 1 pC. AC tests showed five distinct stages of tree growth compared to four stages seen in DC biased tests. In particular, trees growing in the ‘reverse direction’ from the planar to the point electrode, which is observed in the latter stages of AC tests, is not seen in the DC biased tests. It is concluded that for composite voltages, the AC component is the essential driver of tree growth but the DC component can accelerate propagation. AC noise may therefore compromise the reliability of insulation in HVDC networks.

Modelling of partial discharge characteristics in electrical tree channels: Estimating the PD inception and extinction voltages

Partial discharge (PD) characteristics are inherently linked to associated electrical tree growth characteristics. This paper employs both phase-resolved patterns and pulse-sequence analysis (PSA) to study PDs in a short branch-tree grown from a metallic needle tip. It is found that the shape of PSA dV-dV patterns vary with applied voltage; in particular the values of characteristic voltage difference increase with applied voltage. At 10 kV the value of characteristic voltage difference decreases with tree growth, the average number of PDs per cycle increases, the PD magnitudes decrease, and the characteristic wing-like phase-resolved pattern evolves into a turtle-like pattern. This paper proposes a physical model relating PD activity to tree development, considering space charge accumulation inside the tree channel. All key features of experimental results are explained by the model. It is shown that the characteristic voltage differences are determined by the local PD inception and extinction electric fields. These key parameters determining the PD events can be calculated giving a clear physical interpretation of PD measurements, which is invaluable to those using PD as a condition monitoring or asset management tool.

Tree Growth Characteristics of Epoxy Resin in LN2 Under DC Superimposed Pulse Voltage

Chosen as an important insulation material of high temperature superconducting dc cable terminals, epoxy resin needs to address the challenge of the liquid nitrogen (LN 2 ) temperature and various kinds of voltage due to the special operating environment. The switching over voltage, lightning strike, and current impact will introduce the pulse voltage into the operation of the dc equipment, which makes it necessary to study the insulating characteristics of epoxy resin under dc superimposed pulse voltage condition. This paper investigates the electrical tree growth characteristics of epoxy resin with application of dc superimposed pulse voltage in LN 2 . The tested samples were stressed with combined dc-pulse voltage with different amplitudes and polarities in a needle-plate geometry electrode system at -196 °C. Tree length and accumulated damage are employed to characterize the electrical tree. The experimental results indicate that when the dc and pulse voltage are of the same polarity, the rate of tree growth increases with the amplitude of dc voltage rising but would slow down after a period time of treeing. When the polarity of dc and pulse voltage is opposite, the growth rate decreases with the dc voltage amplitude increasing, while it could maintain a relatively high level during the whole treeing time.

Effects of mechanical stretching on electrical treeing characteristics in EPDM

Effects of mechanical stretching, produced after the expansion of cable accessories, on the electrical tree growth characteristics of EPDM (Ethylene Propylene Diene Monomer) are reported in the present work. FFV (fractional free volume) and CED (cohesion energy density), which were respectively related to free path of electrons and molecular chains splitting during electrical tree propagation process, were calculated through molecular dynamics simulations under different stretching ratios. A needle-plane electrode system with the stretching ratio ranging from 0 to 30% was used to investigate the electrical treeing characteristics in the experimental study. The simulation results show that the FFV becomes larger at higher stretching ratio, which enhances the collision between the hot electron and the molecular chain during the treeing process by extending the free path of the hot electron. While CED tends to become smaller at higher stretching ratio, reducing the cohesion strength for the tree channel crack to grow. Moreover, FFV and CED are investigated during the stretching recovery process, where FFV tends to decrease and CED turns to be larger. The experimental results indicate that mechanical stretching affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes shorter as stretching ratio increases from 0 to 30%. Tree length and accumulated damage become larger with increasing the stretching ratio. Electrical tree propagation mechanism considering chemical and physical traps, which are generated under mechanical stretching, are also discussed and analyzed.

Compressive stress dependence of electrical tree growth characteristics in EPDM

To ensure a certain interface electrical strength resistance, the cable accessories are densely coated onto the main cable insulation. Compressive stress is generated in the normal direction of high voltage cable by the spring clamp device or the banding force after the expansion of EPDM (Ethylene Propylene Diene Monomer) stress cone. This paper presented a compressive stress experimental study considering electrical treeing propagation in EPDM by employing a needle-plane electrode system. The results indicate that compressive stress affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes longer as compressive rate increases from 0 to 30%. Tree length and accumulated damage become smaller with increasing the compressive rate. Electrical tree structure is also depended on compressive stress, and branch tree structure becomes the main electrical tree morphology at higher compressive rate. For better understanding the effects of compressive stress on electrical tree propagation characteristics, FFV (fractional free volume) and CED (cohesion energy density) were calculated and analyzed through molecular dynamics simulations under different compressive rates. Results show that the FFV becomes smaller at higher compressive rate, which shortens the free path of the hot electron and weakens the collision with the molecular chain during the treeing process. While CED tends to be lager under higher compressive rate, which improves the cohesion strength for the crack to grow in the tree channel tips and restrains the propagation of electrical tree.

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.

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.

Electrical tree growth characteristics in XLPE cable insulation under DC voltage conditions

The growth characteristics of electrical trees in XLPE cable insulation under DC voltage conditions were studied by a combination of treeing test and microscopic observation system. In the treeing test samples, the tip radius of the needle electrode was 5 μm, and the pin-plane distance was 2 mm. When a 70 kV DC voltage was applied on the samples for totally 10 hours, either continuously or intermittently, no trees appeared. So the trees for growth study were all initially triggered by a 50 kV standard lightning impulse. The DC voltages were applied periodically, so as to make it possible for the required observation and recording. Under the DC voltages, the electrical trees developed gradually from a single slender branch to a sparse bushlike structure, and tree channels were found to be non-conductive. The growth rates were rather slow compared with AC trees under the same condition. Grouping experiments showed that the tree length increased with the increasing number of cycles and amplitude of DC voltage, and it was also related to the voltage duration in each cycle at 60 kV. This tree growth pattern has been well explained by a nonconductive tree model and the equivalent circuit, and the discharges occurred in the tree channels are thought to be the driving force for tree growth. Under the condition of an unexpected grounding of the needle electrode, a new tree would initiate instantly from the needle tip, while an existed tree would grow very rapidly. This is correlated to the material damage caused by the sudden release of the injected charges. The work is useful for the recommendations on test arrangement and operation strategy of HVDC cables.

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.

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.

Investigating the impact of harmonics on the breakdown of epoxy resin through electrical tree growth

Many studies have investigated the influence of harmonics on insulation life. A limited number of these studies address electrical tree growth characteristics while even fewer employ harmonics maintaining a constant peak voltage. This experimental study sought to quantitatively identify any distinguishing features of electrical tree growth characteristics as a consequence of harmonic content. The total harmonic distortion (THD) and waveshape (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) indices were varied to a maximum of 40% and 1.6 respectively. Electrical trees were developed in point-plane geometry using 2 μm tip radius hypodermic needles and a 2 mm gap in epoxy resin (LY/HY 5052) samples. Upon electrical tree initiation, 50 Hz composite waveforms incorporating harmonic content were applied as the excitation waveform at a constant voltage of 14.4 kV peak for 2.5 hours. No change in the electrical tree growth characteristics was identified using the width/length ratio. Electrical tree growth resulting from low power quality reduced operational insulation life and decreased the Weibull shape parameter β with the 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> harmonic displaying particular characteristics worthy of further investigation.

Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage AC applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.

Propagation mechanism of electrical tree in XLPE cable insulation by investigating a double electrical tree structure

This paper presents our experiments and analysis of the electrical tree growing characteristics. The relationship between electrical tree propagation and the material morphology in XLPE cable insulation has been studied by researching the structure and growth characteristics of a double structure electrical tree. It has been found that, due to the influence of uneven congregating state, difference in crystalline structure, and the existence of residual stress in semi-crystalline polymer, five types of electrical tree structures (branch, bush, bine-branch, pine-branch, and mixed configurations) would propagate in XLPE cable insulation. Three basic treeing propagation phases (initiation, stagnation, and rapid propagating phases) are presented in electrical tree propagating process. If initiation phase is very active, the single branch tree will propagate while if this phase is weak then the bush tree will occur more easily. There would be a clear double structure of electrical tree when it grows at submicroscopic structure uneven region of the material. A new parameter, the expansion coefficient is introduced to describe the electrical tree propagation characteristics. In addition, two other coefficients being used to describe our experimental results are dynamic fractal dimension and growth rate of electrical tree.

Dynamics model for electrical tree propagation in cross-linked polyethylene cable insulation under high frequency voltage

In this paper we investigated the structures and propagation characteristics of electrical trees observed in the high volatage cross-linked polyethylene (XLPE) cable insulation samples under high volatage of 10kV at frequency of 1000—2000Hz. Based on the specific tree growth mechanism and the fractal nature of tree structures, a dynamic model has been developed to qualitatively predict the electrical tree growth characteristics in XLPE cable insulation when subjected to an electrical stress under high frequency voltage, and then the tree growth rate equation and life formula of tree growth to breakdown are derived.We performed electrical tree growth tests in XLPE cable insulation samples at high frequency and compared the results, which showed that the proposed model was in good agreement with the expreriment.So,it seems that the proposeed model can be applied to the qualitative analysis of aging law of electrical tree in XLPE cable insulation.

The effect of voltage and material age on the electrical tree growth and breakdown characteristics of epoxy resins

Electrical tree growth (a long-term electrical breakdown process) has been investigated in Araldite CT200 and CT1200 epoxy resins as a function of voltage and material age (defined as the time between manufacture and testing of pin-plane samples). Reproducible and predictable electrical tree growth was obtained for both CT200 and CT1200 epoxy resins provided that (i) the essentially random tree initiation time is removed and (ii) the samples tested were of the same age. The tree growth and time to failure (defined as the time to breakdown from a pre-initiated 10 mu m tree) characteristics as a function of both voltage and sample age show large step changes at a critical voltage and critical age. In particular, the resin physical ageing has a large effect on the tree growth behaviour, with the time to failure varying by three orders of magnitude over a time span of 3 years. Measurements of some of the physical properties (residual internal mechanical stress, surface refractive index, glass transition temperature and dielectric loss) of CT200 epoxy resin all indicate the occurrence of physical ageing of the resin, with structural (network) relaxation as the most important ageing process. However, these measurements are unable to account for the step change (critical age effect) found in the time to failure of tree growth. The fractal nature of tree growth and its relationship with voltage and the long-term changes in the properties of the resin are briefly commented upon.