Search

In this report, a pulsed electro-acoustic (PEA) device is modified to test the charge profile in epoxy resin at 77 K (-196°C) and successfully obtains the charge signal. A piezoelectric material suitable for cryogenic application is chosen for the measurement. In the experiment, the polarization and depolarization tests are operated with different voltage supply procedures. After initial wave recovery, the waveforms are discernable enough to identify the charge information. It is found that the charges are much less active at the low temperature with both slow dissipating rate and low injection quantity, comparing to the case at 293 K.

The development of a system to measure simultaneously the current and space charge in a dielectric is described. The results obtained for low density polyethylene samples show that the relationship between space charge accumulation and the time-dependent current is not straightforward. In accord with previous work on space charge limited currents, the transient currents initially decreased, but after a while an increment or peak appeared. Additionally, the generation and extraction of space charge packets caused an impulse-like current peak such as previously observed. By comparing the electric field and current at the anode, it was found that the increment and the peak of the anode current are determined by injected negative carriers reaching the anode rather than the increase of electric field at the anode. It is shown that the dynamical evolution of the current can be divided into three stages similar to transient space charge limited current theory.

As an interface between the different insulating polymers exists in the joints of High Voltage Direct Current (HVDC) cables, the differences in the field-and-temperature dependence of the conductivities of the materials is regarded as a problem leading to interface charge accumulation and field distortion. In this report, using blocking materials to prevent charge injection from one or both sides of the electrodes, the contributions of charge injection and ionization in low-density polyethylene (LDPE), cross-linked polyethylene with nanoparticles (nano-XLPE) and ethylene propylene diene monomer (EPDM) are distinguished and evaluated based on the results from pulsed electro-acoustic (PEA) method. It is found that the contributions of charge injection and ionization behave differently in different materials. The mechanism behind the field-and-temperature dependences of the conductivities of the three materials is explained successfully. This study helps to clarify the charge transfer mechanism, and thus supports the quantitative calculation of the space charge and electric field distribution in cable joints.

This paper proposes a method of distinguishing the contributions of ionized and injected charges to the charge transport in oil immersed paper in triple-layer and double-layer samples. With the double-layer sample, the apparent mobility including the ions contribution is obtained by the calculation according to the speed of accumulated charge density at the interface. The contribution of ions is excluded by subtracting the accumulated charge density in the triple-layer sample from that in the double-layer sample. It is found that the interface charge mainly comes from the ionization charges under a low field, and the contribution of injected charge takes a more important part at higher electric field.

Residual charges produced in previous partial discharges (PDs) are considered as one aspect of memory effects that affect subsequent discharge behavior. In this paper, we review past studies about memory effects induced by residual charges during successive PDs. At first, surface charge dynamics for cavity and needle-to-dielectric discharges are presented. Subsequently, three methods used to unravel the memory effects are described, e.g. conditional distribution, pulse sequence analysis (PSA) and chopped PD (CPD). Then effects of residual charges on PD inception, discharge propagation and collective behavior are summarized. In addition, contributions of residual charges to the transitions between discharge modes and discharge patterns are presented. At last, some problems relevant to the memory effects are discussed, followed by suggestions for future work.

The effect of yttrium on the reaction of tungsten oxide with CO was studied for the preparing various morphologies and size of tungsten carbide. The reaction rate, phase composition, particles size and morphology, surface area of samples were studied by TG (thermogravimetry), XRD (X-ray diffraction), SEM (scanning electron microscopy) and BET (Brunauer–Emmett–Teller), respectively. It was found that the reaction route was following WO3 → W18O49 → WO2 → WC at lower temperature, while at higher temperature, an intermediate phase W was formed and then carbonized to WC. During the reaction of pure WO3 or mixtures (1 wt.% Y(NO3)3 + WO3) with CO at lower temperature, the final products almost maintained the large irregular block shape. With the increase of temperature, the large WC particles were degraded and the obtained sub-particles became much more dispersed. After the addition of yttrium nitrate, the rod-like sub-particles were transformed into more stable polyhedral-shaped sub-particles with a smaller size. Furthermore, the mechanism of effect of yttrium on prepared tungsten carbides were analyzed in detail.

In this research, the equivalence of space charge for coaxial cables and plane samples are analyzed through simulation. Bipolar charge transport models of coaxial cables and plane samples are established with the same material-dependent parameters, the bulk charge and total space charge distributions are simulated and comparatively researched under isothermal and temperature gradient conditions. The presented results indicate that the space charge behavior shows a basic equivalence in terms of charge variation laws and distribution. In addition, few discrepancies in the heterocharge amount and charge waveform amplitude are observed and mainly ascribed to the sample geometry, mobility gradient distribution and the spatial resolution of the measuring system, respectively. Investigations of space charge characteristics with a plane pulsed electro-acoustic measurement system may be accepted as a practice for providing the space charge characteristics of coaxial cable insulation, unless it is necessary to investigate the cable space charge distribution under actual operating conditions.

Epoxy resin is widely deployed as a high voltage electrical insulation material when compounded with inorganic fillers. However, in the laboratory, the filler prevents visual observation of the long-term degradation known as electrical treeing. To date therefore, much laboratory testing has been conducted on unrepresentative unfilled materials. Here, the impact of micro-sized fillers on the treeing phenomenon in an epoxy system has been explored. Sub-micrometre resolution 3D reconstructions of electrical trees are reported from X-ray computed tomography (XCT) using an advanced ‘pink beam’ synchrotron light source imaging system. The role of filler particles between 1 and 10 μm in size on tree channel propagation is reported. In highly filled materials (30% by weight) a radical change in tree growth behavior is seen, leading to bush tree rather than branch tree growth. The dielectric breakdown time at constant stress was also found to increase as the square root of the filler level. The change in geometry of tree growth may explain the extended life of filled materials in high voltage applications.

Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ∼10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments.

This paper reports observations of electrical tree growth by X-ray computed tomography (XCT) in multiple steps. Growth between times of observation has been obtained by subtraction of an image from the subsequent one. Partial discharge (PD) signals were also recorded during tree growth. It is determined that not all PDs support tree extension. By comparing the energy released by PD and the vaporization energy of newly grown tree channels, it is found that only the PDs that reach the tree tips can contribute to the growth of tree length. In such cases, the efficiency of energy conversion was very high, whereas PD resulting in increases in tree channel diameter had a much lower conversion efficiency. Nano XCT images reveal many finer branches along the trunk of the main tree and in the front of the main channel tree tips. It seems that those finer branches led to tree growth, but were not directly associated with local PD. The new imaging techniques have shown great value in resolving the relationship between partial discharge activity and electrical tree growth, providing a platform for deeper understanding of the tree growth process, and interpretation of partial discharge measurements.

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

In this investigation, the Li2CO3-assisted hydrogen reduction method was proposed to produce different morphologies and size of metallic tungsten particles. The effects of temperature and content of Li2CO3 additive were both considered. The experimental results showed that both the temperature and additive had great effects on the morphology and size of produced tungsten particles. As the increases of temperature and fraction of Li2CO3, the morphology of tungsten particles was changed from spherical to polyhedral; while the size of these particles was increased; and the obtained products became more and more dispersed. The addition of Li2CO3 also affected the reduction rate dramatically. It was found that the Li2CO3 additive retarded the initial reduction reaction, but it could obviously improve the later reduction (WO2 to W) at a high temperature, furthermore, the higher fraction of Li2CO3 made this promotion more remarkable. Based on above results, the reduction mechanism was proposed: without additive, the reduction entirely obeys pseudomorphic transformation mechanism at low temperature, and obeys CVT mechanism at high temperature; with additive, the reduction follows a mechanism of mixture of nucleation, pseudomorphic transformation and CVT at low temperature, while at high temperature, the sublimation of WO3, nucleation and CVT mechanisms all play important roles. Keywords: Li2CO3, Nucleation, Reduction, Morphology and size, Tungsten

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.

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Space charge accumulation is an important issue for HVDC cable design. The space charge accumulation are determined by the charge transport property which can be monitored by the space charge and conduction current measurement. In this paper, based on PEA system and three-electrode system, the modified combined measurement system was developed to measure the space charge and current in LDPE samples simultaneously. The results show that the current gradually decreased with the accumulation of homocharges; the generation and movement of space charge packet resulted in the current peaks; the reduction of homocharge accumulation and the increase of heterocharge tends to increase the current.

EPDM, as a high-quality insulation material, is widely used in cable joints, but it bears a large compression force in the working environment, which may affect its insulation performance. We study the effect of pressure on charge transport in EPDM by calculating the change in the key parameter of carrier mobility in EPDM at different pressures. In this paper, by designing a unipolar carrier blocking experiment and combining improved PEA measurement equipment, the mobility of electrons in EPDM at six pressures of 0.2 MPa-1.2 MPa (interval 0.2 MPa) is calculated. In addition, the mobility data is analyzed in-depth through the hopping conduction model. It was found that the mobility in EPDM at different pressures did not change significantly, and the electronic transition distance and energy required for hopping after fitting the hopping conduction model also did not change much under different pressures. It is indicated that under normal working conditions, pressures will not affect the electrical performance of EPDM.

The effect of temperature gradient on space charge behavior is necessary to investigate for HVDC insulation. In this paper, space charge distributions in neat epoxy resin (EP) and EP/SiO 2 nanocomposites (NC) were measured under different DC stresses and temperature gradients. We found that different temperature conditions applied to the electrodes had a great impact on space charge distributions: Only homocharge accumulated near anode at isothermal conditions, on the contrast, at temperature gradient of 60 °C, negative charge injected from cathode (high temperature side) and accumulated in the bulk, heterocharge appeared near anode (low temperature side). Moreover, SiO 2 nano-fillers added to NC could suppress the space charge accumulation significantly, and the pattern of space charge distribution in NC at temperature gradient of 60 °C also shows differently from that of EP. Numerical simulation based on the bipolar charge transport model was employed to study the experimental results. It shows that under temperature gradient, charge extraction plays an important role in heterocharge accumulation near the low temperature side. Moreover, it indicates that unlike the apparently measured conductivity, the charge mobility of NC does not increase rapidly with temperature in the range from 20 °C to 80 °C.

XLPE and its nanocomposites have been subjected to a progressive step voltage until failure occurred, and the results analyzed in terms of an inverse power law ageing expression. The step-stress tests have been conducted at three temperature 20°C,40°C and 60°C, and the exponent n of the inverse power law ageing expression has been estimated for the two materials by a new method, which fully considers the contribution to the failure of the whole sequence of voltage steps, including ageing processes at the lower voltages, the final step and the process of voltage increase. It was found that the value of exponent n increases slightly with increasing temperature for both XLPE and its nano-composites. More importantly the value of n for the nano-composites was found to be substantially lower than that for XLPE at all temperatures. A complementary experiment in which the space charge distribution was measured under the same step-stress protocol as the breakdown tests has also been carried out and the results of both experiments are discussed.

Electrical trees are artifacts resulting from aging of polymeric insulation in high electrical fields. Whilst there is some debate concerning the mechanism by which they grow, there is no doubt that their growth can lead to the ultimate failure of the host insulation. Studying electrical trees is mainly confined to measurement of associated partial discharges and observing the physical growth of the tree structure optically. This paper reviews developments in observations of the growth of trees in the laboratory. In particular, consideration is given to the benefits of generating three-dimensional replicas of real trees from X-ray computed tomography (XCT) and serial block face scanning electron microscopy (SBFSEM), and how these can facilitate better understanding of tree development mechanisms. It is concluded that both two- and three-dimensional imaging are required, and these need correlating with partial discharge measurements to develop models of tree growth and effective asset management tools.

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.

The effects of charge carrier injection and intrinsic ionic carriers on the conduction process in superclean low-density polyethylene and cross-linked polyethylene (LDPE and XLPE) and the associated development of space charge are investigated. The carrier mobility is measured directly by space charge measurements in a double- and triple-layer structure. It is shown that the difference between superclean LDPE and XLPE in the field-dependence of the current density results from the dominance of ions in XLPE introduced by cross-linking as compared to injected carriers in the case of superclean LDPE.

Electrical tree growth through polymeric insulation is a key degradation process in HV cables. Partial discharges (PDs) are considered to be the energy conversion path for the tree growth. Several methods have been employed to study PDs in electrical tree channels, including the phase resolved PD (PRPD) pattern and pulse sequence analysis (PSA). Our recent work proposed a new method of estimating the PD inception and extinction voltages in non-conductive tree channel based on the PD signals. Here a typical non-conductive tree in polyethylene is considered. Both PRPD and PSA have been employed. It is found that after the PD inception, the PD inception and extinction voltages decrease with the tree growth, and become stable before breakdown. It is also found that the decrease of PD inception and extinction voltages causes an increase in the average number of PDs per cycle, and the shrinking of the dV-dV PSA plot. In turn, the average number of PDs per cycle, and the dV-dV plot can be used to monitor the change of PD inception and extinction voltage during the tree growth.

The movement of space charge packets has been reported in polyethylene and epoxy resin under high electric fields by several groups, and has been characterized by the speed of propagation. A number of explanations of the formation and transport of slow space charge packets has been proposed. However, the mechanisms creating fast space charge packets are not clear. Dissado et al have suggested that the formation of fast charge packets is due to discontinuous electromechanical compression and charge injection at the electrode-insulation interface, and the transport of charge packets is related to corresponding relaxation processes. In that model, charges travel in packets because of group polarization. This paper gives an alternative model considering the reduction of charge carrier activation energy due to charge density triggered polymer chain movement and subsequent chain relaxation times. It is shown that the generation and transport of fast charge pulses are readily simulated by a bipolar charge transport model with three additional parameters: reduced activation energy, a charge density threshold, and a polymer chain relaxation time.

Electrical trees are degraded paths in polymeric insulation and are the mechanism of electrical failure of high voltage insulation systems. Previous studies have confirmed the application of X-ray Computed Tomography (XCT) imaging of electrical trees using phase contrast enhancement. This work evaluates the feasibility of X-ray imaging of electrical trees to measure growth at several stages of growth in the same treeing sample. The impact of x-ray dose on the epoxy resin studied is determined experimentally. An example of multi-stage tree imaging using laboratory micro-XCT and reconstructed renderings is provided. Based on the three-dimensional tree models, characteristics such as the degraded volume and surface area can be quantified. Furthermore, a comparison between two stages is presented to show the growth of the electrical tree in a fixed period of time. Insight into the treeing characteristics is also discussed.

In order to restrict the field distortion in the insulation of high voltage direct current (HVDC) cable under temperature gradient, this paper proposes a nano-filler, the addition of which can restrict the space charge accumulation of low density polyethylene (LDPE) under temperature gradient. To reveal the mechanism of the nannocomposite, the trap depth distribution of the pure LDPE and LDPE nannocomposites under different temperatures were tested based on the Pulsed Electro-Acoustic (PEA) method. The parameters of trap depth distribution were achieved by PEA method under different temperatures(20°C,40°C,60°C and 80°C), and the trap depth distribution under different temperatures were all calibrated to that of 20°C. The results indicated that the trap depths of LDPE decrease with increasing temperature, whereas the trap depths of LDPE nanocomposites changes inobviously with temperature variation. It is considered that, the result contributes to the restriction of space charge accumulation under temperature gradient.

Simultaneous imaging of electrical tree growth and partial discharge measurement have allowed 5 stages of tree growth to be identified. Partial discharge (PD) characteristics have been collected from tree growth in needle-plane epoxy resin samples under 15 kV peak AC voltage, and have been shown to behave differently at different stages of tree growth. Initial slow growth is accompanied by large discharges (about 10 pC). Thereafter, growth of fine channels is associated with smaller PDs (about 0.3 pC). It was observed that the tree propagation is further driven by even smaller PDs below the equipment sensitivity. However, when the tree approaches the counter electrode, the PD magnitude increases again and a more substantial tree grows with a low propagation rate. Reverse tree channels then occur from the planar electrode towards the needle, together with higher magnitude PDs characterized by thick and dark branches before breakdown.