With the development of wide-bandgap high-voltage power devices with high power density, such as SiC MOSFET, packaging insulation has attracted increasing attention for securing reliability. Packaging insulation of power modules withstands constant unipolar square wave voltages due to the anti-parallel diode of IGBT or body diode of MOSFET, which makes it vulnerable to space charge accumulation. However, it is hard to observe the space charge dynamic behavior under the unipolar square wave voltage since the rising or falling time of the square wave is relatively short, hence the ultra-fast space charge measurement is required. In this paper, a sub-megahertz space charge measuring method based on a high repetition rate pulsed electro-acoustic method is proposed. The pulse generator uses the Blumlein line and SiC MOSFET to form nanosecond pulse voltage with repetitive frequency up to 3 MHz. Then, the transient space charge waveform of silicone elastomer for packaging insulation under square wave voltage with high dV/dt is obtained. It is found the fast rising and falling edge of the high square wave voltage can generate electromagnetic interference to the acoustic-electric signal, which should be carefully handled. The proposed sub-megahertz space charge measurement method can provide references for the research of transient charge transport behavior and failure mechanism of high-voltage power module packaging insulation.
Web-based simulations are computer simulations through a web browser, which are expected to be used not only for remote work but also for education. We have developed a web-based simulation of particle motion in ion wind flow using three.js that is an open-source JavaScript library for simplify WebGL coding. Ionic wind is an airflow induced by electrostatic forces during corona discharges. In general, finite element method (FEM) is used to calculate an ionic wind. However, it is difficult for the FEM to represent the behavior of charged particles in ionic winds. To simulate the charged particle behavior, we calculate the motion of tracer particles in an ionic wind. The calculation results are good agreement with experimental results using a high-speed camera.
In this work, we present the development of a miniature surface plasmon resonance (SPR) sensor chip using the Kretschmann configuration. The SPR sensor chip incorporates a polymeric prism made from Norland Optical Adhesive 61 (NOA 61). By utilizing NOA 61 instead of a conventional prism, we overcome certain limitations associated with conventional SPR spectroscopy. We employed the confined sessile drop technique to fabricate the NOA 61 polymeric prism, which offers several advantages, including simplicity, low-cost fabrication, and precise control over lens parameters. A liquid NOA 61 was dropped directly onto a flat circular disk with a 6-mm diameter made of poly(dimethyl siloxane) (PDMS). The sharp edge of the PDMS circular disk facilitated controlled formation and the desired amount of the liquid NOA 61, resulting in the formation of a hemispherical prism. Subsequently, UV curing was employed to solidify the NOA 61, then obtaining the NOA 61 polymeric prism. To complete the miniature SPR sensor chip, 3 nm-thick chromium layer, and 47 nm-thick gold layer were evaporated onto the planar side of the NOA 61 polymeric prism and then mounted on a microfluidic cell fabricated using a 3D printer. The resulting SPR excitation curves were observed in the visible region and exhibited a refractive index independence sensitivity of 885.4 nm/RIU. Furthermore, we demonstrated the capability of the miniature SPR sensor chip to detect nanomaterials.
A composite material with higher thermal conductivity and acceptable breakdown strength has been developed. An electrostatic adsorption method produced a thermoplastic polyimide (tpPI)/ hexagonal boron nitride (hBN) composite to create a thermally conductive composite insulating material. The flaky surface of the hBN particles was oriented vertically to the sample thickness direction (CMV sample) or parallel to the sample thickness direction (CMP sample). The transferred/accumulated charge amount was measured using the Q(t) method to understand the insulating property. As a result, the charge amount ratio (Q(300)/Q0(2)) of the CMP sample at 30°C is higher than that of the CMV sample. In addition, the temperature-dependent transferred/ accumulated charge amount of the CMP sample was also comparatively higher than that of the CMV sample. This is probably attributed to the tpPI/hBN interface, a potential electrical weak point, and is parallel to the CMP sample's thickness (electric field) direction.
On the surface of a stress relief cone installed in a prefabricated joint and termination, precipitate would be generated during long-term operation. The precipitate could lead to affect insulation performance for XLPE cable systems. However, the relationship between the state of precipitate, such as the shape, position, amount, and color tone, and insulation performance has not been clarified. Additionally, photography cannot record the state of precipitate without distortion because the stress relief cone mainly consists of curved surfaces. This paper aims to digitize surface condition of a stress relief cone into 3D objects that can be objectively compared with respect to the state of precipitate and deformation on stress relief cones, which are difficult to quantitatively compare each other by photography. Photogrammetry digitizes the outline and color tone of an object into a 3D object from lots of photographs taken from different shooting direction. In this paper, the stress relief cone taken from the dismantled 275 kV prefabricated termination was digitized into a 3D object with the photogrammetry and confirmed its dimensional error of 1 mm or less. This result confirmed that the 3D digitization of stress relief cones using photogrammetry can be useful for quantitative analysis of the state of precipitate.
Luminescence induced by mechanical stress, i.e. mechanoluminescence (ML), has been studied in a cross-linked DGEBA epoxy resin as a function of stress and temperature. The research participates to the understanding of electromechanical ageing processes in insulating materials. At room temperature (RT) and 45°C, before yielding, ML shows an exponential increase vs. stress above a stress threshold. At 65 °C (T g ), the ML quickly reaches a plateau. The luminescence below the glass transition temperature is compatible with the Zhurkov's model and the literature and has been attributed to free radical recombination with oxygen after bond ruptures caused by the applied mechanical stress. This study could help better understand damage mechanisms in epoxy materials under mechanical stress and demonstrates a potential way to diagnose irreversible damage in this polymer.
During the manufacturing process of insulation rods, internal defects inevitably arise, causing partial discharges or breakdowns under electric field. The detection of internal defects before putting into operation can effectively reduce the occurrence of these situations. Ultrasonic method is considered as an effective method for detecting internal defects with its high efficiency and sensitivity. In this paper, different artificial defect samples are identified based on ultrasonic transmission method detection, and it is found that the internal air defects of fiber-reinforced epoxy composite parts at the millimeter thickness level can be sensitively identified, and the size of the defect area can be calculated based on the waveform and the probe wafer size. The problem of mixing the onset waveform with the bottom echo waveform when detecting thinner parts based on ultrasonic reflection method is solved, and the use of small size probes directly applied to the defect detection of GIS insulation rods is important to guide the operational reliability of insulation rods.
This paper incorporates high voltage direct current step-stress and polarity reversal stress for water treeing, leading to breakdown. In this paper, water is utilized as an electrode to investigate the impact of water ingression on breakdown. A setup resembling the water treeing configuration is created, wherein a HVDC step-stress voltage is initially applied, and the corresponding breakdown voltage and characteristics are recorded. Subsequently, a DC polarity reversal voltage is applied, and breakdown is measured once again. Significantly distinct breakdown characteristics are observed between the two tests. These findings provide valuable insights into the effect of water ingression on breakdown phenomena under different stresses, contributing to a better understanding about insulation failure and will further lead to the development of more accurate models incorporating effect of polarity reversal.
It has been reported that the electrical conductivity properties of insulating materials for cables and capacitors can be measured by a Q(t) meter. It is considered that the Q(t) data contains information of transient charge behavior in the sample. If Q(t) meter can be used to measure not only the conduction properties but also the transient charging behavior in the sample, it will be useful for product inspection and product deterioration diagnosis. For film samples, both Q(t) and space charge distribution p(x,t) can measure. In this report, Q(t) and the p(x,t) are simultaneously measured in a polypropylene (PP) film, and the relationship between Q(t) and p(x,t) is discussed.
Thermochromic dielectrics can actively report temperature anomalies by color changes, which provides a novel route toward the intelligentization of next-generation power equipment. However, thermochromism is commonly realized from the structural and phase transition of materials, the effect of which on electrical insulating performances is still unclear, restricting their applications in power equipment. Herein, the role of phase transition on breakdown characteristics of microcapsule-based thermochromic epoxy dielectrics (TEDs) is studied. Differential scanning calorimetry (DSC) results show that the core of thermochromic microcapsules will undergo phase transitions between 50°C and 68°C. The DC and AC breakdown strength of TEDs are both enhanced compared to those of neat epoxy resins in a broad temperature range of 30°C∼90°C, indicating the phase transition in microcapsules will not severely injure the dielectric strength of TEDs. With increased microcapsule content, the DC breakdown strength of TEDs gradually enhances while the AC breakdown strength first improves and then diminishes. The enhanced breakdown strength of TEDs is proposed to derive from the barrier effect at the interfacial regions between the microcapsule shells and epoxy matrix, which could significantly suppress the charge transport in TEDs. The decreased AC breakdown strength under the higher microcapsule content could be ascribed to the raised dielectric relaxation loss from the microcapsule core.
The increasing use of power electronics equipment in various operating environments has raised concerns about the space charge accumulation in insulation materials, especially in high-radiation environments. In order to ensure more robust reliability of power electronics equipment, it is important to have a better understanding of the space charge phenomena in insulation materials. In this study, we investigated the estimation of material properties related to space charge accumulation through the space charge transport simulation using machine learning. As results, the inverse analysis technique for estimating material properties related to charging through the space charge transport simulation was developed. However, some estimated values of material properties with low sensitivity to the space charge simulation had errors as large as approximately 10% compared to the true values. Therefore, it is necessary to consider improving the mathematical model of the space charge transport simulation as well as applying the inverse analysis technique to actual insulation materials in the future.
Recently, polypropylene (PP) has gained considerable attention as a promising next-generation insulation material for power cables, serving as an alternative to crosslinked polyethylene (XLPE), and efforts have been made to make the PP flexible. In this study, we report an insulative PP-based ternary blend with high toughness, in which a high content of elastomeric dispersed phases is distributed in the iPP matrix. The ternary blend is prepared through melt-mixing of the isotactic PP (iPP), polyolefin elastomer (POE), and PE-PP block copolymer (BCP). The tensile modulus of the ternary blend is largely reduced by 228.3 MPa, meeting requirement as power cable insulation material. The ternary blend exhibits a higher tensile strength of 14.1 MPa and elongation at break of 717.8% compared to those of XLPE. Also, the ternary blend shows a high melting temperature of 164.9 °C, whereas XLPE has a melting temperature of 106.9 °C. Interestingly, the ternary blend also demonstrates excellent toughness at -40°C as well as 20°C, whereas the binary blend without BCP shows poor toughness. Moreover, although the ternary blend possesses a large fraction of the amorphous phases arising from the POE, it exhibits volume resistivity and DC breakdown strength values comparable to those of XLPE.
Partial discharge measurement is important for the analysis of the equipment condition by using as a tool to prevent dielectric breakdown. In addition, waveform characteristics of partial discharge, for instance, rise time and fall, can provide some information, such as the conditions of partial discharge occurrence. In this work, the partial discharge measurement is performed to assess the surface resistivity of insulation material using the waveform characteristics. Insulation materials, which are PP (polypropylene) and PMMA (polymethyl methacrylate), are used to investigate. The experimental results clearly show that the fall time of PP is shorter than that of PMMA. The fall time may correspond with the discharge process of surface charge on insulation material, which depends on surface resistivity. From the results, it can be inferred that PP has a lower surface resistivity. The value of surface resistivity is validated by measuring the surface leakage current, and the tendency agrees with our assumption.
Proton beam writing (PBW) has been realized as a micro/nanofabrication technique that uses a focused proton beam to directly pattern on the materials. At Shibaura Institute of Technology (SIT) in Japan, we have utilized the PBW equipment with 1 MeV proton microbeam to precisely create micro-patterns with different shapes (circles, hexagons, pentagons, squares, and triangles) in poly(methyl methacrylate) (PMMA) dielectric films spin-coated on Cu electrodes. In this work, the micro-patterned PMMA films/Cu electrodes were applied as one dielectric surface of the triboelectric nanogenerator (TENG) to generate the electrical energy from the mechanical energy. The results show that the output voltages of the TENG devices were increased by modifying PMMA films with micro-patterns (except the micro-pentagons) and varied depending on the shapes.
Voltage levels used in AC motor drives is continuously increasing and this paper compares the electrical stress appearing in two different motor winding insulation specimens. The aim is to explore the changes in Partial discharge characteristics to evaluate grade 2 motor insulation for different temperature levels and rise times of the applied voltage as well as influence of overshoots. Here it is discussed how much deviation in properties than can be expected for different insulation thicknesses. The paper shows for example that elevated temperature influences the time lag considerably as well as the inception voltage level, which both influence the PD exposure of the insulation system.
Inhomogeneous temperature distribution within the bulk of the cable insulation can lead to unexpected conductivity variations at different radial positions of the insulation. Consequently, when estimating the electric field, considering the cable insulation as a homogeneous bulk may not provide an accurate model. In this paper cable insulation is peeled out from 220kV XLPE cable at equidistant radial position from conductor. Volumetric current at 50kv/mm at 40 °C for 1 hour is measured for all layers and conductivity equation is estimated. This conductivity equation is used for estimation of electric field inside the bulk of cable insulation using FEM based COMSOL Multiphysics software. Estimation of electric field based on spatial conductivity has been done and compared with electric field estimated considering conductivity as bulk property of cable insulation.
The failure that occurs in the underground cable systems leads to losses in many aspects, not only in the power system but also in the commercial and business. The most failure location in medium voltage underground cable systems is cable joint and cable termination, where the defects occur from poor workmanship. This paper represents the mean to investigate the dielectric properties of the 22 kV XLPE underground cable system to assess the insulation's integrity of the underground cable system. In the experiments, the underground cable system specimens were prepared by simulating the defects in cable joints, i.e., the iron powder on the XLPE surface and the incision on the XLPE surface; moreover, the underground cable system with the healthy condition joint was prepared to represent the new (or unused) underground cable system. To compare the dielectric properties in the frequency domain of the 22 kV underground cable systems with defective joints and the healthy condition joint, the frequency domain spectroscopy (FDS), the off-line non-destructive test, was performed in this research. From the test results, the dielectric dissipation factor shows a significantly different value in the case of iron powder when compared with the healthy condition joint. It reveals the loss peak on the curve at a frequency of 1 Hz. However, the case of incision on the XLPE surface is almost similar to the healthy condition case. The real part of the complex capacitance of all three cases has a constant value in the measured frequency ranges using the log-log scale. However, the imaginary part capacitance shows the distinct value of all three cases, of which the trends are the same as the dielectric dissipation factor curve.
Cross linked polyethylene (XLPE) has the excellent insulation properties, but which is easy to accumulate space charge under the electric field owing to its low thermal conductivity. Boron nitride nanosheets (BNNSs) with high thermal conductivity were introduced into XLPE matrix to improve the thermal conductivity and electrical properties of the composite. XLPE/BNNSs nanocomposites were prepared by the melt blending method. The basic characteristics of nanoparticles and composite samples were characterized by FTIR and SEM. Space charge and DC breakdown characteristics of XLPE/BNNSs nanocomposites were studied. The experimental results indicated that the introduction of BNNSs suppressed the accumulation of space charge and enhanced the DC breakdown strength of dielectric. The space charge accumulation in the XLPE/BNNSs nanocomposites was significantly reduced and the charge amounts in the material were 0.67×10−5C and 0.71x10−5C with doping concentration of 0.1 wt% and 0.5wt%. With the increase of doping concentration, the inhibition effect of space charge in the composite weakened. This is mainly due to the formation of deep traps at the interface between the electrode and the composite material, forming a “charge barrier effect” that suppressed charge injection. With the concentration increases, the “charge barrier effect” becomes weakened and charge amounts in the material increased. In addition, the breakdown strength of the XLPE/BNNSs nanocomposites first increases and then decreases with the increase of BNNSs concentration. When BNNSs concentration was 0.5wt%, the DC breakdown strength of XLPE/BNNSs composite reaches the 403.8 kV/mm, which is about 31% higher than that of pure XLPE. This is mainly because the interaction zone between XLPE matrix and nano filler captures a large number of charge carriers and inhibits the carrier transport.
Epoxy resin, used as the insulation material of solid state transformer, has to endure the harsh operating conditions of bipolar square wave voltage with high frequency. It is necessary to study the behavior of electrical tree induced by divergent field under bipolar square wave field. In this work, the morphology, growing properties, fractal dimension and breakdown characteristics of electrical tree in epoxy resin under bipolar square wave field with varied frequency was investigated. The voltage frequencies changed from 4 kHz to 20 kHz at 10 kV and 50% duty cycle. The results showed that electrical trees were all branch-like in the experiment. The electrical tree grew rapidly with treeing time after a period with slow propagation and accelerated by higher frequency. Furthermore, the growing speed was increased while number of branches decreased when frequency increased. There existed a maximum fractal dimension when frequency increased from 4 kHz to 20 kHz, which is ascribed to the synergistic effect of polarity reversal and decay of space charge produced by high voltage needle tip and partial discharge. It is suggested that carbon substance was deposited on the channel wall of electrical tree, which was revealed by Raman spectroscopy. The field-driven tree growth (FDTG) model was utilized to explain the development of darker and wider channel originating from ground to needle tip.
In this study, the characteristics of the space charge at the interface inside the cable joints known as the weak part of the HVDC cable were experimentally identified. For the experiment, a double-layer sample was made of two types of polymer and after applying a DC voltage, the space charge at the interface was measured and analyzed. The polarity of the space charge formed at the interface was consistent with the Maxwell-Wagner model. The time dependence of space charge accumulation and disappearance will be further studied through additional experiments and analysis in the future.
