In the previous part, simulation works were conducted to analyze the field grading effect of nonlinear materials in the application of bushing, and the aim of this paper is to present the experimental investigations. The adaptive field grading (AFG) bushing samples are prepared by a novel method based on wet winding technology, and the assembly quality is verified by scanning electron microscope, thermal gravimetric analyzer, and conductivity test of slice samples with multi-layered structure. The initial partial discharge voltage and phase-resolved partial discharge pattern characteristics of the AFG bushing samples are obtained. Taking S0 without a field grading layer as the control group, it exhibited noticeable internal partial discharge at 36 kV. At the same time, S2 and S3 samples with nonlinear materials had higher initial partial discharge voltages, which are 66.67% and 38.89% higher than those of the former samples, respectively. The insulation strengths under the lightning impulse voltage of the AFG bushing samples are also tested. The S0 sample was broken down under severe high-voltage impulses, while the S2 sample remained well-insulated after 15 positive and negative lightning impulses. It is hoped that this work will provide an innovative technical roadmap for bushing design and manufacturing.

Field grading materials hold significant potential for power equipment application owing to their ability to suppress electric field distortion. However, achieving synergistic regulation of their multi-physical properties remains a challenge beyond merely modulating nonlinear conductivity. Inspired by the "vein-and-flesh" structure of natural leaves, ZnO microvaristor/epoxy composites reinforced with a polyester fiber network via a wetwinding process. We fabricated ZnO microvaristor/epoxy/polyester fiber composites via a wet-winding process. In this biomimetic design, the polyester fiber network acts as a skeleton that regulates filler distribution. By modulating the network density, we achieved the coordinated regulation of the electrical, thermal and mechanical properties of composites. Results show that composites become electrically nonlinear at filler contents above 35 vol%. At a fixed filler concentration, the threshold electric field and nonlinear coefficient can be precisely tuned by the fiber network density. A moderate density (70 g/m2) yielded the optimal thermal conductivity (0.648 W & sdot;m-1 & sdot;K-1 at 35 vol% loading) and mechanical performance, enhancing flexural strength by 41.8% at the same loading. This work provides a novel strategy for designing high-performance field grading materials through fiber network density regulation.

Bushing has become a critical component endangering the safe functioning of power transmission and limiting its development to higher voltage and power levels. The main technical bottleneck that restricts high voltage bushing is the uneven distribution of electric fields due to its “plug-in” structure. In this paper, a novel adaptive field grading (AFG) bushing with a simple multi-layered structure based on nonlinear materials is introduced. A frequency-domain steady-state simulation method considering the field-dependent dielectric and loss characteristics of nonlinear materials to design the AFG bushing is proposed. Referring to the size of the AC transformer capacitive bushing, the finite element models of the 40.5 kV AFG bushing are established, and the electric field distributions are obtained. Optical electric field sensors are used to test the surface electric field distribution of the bushing samples, validating the accuracy of the simulation method. Combining simulation and experimental test results, the influence of doping concentrations and lengths of the field grading layer on the electric field distribution is studied, and the grading mechanism of nonlinear material is summarized. The results demonstrate the potential of nonlinear materials to relieve electric field concentration in bushings and provide a solid basis for its design and application.

With the rapid expansion of emerging applications requiring high efficiency, high power density, and fast switching speeds, (ultra) wide bandgap ((U)WBG) power electronic modules are increasingly replacing conventional silicon-based counterparts due to their superior electrical, thermal, and switching characteristics. As a widely used encapsulation material, the dielectric properties of silicone gel (SG) determine the operational reliability of power modules. Therefore, the breakdown behavior of SG under practical high–slew-rate square-wave stresses warrants investigation. In this paper, the breakdown tests under bipolar square waves with various rise times at a fixed 50 kHz frequency were conducted on SG samples. Results show that the breakdown voltage of SG is positively correlated with the rise time: As the rise time decreases from 250 ns to 50 ns, the mean breakdown voltage drops from 4.60 kV to 2.79 kV, with a 39.35% reduction. A two-parameter Weibull analysis indicates steep slopes (β≫1) across conditions, implying relatively clustered breakdown voltages; shorter rise times tend to preserve high determinism while lowering characteristic strength. The breakdown behavior is interpreted by space charge dynamics. This study highlights the critical importance of incorporating waveform parameters into the insulation design and evaluation process for (U)WBG power modules.

As the adoption of (ultra) wide bandgap ((U)WBG) power modules accelerate, ensuring reliable dielectric performance in soft encapsulation materials such as silicone gel (SG) has become a pressing challenge. Existing studies on SG have primarily focused on partial discharge (PD) and electrical treeing under DC or low-frequency AC excitation, with limited attention to dielectric breakdown—despite its critical role in defining insulation failure thresholds. Moreover, few efforts have explored the effects of high-frequency waveforms that closely mirror the fast-switching transients typical of practical (U)WBG applications. This study investigates the dielectric breakdown characteristics of SG under three voltage stress conditions: DC, 60 Hz sinusoidal AC, and high-frequency bipolar square waveforms (10–50 kHz) with a fixed 50 ns rise time. Experimental results reveal a significant, frequency-dependent reduction in breakdown voltage under square wave stress, underscoring the vulnerability of SG to rapid voltage reversals. A two-parameter Weibull statistical analysis further quantifies this degradation, showing increasingly clustered failure distributions at higher frequencies. The breakdown behavior is interpreted through the interplay of space charge dynamics and the time-dependent self-healing response of SG. These findings highlight the importance of accounting for waveform characteristics—not just field strength—when evaluating insulation reliability in UWBG power modules and provide a data-driven basis for future encapsulation design strategies.

Partial discharge issues at the triple junctions of the copper-ceramic-silicone gel interfaces in power electronic modules have emerged as a critical barrier to further technological advancement. Therefore, accurately calculating the electric field intensity and optimizing the insulation system are essential to ensure module reliability and performance. In this study, 2D and 3D geometries of a commercial power module were modeled in COMSOL Multiphysics to analyze the electric field distribution around triple points. The results showed a maximum electric field intensity of 22.5 kV/mm in the 3D model, compared to 15.0 kV/mm in the 2D simulation. This finding emphasizes the critical importance of employing 3D modeling to accurately represent the intricate electric field distribution at triple-edge regions. This work lays the foundation for accurate electric field calculations within power electronic modules, which is essential for determining the extent of electric field mitigation required.

In power modules, partial discharge (PD) at the triple points of "Copper-Ceramic-Silicone Gel" poses a significant challenge to the development of higher voltage and power density. This article utilized a reliable and controllable method, jet dispensing, to assemble the electric field adaptively controlled structure in power electronic modules to optimize the electric field distribution. The structure was composed of ZnO/epoxy resin nonlinear conductivity composites, with the thickness controlled at 300 +/- 50 mu m. The assembled electric field adaptively controlled structure significantly enhanced the PD inception voltage (PDIV) of the module from 6.0 to 13.2 kV, with a 120.0% increase under the sinusoidal waves, and from 4.0 to 9.6 kV under positive polarity square-wave pulses, with an increased ratio of 140.0%. This article provides a new perspective on the application of nonlinear materials in power electronic modules.

Abstract Recently, ZnO‐based composites have been widely applied in the field of electric power. To meet the diverse application requirements, it is necessary to figure out the I–V characteristics of ZnO composites whose high‐voltage and ground‐voltage electrodes are arranged on the opposite sides with a certain horizontal distance. 30 vol%, 40 vol% and 50 vol% ZnO‐based silicone rubber composites were prepared. The horizontal distance between their electrodes was set as 50, 100, 500 μm, 1 and 2 mm, respectively. Results showed that with the increase of ZnO fillers volume fraction under a fixed horizontal distance of 100 μm, from 30 vol% to 50 vol%, the I–V curves shifted left, the leakage current increased and the switching voltage decreased. When the horizontal distance between electrodes increased from 50 μm to 1 mm under a fixed doping concentration of 40%, the I–V curves shifted to the right, the leakage current dropped and the switching voltage rose. The mathematical and physical models were established to explain the results. This work provides a referential significance for the practical application of ZnO composites, such as 5G folding mobile phones and power electronic modules.

As the electrification of society towards higher power density and efficiency, new generation power and electronic apparatus place urgent demands on insulation strength and electrical stress management. Nonlinear materials have drawn great attention from academia and industry for their field-depended conductivity, which endows them with the potential virtue of electric overstress mitigation. In this paper, a system of the needle-plate electrode, simulating the condition of a conductive tip in the solid composites system, was used as the experimental framework. The conductivity behavior of ZnO/SiR composites with different filler concentrations in inhomogeneous electric field were measured. The percolation phenomenon was discussed based on the micromorphology obtained by scanning electron microscope. To verify the electric stress management capabilities of nonlinear materials, a discharge detection experiment was set up, and a corresponding finite element model was created to analyze. The results show that nonlinear materials can well grade the electric field, which increased the initial discharge voltage of the overall system by more than 58% and reduced the highest electric field by 81% compared to the conventional method.

Recent years have witnessed the widespread application of nonlinear composites in the power electric field. However, little light was thrown into the situation that the high-voltage and ground-voltage electrodes were arranged on opposite sides with a horizontal width. In this paper, 40 vol% ZnO/silicone rubber nonlinear composites with different thicknesses of 200 mu m, 300 mu m, and 400 mu m were prepared and tested the conductivities, whose horizontal width of electrodes were all set as 50 mu m. When the sample thickness increased from 200 mu m to 400 mu m, the DC conductivity curve shifted to the right, I-Leakage dropped from 2.69x10(-8) A to 1.43x10(-9) A, and U-1 mu A rose from 459 V to 962 V. The physical structures of the composites were built to explain the results. This work will hopefully promote the practical application of ZnO-based nonlinear composites further.

SiC IGBT has the advantages of high withstand voltage, high operating temperature, fast switching speed and low loss. However, these characteristics lead to its encapsulated insulation system withstanding more severe electrothermal stress, causing partial discharges. ZnO-based adaptive composites are expected to play a role in power electronics packaging due to their excellent nonlinear conductivity. In this paper, 40 vol% ZnO/Silicone Rubber adaptive composites were prepared with the horizontal distance between electrodes of 50 um, 100 um, 500 um, 1 mm and 2 mm. Keithley 2410 digital source meter was adopted to measure their I-U curves. Results show that only ZnO composites with the horizontal distance between electrodes of 2 mm can’t turn to the conducting state with increasing applied voltage. When the horizontal distance increases from 50 um to 1 mm, the I-U curves shift to the right, ILeakage decreases from 3.06×10-8 A to 8.03×10-9 A, and U10uA increases from 908 V to beyond 1100 V. The results were compared and analyzed by mathematical and physical models. This work provides a pregnant reference for the practical application of ZnO-based composites in power electronic packing.

DC bushing is one of the technical challenges that restrict the development of DC power transmission and transformation technology. With the electrification of society towards higher power density, bushings are exposed to extreme electrical and thermal stress superpositions during operation. The adaptive bushing, with a field grading layer, provides additional benefits compared to the conventional bushing. In this paper, an electro thermal coupling field simulation model of adaptive bushing was established. The determination of the parameters of the nonlinear materials in the field grading layer was discussed. Under the electrothermal coupling filed, the electric field distribution and temperature distribution were simulated. With the increase of load current, the temperature gradient intensifies, and the maximum electric field distributed in the main insulation part reverses from inner to outer. Benefit from the field-depended conductivity, the nonlinear materials can introduce negative feedback in voltage distribution, so that the electric field shows good stability even under the extreme temperature gradient. This work will promote the application of nonlinear composites in ultra-high voltage DC bushings.

The electrical field grading materials are expected to well response to arbitrary electrical excitation. In this paper, a modified Voronoi network model is established to simulate the response of electrical field grading materials under arbitrary excitation. The grain-boundary structure of ZnO microvaristors is described by the Voronoi polygons with a distorted hexagonal structure and the nonlinear conductivity is modeled based on the carrier transport mechanism. The network calculations replicate the resistive contacts between fillers and the micro-region breakdown phenomenon of insulating materials during the formation of the conduction path. The accuracy of this model is evaluated using AC and surge measurements for the ZnO/SiR samples. The model can manage the typical nonlinear time-domain responses and well reflect the effect of filler concentrations as well as the relationship between capacitive and resistive components of the responses. This work is helpful to design field grading materials to self-adapt high electrical fields of electronic and electrical devices and increase their stability and durability.

AbstractWith the increase in applied voltage and the decrease in volume, integrated electronic modules have become electrostatic sensitive. ZnO microvaristors doped silicone rubber (SiR) composite, with electric field‐dependent conductivity, shows promising prospects for solving the electrostatic problem. In this study, 20 vol% to 50 vol% ZnO/SiR composite films, filled with 10–30 μm ZnO microvaristors, were prepared and put into the test of electrostatic discharge. Compared with the insulating and conducting materials, ZnO/SiR composites achieve the lowest initial voltage and a short time constant of electrostatic discharge. With the increase of ZnO microvaristors volume fraction, from 20% to 50%, the initial voltage and the time constant of electrostatic discharge drop from 1.78 to 0.72 kV and 3.1 ms to 0.4 μs. When the ZnO volume fraction is higher than 30%, there is no residual voltage after 0.1 s. To explore the reason behind different electrostatic discharge performances of the ZnO/SiR composites, the conductivities of the composites were measured. It is found that the break‐over voltage of the composite drops with the increase in the ZnO microvaristors volume fraction. When a high voltage impulse is applied on the ZnO/SiR film, the composite will turn to the conducting state and release the electrostatic charge adaptively so that the initial voltage can be controlled. This work supplies a novel electrostatic discharge idea for electric and electronic devices.

ZnO-microvaristor based composites show promising prospect on solving the partial discharge problem in power electronic modules packing. In this paper, 2D simulation model of silicon-based welded 1200 V IGBT SKM50GB12T4 from Infineon was built in COMSOL. Simulation results showed local electric field intensity concentration occurred at the junction of copper plate, ceramic layer and silicone gel. It is very unfavorable in practical operation. The influence of three factors, the shape of the grading field structure, the electrical parameters of adaptive composites and the size of the grading field structure, on the electric field distribution was discussed. It is concluded that the rectangular structure showed a better field grading effect than the triangular structure. ZnO-based adaptive composites all displayed a better field grading effect than semi-conductive materials. 35 vol% ZnO composites showed the best electric field grading effect. The wider the width of the structure was, the better the electric field grading effect was.

The polymer materials doped with ZnO microvaristors show similar nonlinear conductivity characteristics to the ZnO varistors, which is promising to be used in the insulation of power apparatus as function materials. It is possible that the power equipment would suffer the transient process during the operation, and the behavior of ZnO/SiR composites under such conditions is worth investigating. This study prepared ZnO/SiR samples with different filler volumes for the impulse voltage test. SEM and digital source meter were used to characterize the microscopic surface morphology and DC response of the composites, respectively. With the increase of applied impulse voltage, the transition from capacitive to nonlinear resistive response of is obtained. While the response is just capacitive at low voltage, there is obviously a nonlinear resistive component seen in the time development of $I$ (t) at higher voltages.

ZnO microvaristors/epoxy resin composite has drawn great attention from academia and industry for its adjustable non-linear conductivity, high mechanical strength, and good ageing resistance. However, the sedimentation of ZnO microvaristors in epoxy resin during preparation is the key problem, which limits its application in engineering. In this study, a novel method of wet winding with polyester fibre cloth is proposed to prepare the ZnO microvaristors/epoxy resin composite. The anti-settling effect of ZnO microvaristors in the composite is verified by scanning electron microscopy (SEM) and thermal gravimetric analysis (TGA). The microstructure shows that ZnO microvaristors distribute uniformly in the composite, and the content difference of ZnO microvaristors at the top and bottom part is only 0.4%. The composite shows typical non-linear conductivity, and the threshold electric field and the non-linear coefficient decrease with the content of ZnO microvaristors, while the conductivity in the insulating state shows an increasing trend. To verify the field grading effect of the composite with non-linear conductivity (CNC), a finite element model of a needle-plate electrode, simulating the condition of a conductive tip in a solid insulated system, is set up. CNC can adaptively grade the electric field, which reduces the surface electric field of the needle tip by 86.6% and the highest electric field in the system by 82.1%. This wet winding method solidifies the industrial application of CNC in high-voltage power equipment.

In this study, the degradation behavior of silicone rubber (SiR) composites with ZnO microvaristors under impulse voltage was investigated. Typical impulse voltages with different amplitudes were applied to the samples, and the degraded threshold electric field ( E b ) and nonlinear coefficient ( α ) of the samples were obtained by conductivity test after every impulse was applied. Scanning electron microscope and energy dispersive spectroscopy were used to characterize the micro performance of the samples. The failure of the SiR composites was found to be attributed to the aging behavior in the microstructure of ZnO microvaristors, which was divided into four stages caused by the precipitation of Bi 2 O 3 and the dissipation of the honeycomb grain-boundary structure. The mechanism of degradation was internal local heat accumulation and damage of the microelement caused by the thermal stress, generated by impulse voltage. This work is helpful to develop smart composite materials with self-adaptive capability to control high electrical field and increase the stability and durability of power apparatus and electronic devices.

With the increase of voltage grade of electric power system, the insulation problem of high-voltage cables has become more prominent. In recent years, self-adaptive materials have been applied in cable terminals by virtue of their nonlinear conductivity. In this paper, the 35kV AC cable terminal accessory model with a grading field tube was established by CMOSOL. The conductivities of self-adaptive materials at different temperatures and electric field intensities were measured and fitted into the formula. Under different working conditions, the electric field distribution of the 35kV AC cable terminal accessory with a grading field tube was simulated by applying semi-conductive materials and self-adaptive materials respectively. Results verify that self-adaptive materials have stronger electric field grading effect than semi-conductor materials. Based on the principle of not increasing $E_{\max}$ of the major insulation, self-adaptive materials with a switching field of 650V/mm are optimal for the 35kV AC cable terminal accessory with a grading field tube.

As a functional material, ZnO microvaristors/silicone rubber composite, with nonlinear conductivity, will be manufactured to different thicknesses for different applications. However, few study focuses on the thickness effect of the composite. In this paper, ZnO/SiR composites with thickness from 50 mu m to 2800 mu m were prepared. The conductivity results show that the threshold electric field (Eb) of the composites increases from 40 V/mm to a stable value (more than 1000 V/mm) as the thickness increases. A simulation model based on the Voronoi network was carried out to explore the mechanism. For the thickness close to the diameter of fillers, the key element determining Eb is the microstructural nonuniformity of electrical characteristics of microvaristors. As the sample becomes thick, the increasingly tortuous conduction path is considered to trigger off the increase of Eb. The thickness effect discussed in this paper will be helpful in designing electrostatic discharge elements in electronic devices.