This article reports the evaluation system of the insulating properties based on the pulsed electro-acoustic (PEA) method and the current integration charge Q(t) method. According to the basic principles of the methods, the relationship between the charge ratio and dielectric relaxation time in Q(t) data is first derived, by which a new method is proposed to calculate the material conductivity. By further analyzing the charge results of different materials, it is pointed out that the PEA method focuses on the related characteristics of the net charge distribution captured by deep traps, while the Q(t) method reflects the changing process of the total charge amount and related characteristics. No single method can completely analyze all the charge characteristics of the material, and thus the necessity of the combination of the two methods is determined. Meanwhile, based on the theoretical derivation and experimental results, it is indicated that when the charge ratio of Q(t) data exceeds 2, the electrical state inside the sample basically stabilizes. At this stage, the distorted electric field can be analyzed by the PEA method, and the Q(t) method can calculate the leakage current, dielectric relaxation time, and conductivity. Therefore, the combination of the two methods can provide a more reasonable assessment for the insulation property and state.

Different types of lightning protection systems (LPSs) have been developed for wind turbines to protect the blades from lightning strikes. However, severe damages caused by lightning strikes still happen frequently, which creates huge costs. Experiments using a 5-m blade specimen with tip receptors from 1.5-MW wind turbine blades under 3-m air gap were conducted to investigate the lightning attachment manner to the wind turbine blade with LPS in different situations. Factors including the polarity of the lightning strikes, the orientation of the blade, and the lateral distances (LDs) between the wind turbine blade and the lightning downward leader were taken into account. It was found that the types of discharge paths under positive and negative lightning strikes are quite different, and the positive discharges are much more dangerous to wind turbine blade than the negative ones. The LD between the downward leader and the wind turbine blade is a key factor that influences the interception efficiency. Three types of receptor interception failures were discovered. Multiple upward leaders may incept from the blade body as to intercept the downward leader. However, the protection range of the tip receptor is quite limited, and the connection of the tip receptor and the blade body is the most vulnerable position hit by the lightning strikes. The results present the useful reference to the optimal design of the wind turbine blade LPS.

The solid-state transformer (SST) was regarded as the ideal power router in the energy Internet. However, the partial discharge (PD) in tightly packed SST is one of the main causes of interturn multilayer insulation premature failure. The effect of high-frequency (HF) electrical-thermal coupling stress on SST multilayer insulation's PD characteristics and failure mechanism is not clarified, which is investigated in this article. An HF PD measurement system is established, and a comparative study of the PD characteristics of multilayer polyimide (PI) insulation at different test frequencies and temperatures is explored. The experimental results show that when the frequency exceeds 10 kHz, the PD exhibits a rabbit-ear-like discharge pattern. However, the higher temperature will lead to lower PD inception voltage with large PD magnitudes, which may accelerate the degradation of interturn insulation. According to the phase-resolved PD, PD magnitude, PD number, and discharge phenomenon of the PI layer, the failure process of the multilayer PI system is divided into three stages: electric tree development, electric tree breakdown, and point prebreakdown. In addition, we propose the PD development model of multilayer PI insulation. The research results can provide a theoretical reference for elucidating the PD mechanism under HF stresses.

High temperature tends to be one principal failure cause of silicone gel package insulation in high-power electronic devices, even leading to equipment damage under severe conditions. To clarify the aging characteristics and failure mechanism of silicone gel package insulation of high-power IGBT devices under the high-temperature scenario, this paper carries out experiments on silicone gel material samples by accelerated thermal aging procedures at 200 ℃. The experimental results show that temperature has a significant effect on the physicochemical properties of silicone gel. Further, molecular dynamics simulations are conducted to verify the high-temperature degradation characteristics of the gel. Finding the main cause of silicone gel package insulation failure under high-temperature is the random degradation of the silicone backbone into small molecule cyclosiloxanes. Finally, the Lifetime assessment model was established based on the pyrolysis kinetics and insulation failure mechanism. The unaged silicone gel's pyrolysis activation energy and insulation failure temperature were measured at 125.8 kJ/mol and 453°C, respectively. The evaluation results show that the package insulation life decreases by about 60% for every 10°C increase in temperature. The above achievement of this study presents a theoretical and methodological basis for reliable assessment of the insulation status of silicone gel package insulation for high-power electronic devices under high-temperature.

Accurate measurement of the space charge inside insulating materials facilitates reliable assessment of insulation performance. In this paper, an improved calibration scheme based on the pulsed electro-acoustic (PEA) methodology is proposed for space charge measurement. First, according to the transmission-line model of the measurement system, the simulation results indicate that the acoustic impedances of the upper electrode and the sample may affect the recorded waveforms. Then, taking into account the actual situation that the acoustic impedances of the semiconducting electrode and the sample are different, an improved calibration method is presented by analyzing the propagation process of the acoustic waves. Comparison of the results further indicates that the material of the semiconducting electrodes may affect and lower the accuracy of the traditional PEA method. In contrast, the recovered results by the proposed method are more accurate and reliable. The improved method is thereby applied to calibrate the signals from polyimide (PI) samples. Combining with the simulation results, it is concluded that different semiconducting electrodes render sensible impact on the accumulation rate and process of the space charge. Furthermore, two groups of calibrated results under the same electric field are used to verify the effectiveness of the proposed method. Since the electrode material may be an influential factor in the measurement, it should be properly chosen according to the practical applied conditions of the insulation samples to achieve a better evaluation of the charge accumulation process.

IGBT (insulated gate bipolar transistor) module is prone to the along-surface discharge at the silicone gel-metal electrode-AlN ceramic substrate, that is, the “triple bonding point”, resulting in insulation breakdown, so it is important to develop high-performance silicone gel. In this paper, the composite materials with different contents of 4,4’ -dichlorobenzophenone (DCBP) are prepared and their trap distribution and along-surface discharge characteristics are investigated. The results show that the introduction of DCBP reduces the density of deep trap, increases the density of shallow trap and the degree of crosslinking, is more conducive to the detrapping and migration of charge, and accelerates the attenuation of the surface potential of the composite. Under the condition of sinusoidal voltage of 7 kV and frequency of 10 kHz, the composite material can effectively inhibit the development of the along-surface discharge compared with PSG, and the total discharge times are reduced by 65.08%, 48.71% and 18.27%, respectively. The total discharge range decreases by 69.15%, 62.87% and 22.88%, and the average discharge range decreases by 25.61%, 27.63% and 5.64%, respectively. The release of ultraviolet energy during along-surface discharge excites DCBP to a triplet state, promotes the improvement of crosslinking degree and the generation of new re-crosslinking products, prevents the growth of local low-density insulation region, and further inhibits the occurrence of the along-surface discharge, which enhances the insulation performance of the composite.