Abstract：With the increasing voltage level of high-voltage IGBT modules, the package insulation of modules requires higher reliability. Therefore, the silicone gel with excellent insulation performance and low elastic modulus is widely used in the packaging of high-voltage IGBT modules. However, the bubble defect in silicone gel is the weak insulation link of package insulation. In this paper, the effect of bubble defect on the discharge characteristics of silicone gel under positive square wave voltage is investigated. Firstly, the needle plate electrode structure is selected to form a local high field area to prevent the direct breakdown of the sample. The silicone gel samples containing bubbles in the specified position are prepared by layered potting. By analyzing the area of bright white spots in the high-definition images of the samples taken by the microscope, the amount of bubbles in the samples is estimated, and the content of bubbles in the prepared samples is determined to be 0%, 32%, 55%, and 74%, respectively. An experimental platform for partial discharge characteristics of silicone gel based on the partial discharge current pulse measurement method is established for the positive repeated square wave voltage consistent with the module operation condition. Secondly, partial discharge experiments with different bubble contents are carried out under positive repeated square wave voltage. The statistical characteristics of PDIV and PDEV of each sample are analyzed by Weibull distribution. It is found that with the increase of bubble content, the partial discharge inception voltage(PDIV) and partial discharge extinction voltage (PDEV) of silicone gel samples are significantly decreased, especially the PDIV and PDEV of 74% bubble content samples are decreased by 42% and 46%, respectively. Thirdly, a method to extract the discharge current pulse from the displacement current is proposed, and the obtained discharge current pulse has no distortion and no high-frequency noise. By analyzing the discharge pulses of samples with different bubble content for at least 50 cycles, it is found that with the increase of bubble content in silicone gel from 0% to 74%, the charge amount of a single local discharge in the sample shows an overall increasing trend, the proportion of forward discharge decreases from 55% to 37%, the rising and falling rate of the pulse increase by 3~5 times, and the rise time and fall time of the pulse decrease about 50%. Finally, the discharge process with air bubbles in the silicone gel and the mechanical explanation of the effect of air bubbles on the discharge characteristics are analyzed. The simulation results show that the field strength in the bubble is larger than that in the silicone gel under the square wave voltage, while the breakdown field strength of the air is smaller. Therefore, the bubble is more prone to discharge than the silicone gel. In addition, part of the gas molecules will diffuse to the high field strength region through the silicone oil, which will also reduce the overall insulation strength of the silicone gel. In the process of partial discharge development, the electric tree inside the silicone gel will pass through the bubble, and the air components will enter the electric tree airway to participate in the discharge process, resulting in the improvement of the carrier mobility inside the electric tree airway, and then lead to the change of the partial discharge pulse waveform. The results of this paper can guide the improvement of silicone gel encapsulation technology and lay a foundation for the detection of bubble amounts in silicone gel.
李学宝, 刘相辰, 刘思佳, 赵志斌, 崔翔. 高压器件封装用有机硅凝胶内气泡对其正极性方波电压下放电特性的影响[J]. 电工技术学报, 2023, 38(15): 3999-4009.
Li Xuebao, Liu Xiangchen, Liu Sijia, Zhao Zhibin, Cui Xiang. Influence of Bubbles on the Discharge Characteristics of Silicone Gel for High Voltage Module Encapsulation under Positive Square Wave Voltage. Transactions of China Electrotechnical Society, 2023, 38(15): 3999-4009.
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